Power Electronics

Power Semiconductor Devices

The electronics power conversion circuit that convert and control electrical power, use Power Semiconductor devices. These devices operate in the switching mode, which causes the losses to be reduced and therefore the conversion efficiency is to be improved. However the disadvantages of switching mode operation are the generation of harmonics and the fact that the converter system tends to be more complex.
Power Semiconductor Devices are classified
i. Power Diode
ii. Power bipolar junction transistor
iii. Power MOSFET
iv. Insulated gate bipolar Junction transistor
v. Thyristors

Power Diode

A power diode is a two terminal p-n junction device and a p-n junction normally formed by allowing diffusion and epitaxial growth structure of a power diode and symbol are shown in figure below.

High power diodes are silicon-rectifiers that can operate at high junction temperatures.Power diodes have larger Power,Voltage and Current handling capabilities than ordinary signal diodes.In addition,the switching frequencies of power diodes are low as compared to signal diodes.

The voltage current characteristics of power diodes is shown in figure below

When the anode potential is positive with respect to cathode,the diode is said to be forward biased,the diode conducts and behaves essentially as a closed switch.A conducting diode has a relatively small forward voltage drop across it and the magnitude of the drop would depend on the manufacturing process and temperature.When cathode potential is positive with respect to anode,the diode is said to be reversed.It behaves essentially as an open circuit.Under reverse biased condition,a small reverse current is known as leakage current in the range of μA or mA flows and leakage current increases slowly in magnitude with the reverse voltage until the avalanche voltage is reached .The forward voltage drop when it conducts current ,is in the range of 0.8 to 1V.Diodes with ratings as high as 4000V and 2000A are available.

Following the end of forward conduction in diode,a reverse current flows for a short time.The device doesn’t attains its full blocking capability until the reverse current ceases.The reverse current flows in the interval called rectifier recovery time.During this time,charge carriers stored in the diode at the end of forward conduction are removed.The recovery time is in range of a few μs(1-5)μs in a conventional diode to several hundred nanoseconds in fast recovery diodes.This recovery time is of great significance in high frequency operation.The recovery characteristics of conventional and fast recovery diodes are shown in figure below.

The application of these devices includes electric traction,battery charging,electro plating,electro metal processing,power supplies,welding ups etc

Power Transistor

Power transistors are finding increasing popularity in low to medium power applications.A power transistor has low current gain and requires continuous base drive during on-state conditions but doesn’t require forced commutations.Circuitry power transistors can be used in high switching frequency,permitting size reduction of electromagnetic components and can provide current limit protection by base drive circuit.Power transistor cannot withstand reverse voltage and application is limited to dc voltage for inverters and choppers.

A transistor is a three layer pnp or npn semiconductor device having two junctions.This type of transistors is know as bipolar junction transistor(BJT).The structure and symbol of npn transistor is shown below.

The three terminals of the device are called the collector (c), the base (B) and the emitter (E). The collector ans Emitter terminals are connected to the main power circuit, the base terminal is connected to control signal.

Transistors can be operated in the switching mode. If base current I_B is zero transistor is in an off state and behaves as a switch.On the other hand,if the base is driven hard,ie if the base current I_B is sufficient to drive the transistor into saturation,then the transistor behaves as a closed switch.

The transistor is a current driven device.The base current determines whether it is in on state or off state.To keep the device in on state,there should be a sufficient base current.
Transistor with high voltage and current ratings are known as power transistors.The current gain of power transistors(I_C/I_B)can be as low as 10.For eg. base current of 10A is required for 100A of collector current.

Power transistors switch on and off much faster than thyristors.They may switch on in less than 1μs and turnoff in less than 2μs.Therefore power transistors can be used in applications where frequency is as high as 100kHz.These devices are very delicate.They fail under certain high voltage and high current conditions.They should be operated within its specified limits,known as safe operating area(50A).

The output characteristics of CE configuration is as shown below

The characteristics depict the relation between collector current(I_C) and collector to emitter voltage(V_CE) for different values of base current.These characteristics is of npn transistor.These characteristics have three regions ie active region,saturation and cut off region.

The input characteristics depicts the relationship between base current I_B and emitter to base voltage for different values of collector to emitter voltage V_CE.

The input characteristics is shown in the figure below

The V-I characteristics(output characteristics of power npn transistor is shown in figure below

As in the case of lower power BJT,this characteristics depict the relationship between collector current(I_C) and collector to emitter voltage(V_CE) for different values of base current I_B.

It is seen that these characteristics have some special features very different from those for lower power BJT.These features are as follows.

1. For substantial values of collector current,there is maximum value of collector emitter voltage which the device can sustain,it is denoted as BVsus in above figure.If I_B= 0 the maximum voltage which can be sustained by the device increases to BV_CEO.(The voltage (V_CE)when the base is open circuited).The voltage BV_CBOis the breakdown voltage when the emitter is open circuited.

2. The primary breakdown is due to the avalanche breakdown of C-B junction.In this region the current and the power dissipation can be very high.Therefore this region should be avoided.

3. In the region marked second breakdown,The C-E voltage decreases substantially and the collector current is high.This region is due to thermal runaway.A cumulative process occurs in this region and the device gets destroyed.In this breakdown,power dissipation is not uniformly spread over the entire volume of the transistor but is rather restricted to highly localized areas.Therefore the chances of the device getting destroyed are high.

4. A quasi saturation(between saturation and active region)region exists.This region is due to the lightly doped drift collector region.

Power MOSFET(Power Metal Oxide Semiconductor Field Effect Transistor)

Power MOSFET is a voltage controlled device and it requires only small input current.It has extremely high input impedance and is widely used in switching devices.The switching is high and the switching time is in the order of nanoseconds.They are used in low power high frequency converters.MOSFETs are of two types.

i) Depletion Type MOSFET ii) Enhancement type MOSFET

i) Depletion Type MOSFET

A depletion type n-channel MOSFET has a lightly doped p-type substrate into which two highly doped n-region(n+) are diffused.These two regions acts as drain and source.A thin insulating silicon dioxide layer is grown across the semiconductor surface.

The two holes cut into the oxide layer allow contact with the source and drain.A metal layer is then deposited on the oxide.This layer forms the gate.Due to the presence of silicon dioxide layer,the gate is insulated from the semiconductor.This layer results in very high input resistance.

A p-channel depletion type MOSFET consists of an n-type substrate into which two highly dopped p-regions and a p-channel are diffused.These two regions acts as drain and source.

Principle of Operation of Depletion type MOSFET

i) Negative Gate Operation

In the figure given below a negative bias is applied to the gate

The negative voltage depletes the conducting channel of majority carriers electrons and controls their flow.

The gate and channel forms a parallel plate capacitor with silicon dioxide layer as dielectric.The negative gate bias causes concentration of electrons on the gate.These electrons repel the conduction band electrons in the n-channel leaving a positive ions layer as shown.More negative gate voltage,the greater is the depletion of electrons in n-channel.

ii) Positive Gate Operation

A depletion mode MOSFET when gate bias is positive is shown in figure below.

The gate and channel can again be thought of as a capacitor positive charge on gate induce negative charge in n-channel.These negative charges add to those already present in the channel.Thus positive gate voltage increases or enhances the conductivity of the channel.More positive the gate voltage,greater the conduction from source to drain.

In both positive and negative operations,the gate current is negligible because the gate is insulated.Therefore input resistance is very high.It is necessary to connect the substrate of an n-channel MOSFET to the negative terminal of a battery.So that the p-n junction doesnot becomes forward biased.If this happens the device will cease to act as MOSFET.

In p-channel device,the substrate is connected to the negative terminal of the battery.

Symbols

The four terminal gates,source,drain and substrate are denoted by G,S,D,B respectively.Sometimes substrate is intensively connected to source so that only three terminals G,S and D are brought out.

MOSFET Characteristics

Drain characteristics of an n-channel MOSFET is shown in figure below.

V_GS can be positive and negative.The characteristics for p-channel are similar to the above figure excepts signs of current and voltage are reversed.

The transconductance curve(transfer characteristics)of depletion type MOSFET is shown in figure below.

This characteristcs shows the variation of I_D with V_GS.I_DSS denoteS the drain current with shorted gate.The curve extends on both sides ie V_GScan be negative as well as positive.Since V_GScan be positive also I_DSS is not maximum value of drain current.

This device has three regions.The ohmic region,active region and breakdown region.The rising position of the drain characteristics is the ohmic region.The device acts as resistor.The drain current is nearly constant in the active region.When V_DSexceeds the rated value,avalanche breakdown occurs.This device has two applications ie as a resistor or a current source.

Mosfet can be used to make amplifiers – an example of a 18watt Mosfet amplifier is given here.

Enhancement Type Power MOSFET

This type of power MOSFET is widely used in digital computers.

The construction of enhancement type power MOSFET is shown in figure above .This is an n-channel device.However as seen in the figure,the p-substrate extends right up to the silicon dioxide layer.Thus there is no n-channel between drain and source.

The above figure shows the connections.When the gate voltage is zero,drain battery tries to push free electrons from the source to the drain.However the p-substrate has only a few thermally produced free electrons and some electrons due to surface leakage.Therefore the drain current is almost zero.When gate is positive(V_GS>0)the gate attracts free electrons into the p-substrate region these free electrons recombine with holes near silicon dioxide layer.When V_GS is large enough all these holes near the silicon dioxide layer have recombined and electron starts flowing from source to drain.The effect is as though a n-type layer between source and drain has been created.This n-type layer is n-type inversion layer.When this layer is formed,the transistor is turned on and free electron can easily flow from source to drain.This minimum value of V_GS which can create inversion layer is known as gate threshold voltage(V_GS(th)).

Thus the transistor is off when V_GS<V_GS(th) and on when V_GS>V_GS(th).Thus the conducting capability of enhancement type mosfet depends on the action n-type inversion layer.Vgsth varies from about 1to 5 V for these devices.

An enhancement mode mosfet is used in digital computer since it acts as a switch.When gate voltage is less than threshold it is off but when gate voltage is more than threshold it turns on like a switch.

Symbol

The channel is shown as a broken line to distinguish it from depletion mode also to indicate that normally it is in offstate

Characteristics of Enhancement Type MOSFET

The drain characteristics of enhancement type MOSFET is given below.This depicts the variation of drain current(ID)with drain to source voltage(VDS)for different values of gate to source voltage(VGS).

The lower most curve is for VGS(Th).When VGS < VGS(Th)drain current is almost zero.When VGS >VGS(Th) the device is ON.As in the case of other FET,the device can operate in the ohmic,active or cutoff(break down)region.The rising part of curve (fromVDS=0 to VDS=few volts)is the ohmic region.The device behaves as a resistor,when operated in this region.The drain current is almost constant when the device operates in the active region.when VDS exceeds the rated value,avalanche breakdown occurs and the device is in the breakdown region.

Transconductance curve of enhancement MOSFET is shown below

This curve start from VGS(Th) because the device is off and drain current is zero when VGS<VGS(Th).In addition two other quantities specified are VGS(ON) and ID(ON).Drain current is given by

ID =[(VGS -VGS(TH)) / (VGS(ON)- VGS(TH))] × ID(ON)

MOSFEts have a very thin silicon dioxide layer.This layer is kept very thin to ensure that the gate has good control over the gate current.This layer could be destroyed if a voltage higher than rated value is applied to the gate.If the MOSFET has a rated VGS of -30V,we should never apply a voltage higher than +30V or lower than -30V.Moreover they should not be connected or disconnected in the circuit when the circuit is ON.

Insulated Gate Bipolar Junction Transistor(IGBT)

Bipolar junction transistor have low power losses but have long switching time.(especially at turnoff).MOSFETs have very fast switching characteristics(low turn on and turn off times)but have higher power losses.IGBT combines the advantages of MOSFET and BJT.Thus an IGBT has low switching times as well as low power losses.Its called as IGBT or GEMET or COMFET(conductivity modulated field effect transistor).

Configuration
The configuration of an IGBT is shown in the figure below

In many respects it is similar to a vertical diffused MOSFET(VDMOS).Main difference is the presence of p+ as injecting layer.Next is n+ layer(buffer layer).There is a p-n junction (j1) between these layers and two more junctions(j2 and j3).Thus this IGBT configurations has a parasitic SCR.Turn on of this SCR is undesirable and the body region of the IGBT is made to avoid turn on.The body source short also helps in minimizing the possible turn on.Some IGBTs do not have n+ layer.If n+ layer is absent ,the device is called non punch through IGBT(NPT-IGBT).If this n+ layer is present its punch through IGBT(PT-IGBT)

VI Characteristics
The drain characteristic of IGBT is given below

It shows the relation between drain current iD and drain source voltage VDS for different values of gate source voltageVGS.

The junction j1 blocks reverse voltage.An IGBT without n+ buffer layer has higher reverse blocking capability.Therefore an IGBT required for blocking high reverse voltage does not have n+ buffer layer..The reverse blocking voltage is shown as the VRM on the VI characteristics.The junction j2 blocks the forward voltage when the IGBT is off.BVDSS denotes the break down voltage in the forward direction.The applied voltage drain and source must be less than BVDSS .

The transfer characteristics of IGBT is shown below

When VGS is less than VGS(Th) ,the device is in off state.This curve is almost linear except when drain current is very low.

Gate Turn Off Thyristor(GTO)

GTO is a special thyristor which can be turned on by a positive gate signal and can be turned off by a neagative signal.Evidently the use of GTO in power electronic circuit eliminates the need of forced commutation circuit because turnoff is achieved by applying a negative circuit.

The two transistor analogy of a GTO

Two transistor analogy of transistor is shown in figure below

When a positive signal is applied,a GTO switches into conduction state like the ordinary thyristor.However in ordinary thyristor the current gains of NPN and PNP transistors are very high so that gate sensitivity for

turn on is very high and on state voltage drop is low.However in GTO,the current gai of PNP transistor is low so that turn of fis possible if significant current is drawn from the gate.When a negative gate signal is applied the excess carriers are drawn from the base region of NPN transistor and collector current of PNP is diverted to the gate.Thus the base drive of NPN transistor is removed and this inturn removes the base drive of PNP transistor and turnoff is achieved.

Turn on and Turnoff Characteristics of GTO

The voltage and current wave forms of a GTO is show in figure below.The positive and negative pulses are shown.

When a positive signal is applied,GTO starts conducting.Before initiation of conduction anode current(iA)is zero and anode- cathode voltage VAK is the peak reverse voltage.When conduction starts rises iA to full value and the VAK becomes very small(equal to on state voltage drop which is about 1V or so).when a negative gate signal is applied,the anode current becomes zero and the VAK rises to peak reverse voltage.

The total turnoff time is composed of three distinct times,storage time(ts ),fall time(tf) and tail time(tt).

Initiation of turn off process starts immediately on the application of negative gate signal. The time elapsing between application of negative gate current till this current reaches its negative peak value known as storage time (ts ). During this period, the excess charge are removed by the -ve gate currentt and GTO gets ready to turn off. During fall time tf,the anode current decreases rapidly and anode cathode voltage rises.This time tf in most GTO is about 1μs.After tf is over,the current falls slowly to zero value durung tail time tt..The voltage gets a spike because of the present of elements R and C of the snubber circuit(for protection of GTO).At the end of the tail time,the anode current iA becomes zero and VAK becomes equal to peak reverse voltage.

Advantages and Disadvantages of GTO

Advantages of GTO over Thyristor

Commutation circuit is not needed
Fast switching speed
More di/dt at turn on
Higher efficiency because losses in commutation circuit is eliminated
Circuits using GTO are compact
Lesser acoustical and electromagnetic noise due to elimination of choke of commutation

Disadvantages of GTO

Higher latching and holding current
Higher on state voltage drop and power losses
Higher gate current
Higher gate circuit losses
Lower reverse voltage blocking capacity

Inspite of all above disadvantages,GTOs are being used in a variety of application such as variable frequency inverter circuits,electric traction and steel mills.Rating available are upto about 6kV and 6kA.

Static V-I Characteristics of GTO

From the above characteristics,latching current for large power GTO is several amperes here 2A as compared to 100-500mA for conventional thyristors of same rating.If gate current is not able to turn on the GTO,it behaves like a high voltage,low gain transistor with considerable anode current.This leads to a noticable power loss under such conditions.

Diac(Bidirectional Thyristor Diode)

The cross sectional view of a diac showing all its layers and junction is shown above figure.Diac is a two electrode device,it can conduct in either direction.Terminals are denoted by T1 and T2.word diac stands for ‘diode for ac‘.The four layer are pn pn and pn pn’.
Symbol

Principle of Operation

When T1 is positive with respect to T2,the layers p-n-p-n starts conducting.This happens when voltage of T1 is more than break over voltage VB01.Once the conduction starts,the current through the diac becomes very large and has to be limited by the external resistance in the circuit.When T2 is positive with respect to T1 the layers p-n-p-n’ conducts.This happens when the voltage of T2 exceeds break over voltage VBO2.In both the cases the current during blocking regions are small leakage currents.The behaviour in both the directions are is similar because doping level is same in all the layers in two directions.The break over voltage for commonly used diac is about 30V.

V-I characteristics

When T1 is positive and voltage is less than VBO1 only a small leakage current flows through the device.When voltage exceeds VBO1 it starts conducting and current becomes large.As the current increases,the voltage drop across diac decreases.Thus it exhibit negative resistance characteristics.The characteristics in the reverse direction(when T2 is positive)lies in the third quadrant and is exactly similar to that in the first quadrant.The breakover voltage VBO1 and VBO2 are exactly equal in magnitude.In both the cases,the device exhibits negative resistance behaviour during conduction region.

Application

Diac is mainly used for triggering triacs.A triac requires either positive or negative gate pulse for turning it on.This is provided by a Diac.Matched Diac-Triac combinations are manufactured for various control circuits.

MOS-CONTROLLED THYRISTOR(MCT)

An MCT is a new device in the field of semiconductor-controlled devices. It is basically a thyristor with two MOSFETs built into the gate structure. One MOSFET is used for turning on the MCT and the other for turning off the device. An MCT is a high-frequency, high_:power, low-conduction drop switching device.

An MCT combines into it the features of both conventional four-layer thyristor having regenerative action and MOS-gate structure. However, in MCT, anode is the reference with respect to which, all gate signals are applied. In a conventional SCR, cathode is the reference terminal for gate signals.

The basic structure of an MCT is shown in Fig. 2.20. A practical MCT consists of thousands of these basic cells connected in parallel, just like a power MOSFET (

7, 8). This is done in order to achieve a high-current carrying capacity of the device.

The equivalent circuit of MCT is shown in Fig. 2.21 (a). It consists of one on-FET, one off-FET and two transistors. The on-FET is ap-channel MOSFET and off-FET is an rc-channel MOSFET. An arrow towards the gate terminal indicates n-channel MOSFET and the arrow away from the gate terminal as the p-channel MOSFET. The two transistors in the equivalent circuit indicate that there is regenerative feedback in the MCT just as it is in an ordinary thyristor. Fig. 2.21 (6) gives the circuit symbol of an MCT

An MCT is turned-on by a negative voltage pulse at the gate with respect to the anode and is turned-off by a positive voltage pulse. Working of MCT can be understood better by referring to Fig. 2.21 (a).

Turn-on process. As stated above, MCT is turned on by applying a negative voltage pulse at the gate with respect to anode. In other words, for turning on MCT, gate is made negative with respect to anode by the voltage pulse between gate and anode. With the application of this negative voltage pulse, on-FET gets >turned-on and off-FET is off. With on-FET on, current begins to flow from anode A, through on-FET and then as the base current and emitter current of npn transistor and then to cathode C. This turns on npn transistor. As a result, collector current begins to flow in npn transistor. As off-FET is off, this collector current of npn transistor acts as the base current of pnp transistor. Subsequently, pnp transistor is also turned on. Once both the transistors are on, regenerative action of the connection scheme takes place and the thyristor or MCT is turned on.

Note that on-FET and pnp transistor are in parallel when thyristor is in conduction state. During the time MCT is on, base current of npn transistor flows mainly through pnp transistor because of its better conducting property.

Turn-off process. For turning-off the MCT, off-FET (or n -channel MOSFET) is energized by positive voltage pulse at the gate. With the application of positive voltage pulse, off-FET is turned on and on-FET is turned off. After off-FET is turned on, emitter-base terminals of pnp transistor are short circuited by off-FET So now anode current begins to flow through off’-FET and therefore base current of pnp transistor begins to decrease. Further, collector current of pnp transistor that forms the base current of npn transistor also begins to decrease.

As a consequence, base currents of both pnp and npn transistors, now devoid of stored charge in their n and p bases respectively, begin to decay. This regenerative action eventually turns off the MCT.

An MCT has the following merits :

(i) Low forward conduction drop,

(ii) fast turn-on and turn-off times,
(iii) low switching losses and

(iv) high gate input impedance, which allows simpler design of drive circuits.

An MCT is a brand-new device which is likely to be available commercially very soon. As it possesses highly adaptable features for its use as a switching device, it seems to have tremendous scope for its widespread applications. Its potential applications include dc and ac motor drives, UPS systems, induction heating, dc-dc converters, power line conditioners etc. It may, in the near future, challenge the existence of most of the available devices like - resistors, GTOs, BJTs, IGBTs (7).

NEW SEMICONDUCTING MATERIALS

At present, silicon enjoys monopoly as a semiconductor material for the commercial production of power-control devices. This is because silicon is cheaply available and semiconductor devices of any size can be easily fabricated on a single silicon chip. There are, however, new types of materials like gallium arsenic (GaAs), silicon carbide and diamond which possess the desirable properties required for switching devices. At present, state-of-the-art technology for these materials is primitive compared with silicon, and many more years of research investment are required before these materials become commercially viable for the production of power-controlled devices. Superconductive materials may also be used in the manufacture of such devices, but work in this direction has not yet been reported.

Germanium is not used in the fabrication of thyristors because of the following reasons:

Germanium has much lower thermal conductivity; its thermal resistance is, therefore, more. As a consequence, germanium thyristors suffer from more losses, more temperature rise and therefore lower operating life.
Its breakdown voltage is much less than that of silicon. It means that germanium thyristor can be built for small voltage ratings only.
Germanium is much costlier than silicon.

Thyristors

As stated before, Bell Laboratories were the first to fabricate a silicon-based semiconductor device called thyristor. Its first prototype was introduced by GEC (USA) in 1957. This company did a great deal of pioneering work about the utility of thyristors in industrial applications. Later on, many other devices having characteristics similar to that of a thyristor were developed. These semiconductor devices, with their characteristics identical with that of a thyristor, are triac, diac, silicon-controlled switch, programmable unijunction transistor (PUT), GTO, RCT etc. This whole family of semiconductor devices is given the name thyristor. Thus the term thyristor denotes a family of semiconductor devices used for power control in dc and ac systems. One oldest member of this thyristor family, called silicon-controlled rectifier (SCR), is the most widely used device. At present, the use of SCR is so vast that over the years, the word thyristor has become synonymous with SCR. It appears that the term thyristor is now becoming more common than the actual term SCR. In this book, the term SCR and thyristor will be used at random for the same device SCR. Other members of thyristor family are also discussed in this category.

A thyristor has characteristics similar to a thyratron tube. But from the construction view point, a thyristor (a pnpn device) belongs to transistor (pnp or npn device) family. The name ‘thyristor’, is derived by a combination of the capital letters from THYRatron and transISTOR. This means that thyristor is a solid state device like a transistor and has characteristics similar to that of a thyratron tube. The present-day reader may not be familiar with thyratron tube as this is not being taught these days.

TERMINAL CHARACTERISTICS OF THYRISTORS

Thyristor is a four layer, three-junction, p-n-p-n semiconductor switching device. It has three terminals ; anode, cathode and gate. Fig. 4.1 (a) gives constructional details of a typical thyristor. Basically, a thyristor consists of four layers of alternate p-type and n-type silicon semiconductors forming three junctions J1, J2 and J3 as shown in Fig. 4.1 (a). The threaded portion is for the purpose of tightening the thyristor to the frame or heat sink with the help of a nut. Gate terminal is usually kept near the cathode terminal Fig. 4.1 (a). Schematic diagram and circuit symbol for a thyristor are shown respectively in Figs. 4.1 (b) and (c). The terminal connected to outer p region is called anode (A), the terminal connected to outer n region is called cathode and that connected to inner p region is called the gate (G). For large current applications, thyristors need better cooling ; this is achieved to a great extent by mounting them onto heat sinks. SCR rating has improved considerably since its introduction in 1957. Now SCRs of voltage rating 10 kV and an rms current rating of 3000 A with corresponding power-handling capacity of 30 MW are available. Such a high power thyristor can be switched on by a low voltage supply of about 1 A and 10 W and this gives us an idea of the immense power amplification capability (= 3 x 106) of this device. As SCRs are solid state devices, they are compact, possess high reliability and have low loss. Because of these useful features, SCR is almost universally employed these days for all high power-controlled devices.

An SCR is so called because silicon is used for its construction and its operation as a rectifier (very low resistance in the forward conduction and very high resistance in the reverse direction) can be controlled. Like the diode, an SCR is an unidirectional device that blocks the current flow from cathode to anode. Unlike the diode, a thyristor also blocks the current flow from anode to cathode until it is triggered into conduction by a proper gate signal between gate and cathode terminals.

For engineering applications of thyristors, their terminal characteristics must be known. In this article, their static V-I characteristics, dynamic characteristics during turn-on and turn-off processes and their gate characteristics are discussed.

Static V-I Characteristics of a Thyristor

An elementary circuit diagram for obtaining static V-I characteristics of a thyristor is shown in Fig. 4.2 (a). The anode and cathode are connected to main source through the load. The gate and cathode are fed from a source E_s which provides positive gate current from gate to cathode.

Fig. 4.2 (b) shows static V-I characteristics of a thyristor. Here V_a is the anode voltage across thyristor terminals A, K and I_a is the anode current. Typical SCR V-I characteristic shown in Fig. 4.2 (b) reveals that a thyristor has three basic modes of operation ; namely, reverse blocking mode, forward blocking (off-state) mode and forward conduction (on-state) mode. These three modes of operation are now discussed below :

Reverse Blocking Mode: When cathode is made positive with respect to anode with switch S open, Fig. 4.2 (a), thyristor is reverse biased as shown in Fig. 4.3 (a). Junctions J1 J₃ are seen to be reverse biased whereas junction J₂ is forward biased. The device behaves as if two diodes are connected in series with reverse voltage applied across them. A small leakage current of the order of a few milliamperes (or a few microamperes depending upon the SCR rating) flows. This is reverse blocking mode, called the off-state, of the thyristor. If the reverse voltage is increased, then at a critical breakdown level, called reverse breakdown voltage VBR, an avalanche occurs at J1 and J3 and the reverse current increases rapidly. A large current associated with VBR gives rise to more losses in the SCR. This may lead to thyristor damage as the junction temperature may exceed its permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage across a thyristor does not exceed VBR. When reverse voltage applied across a thyristor is less than VBR, the device offers a high impedance in the reverse direction. The SCR in the reverse blocking mode may therefore be treated as an open switch.

Note that V-I characteristic after avalanche breakdown during reverse blocking mode is applicable only when load resistance is zero, Fig. 4.2 (b). In case load resistance is present, a large anode current associated with avalanche breakdown at V_BR would cause substantial voltage drop across load and as a result, V-I characteristic in third quadrant would bend to the right of vertical line drawn at V_BR.

Forward Blocking Mode : When anode is positive with respect to the cathode, with gate circuit open, thyristor is said to be forward biased as shown in Fig. 4.3 (b). It is seen from this figure that junctions J1_, J₃ are forward biased but junction J₂ is reverse biased. In this mode, a small current, called forward leakage current, flows as shown in Figs. 4.2 (b) and 4.3 (b). In case the forward voltage is increased, then the reverse biased junction J₂ will have an avalanche breakdown at a voltage called forward breakover voltage VB0. When forward voltage is less than VBO, SCR offers a high impedance. Therefore, a thyristor can be treated as an open switch even in the forward blocking mode.

Forward Conduction Mode : In this mode, thyristor conducts currents from anode to cathode with a very small voltage drop across it. A thyristor is brought from forward blocking mode to forward conduction mode by turning it on by exceeding the forward breakover voltage or by applying a gate pulse between gate and cathode. In this mode, thyristor is in on-state and behaves like a closed switch. Voltage drop across thyristor in the on state is of the order of 1 to 2 V depending on the rating of SCR. It may be seen from Fig. 4.2 (b) that this voltage drop increases slightly with an increase in anode current. In conduction mode, anode current is limited by load impedance alone as voltage drop across SCR is quite small. This small voltage drop v_T across the device is due to ohmic drop in the four layers.

THYRISTOR TURN-ON METHODS

With anode positive with respect to cathode, a thyristor can be turned on by any one of the following techniques :

(a) Forward voltage triggering (b) gate triggering

(e)Light triggering.

These methods of turning-on a thyristor are now discussed one after the other.

(a) Forward Voltage Triggering: When anode to cathode forward voltage is increased with gate circuit open, the reverse biased junction J₂ will break. This is known as avalanche breakdown and the voltage at which avalanche occurs is called forward breakover voltage V_B0. At this voltage, thyristor changes from off-state (high voltage with low leakage current) to on-state characterised by low voltage across thyristor with large forward current. As other junctions J_1, J₃ are already forward biased, breakdown of junction J₂ allows free movement of carriers across three junctions and as a result, large forward anode-current flows. As stated before, this forward current is limited by the load impedance. In practice, the transition from off-state to on-state obtained by exceeding V_B0 is never employed as it may destroy the device.

The magnitudes of forward and reverse breakover voltages are nearly the same and both are temperature dependent. In practice, it is found that V_BR is slightly more than V_B0. Therefore, forward breakover voltage is taken as the final voltage rating of the device during the design of SCR applications.

After the avalanche breakdown, junction J₂ looses its reverse blocking capability. Therefore, if the anode voltage is reduced below V_B0 SCR will continue conduction of the current. The SCR can now be turned off only by reducing the anode current below a certain value called holding current (defined later).

(6) Gate Triggering : Turning on of thyristors by gate triggering is simple, reliable and efficient, it is therefore the most usual method of firing the forward biased SCRs. A thyristor with forward breakover voltage (say 800 V) higher than the normal working voltage (say 400 V) is chosen. This means that thyristor will remain in forward blocking state with normal working voltage across anode and cathode and with gate open. However, when turn-on of a thyristor is required, a positive gate voltage between gate and cathode is applied. With gate current thus established, charges are injected into the inner p layer and voltage at which forward breakover occurs is reduced. The forward voltage at which the device switches to on-state depends upon the magnitude of gate current. Higher the gate current, lower is the forward breakover voltage

When positive gate current is applied, gate P layer is flooded with electrons from the cathode. This is because cathode N layer is heavily doped as compared to gate P layer. As the thyristor is forward biased, some of these electrons reach junction J₂. As a result, width of depletion layer around junction J₂ is reduced. This causes the junction J₂ to breakdown at an applied voltage lower than forward breakover voltage V_B0. If magnitude of gate current is increased, more electrons will reach junction J₂,as a consequence thyristor will get turned on at a much lower forward applied voltage.

Fig. 4.2 (b) shows that for gate current I_g = 0, forward breakover voltage is V_B0. For I_gl, forward breakover voltage, or turn-on voltage is less than V_B0 For I_g2 > I_g_{1 ,} forward breakover voltage is still further reduced. The effect of gate current on the forward breakover voltage of a thyristor can also be illustrated by means of a curve as shown in Fig. 4.4. For I_g < oa, forward breakover voltage remains almost constant at V_B0. For gate currents I_g₁, I_g2 and I_g3 the values of forward breakover voltages are ox, oy and oz, respectively as shown. In Fig. 4.2 (b), the curve marked I_g = 0 is actually for gate current less than oa. In practice, the magnitude of gate current is more than the minimum gate current required to turn on the SCR. Typical gate current magnitudes are of the order of 20 to 200 mA.

Once the SCR is conducting a forward current, reverse biased junction J₂ no longer exists. As such, no gate current is required for the device to remain in on-state. Therefore, if the gate current is removed, the conduction of current from anode to cathode remains unaffected. However, if gate current is reduced to zero before the rising anode current attains a value, called the latching current, the thyristor will turn-off again. The gate pulse width should therefore be judiciously chosen to ensure that anode current rises above the latching current. Thus latching current may be defined as the minimum value of anode current which it must attain during turn-on process to maintain conduction when gate signal is removed.

Once the thyristor is conducting, gate loses control. The thyristor can be turned-off (or the thyristor can be returned to forward blocking state) only if the forward current falls below a low-level current called the holding current. Thus holding current may be defined as the minimum value of anode current below which it must fall for turning-off the thyristor. The latching current is higher than the holding current. Note that latching current is associated with turn-on process and holding current with turn-off process. It is usual to take latching current as two to three times the holding current . In industrial applications, holding current (typically 10 mA) is almost taken as zero.

(d) Temperature Triggering : During forward blocking, most of the applied voltage appears across reverse biased junction J₂. This voltage across junction J₂ associated with leakage current may raise the temperature of this junction. With increase in temperature, leakage current through junction J₂ further increases. This cumulative process may turn on the SCR at some high temperature.

(e) Light Triggering: For light-triggered SCRs, a recess (or niche) is made in the inner p-layer as shown in Fig. 4.5 (a). When this recess is irradiated, free charge carriers (holes and electrons) are generated just like when gate signal is applied between gate and cathode. The pulse of light of appropriate wavelength is guided by optical fibres for irradiation. If the intensity of this light thrown on the recess exceeds a certain value, forward-biased SCR is turned on. Such a thyristor is known as light-activated SCR (LASCR).

LASCR may be triggered with a light source or with a gate signal. Sometimes a combination of both light source and gate signal is used to trigger an SCR. For this, the gate is biased with voltage or current slightly less than that required to turn it on, now a beam of light directed at the inner p-layer junction turns on the SCR. The light intensity required to turn-on the SCR depends upon the voltage bias given to the gate. Higher the voltage (or current) bias, lower the light intensity required.

Light-triggered thyristors have now been used in high-voltage direct current (HVDC) transmission systems. In these several SCRs are connected in series-parallel combination and their light-triggering has the advantage of electrical isolation between power and control circuits.

SWITCHING CHARACTERISTICS OF THYRISTORS DURING TURN-ON

Static and switching characteristics of thyristors are always taken into consideration for economical and reliable design of converter equipment. Static characteristics of a thyristor have already been examined. In this part of the section; switching, dynamic or transient, characteristics of thyristors are discussed.

During turn-on and turn-off processes, a thyristor is subjected to different voltages across it and different currents through it. The time variations of the voltage across a thyristor and the current through it during turn-on and turn-off processes give the dynamic or switching characteristics of a thyristor. Here, first switching characteristics during turn-on are described and then the switching characteristics during turn-off.

Switching Characteristics during Turn-on

A forward-biased thyristor is usually turned on by applying a positive gate voltage between gate and cathode. There is, however, a transition time from forward off-state to forward on state. This transition time called thyristor turn-on time, is defined as the time during which it changes from forward blocking state to final on-state. Total turn-on time can be divided into three intervals ; (i) delay time t_d, (ii) rise time t_r and (iii) spread time t_{p ,} Fig. 4.8.

(i) Delay time t_d : The delay time t_d is measured from the instant at which gate current reaches 0.9 I_g to the instant at which anode current reaches 0.1I_a. Here I_g and I_a are respectively the final values of gate and anode currents. The delay time may also be defined as the time during which anode voltage falls from V_a to 0.9V_a where V_a = initial value of anode voltage. Another way of defining delay time is the time during which anode current rises from forward leakage current to 0.1 I_a where I_a = final value of anode current. With the thyristor initially in the forward blocking state, the anode voltage is OA and anode current is small leakage current as shown in Fig. 4.8. Initiation of turn-on process is indicated by a rise in anode current from small forward leakage current and a fall in anode-cathode voltage from forward blocking voltage OA. As gate current begins to flow from gate to cathode with the application of gate signal, the gate current has non-uniform distribution of current density over the cathode surface due to the p layer. Its value is much higher near the gate but decreases rapidly as the distance from the gate increases, see Fig. 4.6 (a). This shows that during delay time t_d,anode current flows in a narrow region near the gate where gate current density is the highest.

The delay time can be decreased by applying high gate current and more forward voltage between anode and cathode. The delay time is fraction of a microsecond.

(ii) Rise time t_r: The rise time t_r is the time taken by the anode current to rise from 0.1 I_a to 0.9 I_a. The rise time is also defined as the time required for the forward blocking off-state voltage to fall from 0.9 to 0.1 of its initial value OA. The rise time is inversely proportional to the magnitude of gate current and its build up rate. Thus t_r can be reduced if high and steep current pulses are applied to the gate. However, the main factor determining t_r is the nature of anode circuit. For example, for series RL circuit, the rate of rise of anode current is slow, therefore, t_r is more. For RC series circuit, di/dt is high, t_r is therefore, less.

From the beginning of rise time t_r anode current starts spreading from the narrow conducting region near the gate. The anode current spreads at a rate of about 0.1 mm per microsecond . As the rise time is small, the anode current is not able to spread over the entire cross-section of cathode. Fig. 4.6 (b) illustrates how anode current expands over cathode surface area during turn-on process of a thyristor. Here the thyristor is taken to have single gate electrode away from the centre of p-layer. It is seen that anode current conducts over a small conducting channel even after tr -this conducting channel area is however, greater than that during td.During rise time, turn-on losses in the thyristor are the highest due to high anode voltage (V_a) and large anode current (I_a) occurring together in the thyristor as shown in Fig. 4.8. As these losses occur only.over a small conducting region, local hot spots may be formed and the device may be damaged.

(iii) Spread time t_p : The spread time is the time taken by the anode current to rise from 0.9 I_a to I_a. It is also defined as the time for the forward blocking voltage to fall from 0.1 of its value to the on-state voltage drop (1 to 1.5 V). During this time, conduction spreads over the entire cross-section of the cathode of SCR. The spreading interval depends on the area of cathode and on gate structure of the SCR. After the spread time, anode current attains steady state value and the voltage drop across SCR is equal to the on-state voltage drop of the order of 1 to 1.5 V, Fig. 4.8.

Total turn-on time of an SCR is equal to the sum of delay time, rise time and spread time. Thyristor manufacturers usually specify the rise time which is typically of the order of 1 to 4 µ-sec. Total turn-on time depends upon the anode circuit parameters and the gate signal waveshapes.

During turn-on, SCR may be considered to be a charge controlled device. A certain amount of charge must be injected into the gate region for the thyristor conduction to begin. This charge is directly proportional to the value of gate current. Therefore, higher the magnitude of gate current, the lesser time it takes to inject this charge. The turn-on time can therefore be reduced by using higher values of gate currents. The magnitude of gate current is usually 3 to 5 times the minimum gate current required to trigger an SCR.

When gate current is several times higher than the minimum gate current required, a thyristor is said to be hard-fired or overdriven. Hard-firing or overdriving of a thyristor reduces its turn-on time and enhances it di/dt capability. A typical waveform for gate current, that is widely used, is shown in Fig. 4.7. This waveform has higher initial value of gate current with a very fast rise time. The initial high value of gate current is then reduced to a lower value where it stays for several microseconds in order to avoid unwanted turn-off of the device

SWITCHING CHARACTERISTICS OF THYRISTORS DURING TURN OFF

Switching Characteristics during Turn-off

Thyristor turn-off means that it has changed from on to off state and is capable of blocking the forward voltage. This dynamic process of the SCR from conduction state to forward blocking state is called commutation process or turn-off process.

Once the thyristor is on, gate loses control. The SCR can be turned off by reducing the anode current below holding current . If forward voltage is applied to the SCR at the moment its anode current falls to zero, the device will not be able to block this forward voltage as the carriers (holes and electrons) in the four layers are still favourable for conduction. The device will therefore go into conduction immediately even though gate signal is not applied. In order to obviate such an occurrence, it is essential that the thyristor is reverse biased for a finite period after the anode current has reached zero.

The turn-off time t_q of a thyristor is defined as the time between the instant anode current becomes zero and the instant SCR regains forward blocking capability. During time t_q ,all the excess carriers from the four layers of SCR must be removed. This removal of excess carriers consists of sweeping out of holes from outer p-layer and electrons from outer n-layer. The carriers around junction J₂ can be removed only by recombination. The turn-off time is divided into two intervals ; reverse recovery time t_rr and the gate recovery time t_{g r}; i.e. t_q = t_rr + t_gr.

Fig. 4.8. Thyristor voltage and current waveforms during turn-on and turn-off processes.. The thyristor characteristics during turn-on and turn-off processes are shown in one Fig. 4.8 so as to gain insight into these processes.

At instant t _l ,anode current becomes zero. After t _l anode current builds up in the reverse direction with the same di/dt slope as before t _l The reason for the reversal of anode current after t _l is due to the presence of carriers stored in the four layers. The reverse recovery current removes excess carriers from the end junctions J₁ and J₃ between the instants t _l and t ₃. In other words, reverse recovery current flows due to the sweeping out of holes from top p-layer and electrons from bottom n-layer. At instant t ₂, when about 60% of the stored charges are removed from the outer two layers, carrier density across J₁ and J₃ begins to decrease and with this reverse recovery current also starts decaying. The reverse current decay is fast in the beginning but gradual thereafter. The fast decay of recovery current causes a reverse voltage across the device due to the circuit inductance. This reverse voltage surge appears across the thyristor terminals and may therefore damage it. In practice, this is avoided by using protective RC elements across SCR. At instant t₃, when reverse recovery current has fallen to nearly zero value, end junctions J₁ and J₃ recover and SCR is able to block the reverse voltage. For a thyristor, reverse recovery phenomenon between t₁ and t₃ is similar to that of a rectifier diode.

At the end of reverse recovery period (t₃ -the middle junction J₂ still has trapped charges, therefore, the thyristor is not able to block the forward voltage at t₃ The trapped charges around J₂, i.e. in the inner two layers, cannot flow to the external circuit, therefore, these trapped charges must decay only by recombination. This recombination is possible if a reverse voltage is maintained across SCR, though the magnitude of this voltage is not important. The rate of recombination of charges is independent of the external circuit parameters. The time for the recombination of charges between t₃ and t₄ is called gate recovery time t_gr At instant t ₄, junction J₂ recovers and the forward voltage can be reapplied between anode and cathode. The thyristor turn-off time t_q is in the range of 3 to 100 µsec. The turn-off time is influenced by the magnitude of forward current, di/dt at the time of commutation and junction temperature. An increase in the magnitude of these factors increases the thyristor turn-off time. If the value of forward current before commutation is high, trapped charges around junction J₂ are more. The time required for their recombination is more and therefore turn-off time is increased. But turn-off time decreases with an increase in the magnitude of reverse voltage, particularly in the range of 0 to – 50 V. This is because high reverse voltage sucks out the carriers out of the junctions J_{l ,} J₃ and the adjacent transition regions at a faster rate. It is evident from above that turn-off time t_q is not a constant parameter of a thyristor.

The thyristor turn-off time t_q is applicable to an individual SCR. In actual practice, thyristor (or thyristors) form a part of the power circuit. The turn-off time provided to the thyristor by the practical circuit is called circuit turn-off time t_c. It is defined as the time between the instant anode current becomes zero and the instant reverse voltage due to practical circuit reaches zero, see Fig. 4.8. Time t_c must be greater than t_q for reliable turn-off, otherwise the device may turn-on at an undesired instant, a process called commutation failure.

Thyristors with slow turn-off time (50 – 100 (usee) are called converter grade SCRs and those with fast turn-off time (3 – 50 µsec) are called inverter-grade SCRs. Converter-grade SCRs are cheaper and are used where slow turn-off is possible as in phase-controlled rectifiers, ac voltage controllers, cycloconverters etc. Inverter-grade SCRs are costlier and are used in inverters, choppers and force-commutated converters.

THYRISTOR GATE CHARACTERISTICS

The forward gate characteristics of a thyristor are shown in Fig. 4.9 in the form of a graph between gate voltage and gate current. Here positive gate to cathode voltage Vg and positive gate to cathode current Ig represent dc values. As gate-cathode circuit of a thyristor is a p-n junction, gate characteristics of the device are similar to that of a diode. For a particular type of SCRs, Vg-Ig characteristic has a spread between two curves 1 and 2 as shown in Fig. 4.9. This spread, or scatter, of gate characteristics is due to difference in the low doping levels of p and n layers. The gate trigger circuitry must be suitably designed to take care of this unavoidable scatter of characteristics. In Fig. 4.9, curve 1 represents the lowest voltage values that must be applied to turn-on the SCR. Curve 2 gives the highest possible voltage values that can be safely applied to gate circuit.

Each thyristor has maximum limits as V_gm for gate voltage and I_gm for gate current. There is also rated (average) gate power dissipation P_gav specified for each SCR. These limits should not be exceeded in order to avoid permanent damage of junction J₃, Fig. 4.3. There are also minimum limits for V_g and I_g for reliable turn-on, these are represented by oy and ox and respectively in Fig. 4.9. As stated before, if V_gm, I_gm and P_gavare exceeded, the thyristor can be destroyed. This shows that preferred gate drive area for an SCR is bcdefghb as shown in Fig. 4.9.

A non-triggering gate voltage is also prescribed by the manufacturers of SCRs. This is indicated by oa in Fig. 4.9. If firing circuit generates positive gate signal prior to the desired instant of triggering the SCR, it should be ensured that this unwanted signal is less than the non-triggering gate voltage oa. At the same time, all spurious or noise signals should be less than the voltage oa.

The design of the firing circuit can be carried out with the help of Figs. 4.10 and 4.11. In Fig. 4.10 (a) is shown a trigger circuit feeding power to gate-cathode circuit. For this circuit,

E_S = V_g + I_gR_S……..(4.1a)

where E_s= gate source voltage

V_g = gate-cathode voltage

I_g = gate current
and R_s = gate-source resistance

The internal resistance R_s of trigger source should be such that current (E_s/R_s) is not harmful to the source as well as to the gate circuit when SCR is turned on. In case R_s is low, an external resistance in series with R_s must be connected.

A resistance R₁ is also connected across gate-cathode terminals, Fig. 4.10 (b), so as to provide an easy path to the flow of leakage current between SCR terminals. If I_gmn and V_gmn are the minimum gate current and gate voltage to turn-on SCR, then it is seen from Fig. 4.10. (b) that current through R₁ is V_gmn/R₁ and the trigger source voltage E_s is given by

E_s= [I_gmn+ V_gmn/R₁] R_s + V_gmn …(4.1b)

For low-power circuits, operating point is obtained by utilizing the source V-I characteristic and the device V-I characteristic. In view of this, for selecting the operating point for the circuit of Fig. 4.10, a load line of the gate source voltage E_s = OA is drawn as AD in Fig. 4.11. Here OD = trigger circuit short circuit current =E_S/R_S. Let us consider a thyristor whose V_g-I_g characteristic is given by curve 3. Intersection of load line AD and V_g-I_g curve 3 gives the operating point S. Thus, for this SCR, gate voltage =PS and gate current = OP. In order to minimise turn-on time and jitter (unreliable turn-on), the load line and hence the operating point S, which may change from S₁ to S₂, must be as close to the P_gav curve as possible. At the same time, the operating point S must lie within the limit curves 1 and 2. The gradient of the load line AD (= OA/OD) will give the required gate source resistance R_s The minimum value of gate source series resistance is obtained by drawing a line AC tangent to P_gav curve

Gate drive requirements in terms of continuous dc signal can be obtained from Fig. 4.11. However, it is common to use a pulse to trigger a thyristor. For pulse widths beyond 100 µsec, the dc data apply . For pulse widths less than 100 µsec, magnitudes of gate voltage and gate current can be increased.

As stated before, thyristor is considered to be a charge controlled device. Thus, higher the magnitude of gate current pulse, lesser is the time to inject the required charge for turning-on the thyristor. Therefore, SCR turn-on time can be reduced by using gate current of higher magnitude. It should be ensured that pulse width is sufficient to allow the anode current to exceed the latching current. In practice, gate pulse width is usually taken as equal to, or greater than, SCR turn-on time. If T is the pulse width as shown in Fig. 2.10 (a), then

T > t on

With pulse triggering, greater amount of gate power dissipation can be allowed ; this should, however, be less than the peak instantaneous gate power dissipation P_gm as specified by the manufacturers. Frequency of firing (or pulse width) for trigger pulses can be obtained by taking pulse of (i) amplitude P_gm (ii) pulse width T and (iii) periodicity T_1. Therefore,

P_gm T/ T₁ > P_gavor P_gm.T. f > P_gav

P_g_av / f T < P_gm

where f = 1/ T₁ = frequency of firing, or pulse repetition rate, in Hz,

T = pulse width in sec

In the limiting case, P_g_av / f T = P_gmor f = P_g_av/T. P_gm

A duty cycle is defined as the ratio of pulse-on period to periodic time of pulse. In Fig. 4.12 (a), pulse-on period is T and periodic time is T_1. Therefore, duty cycle δ is given by

δ = T/ T₁ = f T

From Eq. 4.2 (a), P_g_av / δ < P_gmor P_g_av / δ = P_gm…(4.2 b)

Sometimes the pulses of Fig. 4.12 (a) are modulated to generate a train of pulses as shown in Fig. 4.12 (b). This technique of firing the thyristor is called high-frequency carrier gating. The advantages offered by this method of firing the SCRs are lower rating, reduced dimensions and therefore an overall economical design of the pulse transformer needed for isolating the low power circuit from the main power circuit.

For an SCR, V_am and I_gm are specified separately. If both of these are used for pulse firing,then P_gm may be exceeded and the thyristor would be damaged. For example, GE-C35 thyristor has V_gm = 10 V and I_gm = 2 A. If both these limits are placed on C35, the power dissipation is 20 W. But this is far excess of the specified P_gm = 5 W. It should be ensured that (pulse voltage amplitude) (pulse current amplitude) < P_gm.

There is also prescribed a peak reverse voltage (gate negative with respect to cathode) that can be applied across gate-cathode terminals. Any voltage signal, given by the trigger circuit (or by any interference), exceeding this prescribed limit of about 5 to 20 V may damage the gate circuit. For preventing the occurrence of such hazards, a diode is connected either in series with the gate circuit or across the gate-cathode terminals as shown in Fig. 4.12 (c). Diode across the gate-cathode terminals, called clamping diode, prevents the gate-cathode voltage from becoming more than about 1 V. Diode in series with gate circuit prevents the flow of negative gate source current from becoming more than small reverse leakage current.

The magnitude of gate voltage and gate current for triggering an SCR is inversely proportional to junction temperature. Thus, at very low temperatures, gate voltage and gate current must have high values in order to ensure turn-on. But P_gm should not be exceeded in any case.

The resistor R_l, connected across gate-cathode terminals, Fig. 4.10 (b), also serves to bypass a part of the thermally-generated leakage current across junction J₂ when SCR is in the forward blocking mode ; this improves the thermal stability of SCR.

TWO-TRANSISTOR MODEL OF A THYRISTOR

The principle of thyristor operation can be explained with the use of its two-transistor model (or two-transistor analogy). Fig. 4.15 (a) shows schematic diagram of a thyristor. From this figure, two-transistor model is obtained by bisecting the two middle layers, along the dotted line, in two separate halves as shown in Fig. 4.15 (b). In this figure, junctions J₁ – J₂ and J₂ -J3 can be considered to constitute pnp and npn transistors separately. The circuit representation of the two-transistor model of a thyristor is shown in Fig. 4.15 (c).

In the off-state of a transistor, collector current I_c is related to emitter current I_E as

I_C = αI_E + I_CBO

where α is the common-base current gain and I_CB0 is the common-base leakage current of collector-base junction of a transistor.

For transistor Q₁ in Fig. 4.15 (c), emitter current I_E = anode current I_a and I_C = collector current I_C1. Therefore, for Q₁

I_C1 = α₁ I_a + I_CBO1 ……..(4.3)

where α₁ = common-base current gain of Q₁

and I_{CBO1 =}common-base leakage current of Q₁

Similarly, for transistor Q₂, the collector current I_C2 is given by

I_C2 = α₂ I_k + I_CBO2 …(4.4)

where α₂ – common-base current gain of Q₂,I_CBO2 =common-base leakage current of Q₂ and

I_k = emitter current of Q₂.

The sum of two collector currents given by Eqs. (4.3) and (4.4) is equal to the external circuit current I_αentering at anode terminal A.

There fore I_a = I_{C1 +}I_C2

I_{a =} α₁ I_{a +}I_CBO1+ α₂ I_k + I_CBO2…(4.5)

When gate current is applied, then I_k = I_a + I_g . Substituting this value of I_k in Eq. (4.5) gives

I_{a =} α₁ I_{a +}I_CBO1+ α₂ (I_a + I_g) + I_CBO2

_or

I_{a =}α₂ I_{g +}I_{CBO1 +}I_CBO2/[1-( α₁₊α₂)]

For a silicon transistor, current gain α is very low at low emitter current. With an increase in emitter current, a builds up rapidly as shown in Fig. 4.16. With gate current I_g = 0 and with thyristor forward biased,( α₁₊α₂)is very low as per Eq (4.6) and forward leakage current somewhat more than I_{CBO1 +}I_CBO2 flows. If, by some means, the emitter current of two component transistors can be increased so that α₁₊α₂approaches unity, then as per Eq. (4.6) I_a would tend to become infinity thereby turning-on the device. Actually, external load limits the anode current to a safe value after the thyristor begins conduction. The methods of turning-on a thyristor, in fact, are the methods of making α₁₊α₂to approach unity. These 0.25 various mechanisms for turning-on a thyristor are now discussed below :

(i) GATE Triggering : With anode positive with respect to cathode and with gate current I_g = 0, Eq. (4.6) shows that anode current, equal to the forward leakage current, is somewhat more than I_{CBO1 +}I_CBO2,Under these conditions, the device is in the forward blocking state.

Now a sufficient gate-drive current between gate and cathode of the transistor is applied. This gate-drive current is equal to base current I_{B2 =}I_g and emitter current I_k of transistor Q₂. With the establishment of emitter current I_k of Q₂, current gain α₂ of Q₂ increases and base current I_B2 causes the existence of collector current I_C2 = β₂I_B2 = β₂ I_g. This amplified current I_C2 serves as the base current I_B1 of transistor Q₁ With the flow of I_B1 collector current I_C1 = β₁ I_B1 = β₁ β₂ I_g of Q₁ comes into existence. Currents I_B1 and I_C1 lead to the establishment of emitter current I_a of Q₁ and this causes current gain α₁ to rise as desired. Now current I_g + I_CI = (1 + β₁ β₂) I_g acts as the base current of Q₂ and therefore its emitter current I_k = I_CI + I_g With the rise in emitter current I_k α₂ of Q₂ increases and this further causes I_C2 = P₂ (1 + β₁ β₂) I_g to rise. As amplified collector current I_C2 is equal to the base current of Q₁ current gain α₁ eventually rises further. There is thus established a regenerative action internal to the device. This regenerative or positive feedback effect causes α₁₊α₂to grow towards unity. As a consequence, anode current begins to grow towards a larger value limited only by load impedance external to the device. When regeneration has grown sufficiently, gate current can be withdrawn. Even after I_g is removed, regeneration continues. This characteristic of the thyristor makes it suitable for pulse triggering. Note that thyristor is a latching device

After thyristor is turned on, all the four layers are filled with carriers and all junctions are forward biased. Under these conditions, thyristor has very low impedance and is in the forward on-state.

(ii) Forward-voltage triggering : If the forward anode to cathode voltage is increased, the collector to emitter voltages of both the transistors are also increased. As a result, the leakage current at the middle junction J₂ of thyristor increases, which is also the collector current of Q₂ as well as Q₁ With increase in collector currents I_C1 and I_C2 due to avalanche effect, the emitter currents of the two transistors also increase causing α₁₊α₂to approach unity. This leads to switching action of the device due to regenerative action. The forward-voltage triggering for turning-on a thyristor may be destructive and should therefore be avoided.

(iii) dv/dt triggering : The reversed biased junction J₂ behaves like a capacitor because of the space-charge present there. Let the capacitance of this junction be Cj. For any capacitor, i = C dv/dt.In case it is assumed that entire forward voltage v_a appears across reverse biased junction J₂ then charging current across the junction is given by

i = Cj dv_a /dt

This charging or displacement current across junction J₂ is collector currents of Q₂ and Q₁ Currents I_C2, I_C1 will induce emitter current in Q₂, Q₁ In case rate of rise of anode voltage is large, the emitter currents will be large and as a result, α₁₊α₂will approach unity leading to eventual switching action of the thyristor.

(iid Temperature triggering : At high temperature, the forward leakage current across junction J₂ rises. This leakage current serves as the collector junction current of the component transistors Q₁ and Q₂. Therefore, an increase in leakage current I_CI, I_C2 leads to an increase in the emitter currents of Q_l Q₂. As a result, (α₁₊α₂) approaches unity. Consequently, switching action of thyristor takes place.

(v) Light triggering : When light is thrown on silicon, the electron-hole pairs increase. In the forward-biased thyristor, leakage current across J₂ increases which eventually increases α₁₊α₂to unity as explained before and switching action of thyristor occurs.

As stated before, gate-triggering is the most common method for turning-on a thyristor. Light-triggered thyristors are used in HVDC applications.

The operational differences between thyristor-family and transistor family of devices may now be summarised as under :

i) Once a thyristor is turned on by a gate signal, it remains latched in on-state due to internal regenerative action. However, a transistor must be given a continuous base signal to remain in on-state.

ii) In order to turn-off a thyristor, a reverse voltage must be applied across its anode-cathode terminals. However, a transistor turns off when its base signal is removed.

THYRISTOR PROTECTION

Reliable operation of a thyristor demands that its specified ratings are not exceeded. In practice, a thyristor may be subjected to overvoltages or overcurrents. During SCR turn-on, di/dt may be prohibitively large. There may be false triggering of SCR by high value of dv/dt. A spurious signal across gate-cathode terminals may lead to unwanted turn-on. A thyristor must be protected against all such abnormal conditions for satisfactory and reliable operation of SCR circuit and the equipment. SCRs are very delicate devices, their protection against abnormal operating conditions is, therefore, essential. The object of this section is to discuss various techniques adopted for the protection of SCRs.

(a) di/dt protection. When a thyristor is forward biased and is turned on by a gate pulse, conduction of anode current begins in the immediate neighbourhood of the gate-cathode junction, Fig. 4.6 (a). Thereafter, the current spreads across the whole area of junction. The thyristor design permits the spread of conduction to the whole junction area as rapidly as possible. However, if the rate of rise of anode current, i.e. di/dt, is large as compared to the spread velocity of carriers, local hot spots will be formed near the gate connection on account of high current density. This localised heating may destroy the thyristor. Therefore, the rate of rise of anode current at the time of turn-on must be kept below the specified limiting value. The value of di/dt can be maintained below acceptable limit by using a small inductor, called di/dt inductor, in series with the anode circuit. Typical di/dt limit values of SCRs are 20-500 A/µ sec.

Local spot heating can also be avoided by ensuring that the conduction spreads to the whole area as rapidly as possible. This can be achieved by applying a gate current nearer to (but never greater than) the maximum specified gate current.

(b) dv/dtprotection. With forward voltage across the anode and cathode of a thyristor, the two outer junctions are forward biased but the inner junction is reverse biased. This reverse biased junction J₂, Fig. 4.3 (b), has the characteristics of a capacitor due to charges existing across the junction. In other words, space-charges exist in the depletion region around junction J₂ and therefore junction J₂ behaves like a capacitance. If the entire anode to cathode forward voltage V_a appears across J₂ junction and the charge is denoted by Q, then a charging current i given by Eq. (4.6) flows

i = dQ/dt =d(C_j V_a )/dt

=C_j (d V_a /dt) + V_a(d C_j /dt) …………..(4.6 a)

As C_j the capacitance of junction J₂ is almost constant, the current is given by

i = C_j (d V_a /dt) …………..(4.6 b)

If the rate of rise of forward voltage dV_a/dt is high, the charging current i will be more.This charging current plays the role of gate current and turns on the SCR even when gate signal is zero. Such phenomena of turning-on a thyristor, called dv/dt turn-on must be avoided as it leads to false operation of the thyristor circuit. For controllable operation of the thyristor, the rate of rise of forward anode to cathode voltage dV_a/dt must be kept below the specified rated limit. Typical values of dv/dt are 20 – 500 V/µsec. False turn-on of a thyristor by large dv/dt can be prevented by using a snubber circuit in parallel with the device

Design of Snubber Circuits for Thyristor Protection

A snubber circuit consists of a series combination of resistance R_s and capacitance C_s in parallel with the thyristor as shown in Fig. 4.25. Strictly speaking, a capacitor C_s in parallel with the device is sufficient to prevent unwanted dv/dt triggering of the SCR. When switch S is closed, a sudden voltage appears across the circuit. Capacitor C_s behaves like a short circuit, therefore voltage across SCR is zero. With the passage of time, voltage across C_s builds up at a slow rate such that dv/dt across C_s and therefore across SCR is less than the specified maximum dv/dt rating of the device. Here the question arises that if C_s is enough to prevent accidental turn-on of the device by dv/dt, what is the need of putting R_s in series with C_s ? The answer to this is as under.

Before SCR is fired by gate pulse, C_s charges to full voltage V_s. When the SCR is turned on, capacitor discharges through the SCR and sends a current equal to V_s/ (resistance of local path formed by C_s and SCR). As this resistance is quite low, the turn-on di/dt will tend to be excessive and as a result, SCR may be destroyed. In order to limit the magnitude of discharge current, a resistance R_s is inserted in series with C_s as shown in Fig. 4.25. Now when SCR is turned on, initial discharge current V_s/R_s is relatively small and turn-on di/dt is reduced.

In actual practice ; R_s, C_s and the load circuit parameters should be such that dv/dt across C_s during its charging is less than the specified dv/dt rating of the SCR and discharge current at the turn-on of SCR is within reasonable limits. Normally, R_s C_s and load circuit parameters form an underdamped circuit so that dv/dt is limited to acceptable values.

The design of snubber circuit parameters is quite complex.. In practice, designed snubber parameters are adjusted up or down in the final assembled power circuit so as to obtain a satisfactory performance of the power electronics system.

Overvoltage Protection in Thyristors

Thyristors are very sensitive to overvoltages just as other semi-conductor devices are. Overvoltage transients are perhaps the main cause of thyristor failure. Transient overvoltages cause either maloperation of the circuit by unwanted turn-on of a thyristor or permanent damage to the device due to reverse breakdown. A thyristor may be subjected to internal or external overvoltages ; the former is caused by the thyristor operation whereas the latter comes from the supply lines or the load circuit.

(i) Internal overvoltages. Large voltages may be generated internally during the commutation of a thyristor. After thyristor anode current reduces to zero, anode current reverses due to stored charges. This reverse recovery current rises to a peak value at which time the SCR begins to block. After.this peak, reverse recovery current decays abruptly with large di/dt. Because of the series ifiductance L of the SCR circuit, large transient voltage

L di/dt is produced. As this internal overvoltage may be several times the breakover voltage of the device, the thyristor may be destroyed permanently.

{ii) External overvoltages. External overvoltages are caused due to the interruption of current flow in an inductive circuit and also due to lightning strokes on the lines feeding the thyristor systems. When a thyristor converter is fed through a transformer, voltage transients are likely to occur when the transformer primary is energised or de-energised. Such overvoltages may cause random turn on of a thyristor. As a result, the overvoltages may appear across the load causing the flow of large fault currents. Overvoltages may also damage the thyristor by an inverse breakdown. For reliable operation, the overvoltages must be suppressed by adopting suitable techniques.

Suppression of overvoltages. In order to keep the protective components to a minimum, thyristors are chosen with their peak voltage ratings of 2.5 to 3 times their normal peak working voltage. The effect of overvoltages is usually minimised, by using RC circuits and non-linear resistors called voltage clamping devices.

The RC circuit, called snubber circuit, is connected across the device to be protected, see Fig. 4.29. It provides a local path for internal overvoltages caused by reverse recovery current. Snubber circuit is also helpful in damping overvoltage transient spikes and for limiting dv/dt across the thyristor. The capacitor charges at a slow rate and thus the rate of rise of forward voltage (dv/dt) across SCR is also reduced. The resistance R_s damps out the ringing oscillations between the snubber circuit and the stray circuit inductance. Snubber circuits are also connected across transformer secondary terminals to suppress overvoltage transients caused by switching on or switching off of the primary winding. As snubber circuits provide only partial protection to SCR against transient overvoltages, thyristor protection against such overvoltages must be upgraded. This is done with the help of voltage-clamping devices.

A voltage-clamping (V.C.) device is a non-linear resistor connected across SCR as shown in Fig. 4.29. The V.C. device has falling resistance characteristic with increasing voltage, Fig. 4.27 (a). Under normal working conditions of voltage below the clamping level, the device has a high resistance and draws only a small leakage current, When a voltage surge appears, the V.C. device operates in the low resistance region and produces a virtual short circuit across the SCR. The increased current associated with virtual short circuit produces an increased voltage drop in the source and line impedances and as a result, voltage across SCR is clamped to a safe value. After the surge energy is dissipated in the non-linear resistor, the operation of the V.C. device returns to its high resistance region. Selenium thyrector diodes, metal oxide varistors or avalanche diode suppressors are commonly employed for protecting the thyristor circuit against overvoltages. As the voltage clamping ability of a thyrector is inferior to those of metal oxide varistor and avalanche-diode suppressor, use of thyrector is on the decline.

It has already been stated that RC snubber is not enough for overvoltage protection of SCR. In practic, therefore, a combined protection consisting of RC snubber and V.C. device is provided to thyristors

Overcurrent Protection in Thyristors

Thyristors have small thermal time constants. Therefore, if a thyristor is subjected to overcurrent due to faults, short circuits or surge currents ; its junction temperature may exceed the rated value and the device may be damaged. There is thus a need for the overcurrent protection of SCRs. As in other electrical systems, overcurrent protection in thyristor circuits is achieved through the use of circuit breakers and fast-acting fuses as shown in Fig. 4.29.

The type of protection used against overcurrent depends upon whether the supply system is weak or stiff. In a weak supply network, fault current is limited by the source impedance below the multi-cycle surge current rating of the thyristor. In machine tool and excavator drives, if the motor stalls due to overloads, the current is limited by the source and motor impedances. The filter inductance commonly employed in dc and ac drives may limit the rate of rise of fault current below the multicycle surge current rating of the thyristor. For all such systems, overcurrent can be interrupted by conventional fuses and circuit breakers. However, proper co-ordination is essential to guarantee that (i) fault current is interrupted before the thyristor is damaged and (ii) only faulty branches of the network are isolated.

Conventional protective methods are, however, inadequate in electrical stiff supply networks. In such systems, magnitude and rate of rise of current is not limited because source has negligible impedance. As such, fault current and therefore junction temperature rise within a few milliseconds. Special fast-acting current-limiting fuses are, therefore, required for the protection of thyristors in these stiff supply networks.

The operation of fast-acting current-limiting fuse is illustrated in Fig. 4.27 (b). These fuses and thyristors are found to have similar thermal properties, there co-ordination is therefore simpler. The current-limiting fuse consists of one or more fine silver ribbons having very short fusing time. In Fig. 4.27 (b), fault is shown to occur at zero crossing of the ac sine wave, i.e. at t = 0, Without fuse, the fault current would rise upto A and then would follow dotted curve. A properly selected current limiting fuse melts at A. An arc is then struck. For a brief interval after A, the current continues to rise depending upon the circuit parameters and the fuse design. This current reaches a peak value, called peak let through current, which is indicated by point B in Fig. 4.27 (b). Note that peak let through current is considerably less than the peak fault current without the fuse, the latter is indicated by point D. After the point B, arc resistance increases and fault current decreases. At point C, arcing stops and the fault current is cleared. The total clearing time t_c is the sum of melting time t_m and arcing time t_a, i.e. t_c = t_m + t_a.

Proper co-ordination between fast-acting current-limiting fuse and thyristor is essential. A fuse carries the thyristor current as both are placed in series. Therefore, the fuse must be rated to carry full-load current plus a marginal overload current for an indefinite period. But the peak let through current of fuse must be less than the subcycle surge current rating of the SCR. The voltage across the fuse during arcing period is known as arcing, or recovery, voltage. This voltage is equal to the sum of source voltage and the emf induced in the circuit inductance during arcing time t_a. If the fuse current is interrupted abruptly, induced e.m.f.

L – may be high; as a result arcing voltage would be excessive. It should therefore be ensured during fuse design and co-ordination that arcing voltage is limited to less than twice the peak supply voltage. In case voltage rating of the fuse is far in excess of circuit voltage, an abrupt current interruption would lead to dangerous overvoltages.

When both circuit breaker and fast-acting current-limiting fuse are used for overcurrent protection of SCR, Fig. 4.29, the faulty circuit must be cleared before any damage is done to the device. A circuit breaker has long tripping time, it is therefore generally used for protecting the semiconductor device against the continuous overloads or against surge currents of long duration. A fast-acting C.L. fuse is used for protecting thyristors against large surge currents of very short duration. The tripping time of the circuit breaker, the fusing-time of the fast-acting fuse must be properly co-ordinated with the rating of a thyristor. In order that fuse protects the thyristor reliably, the I²t rating of the fuse must be less than that of the SCR.

Electronic crowbar protection

As thyristor possesses high surge current capability, it can be used in an electronic crowbar circuit for overcurrent protection of power converters using SCRs. An electronic crowbar protection provides rapid isolation of the power converter before any damage occurs

Fig. 4.28 illustrates the basic principle of electronic crowbar protection. A crowbar thyristor is connected across the input dc terminals. A current sensing resistor detects the value of converter current. If it exceeds preset value, gate circuit provides the signal to crowbar SCR and turns it on in a few microseconds. The input terminals are then short-circuited by crowbar SCR and it shunts away the converter overcurrent. The crowbar thyristor current depends upon the source voltage and its impedance. After some time, main fuse interrupts the fault current. The fuse may be replaced by a circuit breaker if SCR has adequate surge current rating.

Gate Protection in Thyristors

Gate circuit should also be protected against overvoltages and over currents. Overvoltages across the gate circuit can cause false triggering of the SCR. Overcurrent may raise junction temperature beyond specified limit leading to its damage. Protection against over-voltages is achieved by connecting a zener diode ZD across the gate circuit. A resistor R₂ connected in series with the gate circuit provides protection against overcurrents.

A common problem in thyristor circuits is that they suffer from spurious, or noise, firing. Turning-on or turning-off of an SCR may induce trigger pulses in a nearby SCR. Sometimes transients in a power circuit may also cause unwanted signal to appear across the gate of a neighbouring SCR. These undesirable trigger pulses may turn on the SCR leading to false operation of the main SCR. Gate protection against such spurious firing is obtained by using shielded cables or twisted gate leads. A varying flux caused by nearby transients cannot pass through twisted gate leads or shielded cables. As such no e.m.f. is induced in these cables and spurious firing of thyristors is thus minimised. A capacitor and a resistor are also connected across gate to cathode to bypass the noise signals, Fig. 4.29. The capacitor should be less than 0.1 µF and must not deteriorate the waveshape of the gate pulse.

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Tayyab Siddiqui