CHAPTER 1Introduction

THYRISTOR FIRING CIRCUITS

THYSITORS: -Modern power electronics truly begin with the advent of thyristor. Thyristor is the generic term, which comes under the class of power semiconductor that has four layers of alternating P and N material. Large number of power semiconductor devices belongs to the family such as SCR, TRIAC, RCT (Reverse conducting SCR, Asymmetric SCR (ASCR) , Light activated SCR (LASER),Gate turn off Thyristor(GTO).Conduction of these power devices are controlled by the current supplied to the gate terminal except GTO and also all the devices except TRIAC are unidirectional therefore current mainly flows from the anode to the cathode.

SCR (Silicon controlled rectifier)-SCR was invented in 1957 at bell laboratories in USA. The first prototype was introduced by GEC (USA) in 1957.The word Thyristor is a combination of two words (THYRatron + transISTOR). So word thyristor can be expressed as a solid-state device like a transistor and has a characteristic of a thyratron tube.SCR or Silicon Controlled Rectifier is a 3-pin gadget, having three essential terminals-anode, cathode and gate. The gate terminal is the control terminal for the use of anode-cathode voltage. Commonly Silicon is utilized in because of its low leakage current. The extremity of the voltages connected to the cathode and anode chooses whether the gadget is in forward biased or reversed biased and the gate voltage chooses the conduction of the SCR. At the end of the day, when forward bias is connected to the SCR, after appropriate positive gate voltage is connected, the gadget conducts but turns off when the current through the gadget is made not as much as the holding current. In this manner the SCR can be utilized as a switch.

Fig 1. Symbolic representation of thyristorSince we know that SCR is a 4-layer device that has got alternative layers of P and N type of materials with three junction j1, j2, j3. Alternating layer of P and N type of material gives SCR two type of structure PNPN or NPNP structure.. For the PNPN structure, the anode is connected to P type material and the cathode is connected to N type material. The gate is connected to the P type material near the cathode. SCR are typically used for controlling high power in terms with high voltage because of this, SCR find its application in the high voltage AC power control system, lamp dimmer circuit, regulator circuit. It also finds its application in the rectification of high power AC in high volt DC power transmission.The operation of PNPN devices can be visualised as specially coupled pair of transistor shown in the figure 2.The PNPN transistor can be defined as two-transistor PNP and NPN connected to each other as shown in the figure 2.The connection of these two transistor is such that it enables the regenerative action when a suitable gate signal is applied to the base of NPN transistor. Initially

the leakage current of specially coupled two transistors feed back amplifier is low (so combined hfe is less than unity) that keeps the circuit in an off state conduction. A positive pulse applied to the gate sets the NPN transistor into conduction which inturn the PNP transistor into conduction. Once both the transistors conducts the effective hfe momentarily becomes greater than unity so that specially coupled two transistors gets saturated and once the device gets saturated, the current through the transistors is enough to keep the combined hfe greater than unity. The circuit will remain ‘on” until its turned off by reducing the anode to cathode current It so that the combined hfe is less than unity and regeneration ceases. This threshold anode current is the holding current of SCR.

Figure 2-Coupled pair of transistorStatic VI characteristics of Thyristor

For obtaining the static VI characteristics of the a thyristor, we use the elementary circuit of SCR shown below in figure 3.In the figure shown below, the anode and cathode is connected to main source through load and gate and cathode gets supply from another source load “Eg”

Figure 3-Elemantary circuit diagram of SCR

The Static VI characteristic of thyristor is shown below in figure 4.The graph is plotted between voltage and current. The working of thyristor can be divided into three-region.1.Forward blocking mode – In this mode, when the thyristor anode voltage is made positive with respect to cathode, the junction J1 and J3 gets forward biased and the junction J2 is reversed biased and only a small leakage current flows from anode to cathode. In this state, the thyristor is said to be in forward blocking or off state condition whereas the leakage current is known as off state current (ID). In this mode, the SCR is treated as open switch. The point from O to M shows the forward blocking mode of SCR.

2.Forward conduction mode –In this mode, when VAK (Anode to cathode voltage) is increased with gate circuit open to a large value such that junction J2 which was reversed biased breaks. This is known as avalanche breakdown and voltage corresponding to this is known as forward breakdown voltage VBO.Thyristor turns ON with point N shifting to N after the breakdown and starts conducting. The point NK is referred as forward conduction mode.Thyristor can be shifted from forward blocking mode to forward conduction mode by two ways1. By applying a gate pulse between gate and cathode.2. By applying forward breakdown voltage VBO across anode and cathode.Latching current-It is given by IL and is associated with turn on process. Latching current is defined as min value of anode current, which must be attained during turn on process to maintain conduction when gate signal is removed or in simple words this can be defined as minimum current required to latch thyristor into on state.Holding current –It is written as IH and is associated with turn off process. Holding current is defined as minimum value of anode current below which the SCR gets turned off.Latching current is more than holding current.3.Reverse blocking mode-In this mode, the cathode of SCR is made positive w.r.t anode with no gate current. Junction J1 and J3 are reversed biased and Junction J2 is forward biased. This mode (reverse blocking mode) is also known as off state thyristor or reserve blocking state and reserve leakage current known as reserve current IB flows through the device. Reverse breakdown voltage VBR must not be exceeded.

Figure 4 -Static VI characteristics of SCRThyristor Turn on or triggering methods-The word triggering is defined a process by which a thyristor or SCR is turned on. Keeping anode positive with respect to cathode can turn on the thyristor. (i.e. by increase the anode current). Following are the techniques, which can be used to turn on a thyristor-1.Forward voltage triggering- In this method of triggering, a forward biased voltage is applied to SCR anode and cathode (i.e. anode is connected to positive and cathode is connected to negative if the applied voltage). This is known as Voltage triggering.So when the thyristor is forward biased and applied voltage is increased across anode and cathode, the depletion layer of reversed biased junction is totally destroyed. When breakdown occurs, depletion layer is totally destroyed because of which thyristor triggers and come to on state. Since the SCR is turned on by voltage so this process is called voltage triggering.2.dV/dT triggering-This triggering method use the high rate of increase of voltage (or dV/dT i.e. rate of change of voltage with respect to time. Since we know that an SCR is optimised for a critical rate of rise of voltage, if we increase the rate of rise of voltage, the thyristor will be triggered and will start conducting.

3.Thermal triggering – In this method of triggering, thyristor is turned on by thermal effect (i.e. by increasing the junction temperature) called as thermal triggering.Semi conductor materials conductivity increases with increase in the temperature .we use this property of the thyristor to trigger it. As we increase the temperature, it increases the number of electrons-holes pair, which in turn increases the leakage current. This increase in the leakage current cause alpha1 and alpha2 to increase, due to regenerative action (alpha1 and alpha 2) may tend to unity and thyristor is turned on. The one thing to remember in this triggering method is that temperature should not be increased to a high value because it might damage the device. 4.Light triggering-In this method triggering is done by radiation i.e. bombardment of energy particles such as neutron photon etc. In this method, light strikes the junction of a thyristor, causing electron-hole pairs increase that causes the thyristor to turn on. Allowing light to strike the silicon wafers turns on these light activated thyristor.5.Gate triggering –Gate triggering is defined as turning on the thyristor or SCR by applying a signal between gate and cathode. In this method, when thyristor is forward biased, a positive signal is applied between gate and cathode terminal. By this process of triggering, we can trigger the device much before its breakdown voltage, so we can say that by increasing the gate current, the forward blocking voltage is decreased as it can be seen in the figure 5 below

Figure 5-Effects of gate current on forward blocking voltage

Thyristor turn on gate characteristics (paraphrase )The figure-6 shows us the waveform of anode current in contrast with gate current. There is a time delay known as turn on time ‘tON” between application of gate signal and the conduction of thyristor.

-Figure 6-Turn on characteristics

The turn on time “tON” is defined as a time interval between 10% of steady state gate current (0.1iG) and 90% of the steady state thyristor on State current (0.9IT).

tON =td (delay Time) + tr (Rise time)

Delay time (td)- Delay time is defined as the time interval between 10% of gate current (0.1IG) and 10% of the thyristor on state current (0.1IT).

Rise time (tr)- Rise time is defined as time required for anode current to rise from 10% of on state current (0.1IT) to 90% of on state current (0.9IT). Following point should be considered in designing the gate control current-1. Gate signal should be removed after thyristor is turned on. A continuous gating signal would

increase the power loss in the gate junction.2. When thyristor is reversed biased, there should be no gate signal; otherwise thyristor may fail

due to increased leakage current. 3. The width of gate pulse tG must be longer than time required for the anode current to rise to

the holding current value (IH). In practice, the pulse width tG is normally made more than the turn on time tON of the thyristor.

Thyristor turn off The word commutation refers to turning off an SCR. A thyristor which is on state can be turned off by reducing the forward current (anode current). To a level which is less than holding current (iH) falling are the two conditions that must be satisfied for the commutation.

1. Anode current or forward current of an SCR must be bought down to a level which is below the holding current.

2. Enough reverse voltage must be applied across the SCR so that it can regain its forward blocking state.

Methods of turning off an SCR or commutation The use of reverse voltage causes the SCR to commutate, this reverse voltage is known as commutation voltage. Depending upon where the commutation voltage is applied commutation can be divided into two major parts.

1. Natural commutation- In natural commutation the voltage applied to SCR is termed as commutation voltage. If SCR is supplied by an AC, at every end of a positive half

cycle the anode current goes through the natural current zero and also immediately a reverse voltage is applied across an SCR these are the conditions to turn off an SCR.

The natural commutation is also known as sourced commutation or line commutation or class F commutation. This type of commutation is possible with line commutated inverters, controlled rectifiers, cyclo converters and AC voltage regulators because the supply source is AC in all the converters.

2. Forced commutation- Since the direct current is constant in nature so there is, no natural current zero to turn off the SCR. In such cases the forward current must be forced to zero with an external circuit to commutate, so this is the reason this type of commutation is referred as forced commutation.

The circuit used for the forced commutation uses components such as capacitors and inductors also known as commutated components. The major task of these commutating components is to apply reverse voltage across the SCR to bring down the current to zero. The forced commutation is mainly used in choppers and inverter circuits. Depending on the arrangement of the commutating components and how the zero current is achieved, the forced commutation is classified into class A, B, C, D and F.

Dynamic turn off switching characteristicsSince commutation refers to the change of an SCR from forward conducting state to forward blocking state and we know that once an SCR starts conducting the gate does not have any control to bring back to forward blocking or off state. As discussed above, for turning off the SCR current must be reduced to a level below the holding current. One of the simplest method of turning off an SCR is to reduce forward current to zero but if we apply forward voltage after the current zero after SCR it starts conducting again even without gate triggering which is because of the presence charge carriers in the four layers so it makes it obvious to apply reverse voltage for a finite time across SCR to remove charge carriers.

Hence turn off time of an SCR is defined as the time between the instant the anode current becomes zero and the instant which SCR retains the forward blocking capability. The excess charge carriers from four layers must be removed to bring back the SCR to forward blocking mode. This process takes place in two stages.

1. First stage – in this stage the excess charge carriers from the outer layers are removed and in the second stage the excess carriers in the inner layer are to re combined hence the total turn off time (tq ) is divided into intervals: reverse recovery time (trr) and the gate recovery time (tqr). tq = trr + tgr.

The figures shown below show us switching characteristics of an SCR during a turn on and turn off. The time T1 to T3 is called reverse recovering time. At an instant t1 the anode current is zero and builds up in a reverse direction, which is called as reverse recovery current. This current removes the excess charge carriers from the outer layer during the time t1 to t3 .

Figure:

At instant J1 and J3 are able to block the reverse voltage but SCR is not yet ale to block the forward voltage due to the presence of excess charge carriers injunction J2. These carriers can disappear only by the way of recombination and this can be achieved by maintaining a reverse voltage across the SCR. Hence during the t3 to t4, the recombination of charge takes place and at the instant t4 the junction J2 completely recovers. This time is called gate recovery time.

Point to remember from the above figure 1. Turn off time is between t1 to t4. Generally this time varies from 10 to 100

microseconds. This turn off time tq is applicable to individual SCR 2. Circuit turn off time (tc) is defined as the time required by the commutation circuit to

apply the reverse voltage to commutate the SCR circuit. For safety reasons, the circuit turn off time must be greater than the turn off time of an SCR otherwise commutation failure occurs.

3. SCR with slow turn off time that is between 50 to 100 micro seconds are called convertor grade SCR and are used in face controlled rectifiers, cyclo converters, AC voltage regulators etc.

4. SCR with fast turn off time that is 3 to 50 micro seconds are termed as invertor grade SCR which are costlier as compared to the convertor grade SCR. These SCRs are used in choppers, forced commutated invertors and converters.

Thyristor Protection

For reliable operations of the thyristor or SCR the specified rating must not the exceeded but SCR are subjected to over current and over voltages. During the turn of an SCR the rate of change of current with respect to time (di/dt) maybe large. Maybe a false triggering of a SCR by high value of (dv/dt). Since SCRs are very delicate devices they must be protected against abnormal conditions. Since SCR is exposed to both over current and over voltage therefore it needs to be protected from both. (di/dt) protection and (dV/dt) protection. The (di/dt) protection can be achieved by adding a small inductor whereas (dv/dt) protection can be achieved by using snubber circuit parallel to the device.

(Figure)

Heating, cooling and mounting of Thyristor In some power losses occurs when the thyristor is turned on or while it’s working. Following are the types of losses that occur during the operation are

1. Forward conduction losses. 2. Losses due to leakage current during forward and reverse blocking 3. Switching losses at turn on or turn off

Mostly the heat produced in the SCR is by electrical loses, that heat is dissipated in the sink. When the heat due to the losses is equal to the heat dissipated by the heat sick, a study junction temperature is reached. Heating and the junction temperature rise after thyristor is depended upon the current handle by the device during the operation.

Thyristor mounting techniques

Depending the high and low power rates of thyristor five major mounting techniques of SCR are given below. SCR mounting should be searched that it facilitates heat flow from junction to the case.

1. Lead mounting- for SCRs, which have a low current rating of one ampere we use, lead mounting. The SCRs which are mounted by the technique are doesn’t require a heat sink or additional cooling. The housing dissipates the energy by radiation and convections.

2. Stud mounting- The stud mounting is widely used due to its flexibility and ruggedness. The SCR in this case is attached to the heat sink by the means of threaded nuts and studs. Anode gets electrically connected to the heat sink.

3. Bolt down mounting- This mounting is also known as flat pack mounting. This type of device mounting has tabs with one or more holes, bolts are pushed in those holes to mount the device on the heat sink with nuts. This device needs isolation from the heat sink. Bolt down mounting is used for small and medium ratings.

4. Press fit mounting - this type of mounting is used for insertion into an approximate sized hole in the heat sink. This insertion can be performed by using a vice hand pressing device into the hole using a wooden block for larger sizes it’s carried out by hydraulic ram. This type of mounting is used for large rated SCRs.

5. Press- pack mounting- This is also known as disc or hockey puck mounting because of the shape. In this case SCR is fixed between two heat sinks and external pressure is applied evenly to avoid deformation. These types of mounting are used for high current rated SCRs.

Firing circuits for Thyristor

SCR can be turned on from off state to on state in several ways such as forward voltage triggering, dv/dt triggering, temperature triggering, light triggering and gate triggering. Gate triggering is one of the most common method and most reliable method for SCR because it’s efficient and itself accurately for turning on the SCR at the desired instant of time.

Main features of firing circuits-

The most common method for turning on an SCR is by gate voltage control. The gate control circuit is called firing or triggering circuit. These gating circuits are usually low power electronic circuit. A firing circuit should fulfil the following two functions

1. If a power circuit has more than one SCR, the firing circuit should be able to produce gating pulses for each DCR at desired instant for proper operation of the power circuit. The pulses produced should be in periodic in nature and the sequences od the firing must be correspond with the thyristorised power controller.

2. The control signal generated by a firing circuit may or may not be able to turn on a SCR.IT is therefore common to feed the voltage pulses to a driver circuit and then to the gate cathode circuits .A driver circuit consist of a pulse amplifier and pulse transformer.

General Layout of firing circuit scheme for Silicon controlled rectifier –

Figure

The word firing angle refers to the start point of SCR where it start conducting on the ac source that start point is defined as Firing angle. . Firing circuit schemes in general

consist of the component shown in the above figure .A regulated Dc power supply is obtained from an alternating voltage source. Pulse generator, which is supplied from both AC and DC source, gives out pulses (voltage), which are fed to pulse amplifier for amplification. Shielded cables transmit the amplified pulses to the pulse transformer. This function of pulse transformer is to isolate the low voltage gate cathode circuit from the high voltage anode cathode circuit.

Firing angle control- the control of ‘firing angle’ is used to control the speed of fan, motors, intensity of bulb by controlling the power to SCR. By changing the time of application of gate pulses to the SCR controls this firing angle. Variation in firing angle of SCR by delaying the application of gate current can be done by two ways.

1.Phase shift gate control-This control causes 0 to 180 degree delay of conduction. The phase gate voltage is changed with respect to anode cathode voltage. Usually capacitor or inductor is used for this purpose.

2.Pulse triggering –The gate voltage can also be applied by giving pulses to the gate terminal. The duty cycle of the pulses can be varied to provide variation in the conduction. Pulses can be generated either by using UJT or using 555 timers.

Type of firing circuits

Resistance firing circuits:- This triggering circuit (Resistance firing circuit) simplest and most economical but has a very limited rage of firing angle control(i.e. 0 to 90 degrees).The performance of resistance firing circuit greatly depends on temperature and is different for different SCRs. The basic diagram for resistance triggering circuit is shown below. In case gate R2 is zero, current may flow from the source through load followed by R1 and diode and gate to cathode. The current should not exceed from max value of gate current (Igm). The R1 function is to limit the value of gate current to safe value as R2 is varied. The resistance R should be of such a value that maximum voltage drop should not exceed maximum possible gate voltage drop Vgm that is only possible when the value of R2 is zero. Since the value of R1 and R2 are large, gate trigger circuit draws a small current. Diode D allows the flow of current during positive half cycle only i.e. gate voltage Vg is half wave dc pulse). Varying variable resistance can control the amplitude of this dc pulse.

Figure - Resistance firing circuit.

Waveform

The value of R2 determines the gate voltage amplitude. So if the value of R2 is large then it will draw small amount current and the voltage across that will be also small as shown in figure (a) where Vgt (gate trigger voltage) and Vgp(peak of gate voltage).Since Vgp is less tha Vgt ,SCR will not turn on. So output voltage and current (Vo and Io ) are zero and supply voltage Vs appears across Vt. Since we are just using the resistance, gate voltage is in phase with the source voltage. R2 is varied as such that now Vgp=Vgt, the value of firing angle is 90 degree. The waveform for current and voltage for this case is shown in figure …. Part (b).In case when Vg is greater than Vgt, as soon as Vg becomes equal to Vgt for the first time the SCR is turned on .The resistance triggering circuit cannot give firing angle beyond 90 degree. When Vg increases above Vgt SCR is turned on at firing angle less than 90 degree. When Vg reaches Vgt for the first time, SCR fires and gate loses control and Vg is reduced to almost zero value as shown in figure part (c).From the circuit we get to know that firing angle depends upon the resistance R2.So increasing the R2 from small value firing angle increases but in this case firing angle cannot be more than 90 degree.

Figure -Resistance triggering circuit of a SCR in half wave circuit with Dc load

(a.) No triggering of SCR (b.) firing angle =90 degree (c.) firing angle greater than 90 degree

RC firing circuit-

RC firing circuit can overcome the limiting range of firing angle controlled by Resistance firing circuit. In this case we will just study two different variations of the RC triggering circuit.

1.RC half wave trigger circuit-The figure shown below is RC half wave trigger circuit. The firing angle can be controlled by varies the value of R i.e. from 0 to 180 degree. The capacitor used to shift the phase of gate voltage and D1 is used to prevent negative voltage from reading the gate cathode of SCR.

During negative half cycle, the capacitor charges to the peak value of supply (Vm) through the diode D2. This voltage is maintained across the capacitor till the supply voltage crosses zero. As the supply becomes positive, the capacitor charges through resistor R from initial voltage of Vm to a positive value. When the capacitor voltage is equal to the gate trigger voltage of the SCR, the SCR is fired and the capacitor voltage is clamped to a small positive value.

Case 1 R is large –When the value of resistance is large, time taken by the capacitor to charge from Vm to Vgt is large, which result in large firing angle and lower load voltage.

Case2 R is small- when the value of R is small, capacitor charges at faster rate towards Vgt and result in early triggering of SCR and hence Vl is more. When SCR triggers, the voltage drop across it falls to 1-1.5V.This in turn lower the voltage across R and C. Low voltage across the SCR during conduction period keeps the capacitor discharged during this positive half cycle.

Figure- Waveforms for RC half-wave trigger circuit

a.) High value of R b.) Low value of R

B.RC full wave-For this circuit given below, Initial voltage from which capacitor C charges is essentially zero. The capacitor C is reset to this voltage by clamping action of the thyristor .For this reason the charging time constant RC must be chosen longer than the half wave RC circuit in order to delay the triggering.

figure-RC full wave trigger circuit

Figure-waveform of the RC full wave trigger circuit

3.UJT (Uni junction transistor)

A unijunction transistor is composed of a bar of N type section having a P type connection in middle connection at the end is known as bases B1 and B2.The P type mid point is the emitter.( with emitter disconnected, the total resistance Rbbo is the sum of RB1

and RB2)

VI characteristics of UJT or Emitter characteristics curve-

The UJT emitter current vs Voltage characteristics Curve shown that as VE increases, current IE also increases until IP at the peak value or point. Beyond this peak point, the current increases as voltage decreases in the negative resistance region. The voltage reaches its minimum value at value point. The resistance of RB1,the saturation is lowest at the valley point.

UJT oscillator triggering

Unijunction transistor is a highly efficient switch i.e. switching time is in the range of nanoseconds. UJT exhibits negative resistance characteristics; it can be used as a relaxation oscillator(emiiter.

Below diagram with UJT working in the oscillator mode. External resistance R1 and R2 are small in comparison with the internal resistance RB1, RB2 of UJT bases. Charging resistance R should be such that its load line intersect the device characteristics only in the negative resistance region. When source voltage VBB is applied, capacitor C begins to charge through R exponentially toward VBB. During this charging, emitter circuit of UJT is an open circuit.

Vc(capacitor voltage )=VE(emitter voltage)

Vc=VE=VBB(1-e-t/RC)

When this emitter voltage VE or VC reaches the peak point voltage Vp, the unijunction between E-B1 breaks down. As a result, UJT turns on and capacitor (C) rapidly discharge through low resistance R1 with time constant (R1C). Here time constant 2 is smaller than time constant1.when the emitter voltage decays to valley point voltage Vv, UJT turns off.

The time ‘T’ required for capacitor ‘c’ to charge from initial voltage Vv to peak point voltage Vp through large resistance R,

Vp=n(VBB)+VD

Vp=Vv+VBB (1-e-t/RC)

VD=Vv

n=(1-e-t/RC)

T=1/F=RC ln(1/(1-n))

In case T is taken as time period of output pulse duration, then value of firing angle is given by wt,

WRC ln(1/(1-n))

W=angular frequency of UJT oscillator.

USE OF PULSE TRANSFORMER IN FIRING CIRCUIT-

The pulse transformer is often used in firing circuit for thyristor and GTO. Transformer usually has two secondaries. These transformer are designed to have low winding resistance, low leakage reactance and low inter winding capacitance.

Following are the advantages of pulse transformer in triggering semi conductor devices.

1. Isolation of low voltage gate circuit from the high voltage anode current.2. Triggering of two or more devices from the same trigger source.