p120Another name: SCR—silicon controlled rectifierThyristor Opened the power electronics era–1956, invention, Bell Laboratories–1957, development of the 1st product, GE–1958, 1st commercialized product, GE–Thyristor replaced vacuum devices in almost every power processing

area.

Still in use in high power situation. Thyristor till has thehighest power-handling capability.

THE THYRISTORS

Appearance and symbol of thyristors

(a) Constructional details (b) Schematic diagram and (c) circuit symbol of a thyristor.

Structural details of conventional centre-gate thyristor.

Static I-V Characteristics of a Thyristor (a) Elementary circuit for obtaining thyristor I-V characteristics (b) Static I-V characteristics of thyristor.

Static characteristics of thyristor Blocking when reverse biased, no matter if there is gate current applied. Conducting only when forward biased and there is triggering current applied to the gate. Once triggered on, will be latched on conducting even when the gate current is no longer applied.

Structure and equivalent circuit of thyristor

P 1

A

G

K

N 1

P 2 P 2

N 1

N 2

a)

NPN

PNP

A

G

K

I G

I K

I c2

I c1

I A

V 1

V 2

b)

Structure

Equivalent circuit

Equivalent circuit

R

NPN

PNP

A

G

S

K

E G

I G

E A I K

I c2

I c1

I A

V 1

V 2

Equivalent circuit: A pnp transistor and an npn

transistor interconnected together

Positive feedback Trigger

Can not be turned off by control signal

Half-controllable

I c1=α 1 IA + I CBO1 （1-1） I c2= α 2 IK + I CBO2 （1-2）

IK=IA+IG （1-3）

IA=Ic1+Ic2 （1-4）

(

I I I

) 1 2 1

CBO2 CBO1 G 2 A

I ( 1-5 )

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I c1=α 1 IA + I CBO1 （1-1） I c2= α 2 IK + I CBO2 （1-2）

IK=IA+IG （1-3）

IA=Ic1+Ic2 （1-4）

(

I I I

) 1 2 1

CBO2 CBO1 G 2 A

I ( 1-5 )

Typical variation of current gain with emitter current of a thyristor.

Methods to trigger thyristor on Gate current trigering High voltage across anode and cathode—avalanche breakdown High rising rate of anode voltage dv/dt too high High junction temperature Light activation

Effect of gate current on forward breakover voltage.p142

Switching characteristics of thyristor

(a) Distribution of gate and anode currents during delay time (b) Conducting area of cathode (i) during td (ii ) after tr (iii) after tp

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. .

Switching Characteristics during Turn-on The delay time td

is measured from the instant at which gate current reaches 0.9 Ig to the instant at which anode current reaches 0.1 Ia

the time during which anode voltage falls from Va to 0.9 Va the time during which anode current rises from forward leakage current to 0.1 Ia

Rise time tr isthe time taken by the anode current to rise from 0.1 Ia to 0.9 Ia the time required for the forward blocking off –state voltage to fall from 0.9 to 0.1 of its initial value.

inversely proportional to the magnitude of gate current and its build up rate. Thus tr can be reduced if high and steep current pulses are applied to the gate. The main factor determining tr is the nature of anode circuit. For example,for series RL circuit, the rate of rise of anode current is slow, therefore, tr is more. for RC series circuit, di/dt is high, tr is therefore, less.During rise time,turn-on losses in the thyristor are the highest due to high anode voltage (Va) and large anode current (Ia) occurring together in the thyristorAs these losses occur only over a small conducting region, local hot spots may be formed and the device may be damaged.

Spread time tp is the time taken by the anode current to rise from 0.9 Ia to Ia

the time for the forward blocking voltage to fall from 0. 1 of its initial 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 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 µs.Total turn on time depends upon the anode circuit par ameters and the gate signal waveshapes. higher the magnitude of gate current , the lesser time it takes to inject the sufficient charge. The turn·on time can be reduced by using higher values of gate currents.When gate current is several times higher than the minimum gate current required, a thyristor is said to be hard-fired or overdriven.

Reduces its tum-on time and enhances i ts di/dt capability.

Typical waveform for gate current.

Switching Characteristics during Turn-off The dynamic process of the SCR from conduction state to forward blocking state is called commutation process or turn -off process . The SCR can be turned off by: reducing the anode current below holding current IH· IA < IH

reversing the voltage across the SCR. Vak < 0

The turn-off time tq 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 tq, all the excess carriers from the four layers of SCR must be removed.

sweeping out of holes from outer p-Iayer and electrons from outer n-layer. the carriers around junctionJ2 can be removed only by recombination.

The tum-off time is divided into two intervalsreverse recovery time trr and the gate recovery time tgr .

tq = trr + tgr

Gate characteristics of thyristor.

represents the lowest voltage values that must be applied to turn-on the SCR.

represents the highest possible voltage values that can be safely applied to gate circuit.

rated (average) gate power dissipation Pgav specified for each SCR These limits should not be exceeded in order to R.void permanent damage of junction J3

Forward gate characteristics of thyristor.

p133

THYRISTOR RATINGS

Thyristor ratings indicate voltage, current, power and temperature limits within which a thyristor can be used without damage or malfunction. Ratings and specifications serve as a link between the designer and the user of SCR systems.Subscripts are associated with voltage and current ratings for convenience in identifying them.

First subscript letter indicates the direction or the state : D = forward-blocking region with gate circuit open; T = on-state; R = reverse F = forward. Second subscript letter denotes the operating values. W = working value;R = repetitive value ;S = surge or non-repetitive value;T = trigger Third subscript letter indicates the maximum or peak value. Ratings with less than three subscripts may not follow these rules. Gate ratings involve the subscript G. Subscript A usually stands for anode and subscript AV for average

Anode Voltage Ratings (i) VDWM : Peak working forward-blocking voltage. It specifies the maximum forward-blocking voltage that a thyristor can withstand during its working. ( equal to the maximum value of the sine voltage wave. ( ii ) VDRM : Peak repetitive forward-blocking voltage. It refers to the peak transient voltage that a thyristor can withstand repeatedly or periodically in its forward-blocking mode. The rating is specified at a maximum allowable junction temperature with gate circuit open or with a specified biasing resistance between gate and cathode. Voltage VDRM is encountered when a thyristor is commutated or turned-off. During turn-off process, an abrupt change in reverse recovery current is accompanied by a spike voltage L dildt ; this is responsible for the appearance of VDRM across thyristor terminals. (iii) VDSM : Peak surge (or non-repetitive) forward-blocking voltage. It refers to the peak value of the forward surge voltage that does not repeat. Its value is about 130% of VDRM, but VDSM is less than forward breakover voltage VBO

(iv) VRWM : Peak working reverse voltage. It is the maximum reverse voltage that a thyristor can withstand repeatedly. Actually, it is equal to the peak negative value of a sine voltage wave. (v) VRRM : Peak repetitive reverse voltage . It specifies the peak reverse transient voltage that may occur repeatedly in the reverse direction at the allowable maximum junction temperature. The transient lasts for a fraction of the time of one cycle. The reason for the periodic appearance of VRRM is the same as for VDRM (vi) VRSM : Peak surge (or non-repetitive) reverse voltage. It represents the peak value of the reverse surge voltage that does not repeat. Its value is about 130% of VRRM

But VRSM is less than reverse breakover voltage VRRM . (vii) VT : On-state voltage drop. It is the voltage drop between anode and cathode with specified forward on-state current and junction temperature.Its value is of the order of 1 to 1.5 V.

( ix) Voltage safety factor (VSF) : It is defined as the ratio of peak repetitive reverse voltage (VRRM) to the maximum value of input voltage.

Voltage safety factor is usually taken between 2 to 3. (x) Finger voltage: It is the minimum value of forward bias voltage between anode and cathode for turning-on t he device by gate triggering. The magnitude of finger voltage is somewhat more than the normal on-state voltage drop in the thyristor.

(viii ) Forward dv /dt rating. If rate of rise of forward anode-to-cathode voltage is high, thyristor may turn on even when (a ) there is no gate signal and (b) Anode to cathode voltage is less than fonward breakover voltage. A high value of dv/dt , at which a thyristor just gets turned on is called critical rate of rise of anode voltage or forward dv / dt rating of the device. If applied du/ dt exceeds this critical value, thyristor gets turned on. For applied du / dt lower than forward dv / dt rating, thyristor remains in forward blocking mode. The forward dv / dt rating depends on the junction temperature; higher the junction temperature, lower the forward dv / dt rating of the device. In practice, dv /dt triggering is never employed as it gives random turn-on of a thyristor. This type of triggering also leads to destruction of the device through high junction temperature.

Current Ratings Average on-stote current (lTAV) : The forward voltage drop across conducting SCR is low, therefore power loss in a thyristor depends primarily on forward average on- state current ITAV.

Variation of junction temperature with constant anode current ic and with rectangular wave of ia

The rms current for an SCR is constant whatever the conduction angle may be.

ITAV = Irms / FF

For the same conduction angle,

FFsine wave > FFrectangular wave

This means for the same dc (or rms ) current

ITAV (sine wave) < ITAV (rectangular wave)

The derating of the SCR is therefore more for sine waves than for the square or rectangular waves .

Average on state power dissipation Pav as a function of ITAV for (a) rectangular wave and (b) half wave sinusoidal (R-Load).For inductive load: waveforms become more smooth, the form factor

decreases and as a consequence, higher average on-state current ITAV can be handled by the device.

The derating of the SCR is more for sine waves than for the square or rectangular waves .For example: for γ =30˚ ITAV =Idc /3.464 for rectangular wave and Idc = 3.979 for sine wave

RMS on state current (IRMS):Heating of the resistive elements of a thyristor, such as metallic joints, leads and interfaces depends on the forward rms currentis used as an upper limit for constant as well as pulsed anode current ratings of the thyristor.Its value remains the same for different conduction angles.Example:IRMS = 35 A , Sin-wave FF for 120˚ = 1.875 , Sin-wave FF for 30˚ = 3.9812

ITAV = Irms / FF ITAV = 35 A/ 1.875 = 18.637 A at γ = 120˚

If ITAV is hold constant, but γ decreased to 30˚ then IRMS becomes equal 18.875 A x 3.9812 = 74.2 A . This leads to large ohmic losses and destroys the SCR

Surge Current Rating ITSM (peak non-repetitive on-state current):Surge current occurs, when a thyristor is subjected to abnormal operating conditions due to faults or short circuits.A surge current rating indicates the maximum possible non-repetitive, or surge, current which the device can withstand. Surge currents are assumed to be sine waves with frequency of 50, or 60, Hz depending upon the supply frequency. This rating is specified in terms of the number of surge cycles with corresponding surge current peak. Surge current rating is inversely proportional to the duration of the surge. It is usual to measure the surge duration in terms of the number of cycles of normal power frequency of 50 or 60 Hz.For example,ITSM = 3000 A for 2 cycle,ITSM =.2109 A for 3 cycles and ITSM = 1800 A for 5 cycles.

For duration less than half-cycle i.e. 10 ms, a sub-cycle surge current rating is also specified. This rating for 50 or 60 Hz supply is the peak value for a part of the half-sine wave. The sub-cycle surge current rating Isb can be determined by equating the energies involved in one cycle surge and one sub-cycle surge as follows :

T = time for one half cycle of supply frequency, s I = one-cycle surge current rating, A Isb = sub-cycle surge current rating, A t = duration of sub-cycle surge, sec

I2t rating: employed in the choice of a fuse or other protective equipment for thyristors. specifies the energy that the device can absorb for short time before the fault is cleared. usually specified for overloads lasting for less than, equal to one-half cycle of 50 or 60 Hz supply. I2t = (rms value of one-cycle surge current )2 x time for one cycle

In order that a fuse (or other protective equipment) protects a thyristor reliably, the I2t rating of the fuse must be less than the I2t rating of the series-connected thyristor.

di / dt rating: indicates the maximum rate of rise of current from anode to cathode without any harm to the device. Typical values of di/dt are 20 to 500 A/µs.

THYRISTOR PROTECTION A thyristor must be protected against all abnormal conditions for satisfactory and reliable operation of SCR circuit and the equipment.

di/dt protection: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.

dv/dt protection: False turn-on of a thyristor by large dv/ dt can be prevented by using a snubber circuit in parallel with the device.A snubber circuit consists of a series combination of resistance Rs and capacitance Cs, in parallel with the thyristor .

Rs is to limit the magnitude of discharge current.

Example :The following Fig . shows a thyristor controlling the power in a load resistance RL. The supply uoltage is 240 V dc and the specified limits for di/dt and dvldt for the SCR are 50 A/µs and 300 V/µs respectively. Determine the values of the di/dt inductance and the snubber circuit parameters Rs, and Cs. Solution:

IA (at turn on) = (Vs/Rs + Vs/RL ) < ITRM Rs should be greater than calculatedCs should be less than calculated

Example:A thyristor operating from a peak supply voltage of 400 V has the following specifications : . Repetitive peak current. Ip = 200 A, (di/dt)max = 50 A/µs, (dv/dt)max = 200 V/µs. Choosing a factor of safety of 2 for Ip, (di/dt)max and (dv/dt)max design a suitable snubber circuit . The minimum value of load resistance is 10 Ω. Solution. For a factor of safety of 2, the permitted values are:Ip = 200 A/2 = 100A(di/dt)max = 50 A/µs/2 = 25 A/µs(dv/dt)max = 200 V/µs/2 = 100 V/µsIn order to restrict the rate of rise of current beyond specified value, (di/dt) inductor must be inserted in series with thyristor

Before thyristor is turned on, C, is charged to 400 V. 'When thyristor is turned on, the peak current through the thyristor is

As this peak current through SCR is more than the permissible peak current of 100 A, the magnitude of Rs must be increased. Taking R, as 8 n, the peak current through the SCR

Ip less than the allowable peak current.

At the instant switch S is closed, see figure, thyristor is open circuited and the rate of change of the thyristor voltage is

Since designed value of(dv/ dt) is much less than the specified maximum value of 100 V/µs,We choose a smaller capacitor say 0.3 µF, the corresponding dv/ dt =74.07 V/µsSo choose L = 16 µH, Rs = 8 Ω and Cs = 0.3 µF.

sVsVdt

dvdt

dv

RR

V

dt

dvc

Ls

ss

/100/6.52104225.0

222.22810

400104225.0

6

6

Overvoltage Protection Overvoltage transients are perhaps the main cause of thyristor failure. Transient overvoltages cause either maloperation of the circuit or permanent damage to the device

internal overvoltages : genera ted internally during the commutation of a thyristor. external overvoltages : caused due to the interruption of current flow in an inductive circuit and also due to lightning strokes an the lines feeding the thyristor systems.

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 called snubber circuits (not enough for overvoltage protection of SCR ) non-linear resistors called voltage clamping devices (V.C): (Selenium thyrector diodes, metal oxide varistors or avalanche diode suppressors )

Volt-ampere and yolt-resistance characteristics of voltage-clamping device

Overcurrent Protection due to faults, short circuits or surge currents overcurrent protection in thyristor circuits is achieved through the use of circuit breakers and fast-acting fusesSupply systems: In a weak supply network, fault current is limited by the source impedance below the multi-cycle surge current rating of the thyristor.In stiff 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 peak let through current of fuse must Be less than the subcycle surge current Rating of the SCR.

Action of currant · limiting fuse in en !I.e circuit.

Electronic crowbar protection:An electronic crowbar protection provides rapid isolation of the power converter before any damage occurs.

Elementary electronic crowbar circuit.

Gate Protection Overvoltages across the gate circuit can cause false triggering of the SCR.Protection against overvoltages is achieved by connecting a zener diode ZD across the gate circuit . Overcurrent may raise junction temperature beyond specified limit leading to its damage.A resistor R2 connected in series with the gate circuit provides protection against overcurrents. 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 capacitor (< 0.1µF) and a resistor are also connected across gate to cathode to bypass the noise signals.

Circuit component showing the thyristor protection. C.B Circuit breaker ; F.A.C.L.F. Fast actingCurrent limiting fuse ; H.S Heat sink; ZD Zener diode.

FIRING CIRCUITS FOR THYRISTORS The instant of turning on the SCR cannot be controlled by the first three methods listed above. Light triggering is used in some applications , particularly in a series-connected string.Gate triggering is, however, the most common method of turning on the SCRs, because this method lends itself accurately for turning on the SCR at the desired instant of time. In addition, gate triggering is an efficient and reliable method. Main Features of Firing Circuits Gating circuits are usually low-power electronic circuits. A firing circuit should fulfil the following two functions

A ge::e: al lnyou t of t~e firing circuit scheme fo r SCR., .(i) If power circuit has more than· one SCR, the flring circuit should produce gating pulses for each SCR at the desired instant for proper operation of the power circuit. These pulses must be periodic in nature and the sequence of firing must correspond with the type of thyristorised power controller. For example, in a single-phase semiconverter using two SeRs, the triggering circuit must produce one firing pulse in each balf cycle; in a 3-phase full converter using SLX SeRs, gating circuit must produce one trigger pulse after every 60° interval. ( ii) The control signal generated by a firing circuit may not be able to turn-on an SCR It is therefore common to feed the voltage pulses to a driver circuit and then to gate-cathode circuit. A driver circuit consists of a pulse amplifier and a pulse transfonner. A firing cir cuit scheme, in general, consists of the components shown in Fig. 4.63. A r egulated dc power supply is obtained from an alternating voltage source. Pulse generator, supplied from both ac and dc sources, gives out voltage pulses which are then fed to pulse amplifier for their amplification. Shielded cables transmit the amplified pulses to pulse transformers . The function of pulse transformer is to isolate the low-voltage gate-cathode circuit from the' high-voltage anode-cathode circuit. Some firing circuit schemes are described in this section .

Resistance firing circuit.

RC half-wave trigger circui t.

Waveiorms for RC bali·wave trigger circuit of Fig. 4.66 (a) high value of R (b) low value of R. ' I I

Waveforms for RC half·wave trigger ci r C~lit of Fig. 4.68 (0 ) high value of R (b ) low value of R.

RC full·wave trigger circuit.