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THE UNIVERSITY OF NOTTINGHAM

School of Electrical and Electronic Engineering

THYRISTOR BASICS

Thyristors are 4 layer (p-n-p-n) devices that have a "latch type"switching action. There are 3 terminals on the device; anode(A),cathode(K) and gate(G) see Figure 1.

VAK

Anode

Cathode

Gate

IA

Figure 1

When the device is reverse biassed (VAK negative) it behaves like adiode and blocks current until a large enough voltage is applied tocause avalanche breakdown (and device destruction). When thedevice is forward biassed (VAK positive) two states are possible:

(a) Forward blocking - the device supports a large applied voltagewith very little current flow (effectively a switch in the OPENposition).

(b) Forward on-state - the device conducts forward current withvery little (1V - 2V) forward voltage drop (effectively a switch in

the CLOSED position).

The transition from (a) to (b) can be made to occur by applying avoltage between the gate and cathode so that sufficient gatecurrent for "turn-on" flows - this is the normal mode of operation.The device will also switch from (a) to (b) if the forward voltageexceeds a certain maximum value known as the breakover voltage- this causes device destruction and must be avoided.

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Unfortunately the transition from (b) to (a) is much more difficultto arrange and cannot be achieved with conventional thyristors byreversing the gate current (see Gate-Turn-Off thyristors [1-3]).The only practically usable way in which a conducting thyristor can

be turned off is to reduce the anode current to zero and at thesame time to reverse bias the device for a duration exceeding the"turn-off time" tq. The time tq is a parameter of the device and isgenerally between 10 s and 300s depending on the applicationthat the device is intended for. If the reverse bias time isinsufficient (ie less than tq) then the device will start conductingagain as soon as forward voltage is applied. Failure to achieveforward blocking after turn-off (commutation failure) can also becaused if the voltage across the device is allowed to rise too

rapidly - all thyristors have maximum (dv/dt) rating which thecircuit designer must ensure is not exceeded. An additionalthyristor parameter that the designer must take into account isthe maximum (di/dt) rating - this relates to the maximumallowable rate of rise of current at turn-on. During the early stagesof the turn-on process the current is concentrated into areas ofthe silicon wafer near the gate since these begin to conduct first.If the current rises too rapidly the excessive current density willburn-out parts of the wafer rendering the whole device useless.

In thyristor circuits fed from AC supplies, such as controlledrectifiers, the necessary conditions for turn-off of each deviceoccur at the appropriate instant as a natural consequence of theway in which the supply voltage varies with time; thisphenomenon is known as natural commutation. In thyristorcircuits fed from DC supplies, such as choppers, naturalcommutation cannot occur and auxiliary commutation circuits

must be provided so that the conditions for turn-off can be forcedon the thyristor at the appropriate time; this is known as forcedcommutation see [1-3].

[1] "Power Diode and Thyristor Circuits", R M Davis, PeterPeregrinus Ltd.[2] "Principles of Power Electronics", J G Kassakian, M F Schlechtand G C Verghese, Addison Wesley.[3] "Power Electronics - Converters, Applications and Design" , N

Mohan, T Underland and W Robbins, Wiley.

Dr J C Clare 13/10/03