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

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