Device Protection with Thyristors

di/dt Protection

A minimum time is required for the thyristor to spread the current conduction uniformly throughout the junctions. If this time is not allotted and the rate of rise of anode current is very high compared to the spreading velocity at turn-on, then this could lead to localized "hot-spot" heating and the device may fail as a result of excessive heating.

For an inductive load, when thyristor T₁ is turned off, free-wheeling diode D_m conducts load current. If thyristor T₁ is fired when diode D_m is still conducting, di/dt can be very high. In order to reduce the high di/dt a series inductor L_s is added to the circuit as shown. The forward di/dt is given as

 dv/dt Equivalent

As seen earlier, a high dv/dt may cause damage to a thyristor. In order to protect a thyristor from high dv/dt, the circuits shown in figure above could be used.

The value of the snubber time constant R_SC_S can be found for a known dv/dt. And for a known discharge current I_TD , the value of resistor R_S can be found using the above equation.

Exercise-1

The dv/dt rating of the thyristor in the circuit shown in Figure below is 100V/μs. Determine the minimum value of C so that no accidental turn on of the thyristor occurs due to the dv/dt rise when S is close.

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TRIAC

The bi-directional thyristor or TRIAC conducts current in both directions. It can be considered as two SCRs connected in antiparallel with a common gate connection.

The TRIAC structure, symbol and VI characteristics are shown in figure. In I+ mode, the terminal MT2 is positive and the device is switched on by a positive gate current pulse. In III– mode, the terminal MT1 is positive and the device is turned on by a negative gate pulse.

For ac power control, a triac is more economical than an inverse-parallel thyristor combination, but it has a few drawbacks.

1. The gate current sensitivity is somewhat lower and turn-off time is slower than for a thyristor.
2. Besides, the reapplied dv/dt capability is poor and, therefore, it is difficult to operate with an inductive load.

Triacs are commonly used in incandescent lamp dimming and heating control, where the load is resistive. Figure shows the popular triac light dimmer circuit. The gate of the triac is supplied from an R-C circuit through a diac.

A diac shown in the figure is a symmetric voltage-blocking device.

The variable resistance R₁ controls the dimming level of the lamp, which is connected in series. When the capacitor voltage V_C in either polarity exceeds the threshold voltage ±V_S of the diac, a pulse of current triggers the triac at a firing angle (α), giving phase-controlled, full-wave current to the lamp.

When the triac is on, the R-C circuit is shorted and the line voltage is impressed across the lamp. At the end of every half-cycle, the triac turns off and the next half-cycle begins. An R-C snubber as shown is important to a triac circuit to reduce reapplied dv/dt.

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