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

$\frac{{di}}{{dt}} = \frac{{{V_S}}}{{{L_S}}}$

dv/dt Equivalent

$\frac{{dv}}{{dt}} = \frac{{0.632{V_S}}}{\tau } = \frac{{0.632{V_S}}}{{{R_S}{C_S}}}$

${R_S} = \frac{{{V_S}}}{{{I_{TD}}}}$

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. dv/dt Equivalent of Thyristor

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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Device Protection with Thyristors

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