THYRISTOR PROTECTION
Reliable operation of a thyristor demands that its specified ratings are not exceeded. In practice, a thyristor may be subjected to overvoltages or overcurrents. During SCR turn-on, di/dt may be prohibitively large. There may be false triggering of SCR by high value of dv/dt. A spurious signal across gate-cathode terminals may lead to unwanted turn-on. A thyristor must be protected against all such abnormal conditions for satisfactory and reliable operation of SCR circuit and the equipment. SCRs are very delicate devices, their protection against abnormal operating conditions is, therefore, essential. The object of this section is to discuss various techniques adopted for the protection of SCRs.
(a) di/dt protection. When a thyristor is forward biased and is turned on by a gate pulse, conduction of anode current begins in the immediate neighbourhood of the gate-cathode junction, Fig. 4.6 (a). Thereafter, the current spreads across the whole area of junction. The thyristor design permits the spread of conduction to the whole junction area as rapidly as possible. However, if the rate of rise of anode current, i.e. di/dt, is large as compared to the spread velocity of carriers, local hot spots will be formed near the gate connection on account of high current density. This localised heating may destroy the thyristor. Therefore, the rate of rise of anode current at the time of turn-on must be kept below the specified limiting value. 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. Typical di/dt limit values of SCRs are 20-500 A/µ sec.
Local spot heating can also be avoided by ensuring that the conduction spreads to the whole area as rapidly as possible. This can be achieved by applying a gate current nearer to (but never greater than) the maximum specified gate current.
(b) dv/dtprotection. With forward voltage across the anode and cathode of a thyristor, the two outer junctions are forward biased but the inner junction is reverse biased. This reverse biased junction J2, Fig. 4.3 (b), has the characteristics of a capacitor due to charges existing across the junction. In other words, space-charges exist in the depletion region around junction J2 and therefore junction J2 behaves like a capacitance. If the entire anode to cathode forward voltage Va appears across J2 junction and the charge is denoted by Q, then a charging current i given by Eq. (4.6) flows
i = dQ/dt =d(Cj Va )/dt
=Cj (d Va /dt) + Va(d Cj /dt) …………..(4.6 a)
As Cj the capacitance of junction J2 is almost constant, the current is given by
i = Cj (d Va /dt) …………..(4.6 b)
If the rate of rise of forward voltage dVa/dt is high, the charging current i will be more.This charging current plays the role of gate current and turns on the SCR even when gate signal is zero. Such phenomena of turning-on a thyristor, called dv/dt turn-on must be avoided as it leads to false operation of the thyristor circuit. For controllable operation of the thyristor, the rate of rise of forward anode to cathode voltage dVa/dt must be kept below the specified rated limit. Typical values of dv/dt are 20 – 500 V/µsec. False turn-on of a thyristor by large dv/dt can be prevented by using a snubber circuit in parallel with the device
Written by John on April 17th, 2009 with
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#1. November 16th, 2011, at 9:54 AM.
got required information…….thanks