di/dt protection
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.
To avoid local hot spots we use an inductor in series with the device as it prevents high rate of change of current through it.
dv/dt protection
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 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.
❖ As Cj, the capacitance of junction J2, is almost constant, the current is
given by
❖ 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.
Password: Lec-6 Dynamic
pdf made by Dr M Balasubbareddy