SWITCHING CHARACTERISTICS OF SCR
TURN-ON:
SCR Turn on time is defined as the time during which SCR changes from forward blocking mode to final on-state.
Total turn on time can be divided into three Total turn on intervals;
(i) Delay time (td).
(ii) Rise time (tr)
(iii)Spread time (tp)
Turn on time = ta + tr + tp
Delay time (td):
The delay time (td) is the time between the instant at which gate current reaches 0.9 Ig to the instant at which anode current reaches 0.1 Ia. Here Ig and Ia are respectively the final values of gate and anode currents.
(or)
The delay time (td) may also be defined as the time during which anode voltage falls from Va to 0.9 Va where Va= initial value of anode voltage.
(or)
The time during which anode current rises from forward leakage current to 0.1 Ia where la final = value of anode current.
Rise time (tr):
The time taken by the anode current to rise from 0.1 la to 0.9 Ia.
(or)
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 0A.
During rise time, turn on losses in the thyristor are the highest due to high anode voltage (Va) and large anode current (la) occurring together in the thyristor.
Spread time (tp):
The time taken by the anode current to rise from 0.9 Ia to la
(or)
It is also defined as the time for the forward blocking voltage to fall from 0.1 of its initial value to the on-state voltage drop.
Switching Characteristics during Turn-off:
SCR turn off means that it has changed from ON to OFF state and is capable of blocking the forward voltage. This dynamic process of the SCR from conduction state to forward blocking state is called commutation process or turn-off process.
If forward voltage is applied to the SCR at the moment its anode current falls to zero, the device will not be able to block this forward voltage, as the carriers (holes and electrons) in the four layers are still favorable for conduction. The device will therefore go into conduction immediately even though gate signal is not applied. To solve this problem it is essential that the thyristor is reverse biased for a finite period after the anode current has reached zero.
Thyristor Turn off time (tq):
It is the time between the instant anode current becomes zero and the instant SCR regains forward blocking capability.
During this time (t) all the excess carriers from four layers of SCR are removed.
The turn off time is divided into two intervals.
(a) reverse recovery time (tm)
(b) gate recovery time (tgr)
tq = trr + tgr
After t1 anode current builds up in the reverse direction with the same di/dt slope. The reason for dt the reversal of anode current is due to the presence of charge carriers stored in the four layers.
At instant t3, when reverse recovery current has fallers to nearly zero value, end junctions J, and J3 recover and SCR is able to block the reverse voltage.
at the end of reverse recovery period t3, the middle junction 'J2' still has charges, therefore, the
thyristor is not able to block the forward voltage at t3.
The charge carriers at J2 cannot flow to the external circuit therefore they must decay only by recombination. This is possible if a reverse voltage is maintained across SCR. The time taken for this (t4-t3) is called gate recovery time (tgr).
The thyristor turn off time 't,' is depended up on di magnitude of forward current, di/dt at the time of dt commutation and junction temperature.
Circuit turn off time tc:
It is defined as the time between the instant anode current becomes zero and the instant reverse voltage due to practical circuit reaches zero.
tc > tq for reliable turn off, otherwise the device may turn on at an undesired instant, a process called Commutation failure.
Thyristors with slow turn off time are called - converter grade SCR's.
Ex: phase controlled rectifiers, cyclo converters & ac voltage controllers
SCR's with fast turn off time are called inverter grade SCR
Ex: inverters, choppers and forced commutation converters.
