Dynamic / Switching characteristics of SCR

Switching characteristics are also known as dynamic characteristics or transient characteristics. The time variations of the voltage across the SCR and the current through it during turn-on and turn-off processes give these dynamic or switching characteristics.

Dynamic or Switching characteristics of SCR

Switching characteristics during 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 intervals

• Delay time td
• Rise time tr
• Spread time tp

Delay time: Delay time 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  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 Ia = final value of anode current.

Rise time: The time taken by the anode current to rise from 0.1 Ia to 0.9 Ia. (or)

The rise time is also defined as the time required for the forward blocking off-state anode voltage to fall from 0.9 to 0.1 of its initial value OA.

During rise time, turn on losses in the thyristor are the highest due to high anode voltage and large anode current occurring together in the thyristor.

Spread time: The time taken by the anode current to rise from 0.9 Ia to Ia.  (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 turns 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 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.

Turn off time tq :

It is defined as time between the instant anode current becomes zero and the instant SCR regains forward blocking capability.

During this time all the excess carriers from four layers of SCR are removed. The turn off time is divided into two intervals.

• Reverse recovery time
• Gate recovery time

After t1 anode current builds up in the reverse direction with the same di/dt slope. The reason for the reversal of anode current is due to the presence of charge carriers stored in four layers.

At instant t3, when reverse recovery current has fallers to nearly zero value, end junctions J1 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 is called gate recovery time(tgr).

The thyristor turn off time tq is dependent up on magnitude of forward current, di/dt at the time of commutation and junction temperature.

Circuit turn off time:

• 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. Example: phase controlled rectifiers, cyclo converters and ac voltage controllers.
• SCR's with fast turn off time are called inverster grade SCR. Example: inverters, choppers and forced commutation converters.