Turn off Switching Characteristics of Thyristor
Turn OFF characteristics of Thyristor:
- When the thyristor is on, and its anode current is above the latching current level the gate loses control. It can be turned off only by reducing the anode current below holding current. The turn off time tq of a thyristor is defined as the time between the instant anode current becomes zero and the instant the thyristor regains forward blocking capability. If forward voltage is applied across the device during this period the thyristor turns on again.
- During turn off time, excess minority carriers from all the four layers of the thyristor must be removed. Accordingly tq is divided in to two intervals, the reverse recovery time (trr) and the gate recovery time (tqr). The variation of anode current and anode cathode voltage with time during turn off operation on an expanded scale.
- The anode current becomes zero at time t1 and starts growing in the negative direction with the same diA/dt till time t2. This negative current removes excess carriers from junctions J1 & J3. At time t2 excess carrier’s densities at these junctions are not sufficient to maintain the reverse current and the anode current starts decreasing. The value of the anode current at time t2 is called the reverse recovery current (Irr).
- The reverse anode current reduces to the level of reverse saturation current by t3. Total charge removed from the junctions between t1 & t3 is called the reverse recovery charge (Qrr). Fast decaying reverse current during the interval t2 t3 coupled with the di/dt limiting inductor may cause a large reverse voltage spike (Vrr) to appear across the device.
- After the reverse recovery time, the thyristor regains reverse blocking capacity and VAK starts following supply voltage vi. At the end of the reverse recovery period (trr) trapped charges still exist at the junction J2 which prevents the device from blocking forward voltage just after trr. These trapped charges are removed only by the process of recombination. The time taken for this recombination process to complete (between t3 & t4) is called the gate recovery time (tgr). The time interval tq = trr tgr is called “device turn off time” of the thyristor.
- No forward voltage should appear across the device before the time tq to avoid its inadvertent turn on. A circuit designer must provide a time interval tc (tc > tq) during which a reverse voltage is applied across the device. tc is called the “circuit turn off time”.
- Thyristor is the device of choice at the very highest power levels. At these power levels (several hundreds of megawatts) reliability of the thyristor power converter is of prime importance. Therefore, suitable protection arrangement must be made against possible overvoltage, overcurrent and unintended turn on for each thyristor.
Questions of this topic
In the ideal single phase fully controlled converter T1 & T2 are fired at a firing angle ∝ after the positive going zero crossing of Vi while T3 & T4 are fired ∝ angle after the negative going zero crossing of Vi, If all thyristors have a turn off time of 100 μs, find out maximum allowable value of ∝.