Turn OFF Switching Characteristics
Once the thyristor is ON and its anode current is above the latching current level, the gate losses control. It can be turned OFF only by reducing the anode current below the holding current. The 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 into two intervals, the reverse recovery time(tRR) and the gate recovery time(tGR), figure shows the variation of the anode current and the anode to cathode voltage with time during turn OFF operation for an applied sinusoidal voltage(VI).
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 the junctions J1 and J3. At time t2 excess carriers densities at these junctions are not sufficient to maintain the reverse current. The value of the anode current at time t2 is called as the reverse recovery current(IRR). The reverse anode current reduces to the level of reverse saturation current. The total charge removed from the junctions between t1 and 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 to appear across the device. This voltage must be limited below the VRRM rating of the device. Up to time t2 the voltage across the device(VAK) does not change substantially from its state value. However, after the reverse recovery time, the thyristor regions the reverse blocking capacity and VAK starts following the supply voltage. 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 and 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 in order to avoid its inadvertent turn ON. While designing an SCR circuit, one must provide a time interval (tc > tq) during which a reverse voltage is applied across the device, where tc is the circuit turn OFF time.
Silicon Controlled Rectifier
The SCR or thyristor is one type of semiconductor device and using in high-power switching applications is exceptionally planned. The working of this device should be possible in a switching mode only and acts as a switch. When the SCR is triggered by its gate terminal into the transmission, then it will supply the current constantly. While planning a SCR or Thyristor circuit, exceptional focus ought to be expected for enacting the circuit. This article examines various techniques for SCR setting off or SCR turn ON strategies or setting off of Thyristors. There are different setting off strategies are accessible in light of different substances which incorporate temperature, voltage, and so on.
Table of Content
- Silicon Controlled Rectifier
- Key Terminologies
- Construction
- Operation
- Modes of Operation
- Types
- Advantages of SCR