Thyristor

Thyristors, or silicon-controlled rectifiers (SCRs) have been the traditional workhorses for bulk power conversion and control in industry. The modern era of solid-state power electronics started due to the introduction of this device in the late 1950s.

Volt-Ampere Characteristics

The figure shows the thyristor symbol and its volt-ampere characteristics. Basically, it is a three-junction P-N-P-N device, where P-N-P-N and N-P-N component transistors are connected in regenerative feedback mode. The device blocks voltage in both the forward and reverse directions. When the anode is positive, the device can be triggered into conduction by a short positive gate current pulse; but once the device is conducting , the gate losses its control to turn off the device. A thyristor can also turn on by excessive anode voltage. Its rate of rise (dv/dt), by a rise in junction temperature ( TJ ), or by light shining on the junctions. 

The volt-ampere characteristics of the device indicate that at gate current IG = 0 , if forward voltage is applied on the device, there will be a leakage current due to blocking of the middle junction. If the voltage exceeds a critical limit (break over voltage), the device switches into conduction. With increasing magnitude of IG, the forward break over voltage is reduced. And eventually at IG3 , the device behaves like a diode with the entire forward blocking region removed. The device will turn on successfully if a minimum current, called a latching current, is maintained. During conduction, if the gate current is zero and the anode current falls below a critical limit, called the holding current, the device reverts to the forward blocking state. With reverse voltage, the end P-N junctions of the device become reverse-biased and the V-I curve becomes essentially similar to that of a diode rectifier. Modern thyristors are available with very large voltage (Several KV) and current (Several KA) ratings.

Switching Characteristics

Initially, when forward voltage is applied across a device, the off-state, or static (dv/dt), must  be limited so that it does not switch on spuriously. The (dv/dt) creates displacement current in the depletion layer capacitance of the middle junction, which induces emitter current in the component transistors and causes Switching action. When the device turns on, the anode current (di/dt) can be excessive, which can destroy the device by heavy current concentration. During conduction, the inner P-N regions remain heavily saturated with minority carries and the phenomena are similar to that of a diode. However, when the recovery current goes to zero, the middle junction still remains forward-biased. This junction eventually blocks with an additional delay when the minority carries die by the recommendation process. The forward voltage can then be applied successfully, but the reapplied (dv/dt) will be somewhat less than the static (dv/dt) because of the presence of minority carriers. For example, POWEREX SCR/diode module CM4208A2 (800 V , 25 A) has limiting (di/dt)=100 A/m

and off-state dv/dt =500 V/ parameters. A suitably-designed snubber circuit (discussed later) can limit di/dt and dv/dt within acceptable limits. In a converter circuit, a thyristor can be turned off (or commutated) by a segment of reverse AC line or load voltage (defined as line or load commutation, respectively), or by an inductance capacitance circuit-induced transient reverse voltage (defined as forced commutation).

To Follow Us : 

https://www.youtube.com/channel/UC8uzr-PXQxLpAiVKA7S1rRQ/videos?view_as=subscriber

https://www.facebook.com/Engi.Prog/