Thyristor/SCR Applications

Thyristors

Thyristors are also called SCRs which means Silicon Controlled Rectifiers

Most of  beginner electronics hobbyists avoid the use of thyristor in their designs. Like me when I was starting new, I avoided anything that has to do with Thyristor, Diac, Triac and their families because of some reasons beyond my explanations but which now, as I have known many things in electronics circuit, I called ignorance or fear of components. Another factor that made me ignore the use of these semiconductors the way professionals and engineers describe their operations how to turn it on and turning it off. They use professional jargon and higher explanations beyond the understanding of a beginner. For example what I read about turning off methods for SCRs repelled me away from them but when I actually experimented with thyristors they weren't as I was made to understand them! They turn on and off just like IGBTs and MOFETs

They say that:

"Once the  SCR is fired, it remains on even when triggering pulse is removed. This ability of the SCR to remain on even when gate current is removed is referred to as latching. So SCR cannot be turned off by simply removing the gate pulse."

They also say that:

"There are three methods of switching off the SCR, namely natural commutation, reverse bias turn-off, and gate turn-off.

(a) Natural Commutation

When the anode current is reduced be­low the level of the holding current, the SCR turns off. However, it must be noted that rated anode current is usually larger than 1,000 times the holding value. Since the anode voltage remains positive with respect to the cathode in a dc circuit, the anode current can only be reduced by opening the line switch S, increasing the load impedance R_L or shunting part of the load current through a circuit parallel to the SCR, i.e. short-circuiting the device.

(b) Reverse-bias Turn-off

A reverse anode to cathode voltage (the cathode is positive with respect to the anode) will tend to interrupt the anode current. The voltage reverses every half cycle in an ac circuit, so that an SCR in the line would be reverse biased every negative cycle and would turn off. This is called phase commutation or ac line commutation. To create a reverse biased voltage across the SCR, which is in the line of a dc circuit, capacitors can be used. The method of discharging a capacitor in parallel with an SCR to turn-off the SCR is called forced commutation.

In power electronic applications one advantage of using SCRs is that they are compact. The control equipment is also compact if integrated circuits are used. There has also been an attempt to miniaturize capacitors used for forced commutation and for filtering. The former use is important because the currents can be high and thermal dissipation takes high priority in design considerations. Small sizes of capacitors are at present being achieved by the use of metalized plastic film or a plastic film and aluminium foil.

(c) Gate Turn Off

In some specially designed SCRs the characteristics are such that a negative gate current increases the holding current so that it exceeds the load current and the device turns-off. The current ratings are presently below 10 A and this type will not be considered further."

This is the best component I have found very useful as a high current battery charger in my homemade Inverter. I use it as it is called - SCR - Silicon Controlled Rectifier!

I have been using Mosfets as switch in my battery charger designs but it became more expensive and complex for me since I found out how I could use SCRs! In SCR design, a pair of IDENTICAL SCR MODULES serves as the high current rectifier and also serve as the high current switch! What you need to do is to build yourself a control circuit that stops the supply of voltage to the gate of the SCR when the battery voltage reaches the pre-setted threshold typically 13.80v to 14.40v

How to design this charger will be explained in this blog.


This SCRs turn on/off just like IGBTs.

 SEMIKRON  SKKH 162/16 E  THYRISTOR MODULE, 160A, 1600V (Semikron)

See Picture Below:

See the symbol:

Semikron SKKH SEMIPACK 2 series are thyristor and diodes. They comprise one diode and one thyristor embedded in one module and I love these series  best because you only need to fire one GATE for conductance.

SKKT 162/16E - 1600V 160A Dual SCR (Semikron)

 Semikron SKKT SEMIPACK 2 series are thyristor and diodes. They Dual thyristor embedded in one module and you need to fire two GATEs for conductance

The configuration of two SCR Modules as in Rectification mode.

Though the picture above is not the actual charger bot it is only to show how they are paired up in the design. High leakage  capacitor needs to be attached to the positive and negative terminals for filter and stability.