We all are familiar with diodes and how they work. A diode is a semiconductor which is made up of n-type and p-type materials. they are mainly used in rectifier circuits and the rectifier is also uncontrolled. we also know the forward bias and reverse bias conditions of diodes. but when we need high current capacity devices we prefer thyristor over diodes. a thyristor is a name given to the family of power semiconductor devices. for example LASCR, SCR, SUS, IGBT, etc. SCR(silicon controlled rectifier) is a commonly used device from the thyristor family. so, SCR is commonly called as a thyristor.
What is controlled and uncontrolled devices
when we use diodes in the rectifier circuit the output which we get from the rectifier circuit is uncontrolled. which means we cannot change the output waveform of the rectifier. But if we use SCR we can achieve the desired output from the rectifier by controlling the gate of SCR. we will get this in working of a thyristor.
Working of SCR
In a regular diode, we have one n-type and one p-type material. also, we have two terminals and one junction barrier. but in SCR we have two n-type and two p-type materials. we have three junctions J1, J2, J3 .also we have an additional gate terminal is at p layer near cathode(K). A and K carry the main supple but gate carries the low power signal.
when A is made positive w.r.t K two outer junctions J1 and J3 are forward biased but inner junction J2 is reversed biased. due to the presence of the depletion layer, J2 doesn't allow the passage of current. only leakage current can pass the junction due to the drift of mobile charges. this is called forward blocking state. If we made A negative w.r.t K two outer junctions J1 and J3 are now reverse bias, but J2 is forward bias. this is called reverse blocking state or off state of SCR.
If we keep on increasing the anode to cathode forward voltage, junction J2 will vanish and avalanche breakdown occurs at J2. now the SCR keeps on conducting, and now our SCR is no more. to save our device we from breakdown and for achieving conduction we have to apply a signal to gate, which makes the junction J2 forward biased and as our all junctions are forward biased our SCR is ready for conduction. now our SCR is in forward conducting state. when the current in SCR is greater than the latching current, no more gate signal is required to keep the SCR in on the state. SCR will also return to its normal forward blocking state if the value of current falls below the holding current.
Holding current is the minimum value of current required to keep SCR in ON state. if the current is minimum than the holding current SCR will automatically come in forward blocking state.
Latching current is the minimum value of current beyond which no more gate signal is required to keep SCR in on state.
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