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Join Date: Oct 2014
Posts: 7

# What is Current/Voltage Controlled Component ?

11/15/2014 5:23 AM

I have confusion that when we say the component is current controlled or voltage controlled then what does it mean?Because if there is some voltage, there will be current as well. So how can say that thyristor (SCR) is a current controlled component and IGBT is voltage controlled component?

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#1

### Re: What is current/voltage controlled component ?

11/15/2014 7:58 AM
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Join Date: Apr 2010
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#2

### Re: What is current/voltage controlled component ?

11/15/2014 9:39 AM

It has to do with input impedance. A high input impedance draws very little current so the device would be voltage controlled. With a current controlled device, the input impedance is relatively low and the input current changes much more than the voltage.

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#3

### Re: What is Current/Voltage Controlled Component ?

11/16/2014 12:09 AM

An SCR can be used in a voltage controlled mode in 2 ways:-

1. Where the voltage across the anode - cathode is increased beyond its avalanche breakdown value, whereupon it will go to full conduction mode without any reference to the gate conditions).

2:- Where the rise speed of the anode-cathode voltage triggers the device.

In both cases the switch off is current controlled where it has to fall to a critical value before the device ceases to conduct, but this can also be called another voltage control mode as it requires a reduction of anode-cathode voltage for this to occur.

It is also possible to shut down an SCR by applying a gate voltage negative WRT cathode, this is a definite voltage control mode as the reverse bias impedance of the gate is substantial, a GTO thyristor uses this method.

The SCR is current controlled when a gate current is utilised to forward bias and therefore trigger the device. Semantics indicate that there must be a voltage applied for such current to flow, but due to the relatively low input impedance of the gate circuit, it is called a current controlled device as a current does flow in the gate circuit.

The IGBT though, is a true voltage controlled device because the gate is totally insulated from the rest of the device and relies on electrostatic charge between the base and the emitter to cause the device to conduct. There is no flow of current in the gate.

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