IGBT is quite abstract to talk about. In terms of functionality, it can be understood as a circuit switch, like the light switch at home, either "on" or "off."

Last time we talked about the internal structure of IGBT, today we will continue to talk about its working principle.

When a positive voltage is applied to the gate of the IGBT, the emitter will keep the drive circuit open.

However, when the gate voltage is zero or less than zero, the circuit application will be turned off.

It can be concluded here that the IGBT is switched on or off by activating or deactivating its gate terminal.

In the previous video, we mentioned that the IGBT can be used as both a BJT and a MOSFET, which means that the amplification it achieves is the ratio between its output signal and the controlled input signal.

IGBT Circuit Turn-On

Let's look at this picture.

When the collector is at a positive potential relative to the emitter, the N-channel IGBT will conduct, and the gate is at a sufficiently positive potential (>VGET) relative to the emitter.

This will cause a depletion layer to form below the gate and form a channel, and then current starts flowing from the collector to the emitter.

The collector current Ic in the IGBT is composed of two components Ie and Ih.

Here is a formula for collector current: Ic = Ie + Ih

Ie is the current flowing from the collector to the emitter due to injected electrons passing through the injection layer, drift layer, and finally forming the channel.

Ih is the hole current flowing from the collector to the emitter through Q1 and the body resistance Rb.

The situation can be seen in this picture:

However, the Ih here can be almost negligible.

Therefore, Ic ≈ Ie.

IGBT Circuit Turn-Off

When preparing to turn off the IGBT, we will encounter a special situation. When the collector current of the IGBT exceeds a certain threshold, the parasitic thyristor will be locked, the gate terminal will lose control of the collector current, and the IGBT cannot be turned off.

At this point, we need to use a typical commutation circuit to force the thyristor to commutate. We will have the opportunity to explore this topic in more depth later.

In short, there is a possibility of damaging the device if it is not turned off quickly.

An additional point about its working principle:

The IGBT only works when there is a voltage supply on the gate terminal.

Once there is a gate voltage (VG), the gate current (IG) will increase, and then it will increase the gate-emitter voltage (VGE).

Therefore, the gate-emitter voltage increases the collector current (IC). And the collector current (IC) reduces the collector-emitter voltage (VCE).