In our previous discussion on IGBTs, we mentioned the term "commutation circuit" when discussing the circuit turn-off of IGBTs.

In that context, we mentioned a situation: when the collector current of the IGBT exceeds a certain threshold, the parasitic thyristor (SCR) is triggered, the gate terminal loses control over the collector current, and the IGBT cannot turn off.

Therefore, we need a commutation circuit to force the thyristor to commutate.

Today, let's delve into the concept of "commutation circuit."

Commutation: The process of transferring current from one branch to another is called commutation, or commutation. During commutation, some branches switch from the conducting state to the off state, while others switch from the off state to the conducting state.

For device commutation, it generally applies to fully controlled devices, such as MOSFETs or IGBTs. In most electronic applications, IGBTs are more commonly used for "commutation."

Application of IGBT Commutation Circuit

The commutation circuit is mainly used to control the conduction and turn-off processes of the IGBT to achieve forward and reverse current flow.

However, it is important to note that not all IGBT switching processes require commutation; it depends on the circuit.

Fundamental Reason: Because the IGBT integrates a freewheeling diode internally, in practical applications, either the IGBT or the internal diode may carry the current.

Analysis of Commutation Circuit Application

To facilitate understanding, we divide commutation into two characteristics: "forced" and "natural." This definition is easier to understand. The issue we mentioned at the beginning falls under the category of "forced." See the diagram below:

[Diagram: Commutation circuit diagram]

A: Process from 1 to 2

When circuit 1 is operating stably, if S1 is to be turned off, D4 will act as a relay. Since there is a load, the load current ia cannot undergo a sudden change. After a short dead time (brief), S4 will open, and the direction of the ia current will not be affected, but the current will gradually decrease.

Note: During the process from 1 to 2, S1 is forcibly turned off, which generates turn-off losses and may produce turn-off voltage spike stress.

B: Process from 2 to 1

When circuit 2 is operating stably, if S4 is to be turned off (with no change in the load current ia), after a short dead time, S1 opens. D4 is forcibly turned off, and S1 acts as a relay.

Note: D4 is forcibly turned off, which generates reverse recovery losses, and there may be a certain voltage spike stress. At the same time, S1 will generate turn-on losses and some current spike stress.

In summary, the role of commutation in IGBT circuits is:

1. Forcibly turning off the IGBT

When the IGBT cannot be turned off normally, the commutation circuit can forcibly turn off the IGBT through a specific control method to avoid circuit faults or damage.

2. Controlling the direction of current

By using the commutation circuit, the direction of both forward and reverse currents can be controlled, enabling control of the current direction.

Conclusion

IGBTs are power transistors that combine the best characteristics of MOSFETs and bipolar transistors. We usually use the basic structure of an N-channel IGBT for illustration purposes.

Its main application scenarios can be classified based on output capacity and operating (switching) frequency.

The following is a recommendation for VBsemi's IGBT products, which mainly have advantages including:

1. Higher voltage and current handling capacity

2. Extremely high input impedance

3. The ability to switch very high currents with very low voltage

4. Voltage-controlled device, meaning it has no input current and low input loss

5. Bipolar nature, enhancing conductivity, and more.

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