Basic theory behind IGBT construction as N-channel MOSFET and PNP transistor.
Fig. 1

Insulated Gate Bipolar Transistor IGBT Circuits Tutorial

by Lewis Loflin

Recently I discovered the advantages of using Insulated Gate Bipolar Transistors (IGBTs) over MOSFETs. In fact I had several left over from my plasma cutter repair days and decided to use them. This is particularly true when used with photovoltaic MOSFET drive opto-couplers such as the VOM1271.

To quote two sources on the advantages of IGBTs:

When compared to the IGBT, a power MOSFET has the advantages of higher commutation speed and greater efficiency during operation at low voltages. ... The IGBT combines the simple gate-drive characteristics found in the MOSFET with the high-current and low-saturation-voltage capability of a bipolar transistor.

And:

The main advantages of using the Insulated Gate Bipolar Transistor over other types of transistor devices are its high voltage capability, low ON-resistance, ease of drive, relatively fast switching speeds and combined with zero gate drive current makes it a good choice for moderate speed, high voltage applications...

Fig. 1 Basic theory behind IGBT construction as N-channel MOSFET and PNP transistor.

IXGH25N100 IGBT.
Fig. 2a

The IXGH25N100 are the ones I used in my test circuits. Mine were salvaged from a plasma cutter inverter board I'll discuss below.

Besides the high voltage capability some have a "maximum rated collector current Ic(max) exceeding 100A".

The symbol for an IGBT is on the left.

The IXGH25N100 does not have internal flywheel diodes.

IGBT with internal flywheel diodes.
Fig. 2b

Fig. 2b illustrates an IGBT with internal flywheel diodes. The FGA25N120 is rated at 1200V, 25A. Voltage C-E sat at 25A is 2.0V.

An important factor is collect-emitter saturation voltage. The following are used in inductive heating circuits.

FGPF4633 is rated 330V a V C-E 1.55V at 70A.

IHW20N120R3 1200V 20A 1.48V.

Example 12VDC to 120VAC IGBT inverter circuit.
Fig. 3

Example 12VDC to 120VAC IGBT inverter circuit.

Example IGBT plasma cutter chopper circuit.
Fig. 4

Inverter plasma cutters

I used to repair portable plasma cutters of this type. To quote Wikipedia:

Inverter plasma cutters rectify the mains supply to DC, which is fed into a high-frequency transistor inverter between 10 kHz to about 200 kHz. Higher switching frequencies allow smaller transformer resulting in overall size and weight reduction.

The transistors used were initially MOSFETs, but are now increasingly using IGBTs. With paralleled MOSFETs, if one of the transistors activates prematurely it can lead to a cascading failure of one quarter of the inverter. A later invention, IGBTs, are not as subject to this failure mode. IGBTs can be generally found in high current machines where it is not possible to parallel sufficient MOSFET transistors.

IGBT switching power supply for microwave oven.
Fig. 5

Fig. 5 uses an IGBT to chop 170v DC for high frequency transformer. This is used in Panasonic microwave ovens.

The higher frequency allows use of smaller (cheaper) transformers. This also reduces weight. D701, D701, C703, and C704 form a voltage doubler. R701 is a HV bleeder resistor.





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