The term IGBT stands for Insulated Gate Bipolar Transistor". It has three terminals (or leads) labeled Gate, Collector and Emitter. Some IGBT's have a built in protection diode. This diode is connected across the Collector and Emitter terminals. It is added for transient voltage protection when the IGBT is switching under load conditions.
When these devices are considered 'suspect' it is necessary to validate their operation through a functional test procedure. Here is one method that can be used:
The first test will validate the condition of the protection diode, (note: not all IGBT's have internal diode protection) and the resistance of the Collector and Emitter terminals. This is due to the fact that most faulty IGBT's show a low resistance or short across the Collector and Emitter terminals when the device is stressed.
To perform this test will require a DVM (Digital Volt-Meter). Set the DVM to the Diode Test position, and confirm that the device tests as follows:
1) Attach the Red Lead of the DVM to the Collector Terminal and Black Lead to the Emitter Terminal. Confirm an open circuit condition.
2) Attach the Red Lead of the DVM to the Emitter Terminal and Black Lead to the Collector Terminal. Confirm an open circuit condition (IGBT without a protection diode). If the DVM displays a forward voltage drop of 0.2 to 0.8 VDC it means that the IGBT has a built in protection diode. Both of these results are normal test conditions and are used to confirm whether the IGBT has a protection diode, and if it does, whether it is functional or not.
To perform the second test, use the DVM to test across any two-phase terminals, looking for shorts or low resistance. This test will show if there is a breakdown of the internal insulation between each phase of the IGBT.
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