The term IGBT is an acronym for Insulated Gate Bipolar Transistor. It can be thought of as a Transistor with a MOSFET front end. 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 Inductive loads.
When these devices are considered 'suspect' it is necessary to validate their operation through a functional test procedure. One of two methods can be used:
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The first test method will validate the condition of the protection diode, and the resistance of the Collector and Emitter terminals. It is not a definitive test, but will find 80% of all faulty devices. 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:
Attach the Red Lead of the DVM to the Collector Terminal and Black Lead to the Emitter Terminal. Confirm an open circuit condition.
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.
The second test method will check the IGBT's functionality, by gating the device. To perform this test will require an IGBT Tester. Confirm that the device tests as follows:
Attach the IGBT Tester leads to the Gate, Collector and Emitter Terminals. Confirm that the device Conduction LED is not lit.
Press the Test Button, and confirm that the Conduction LED lights.
Reverse the Collector and Emitter leads. Do not press the Test Button. If the Conduction LED lights the IGBT has a built in protection diode. This is a normal test condition for this type of device. If the Conduction LED does not light then the IGBT does not have a built in protection diode. This is also a normal test condition for this type of device.
It is important to note that Functional test of the IGBT should always be performed when the device is out-of-circuit. In other words, it should be removed from the product and have no other electrical connections in place, other than those outlined in the Test procedure.
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