An IGBT or the Insulated Gate Bipolar Transistor is an amalgamation of a MOS and a bipolar transistor. It combines the best performances of both devices – the easily driven MOS gate and the low conduction loss of the bipolar. This effective device is quickly displacing most power bipolar transistors that were an obvious choice for high voltage and high current applications. IGBTs offer a balance in tradeoffs between conduction loss, switching speed and ruggedness. Manufacturers are now tweaking IGBTs to work successfully in the areas of high frequency and high efficiency that so long were the sole domain of power MOSFETs. In fact, barring applications that require very low currents, the industry trend is to replace power MOSFETs and power bipolar transistors with IGBTs.
When choosing an IGBT for a specific application, answering a few questions will usually narrow down the selection. Zeroing in on the most appropriate device will require a better understanding of the terms and graphs published by the manufacturers. These questions will be:
• What will be the operating voltage? Select IGBTs with VCES rating of at least 120% of the voltage that has to be blocked.
• Will the switching be hard or soft? A Punch-Through or PT type IGBT is best suited for soft switching because tail current reduces.
• What current does the device require to handle? In the part number of an IGBT, the first two numbers are a rough indication of the usable current. When looking for a device to work with hard switching applications, the selection usually depends on usable frequency versus current graph of the device. However, a certain amount of derating may be needed for which you could start with the IC2 rating.
• What is the speed you require to switch? For maximum possible speeds, a PT type IGBT is more suitable. Again, for hard switching applications, refer to the frequency versus current graph of the device.
• Will the device have to withstand short-circuit conditions? If you are driving motors, the device will certainly have to withstand shorts with low switching frequencies. Most often, short circuit capability is not required for switch mode power supplies.
A generic N-channel IGBT is fundamentally an N-channel MOSFET on a p-type substrate. PT type IGBTs usually have an additional n+ layer. Therefore, the operation of an IGBT is similar to how a power MOSFET works.
When you apply a positive voltage from the emitter to the gate terminal, electrons are drawn towards the gate in the body region. When the gate-emitter voltage is equal to or above the threshold voltage, electrons drawn towards the gate form a conducting channel across the body region, allowing current flow from the collector to the emitter or electron flow from the emitter to the collector.
The flow of electrons causes positive ions or holes to flow from the p-type substrate into the drift region near the emitter. Therefore, IGBTs can have simplified equivalent circuits such as:
The price for lower on-state voltage is the IGBT may latch up if operated outside the datasheet ratings. This is a failure mode where the IGBT cannot be turned off by the gate.
