PowerViews

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January 21, 2013

Driving Paralleled IGBTs

Alan Ball
IGBT Applications Engineering Manger, ON Semiconductor

Power Channels: Automotive Electronics, Communications Power, Digital Power, Energy Efficiency, Power Components, Switch-Mode Power

As the demand for high power electronic equipment grows, it becomes necessary to drive high-voltage, high-current loads. Achieving high levels of both voltage and current in a switching power converter often requires paralleled switching devices, such as IGBTs which are well suited for this type of application.

There are a number of issues to be considered when connecting two or more IGBTs in parallel. One of those issues is the interconnection of the gates. Paralleled IGBTs can use a common gate resistor, separate ones or a combination of those two scenarios. Most discussions on this topic offer the opinion that separate gate resistors are a must. However, there is a strong case for a common gate resistor.

When determining the drive scheme for paralleled IGBTs one of the first things to consider is the total amount of drive current required. If a suitable driver is not available to drive the total base current of the IGBTs that are connected in parallel, the IGBTs will have to be driven by individual drivers. In this case each IGBT will have to have its own gate resistor. Most drivers are sufficiently fast to apply the turn on and turn off pulses within 10s of nanoseconds of each other. This is quite adequate for IGBT timing as they typically switches in a few hundred nanoseconds.

If a single driver is used then the discussion can focus on the configuration of the gate resistors. The downside to individual gate resistors is that mismatches in the timing can be increased since the drive voltage on the gates will not track at turn on and turn off. Even though the gate pulses at the driver end of the resistors will be exactly the same, the variance in gate charge of the IGBTs combined with the gate resistance and circuit board impedances will create varying rise, fall and delay times at the gates of the IGBTs. Still, many sources advocate the use of individual gate resistors as they will minimize the possibility of oscillations between IGBTs.

Oscillations can occur due to the stray inductance (mainly in the emitter circuit) of the layout, combined with the gate capacitance and gain of the IGBTs. Minimizing the inductance in the emitter circuit, will go a long way toward eliminating parasitic oscillations.

A common gate resistor assures that both gates are at the same potential at any given time, with only a very small difference due to parasitic variations in the impedance path. This can reduce differences in losses and help IGBTs to share current more equally during the switching transitions. Common or individual gate resistors have no effect on the DC current levels as all of the IGBT gates will eventually charge up to the gate bias voltage. The use of a common gate resistor has been suggested by other sources also, but is not as common of a guideline as individual gate resistors.

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