Circuit diagram design of isolated half-bridge gate driver

Overview:
Many applications use isolated half-bridge gate drivers to control large amounts of power, from isolated DC-DC power modules that require high power density and efficiency, to solar inverters where high isolation voltage and long-term reliability are critical, and more. And so on. This article will elaborate on these design concepts to demonstrate the superior performance of isolated half-bridge gate driver ICs in small packages for high performance.

A basic half-bridge driver (shown in Figure 1) isolated by optocoupler drives the gates of the high-side and low-side N-channel MOSFETs (or IGBTs) with opposite polarity signals to control the output power. The driver must have low output impedance to reduce conduction losses while also requiring fast switching capability to reduce switching losses. For accuracy and efficiency considerations, high-end and low-side drivers require highly matched timing characteristics to reduce the dead time before the first switch of the half-bridge is turned off and the second switch is turned on.

Figure 1. High-voltage half-bridge gate driver

As shown, a conventional implementation of this function is to isolate with an optocoupler followed by a high voltage gate driver IC. A potential disadvantage of this circuit is that the single isolated input channel relies on a high voltage driver circuit. To achieve the required timing matching and dead time between channels. Another problem is that the high voltage gate driver is not galvanically isolated, but relies on the junction isolation of the IC to separate the high side drive voltage from the low side drive voltage. In low-side switching events, parasitic inductance in the circuit can cause the output voltage VS to drop below ground. When this happens, the high-end drive may latch up and be permanently damaged.
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