Current through a MOSFET between drain and source is controlled by a drive voltage applied to the MOSFET gate. In switching power supplies a pulsed gate drive voltage turns the drain-source current on and off, operating the MOSFET as a current switch. Gate drive transformers are used to deliver the controlling pulses while providing isolation between the MOSFET and the controlling drive circuit. This article discusses gate drive transformers and applications for which Coilcraft off-the-shelf gate drive transformers are best suited.

Gate driver circuits need an isolated (floating) bias supply to maintain the required turn-on bias when the FET source rises to the input voltage. A gate drive transformer isolates the controlling gate drive circuit from the switch node when driving the MOSFET gate, and may also scale the output voltage via an appropriate primary-to-secondary turns ratio.

In some applications, digital isolators or opto-couplers may provide the means to drive MOSFET’s directly, but gate drive transformers are preferred for higher-voltage requirements and have the advantage of much lower turn-on and turn-off delay times, as well as the capability to scale voltage by turns ratio. Therefore, gate drive transformers are often the best performing solution for high-voltage and high-frequency applications where fast and accurate signal timing is critical.

What does a typical gate drive transformer circuit look like?

Figure 1 shows a simplified single-output, transformer-coupled (AC-coupled), high-side gate drive circuit for lower power applications. Depending on duty cycle and other circuit conditions, additional components (capacitors, diodes and resistors) may be used, to prevent:

1. The development of a DC voltage across the transformer which would cause it to saturate
2. The magnetizing inductance and coupling capacitance from resonating with sudden changes in duty cycle

For single-ended (AC coupled) circuits the worst-case duty cycle is 0.50.

Figure 1: Simplified transformer-coupled single-ended gate drive circuit

Half-bridge and full-bridge configurations (such as the transformer-coupled, push-pull half-bridge gate drive circuit shown in Figure 2) are used for higher-power applications.

For double-ended (DC-coupled) bridge configurations, the worst-case is the maximum duty cycle (theoretically 1.0).

Figure 2: Transformer-coupled push-pull half-bridge gate drive circuit

Figure 3 illustrates a typical gate drive transformer solution where a full-bridge power stage is driven by both high-side (Q1 and Q2) and low-side (Q3 and Q4) MOSFETs.

Figure 3: Full-bridge power stage with high-side and low-side primary MOSFETs