With their excellent isolation and reliability, gate-drive transformers (GDTs) are commonly used for driving half-bridge power circuits. However, they pose challenges when you want to have both switches off for an extended period.
This application usually requires the GDT to be driven with an open-collector, and with a short “on” time, the transformer’s field would not store enough energy to ensure the proper turn-off of the device. In this case, a load resistor must be added, which uses more power and provides marginal performance.
The design shown in Figure 1 allows the drive signal to the MOSFET to range from 50% down to 0%, while still offering a fast turn-off that cannot be accomplished with a simple load resistor. When T1’s primary is energized, the secondary delivers current through D1, charging the gate.
When the transformer’s primary is released, the secondary delivers a low-energy negative spike, then goes to zero. While any charge remains in Q2’s gate, Q1 shunts the current directly to Q2’s source, using the transformer’s secondary only as a control signal. This allows the gate capacitance to be discharged rapidly.
To significantly widen the number of transformer choices, the circuit uses a primary drive scheme that does not require a separate reset winding to be included for the GDT. Of course, multiple secondaries may be used for controlling several devices, each with its own secondary circuit.
Figure 2 shows the circuit used in a typical forward-converter application. The back-to-back zener diodes are intended to clamp any voltage spikes that would be caused by leakage and stray inductances. However, the 560-Ω load resistor, together with the gate capacitance and a low-leakage transformer design, is usually enough to suppress such spikes, so the zeners may not be needed.