October / November 2021

30 GATE DRIVERS www.talema.com Issue 4 2021 Power Electronics Europe www.power-mag.com secondary side of the transformer (capacitor & diode) ensures adequate gate drive voltage and restores the original gate drive amplitude on the secondary side of the transformer. Double ended gate drive circuits are used with a double output PWM controller to drive 2 or 4 switches in high power applications (Figure 7). The GDT is driven by a variable pulse width and constant amplitude. OUTA and OUTB are opposite polarity and symmetrical. When OUTA is on, positive voltage is applied. The average voltage across the primary for any two consecutive switching periods is always zero, removing the need for any AC coupling. Pulse response characteristics It is important that a pulse transformer reproduces the shape of input pulse as accurately as possible at its secondary terminals. Performance is specified in terms of its ability to do so. In practice, output response is distorted. Current cannot change instantaneously resulting in finite rise and fall times, and the input voltage is of discontinuous nature (Figure 8). The pulse width varies from less than 1 µs to about 25 µs, with parasitic elements causing overshoot, delay and ringing and non-ideal components (transients) causing deviations in the flat portion. The aim of transformer design is therefore to minimize leakage inductance and distributed capacitance. As illustrated in Figure 9, a typical pulse wave has four regions, and permissible distortion is defined in terms of the illustrated parameters. Conclusion Most popular implementations of isolated gate drives use either magnetic (Gate Drive Transformers, or GDTs) or optical (Opto Coupler) techniques. Advantages of GDTs include a lack of propagation delay in carrying signals from the primary side to the secondary; no requirement for a separate isolated power supply; the provision of a step-up / step-down facility; and high efficiency. There are some disadvantages including their unsuitability for DC, for low frequency AC, for normally on devices, and for high power, high density synchronous rectification applications; the need for AC coupling capacitors and Zener diodes where high duty ratios are necessary, and a complexity and costliness resulting from the requirement for a transformer primary to be driven by a high speed buffer.However, where GDTs can be used, careful design can ensure a highly effective and efficient solution. LEFT Figure 8: Pulse response deviates from the ideal BELOW Figure 9: Pulse response regions and parameters