October / November 2021

www.talema.com GATE DRIVERS 27 www.power-mag.com Issue 4 2021 Power Electronics Europe ground and DC bus potential implying a requirement for both a floating supply, and a level shifter to transmit the PWM control signal to the floating driver circuitry. Most popular implementations of isolated gate drives use either magnetic (GDTs) or optical (Opto Coupler) techniques. Gate drive transformers A transformer coupled gate drive remains the best option for high power applications. Galvanically isolated output windings mean that a single transformer can drive all switches in the bridge and facilitate the driving of parallel switches (MOSFETs/IGBTs). A negative gate bias when the device is off also reduces dv/dt susceptibility to false switching, which can cause permanent damage. Transformer coupled solutions suffer negligible delays, and can operate across comparatively high potential differences. GDTs are optimized to transmit rectangular electrical pulses with fast rise and fall times to activate or deactivate the switching device, handling low power but high peak currents to drive the gate of a power switch. Power ratings range from µW to several kW. They provide both the floating supply and the level shifting of the switching signal eliminating the need for a separate floating power supply. MOSFET/IGBT gates can be driven directly, or an isolated control signal can be applied to a gate driver IC. Impedance matching is provided, and separate primary and secondary windings facilitate isolation and offer the option of voltage scaling. When operating at switching frequencies above 100 kHz, designers of GDTs must guard against the adverse effects of leakage inductance and distributed capacitance. GDTs are also known as pulse, trigger, wide band or signal transformers, mostly dependent on their application. The moniker “gate drive transformer” is used where the transformer directly drives a power device gate, whereas “pulse transformer” references a device used as a means to transmit rectangular voltage signals/pulses to a semiconductor gate. Generally, a pulse transformer transfers a pulse of current/voltage from the primary/generating side of the circuit to the secondary/load side, with shape and other properties maintained. A pulse transformer that initiates an action may be known as a trigger transformer. Basic circuit The basic circuit of a transformer-based isolated gate drive (Figure 5) includes reset components such as a blocking capacitor C, primary resistor R, gate resistor Rg, and a back to back Zener diode. When a square pulse is applied at the primary terminals, it is transmitted by the secondary as a square wave or as a derivative of the input voltage. The blocking capacitor C is placed in series with the primary winding of the transformer to Figure 3: Alternative switch arrangements Figure 4: Isolated gate driver topology