POWER ELECTRONICS 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 100KHz, 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.


Figure 5: Basic circuit


windings, and the design must accommodate the maximum volt-time product. The GDT is driven by a variable pulse width


as a function of the PWM duty ratio. Amplitude may be constant or variable according to configuration.


Single ended and double ended circuits All GDTs operate in both the first and third quadrant of the B-H plane.


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).


Figure 8: Pulse response deviates from the ideal The pulse width varies from less than a


Figure 6: Single ended transformer-coupled gate drive


Single ended circuit Single ended gate drive circuits are used with a single output PWM controller to drive a high side switch. The GDT is driven by a variable pulse width and variable amplitude. This circuit is limited to 50% duty ratio.


Basic circuit The base circuit of a transformer based isolated gate drive 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 provide the reset voltage (negative bias) for the magnetising inductance, preventing transformer saturation. The amplitude of output voltage reduces with the duty ratio increase, hence this circuit limits the duty cycle to less than 50%. This approach works well in Switch Mode


Power supply (SMPS) circuits, where the frequency is high and the duty cycle ratio is small. Gate drive voltage, Vc, changes with duty


ratio. Sudden changes in duty ratio will excite the L-C resonant tank formed by Lm & C, an effect that can be damped by the low value resistor (R). The gate is driven between -Vc and VDRV-Vc levels as opposed to original output voltage range of the driver, 0V and VDRV. A back to back Zener diode is used to clamp the device gate voltage, and gate resistor Rg is used to avoid gate transient surge current Core saturation limits the applied volt-time product across the


Figure 7: Double ended transformer-coupled gate drives


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


For wide duty cycle applications, a DC restoration circuit on the 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 circuit 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.


micro second to about 25 micro seconds, 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 minimise 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.


Figure 9: Pulse response regions and parameters


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.


Talema Group www.talema.com


OCTOBER 2021 | ELECTRONICS TODAY 43


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