October / November 2021

26 GATE DRIVERS www.talema.com Issue 4 2021 Power Electronics Europe www.power-mag.com Designing Transformer Coupled Gate Drive Circuits and Gate Drive Transformers Metal oxide semiconductor field effect transistor (MOSFET) and Insulated gate bipolar transistor (IGBT) are amongst the most popular, efficient semiconductor devices for switching power supplies. For medium to high power switching applications, dedicated gate drivers are essential, because it would take too long to charge the gate capacitance for the gate of a power switch to be driven by the output of a logic IC. So, what are the circumstances under which gate drivers offer the best solution – and how can their design be optimised? Bhuvana Madhaiyan & Sampath Palaniyappan, Design and Development Engineers, Talema Group, Ireland Isolated drivers are essential, for safety and other reasons. Most popular implementations of isolated gate drivers use either magnetic (Gate Drive Transformers, or GDTs) or optical (Opto Coupler) isolation 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. But there are some disadvantages too, including their unsuitability for DC, for low frequency AC, and for “normally on” devices. Gate drivers The MOSFET and IGBT are amongst the most popular, efficient semiconductor devices for medium to high power switching power supplies. Both are driven into conduction by making the gate terminal positive relative to the source/emitter (Figure 1). Charging the gate capacitor turns the power device on, allowing current to flow between its drain and source terminals when the gate voltage reaches the threshold voltage (VTH). Discharging turns the device off. The device is operated as a switch by applying a voltage sufficiently larger than VTH between the gate and source/emitter terminal. In high power applications, it would take too long to charge the gate capacitance for the gate of a power switch to be driven by the output of a logic IC (PWM controller). Instead, dedicated gate drivers are used to apply voltage which can be integrated within PWM controller ICs or implemented as dedicated ICs, discrete transistors, or transformers. Thus gate driver circuit design is critical to the achievement of the required DC/DC converter/SMPS output (Figure 2). Figure 3 shows the two alternative switch arrangements. “Low side drivers” drive ground referenced switches, whereas “high side–low side” drive a floating and a ground referenced switch using a bridge arrangement. Typical applications include solar inverters, converters for wind turbines, welding equipment, electric vehicles, and medical devices. Isolated gate drivers Isolation may be defined as the electrical separation between circuits in a system. Signals and power can pass between isolated circuits by inductive, capacitive or optical means. Isolation is mandated for safety for power inverter and converter gate drive circuits, where it also protects low voltage electronics from any damaging faults. An isolated gate drive circuit is used for power converters where high power density and high efficiency are required. Such a circuit uses high and low switches such that the low side driver cannot directly drive an upper power device. That upper power device requires an isolated gate drive, because its source/emitter is at floating potential. In Figure 4, the source terminal of switch 1 is allowed to float between Figure 2: Schematic of typical power electronic system layout Figure 1: MOSFET (left) and IGBT symbols