POWER ELECTRONICS


Designing transformer coupled gate drive circuits and gate drive transformers


Bhuvana Madhaiyan & Sampath Palaniyappan, design and development engineers at Talema Groupexplore switching power supplies


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. Isolated drivers are essential, for safety and


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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. So, what are the circumstances under


which GDTs offer the best solution – and how can their design be optimised?


Gate drivers Power semiconductor devices are at the heart of modern power electronic systems. The Metal oxide semiconductor field effect transistor (MOSFET) and Insulated gate bipolar transistor (IGBT) are amongst the most popular, efficient semiconductor devices for medium to high power switching power supplies.


etal oxide semiconductor field effect transistor (MOSFET) and Insulated gate


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.


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 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.


Figure 2: Schematic of typical power electronic system layout


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.


Figure 4: Isolated gate driver topology Most popular implementations of isolated


gate drives use either magnetic (Gate Drive Transformers, or GDTs) or optical (Opto Coupler) techniques.


Figure 3: Alternative switch arrangements Typical applications include solar inverters,


converters for wind turbines, welding equipment, electric vehicles, and medical devices.


Figure 1: MOSFET (left) and IGBT symbols 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


42 OCTOBER 2021 | ELECTRONICS TODAY


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


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 optimised 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.


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