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Ultralow Input-output Capacitance PCB-embedded Dual-output Gate-drive Power Supply for 650 V Gallium Nitride-based Half-bridges

PCB-embedded transformer
Fig. 1. PCB-embedded transformer structure illustration
With features such as high switching frequency and high blocking voltage, wide-bandgap (WBG) devices, such as silicon carbide (SiC) metal-oxide semiconductor field-effect transistors (MOSFET) and gallium nitride (GaN) high-electron-mobility transistors (HEMTs), are being used more in power converters to boost the efficiency and power density. A key trade-off of these gains is the increasing electromagnetic inference (EMI) noise. To attenuate the EMI noise from the power loop into the auxiliary sources, the isolation capacitance in the isolated gate drive power supply is expected to be as small as possible. In this paper, a gate drive power supply dedicated to drive two 650 V GaN devices in a phase leg, is presented. It has a PCB-embedded transformer as a substrate, achieving an ultra-low inter-capacitance of 1.6 pF, high efficiency of 83 %, and high power density of 72 W/in3. The input of the power supply is 15 V. There are two isolated outputs; each has output voltage of 7 V and output power of 1 W. To reduce the inter-capacitance (isolation capacitance) from the primary to the secondary side in the transformer, the primary and secondary windings should be located as far away as possible, which increases the volume accordingly. In addition to the above trade-off, efficiency is also a key design variable affected by the power supply volume. The challenge of the work is to find an appropriate design approach, aiming at small converter volume, small inter-capacitance, and high efficiency. To design the targeted gate driver power supply, the paper first presents the circuit design, followed by the transformer optimization. The PCB-embedded transformer structure is then illustrated in Fig. 1, and the core and PCB material selections are addressed. To find a compromise among a low inter-capacitance, low transformer loss, and a small volume, an optimization of the transformer dimensions is performed, based on the transformer models. With the optimized dimensions, the transformer is built and shown in Fig. 2, and the fabrication procedures, including a standard lamination process, are introduced. The transformer and the converter hardware are shown and characterized with experimental waveforms, efficiency, isolation voltage, inter-capacitance, and total volume.
Optimized transformer
Fig. 2. Active-clamp flyback with the PCB-embedded transformer substrate
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