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Towards Integrated GaN Power Converters
Keywords: GaN, integrated circuit, power conversion
GaN integrated circuits have been tested for a few years now [Chen], the proposed work implements advanced power functions such as pre-drivers, current sensing and self-power supply auxiliary transistors, tested at high frequency and high temperature. Recent research has made possible the design of power functions [DiCioccio, Bergogne, Perrin]. This paper presents an integrated circuit made using a GaN on Si Normally-Off technology. The circuit is aimed at offline power supplies applications. The first samples were tested at voltages up to 250V and currents in the 0.1 to 1A range, switching frequency is set to 1MHz and 5MHz. The circuit is then tested at high temperature, 200C. The test circuit embeds several separate circuits. The GaN chips were mounted on daughter boards in order to be able to test them. The boards consists of a four layer Printed Circuit Board (PCB) with a Chip on Board assembly for the GaN and many connectors for testing purposes. The connectors allow access to the sub-functions embedded in the circuit. When necessary, buffers and decoupling capacitors have been placed on the PCB, as close as possible to the chip. Four adjustable bipolar power supplies with reinforced insulation and isolated control signal transmission have been designed and manufactured specifically for use with daughter boards. The control signals are generated by a double output generator, a power supply with current compliance allows high voltage tests. The FHV function (High Voltage Function) includes two high voltage inverter legs connected by an integrated dual coupled inductance also connected to the FLV function (Low Voltage Function). This arrangement makes it possible to test the high and low voltage arms separately, to test a synchronous buck converter with integrated inductance and a DC / DC converter of the Dual Active Bridge type, by choosing a bonding scheme or another. A synchronous Buck configuration is tested, it is a DC / DC converter giving an output of 70V DC approx. under 300mA at 5MHz under 180VDC input. The output filter is implemented outside the integrated circuit. The FLV function is tested in a Buck configuration. The output voltage is 20V DC approx. for a 100mA load. The input voltage is 42VDC and the switching frequency is 5MHz. Low voltage functions are useful to process power efficiently for point of load voltage regulation. In that case, High Voltage Functions may be used to step down the mains voltage to a common low voltage DC bus. A more complex function was also integrated: a power switch with pre-driver transistors, current sensing auxiliary Sources and also low current auxiliary Drains to provide a self-supply function. This the active part of an intelligent and self-powered power switch. The test was made using a 250VDC input voltage switched at 1MHz for a 1A load. Correct current measurement during conduction phases of the power transistor was observed. Conclusion. An integrated GaN power ASIC was developed and tested for offline applications. Future work include the integration of passive components with specific packaging in order to reach a high level of integration.
Dominique BERGOGNE,
Grenoble, France

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