Unlocking SiC MOSFET Switching Performance Through Packaging and Instrumentation
Electrical energy conversion is vital to the distribution and use of electricity. With the increased use of renewable energy sources and the move to cleaner electrified transport methods, the demands of power electronic converters and inverters continue to increase. A major component in any power electronics converter or inverter is the transistor. Advances in wide-bandgap semiconductors such as Gallium Nitride (GaN) and Silicon Carbide (SiC) are leading to improved transistor technologies. These materials allow for smaller transistors with reduced on state resistances and input capacitances, this reduces losses and supports higher switching frequencies. In contrast, packaging and measurement technologies have not significantly evolved to support these emerging high-speed devices. Traditional wire-bonding packaging technologies that were originally developed in the 1950's are still being used today, with the majority of power modules on the market using bond-wires for transistor connections. Wire-bonding introduces significant stray inductances into the transistor connections. When combined with high-speed switching, such inductance can cause device damaging voltage overshoots, unwanted resonances and parasitic turn-on of the transistor. Due to such effects, the switching speed of the semiconductor device has to be restricted in order to ensure reliability and prevent damage. This negates many of the advantages that wide-bandgap devices can offer. Existing switching measurement technologies are also unable to support the high speed potential of wide-bandgap semiconductor devices. Currently available systems either lack bandwidth, have significant bandwidth distortion or introduce a large stray inductance into the circuit under test. Insufficient bandwidth can slew the measured rise-time of the switching edge leading to incorrect switching loss measurement and can attenuate high-frequency resonances reducing their apparent severity. A distorted bandwidth can significantly distort the measured time-domain signal, causing overshoots and tail-currents that are not present in the “true” waveform. The stray inductance of the current measurement system will introduce inductance into the power module, exacerbating the inductance problems previously mentioned. These factors result in many proposed high speed power modules omitting current measurement completely. This leaves the true high-speed switching nature of wide-bandgap semiconductors unknown. In this Thesis, a hybrid PCB-Ceramic half-bridge packaging structure is proposed, developed and electrically optimised. This novel structure combines single layer ceramic with conventional multi layer circuit boards. The structure allows for very low power loop inductances of <0.8 nH whilst integrating auxiliary circuits in order to unlock extremely high device switching speeds in excess of 50 A/nS and 70 V/nS. To characterise the proposed power module, a zero-compensated shunt resistor based current measurement system is proposed, achieving 3 dB bandwidths in excess of 1.6 GHz. Through the integration of this current measurement system within the power module, insertion inductances as low as 20 pH have been achieved. An insertable oscilloscope-probe version of this measurement system is also developed using a novel new connection method known as the multi-layer interconnect (MLI). The MLI structure retains the connection convenience offered by existing current measurement probes whilst utilising multi-layer mutual inductance cancellation to achieve insertion inductances as low as 67 pH. This is considerably lower than a traditional coaxial current shunt which typically has an inductance in the range of 2-7 nH depending on the model.
Wide bandgap, Switching, Transistor, Current measurement, high bandwidth, Semiconductor, packaging, semiconductor packaging, gigahertz bandwidth, low inductance, mutual inductance cancellation, double pulse test, DPT, passive probe, oscilloscope probe, high speed switching, high-speed switching, SiC, silicon carbide, GaN, soldering, thermo-mechanical stress, comsol, ansys, finite element analysis, skin effect
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