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Elucidation of the intrinsic loss arising from switch output capacitance Coss in ultra-high-speed low-loss power converters

Elucidation of the intrinsic loss arising from switch output capacitance Coss in ultra-high-speed low-loss power converters

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Attempts to drive down the switching losses of hard-switched unipolar devices by increasing gate drive current levels are ultimately limited by the device output capacitance, which thus represents an intrinsic loss. The paper presents an in-depth analysis of the charging and discharging of the output capacitance and the associated loss producing mechanisms. It is shown that energy is stored in the depletion layer of the device during turn-off and then dissipated as heat during the turn-on process. The instantaneous power dissipated in the device is thus different to the instantaneous power loss measured at the device terminals, although the total energy loss over one complete turn-off and turn-on cycle is still faithfully reproduced. An analytical approach to determining the intrinsic switching loss due to the output capacitance is presented and verified by experimental results and by TCAD simulation. It is shown that around 20% of the total switching losses arise from the intrinsic output capacitance loss under fast-switched inductive load conditions. A comparison of Si and SiC FET devices demonstrates the potential for SiC power devices to provide an energy efficient option for very high frequency switching.


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