Optimal Frequency of Silicon Carbide Power Correction Circuits for On-Board Chargers
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- Institutt for elkraftteknikk 
Compact high-efficiency power converters is a hot topic in electrical power engineering. So is also the case for electric and plug-in hybrid electric vehicle battery chargers. Increasing the switching frequency in the power converters will reduce the size of passive components, but this also results in lower efficiency. The progress being made in wide bandgap semiconductors introduce possibilities for lower loss, and thus improved efficiency at higher switching frequencies. Silicon carbide is such a semiconductor material, whose electrical and thermal capabilities trumps those of a classical silicon semiconductor.To investigate whether utilizing a silicon carbide semiconductor in an on-board charger for electric and plug-in hybrid electric vehicles can provide lower switching loss, a silicon carbide MOSFET is tested against a silicon MOSFET. The focus is on the hard switched application of the power supply s power factor corrector. A continuous-conduction mode DCDC boost converter is designed and used to perform the test. The converter is designed for an input voltage of 230V, output voltage of 350V and rated for 3500W. Emphasis has been made on parasitic elements and their effect on switching behavior. Three different circuits was developed through the course of the thesis, with improvements in layout implemented for each step. Significant improvements was seen in gate drive circuitry and the overall converter from the first design to the second, and improvements in the gate circuitry was seen from the second design to the third. From this, it is concluded that for best performance, the gate driver circuit should be as compact as possible, with short conduction paths and close to the MOSFET gate pin. The return form the MOSFET source to the gate driver ground should be as large as possible, and directly underneath the gate signal path. SMD components with low parasitic inductance and capacitance should be used in the gate driver circuit. The Si and SiC MOSFET show different switching behavior. The main difference is at turn-off, regarding delay and voltage rise waveform. At the same gate resistance and gate voltage, the turn-off delay time for the Si MOSFET is almost three times longer than the SiC MOSFET delay time. While the SiC MOSFET turn-off voltage showed a linear increase the entire rising period, the Si MOSFET voltage rise was more exponential. Accurately determining losses in a MOSFET, and how well it is capable of operating, is a challenging exercise. Distortion caused by the probes while measuring, bandwidth limitations in the probes and oscilloscope, time delays in the probes and improper components and design can all contribute to erroneous results. Nevertheless, it was concluded that in this circuit, with these MOSFETs, the SiC MOSFET had a switching loss between 106 µJ and 138 µJ, which was between 0,7 and 0,75 times lower than the Si MOSFET switching loss. With the conduction loss for the MOSFETs taken into account, which was found to be 2,2 W for the SiC MOSFET and 7,5 W for the Si MOSFET, at nominal operation and a selected operating temperature of 125 ºC, the optimal switching frequency was found to be between 165 kHz and 215 kHz. This is based on a power loss cap of 25 W.