Reseach Of Two SiC-based High Frequency High Voltage Special Converters

High voltage bus and high voltage devices are widely used in the special converters.SiC devices show better performance over the Si devices owing to their wide band gap and lower FOM in the applications above 1 kV.Moreover,the converter with SiC MOSFETs can operate at higher frequency because of the much lower parasitic capacitance.However,SiC devices are very challenging at high voltage high frequency applications.Pulse power converters demands single high voltage and current pulse shape waveform in high temperature,heavy neutrons and ions radiation environment,where 50%~75%voltage derating margin is typically required for the power devices.SiC MOSFETs are promising to improve radiation reliability owing to wide band-gap.However,the conventional converters with off-the-shelf commercial SiC MOSFETs cannot provide enough voltage margin accordingly.A stack-capacitor pulse converter topology is proposed to reduce the voltage stress of switching devices and capacitors and improve the voltage derating capability significantly.The main idea is to build high voltage with multiple low voltage stacks instead of single high voltage capacitors,so that SiC MOSFETs can be applied with improved voltage margin.The method is to charge the stack-capacitors in parallel with fast charging speed,and discharge in series for high pulse voltage.The converter operates at higher switching frequency,which reduces the weight and size of the magnetic components.A 300 kHz prototype with 18-28 V input and 1 kV/100 A output was built using 650 V SiC devices,demonstrating 75%reduction of the voltage stress of the capacitors and power devices compared with the conventional converters.On the other hand,a 1-kV input SiC LLC converter with the matrix planar transformers is proposed to achieve high efficiency and high power density.With fast switching speed 140 ns of SiC MOSFETs at 1 kV input voltage,high dv/dt of 11.8 kV/?s can cause large displacement current via the parasitic capacitance between the primary and secondary side of the planar transformers.The displacement current can distort the resonant current seriously causing the control MOSFETs not to realize ZVS,and inducing high switching loss at 1 kV.A LLC topology with split resonant tanks is proposed for high input voltage applications.The resonant tank is split into two identical resonant tanks to provide symmetrical resonant current with the same impedance.The input and output current of the resonant tank is symmetrical avoiding the current distortion.Therefore,all of the control MOSFETs can realize ZVS.The transformer interwinding capacitance modeling and a reduction solution are presented,which reduces the waveform distortion by 68%.A prototype of 3 kW SiC converter with input of 1 kV and output of 32 V/3 kW was built.The efficiency is 94.9%at full load with the diode rectification at 300 kHz.The power density is 3.87 kW/L(63.4 W/in~3)and 4.11 kW/kg.