Research On Driver Circuit And Power Circuit Oscillation Problems Of High Power SiC MOSFET Modules

The SiC MOSFET has the characteristics of fast switching speed,high breakdown voltage,small on-resistance,and high thermal conductivity,and has gradually been applied in the fields of rail transit,new energy power generation,smart grid,etc.However,in the applications of the SiC MOSFET,the switching oscillation caused by the interaction of high change rate of the currents(di/dt)or high change rate of the voltages(dv/dt)in the switching transients and the parasitic parameters in the circuit seriously threatens the reliability of the device and system.In addition,the SiC MOSFET also has the problem of weak short-circuit withstand capability.These problems put forward higher requirements for the design of the driver,the protection circuit and main circuit.Based on that,the characteristics of the SiC MOSFET,the design of the driver and the issues of the power loop oscillations of the SiC MOSFET are studied in this thesis.Firstly,the structures and operating characteristics of the SiC MOSFET are studied.And the turn-on and turn-off process are analyzed in detail step by step.The parameter extraction method is given based on the nonlinear characteristics of the transmission capacitance and the output capacitance.Moreover,the switching transient model of the SiC MOSFET is established.And the accuracy of the model is verified by the double pulse experiments.On this basis,the influence of the gate resistance,driving voltage,parasitic inductances of the power loop and gate-source capacitances on switching characteristics of the SiC MOSFET are studied by the proposed model.Secondly,the driving voltage,power supply,power amplification,isolation mode and other key parts of the SiC MOSFET driver circuit are deeply compared,analyzed,designed and tested.And some specific suggestions on circuit design are given.Aiming at the overshoot voltage and oscillation problems caused by the higher switching speed of SiC MOSFET,a multi-level turn-off driving scheme is designed and the mechanism of the multi-level turn-off drive circuit is analyzed.Afterwards,the turn-off voltage spikes and switching losses at different load currents and turn-off intermediate levels are studied.The setting method of the intermediate level and action time of the multi-level turn-off driver is optimized.And the hardware design of a multi-level turn-off driver is completed.Because of the weak short-circuit withstand capability of the SiC MOSFET,the short-circuit protection and soft turn-off schemes are studied.The experiment verifies the performance of the above-mentioned methods and circuits.Finally,taking the oscillation problem of the power loop for full SiC MOSFET auxiliary converter of the electric locomotive as an example,a small-signal model of the SiC MOSFET turn-off oscillation is established based on full consideration of the circuit parasitic parameters.Then the conditions for fully decoupling the wire parasitic inductances by decoupling capacitor are derived.The high-frequency oscillation and low-frequency oscillation also analyzed in the frequency domain.On the basis of these,a low-frequency oscillation resonance analysis model is proposed to accurately analyze low-frequency oscillation in the cascade system.Moreover,an impedance analysis network is established to calculate the low-frequency resonance current.Finally,the simulation and experimental results prove the accuracy of the proposed turn-off oscillation analysis model and the analytical calculation method of the resonance current.These methods provide guidance for the design of system parameters.