Research And Design Of Photovoltaic Inverter Using SiC MOSFET

As the most important component of distributed renewable energy, solar photovoltaic power generation has been increasing the proportion and increment speed in power generation. How to improve the energy conversion efficiency of PV inverter has become the hotspot.In this thesis, the characteristics of SiC power switching devices using wide band gap material are studied. And SiC power MOSFET with high temperature resistant, high voltage level,fast switching speed and low power loss is applied to the single-phase PV off-grid inverter to improve the switching frequency, reduce the switching loss, improve the energy conversion efficiency,small passive device capacity requirements, and promote the miniaturization development of photovoltaic products.In terms of hardware, according to the characteristics of SiC MOSFET and the requirements of its driving circuit, the isolation driving circuit using +18V and -3V driving power supply was designed. According to the requirements of the inverter for control power, Forward isolated DC/DC switching power supply circuit was designed. According to the requirements of the inverter for voltage and temperature closed-loop control, the AC voltage and current sampling circuit and bridge temperature acquisition circuit was designed. According to the requirements of the DC bus voltage, the 380V boost circuit with two push-pull circuit cascaded is designed to provide the high DC power for the inverter. In addition, the inverter control circuit and the boost control circuit are controlled by two STM32F103RCT6.In the software aspect, the improved SPWM control algorithm is used in which the working mode of the single-phase inverter was analyzed, and the formula of SPWM pulse width is deduced. The digital realization method for the improved SPWM control algorithm in STM32 Micro controller was analyzed. And the reason for the inverter's output waveform shock at zero crossing point was analyzed. Then the solution based on master-slave working mode of advanced timer TIM1 and TIM8 was put forward. At the same time, a digital implementation method based on two-step operation mode of STM32 timer for producing driving waveform of push-pull circuit is proposed. Finally,the main program structure of the inverter and push-pull booster circuit is described in detail.Aiming at the above design scheme, a 400W photovoltaic off-grid inverter prototype using SiC MOSFET was designed and developed. The experimental platform was designed and the hardware design scheme and software control algorithm were experimentally verified. The main waveforms and experimental results were recorded and analyzed. Finally, the inverter can output 50Hz frequency AC voltage. After THD analysis, the waveform meets the original design requirements. Experiments show that the hardware design is feasible and the software control algorithm is stable and reliable. And the results certain reference value for the promotion and application of SiC power MOSFET in the miniaturized photovoltaic inverter.