The Research Of High Frequency Photovoltaic Grid-connected Micro-inverter Based On GaN HEMT

Photovoltaic power generation technology is based on the photoelectric effect and can be considered as real green energy.It occupies a large share of renewable energy in the power system,with rapid development in recent years.However,there are obvious shortcomings for the traditional photovoltaic power generation connecting directly with the grid,including poor scalability,low reliability and partial shadows.In the thesis,a novel two-stage photovoltaic grid-connected micro-inverter is proposed and designed.New topological structures are designed for the front and the rear stages.To get better performance,the gallium nitride high electron mobility transistor(GaN HEMT)is adopted in the design to replace traditional silicon power switching devices.The main work content of the thesis includes:(1)The switching characteristics of GaN HEMT are studied.The corresponding Pspice equivalent circuit model is established.A high-frequency and low-power level shift circuit with a logic correction function is proposed,which can convert the high-frequency 1.5 V low-level pulse generated by the control chip converted to a 5 V high-level drive pulse,with its rise and fall delay time 2 ns.(2)Aiming at the problem of the low output power of the traditional DC-DC converters,a high-frequency Sepic-type DC-DC converter based on GaN HEMT is designed as the front stage of the micro-inverter.The Sepic converter includes two improvements on the topology:Firstly,the single GaN HEMT device is used to control two flyback transformers at the same time.The two output capacitors corresponding to the two transformers are alternately charged and discharged.In this way,the output voltage ripple is obviously suppressed.The output power of the new design is increased to be 500 W,with a 150% increment.Secondly,by using the converter boost characteristics,a non-isolated clamp circuit is fulfilled,which not only reduces the switching voltage stress but also forms an absorption loop for the leakage energy of the coil of the transformer,so that the energy efficiency of the converter is increased to 95.6%.A dynamic PWM comparison circuit based on dynamic reference is designed to reduce the energy consumption of the converter voltage feedback link.(3)For the traditional full-bridge inverter with AC bypass,its structure is relatively complex.To solve the problem,a GaN HEMT-based H5 inverter is designed as the latter-stage of the micro-inverter.The inverter utilizes a single GaN HEMT device to replace the traditional two silicon IGBT devices.When the output power is 500 W,the energy efficiency of the latterstage inverter reaches 96%,which is higher than the traditional inverter,and the common-mode leakage current is less than 14 m A.(4)According to the above design,a test prototype of the two-stage micro-inverter is fabricated and characterized.The test results show that the micro-inverter meets the requirements of photovoltaic power generation grid-connected with 500 W output capability.The overall energy efficiency can reach 91.2%,and the total harmonic distortion is only 3.8%.