Research On Hybrid Control And Improved SPWM Algorithm For High-power Flyback Inverter

With the increasing popularity of photovoltaic power generation technology,various grid-connected photovoltaic inverter topologies are proposed.The flyback topology has the advantages of simple circuit structure,low cost and galvanic isolation between the input and output side,and becomes a research upsurge in the field of micro-inverter.However,with the use of interleaving technology and transformer optimization design method,flyback inverters can also be applied in high-power condition.Based on the three-cell interleaved high-power flyback inverter,the efficiency optimization of the inverter is studied in this paper.In the meanwhile,because the main switch is driven by high-frequency sinusoidal pulse width modulation(SPWM)techniqe,the improvement of SPWM control technology in traditional single-phase inverters is also discussed.In this paper,the main work and conclusions are as follows:The key technique of the flyback inverter working in high-power condition is analyzed.The use of three-cell interleaved flyback converter increases the output power levels,and increases the output current ripple frequency,which is beneficial to be filter out.The optimization design technology of the flyback transformer helps for minimizing leakage loss.Choose the right operating mode for the flyback inverter to reduce the complexity of control.The Flyback converter works in DCM to optimize device selection and output optimal power.Base on this,the theoretical and technical objectives of this paper are established.Based on the main energy dissipation devices(the transformer and main power switch MOSFET)of the high-power flyback inverter,the main energy loss mathematical model is established,which regards the system loss as determined by the effective input current.The losses of two-cell and three-cell interleaved flyback converters are compared and analyzed.It is found that there is intersection in the curve of system losses of two-cell and three-cell interleaved flyback converters with respect to current.Based on this,a hybrid control strategy for the three-cell interleaved high-power flyback inverter is proposed,in which two-cell and three-cell interleaved flyback are operated alternately in half grid period.That is to say: 2?-DCM/3?-DCM hybrid modulation.The efficiency optimization scheme of the high-power flyback inverter for all power range is also proposed.The simulation results have verified the feasibility of the proposed hybrid control method.According to the traditional SPWM control technology,because of the sampling interval in digital controller,it is not considered that the mutation of modulation depth in a period of a modulation wave will result in the instantaneous output waveform distortion.This paper presents an improved SPWM algorithm in order to make the output waveform of the inverter keep smooth when the modulation depth changes abruptly.Within the current carrier cycle,the proposed method involves in taking the mean value of the modulation depth of before and after,and then the comparison of modulation wave and carrier decides the driving signals of power devices.The amount of the change of the modulation depth can set a threshold.The improved SPWM algorithm is designed mainly to make the output waveform have a smooth transition when the mutation of modulation depth happens.The improved SPWM algorithm can also be applied in the flyback inverter.The simulation results show the effectiveness of the proposed improved SPWM algorithm.