Research On PWM Technology Based On Random Signals

With the increasing application of Pulse Width Modulation(PWM)technology in power electronic systems,the problem of harmonic electromagnetic interference(EMI)caused by PWM has attracted people's attention.This paper based on the causes of EMI and its own characteristics.The DC/DC converter is used as the research object and adopted a random PWM technique,which can more evenly distribute the harmonic energy concentrated in the original switching frequency and frequency multiplication over the entire frequency band,so that the original discrete harmonic spectrum becomes a uniform continuous spectrum to effectively reduce electromagnetic interference.The main research work of this paper is as follows:(1)The expression of power spectral density of fixed period PWM,random pulse position PWM,random duty ratio PWM and random switching frequency PWM is derived and analyzed.This paper focuses on Random Carrier-Frequency Modulation with Fixed Duty Cycle(RCFMFD)and Random Carrier-Frequency Modulation with Variable Duty Cycle(RCFMVD).The simulation models of RCFMFD and RCFMVD were designed in MATLAB.Comparing the two methods with the fixed-cycle PWM,the discrete harmonic distribution can be made more uniform and continuous.(2)Based on the needs of simulation and experimental research,this study puts forward the basic requirements of the Buck switching power supply,and designs the L and C parameters of the circuit,as well as the selection of switches and diodes.The power spectrum densities of the diode-side voltage and inductor current of the fixed-period PWM,RCFMFD,and RCFMVD modulation methods are analyzed in the simulation model.The results show that the two randomly-modulated harmonic energy are evenly distributed over the entire frequency band,which improves the electromagnetic interference generated by the circuit.However,because the load voltage ripple of RCFMVD is too large and does not meet the design requirements,the final experiment verifies the choice of RCFMFD modulation strategy.(3)This experiment put forward the overall scheme of Buck switching power supply,and designed the hardware circuit and software(including random digit length,PID control algorithm,RCFMFD algorithm,etc.).An experimental test system for near-field interference and conducted electromagnetic interference is built,and its composition and testing parameters are expounded.The experimental results show that two kinds of random modulation strategy can effectively suppress the harmonic energy concentrated in the original switching frequency and frequency multiplication,and make it evenly distributed in the whole frequency range,and improve the influence of the near field interference in the Buck switching power supply.The experiment also shows that the RCFMFD modulation strategy reduces the limit of conduction electromagnetic interference in Buck switching power supply,and effectively improves the electromagnetic interference of DC/DC switching power supply.