Research On Limit Cycles Oscillation Characteristics Of Buck DC-DC Converter System Controlled By Digital PI

DC-DC switching converters are widely used in various power electronics situations,such as in portable electronic devices and audio amplifiers.Compared with the analog control method,the digital control method has some advantages of more flexibility,better noise immunity,more complex control schemes and so on,which has made digitally controlled DC-DC switching converters widely used.The quantization effect of Analog-to-Digital Converter（ADC）and Digital Pulse Width Modulator（DPWM）may cause Limit Cycle Oscillation（LCO）,which has an important impact on the stability of the output voltage of the switching power converter.This article focuses on the LCO of a digital PI controlled DC-DC Buck switching converter system.Firstly,the basic principle,working mode,modulation method and compensation control method of a digitally controlled DC-DC switching converter are described in detail,and the main circuit topology parameters are selected.Secondly,the state space averaging method is used to model a digitally controlled DC-DC switching converter;and the causes,influencing factors and elimination conditions of LCO in the system are analyzed according to N_adc=N_dpwm+2,N_adc=N_dpwm+1 or N_adc=N_dpwm.Thirdly,matlab’s simulink is used to model a digital PI controlled DC-DC Buck switching converter,and simulink simulation is performed under three conditions.Finally,a test platform for digital PI controlled DC-DC Buck converter based on FPGA is set up,and the LCO characteristics of the system are functionally simulated based on Verilog.After research,it finds out that the higher the quantization accuracy of DPWM compared to the quantization accuracy of ADC is,the fewer LCO the system will have.Furthermore,under the condition of keeping K_p constant and increasing K_i,the system will have more duty cycles of LCO,while under the condition of keeping K_i constant and increasing K_p,the system will be more stable.Finally,if the condition that N_adc=N_dpwm is satisfied,properly increasing K_p and decreasing K_i is helpful to avoiding LCO in the system,which is instructive to design of a digital PI controlled DC-DC Buck switching converter system.