Study On The Suppression Of Double-line-frequency Output Voltage Ripple In Quasi-single-stage AC/DC Converter

For a single phase power factor correction(PFC)converter,there is inherent instantaneous power difference between ac input power and constant dc output power,which leads to a significant double-line-frequency output voltage/current ripple.The double-line-frequency output voltage/current ripple will degrade the performance of the back-end DC/DC converter,generate flicker of LED lighting,cause temperature increase inside batteries.Usually bulk electrolytic capacitors with large capacitance are needed to balance out the instantaneous power difference.However,bulk electrolytic capacitors increase converter volume and they are found to have the highest failure rate among all components,thus electrolytic capacitor is the main factor that effects the power density,the lifetime and reliability of the product.Therefore,it is significant to suppress the double-line-frequency ripple and avoide using electrolytic capacitor.The quasi-single-stage AC/DC converter with series compensation structure is composed of a PFC converter and a ripple cancellation circuit(RCC)connected in series.RCC is utilized to generate the same magnitude but 180°phase shifted double-line-frequency voltage ripple,so that constant dc output voltage is obtained.As most of the input power is transferred to the output through PFC converter directly while only small power delivered to the output through RCC has been processed twice,reduced redundant power processing is achieved.Therefore,the quasi-single-stage AC/DC converter with series compensation structure has the advantages of high power conversion efficiency,high power factor and low output double-line-frequency voltage ripple.The control of RCC is critical to the suppression of double-line-frequency voltage ripple.This dissertation focuses on quasi-single-stage AC/DC with series compensation structure and investigates the control stratagies that can achieve the excellent double-line-frequency voltage ripple suppression.In this dissertation,the equivalent small-signal model of quasi-single-stage AC/DC with series compensation structure is established.Based on this,the output impedence model of RCC is proposed and the mechanism of double-line-frequency ripple suppression is revealed from the frequency domain perspective for the first time.The magnitude of output impedence at the double line frequency determinates the double-line-frequency output voltage ripple.The larger magnitude is,the smaller output voltage ripple will be.The ouput impedence in traditional voltage mode control is studied and the effect of circuit parameters,parasitic parameters and control circuit parameters on output impedence is studied.Based on the signal-flow graph,the output impedence shaping methods have been derived systematically.The results show that the proposed voltage feedforward control based on signal-flow graph has lower double-line-frequency output voltage ripple compared with voltage mode control.In order to further reduce the double-line-frequency output voltage ripple,an output impedance shaping method with a virtual impedance is proposed.Different cases of adding virtual impedance into the inductor and capacitor branch have been discussed and compared.It is found that by introducing a virtual impedance in series with the inductor branch can effectively increase the output impedance.In this dissertation,the implementation of the virtual impedance with average current mode control is studied,and the design considersation of the magnitude of virtual impedance is studied.The results show that the proposed average current model control based virtual impedance can achieve excellent double-line-freuqnecy output voltage ripple suppression and the proposed output impedance model can effecticely evaluate the double-line-freuqnecy output voltage ripple suppression with differernt control parameters and different load types.This dissertation provides an insight into the relationship between control scheme and double-line-frequency ripple suppression from the perspective of audio susceptibility model.For the purpose of reducing the double-line-frequency ripple,the closed-loop audio susceptibility of series RCC should be designed relatively low at double line frequency.According to such requirement,quasi-proportional-resonant(QPR)compensator with its resonant poles designed at double line frequency is introduced,and thus magnitude of audio susceptibility at double line frequency is greatly reduced.The stability analysis of QPR compensator control considering digital control delay is analysed and the design consideration of QPR compensator is given.A ripple controller with improved voltage feed-forward scheme and QPR compensator is proposed and magnitude of audio susceptibility at double line frequency is further reduced.The results show that the proposed QPR compensator with voltage feedforward control based on audio susceptibility model can significant suppress the double-line-frequency ripple.Moreover,due to the flexibility of QPR compensator,the proposed ripple controller presents good ripple suppression in different line frequency application.In order to verify the correctness of the theoretical analysis and effectiveness of control strategies,corresponding simulation circuit and prototypes are established,a mass of simulation results are provided and corresponding prototypes in lab are set up to present experimental results.Simulation and experimental results have a good agreement with the theoretical analysis presented in this dissertation.The proposed quasi-single-stage AC/DC converter achieves high efficiency,high power factor,electrolytic capacitor-free and excellent double-line-frequency ripple output voltage ripple suppression.