Modeling And Control Technique Of Tri-state Boost Converter

A tri-state dc-dc converter operating in pseudo continuous conduction mode(PCCM)combines the advantages of continuous conduction mode(CCM)and discontinuous conduction mode(DCM).It eliminates the right half plane(RHP)zeros existing in the boost type and buck-boost type converters,which results in fast transient response and wide load range.Therefore,it is significant to study the tri-state dc-dc converter.Firstly,taking tri-state boost converter as an example,the operating principle of different freewheeling control techniques following fixed freewheeling time(FFT)control,constant discharge time control,constant reference current(CRC)control and dynamic reference current(DRC)control are analyzed in detail.The corresponding small signal models are established using the time average equivalent circuit method,and the control-to-output transfer functions corresponding to the four control techniques are obtained and analyzed in frequency domain.The research results indicate that there is an RHP zero in FFT control that shifts with the converter's operating point,while it is eliminated in the others.Thus,it can be concluded that whether the RHP zero of tri-state boost converter exists or not,it is closely related to the control techniques for the freewheeling switch.Frequency domain and time domain simulation results verify the theoretical analysis.Secondly,the principle of voltage mode DRC controlled tri-state boost converter is analyzed in detail.Besides,its conversion efficiency and load transient performance are compared with those of the tri-state boost converter with voltage mode CRC control.In addition,their corresponding load range formulas are derived respectively.To improve the load transient performance,the current mode control technique is applied to tri-state boost converter,and the operation principles of current mode CRC and DRC control techniques are analyzed in detail.Moreover,the whole small signal models are established respectively,based on which,the corresponding output impedances are deduced and the corresponding low-frequency gains are analyzed and compared.Furthermore,the theoretical results are verified through the simulation results in frequency domain.The research results indicate that the voltage mode DRC controlled tri-state boost converter achieves such advantages as high efficiency at light load condition,wide load range and superior transient performance over voltage mode CRC controlled tri-state boost converter.While comparing with the current mode DRC control,the current mode CRC controlled tri-state boost converter has faster load transient response speed.Experiment results verify the theoretical analysis.Finally,a unified discrete iterative map model of current mode controlled tri-state dc-dc converters,including tri-state boost converters,tri-state buck converters and tri-state buck-boost converters is established.According to this model,the dynamical behaviors of the tri-state dc-dc converters with the variations of circuit parameters are analyzed.In addition,the boundary equation of tri-state dc-dc converters from stable PCCM period-1 state to unstable state is derived.Meanwhile,the operating regions are divided effectively by the boundary equation.The research results indicate that the valley reference current is closely related to the power loss and will affect the stability of tri-state dc-dc converters.The larger the valley reference current,the more stable of the converters but the lower efficiency.Differ from the voltage mode controlled tri-state dc-dc converters,the equivalent series resistance of inductor has little impact on the stability of current mode controlled tri-state dc-dc converters.Afterwards,by using ramp compensation,the stable working range of current mode controlled tri-state dc-dc converter could be broadened.Experiment results verify the theoretical analysis.