Research On Capacitor-Current Hysteresis Control Of Buck Converter

DC/DC converters are widely applied in the end of power conversion system as the power interface of load end, and hence its performance directly determines the load operation metrics. Especially for microelectronics and communication power supplies, it requires high transient response of DC/DC converters to deal with fast and wide load fluctuations. On this issue this paper would explore capacitor-current based ripple control method for DC/DC converters to achieve fast load transient characteristics. Based on the basic capacitor-current hysteresis(CCH) control scheme, the following research work are carried out, including optimization of control parameters, influence of parasitic parameters on control performance and constant frequency realization of the concerned hysteresis control.First of all, circuit design for capacitor-current sensing in a non-invasive way is proposed based on matched transfer function, and with the proposed scheme high precision sensing of capacitor current is achieved. Matching the transfer functions of the sensing circuit and the sensed capacitor in different switching frequencies, different sensing circuits and their system parameters are given, respectively responding to C, RC and RLC cases. Some simulation and experimental results verify the accuracy and rapidity for the RLC case. Moreover, the proposed current sensing scheme using matched transfer function are capable to other circuit components.Secondly, optimized control parameter of CCH controlled Buck converter is determined using load transient analysis, and the impact of parasitic parameters of output capacitor on dynamic performance is described as well. On this basis improved CCH control scheme using output-voltage separated control strategy is proposed to eliminate the parasitic-parameter impact. Moreover, the quantitative relationship between the control parameters of CCH scheme and load transient regulation time is figured out, as well as the failure reason of CCH control. Simulation and experimental results confirm the correctness of the theoretical analysis and the validity of the proposed improved CCH control method. The proposed output-voltage separate control are functioned with separated adjustment for steady state and dynamic performance.Finally, perturbed constant frequency scheme is proposed for our concerned CCH control. The disturbance ramp signals operating at the switching frequency is introduced into the CCH control circuit, so as to achieve constant switching frequency characteristic of the controlled system. Moreover, design conditions and circuit implementation of the proposed ramp signal are given. Simulation and experimental results confirm the validity of the proposed perturbed constant-frequency scheme, which provides a new solution to constant frequency control method for power converters.