Research On The Efficiency Under Light Load And The Robust Large-Signal Control Strategy For Phase-Shifted Full-Bridge DC-DC Converter

The phase-shifted full-bridge DC-DC converter is taken as the research object.Improving the power efficiency under light load condition and ameliorating the dynamic performance of output voltage under the large line regulation and load regulation are two critical issues.Four aspects are being developed,which correspond to power loss model and modulation strategy under light load condition,large signal model and robust large-signal control strategy respectively.(1)In view of the low precision of the power loss model under light load condition,the model based on the boundary constraint of magnetic loss factor and the building of Miller platform is proposed.The magnetic loss factor in the Steinmetz equation is modified.Peak-to-peak value of magnetic induction intensity and related core loss under finite frequency component can be effectively constrained.Then,the initial and final values of current at the primary side of the transformer in the switching process are considered.Furthermore,the corresponding Miller platforms can be constructed.The precision of proposed power loss model is established by the numerical calculation and experimental verification.(2)Given that the power efficiency is relatively low under light load condition,the modulation strategy based on the combinations of the output high frequency filter capacitor and the dead time of lagging leg is proposed.The power loss difference functions are deduced on the bases of the loss model.Then,AC impedance characteristic of the output port is investigated.The combinations are divided into two categories according to the strength of resistance characteristic in the AC impedance.Furthermore,the positive and negative correlations between power loss difference functions and load resistor or equivalent resistance are analyzed.Proposed modulation method is verified by the numerical calculation of power loss and the measured results of efficiency.Efficiency under light load condition can be improved by selecting the modulation strategy related to the combination with lowest power loss.(3)In order to improve the low precision of the second-order large signal model,the step-by-step approach used to approximate the actual situation is proposed.It is related to the voltage source and the damping ratio in the equivalent circuit at the secondary side of transformer.Equivalent circuits are built in accordance with the average switch model method.Then,two key parameters in the equivalent circuit at the secondary side are approached step-by-step.They are the average voltage of equivalent voltage source and the equivalent conduction resistance which affect the damping ratio respectively.Furthermore,the second-order large signal average model is obtained according to the equivalent circuit equations.The numerical calculation results of output voltage and corresponding experimental results are investigated.It is shown that the calculated values of output voltage in the proposed model are closer to the actual values than those in the typical model.Therefore,the precision of the proposed model is effectively ameliorated.(4)Aiming to improve the regulation performance of output voltage within large range,the control strategy based on the high gain observer is proposed under the constraint of uncertain factors.The switching point and the effective duty cycle are improved.The nonlinear controller is initially designed according to the circle criterion theory in the sector area.Then,the gains of observer are designed step-by-step.The influence of input high frequency components on the controller is effectively suppressed by adopting the final gain.Furthermore,under the certain observer,the control strategy is improved for each of the combinations of uncertain factors.The experimental results show that the improved controller can not only enhance the robustness,but also ameliorate the dynamic performance of output voltage.