Research On Control Strategy Of High-frequency Active Clamp Flyback Converter

With the rapid development of portable electronic products,AC-DC portable adapter power supply requires more and more demands on smaller size and faster charging speed,which make power adapters are changed towards to high frequency,high efficiency,high power density and small volume.Increasing switching frequency can improve the power density and reduce the volume,but which increase the power loss of the system,so the widely used flyback AC-DC converter is limited.Active Clamp Flyback(ACF)converter is a kind of soft switching topology derived from flyback converter.It has the advantages of high efficiency,low voltage stress,high switching frequency,wide input range and low EMI noise,etc.It is an important topology scheme for adaptor power supply to meet the demand of high frequency.However,the negative current required by power switch to achieve ZVS in high frequency applications has a great impact on efficiency,and the problems of poor topological model accuracy,high frequency loop performance is prone to cross interference of dual loop control,and low sampling accuracy of primary side limit the further application of ACF converter.In this paper,aiming at the problems caused by high frequency of ACF,the adaptive part ZVS control strategy of power tube,high frequency topological model,decoupling error compensator,constant voltage sampling strategy of primary side and other contents are studied,and the ACF power system with high frequency,high efficiency and high-power density is realized.The main innovations are given as follows.1.A high efficiency predictive hysteresis current-mode control method(PHCM)based on adaptive dead zone and partial ZVS regulation is proposed.Unlike the existing fully soft switch control,the control method will be various losses in the process of soft switch power tube to compromise,got the optimal valley value of the current model,used in each cycle of adaptive clamping tube conduction time adjustment and dead band time adjustment,and in the implementation of soft switch control power tube section at the same time,achieve the optimal efficiency of the system.The test results show that the peak efficiency of the system reaches 93.75%,which is 0.4%higher than the existing control method of full ZVS,while the power tube partial ZVS and dead zone time are adjusted cycle by cycle.2.To solve the problem of poor precision of small signal modeling in high frequencymode,a zero-input resonant equivalent circuit model is derived,which includes PWM and resonant operating characteristics,and then a high-precision topological small signal model is established by extended describing function modeling method.The model is in good agreement with the simulation results in full switching frequency bandwidth.Aiming at the influence of the cross coupling between the output voltage regulating loop and the power tube ZVS regulating loop on the dynamic performance of the system,a better and simpler decoupled error compensator was proposed,which improved the dynamic performance of the system and reduced the voltage overshot without affecting the main control architecture of the system.The test results show that the load dynamic response can reach 500μs,and the voltage overshoot is less than 1V.Compared with the decoupling compensator,the maximum dynamic response can be increased by 340μs,and the maximum voltage overshoot can be reduced by 0.6V.3.Since the nonlinear auxiliary winding voltage waveform caused by the resonance process,high frequency ACF cannot achieve high-precision primary side constant voltage regulation by collecting knee point voltage in traditional flyback.In order to solve this problem,based on the analysis of the resonant voltage waveform,this paper proposes a two point smapling method,the sampling structure is simple,and which can realize the higher accuracy of the output voltage at the same time.In addition,this mehtod cannot affects the power density of system,and can be applied to both primary and secondary resonant ACF converter.The test results show that the voltage accuracy is up to 4%in primary side resonant ACF and 3.5%in secondary side resonant ACF,and the power density of the system is up to 40 W/in~3.Compared with the existing scheme,the voltage accuracy is improved by 1%,and the power density is increased by10 W/in~3.