Research On Design And Control Strategy Of High Frequency Active Clamp Flyback Converter

The booming consumer electronics industry has an urgent demand for AC/DC power converters with high frequency,high efficiency and high power density.As one of hot research topics,ACF converter which is based on traditional flyback converters can realize zero voltage switching to ensure high efficiency and use Ga N devices to achieve high switching frequency.However,there is still room for optimization in terms of dynamic performance,efficiency and stability.Based on digital controlled Cr CM ACF converter,research in this thesis is as follows:Firstly,study the dynamic performance of high frequency ACF converters.Owing to completeness of the Cr CM resonance process,it is not suitable for the CCM small signal modeling method.So the voltage of clamping capacitor is introduced to accurately describe the Cr CM resonance process,and the Cr CM small signal model is derived.Besides,the type of digital compensator is selected according to characteristics of the transfer function and compensator parameters are solved accordingly.In addition,to reduce the digital delay,reasonably trade off between ADC sampling delay and sampling accuracy.To improve program operating efficiency,functions of main program and interrupts are clarified,the interrupt trigger condition is optimized.Secondly,study the efficiency and stability of high frequency ACF converter.The existing ACF control methods with frequency conversion needs to sample high frequency signals.The sampling processes are difficult and control loops are complicated.Therefore,this thesis proposes an adaptive frequency conversion control method which is high efficient and simple controlled.To ensure that the prototype can work at the optimal frequency,the Cr CM period model is derived.The model parameters are accurately solved by using experimental data.Besides,to improve the efficiency of light load conditions,the adapative burst mode control is designed and the problem of sampling accurate output current is solved.In terms of stability optimization,to avoid the interference between sampling signals,dual ADC sampling channels and the sampling queue are reasonably assigned;to improve the stability of mode switching,switching between soft start mode and adaptive frequency conversion mode is optimized.Thirdly,study the frequency response measurement of high frequency ACF converter.This thesis uses a measurement method by adding PRBS disturbance.During the optimization process,pre-emphasis,deemphasis,truncation and spectral smoothing techniques are adopted to obtain the relatively smooth and accurate frequency response data.The proposed method is suitable for digital switching power supplies and less dependent on specialized experimental instruments which means this method has broad application prospects.Based on above researches,a prototype with 100VAC～220VAC input,19.5V/0.33A～3.33 A output is implemented to verify the proposed control methods.After adopting the dynamic performance optimization methods,the average adjustment time of the prototype switching between half load and full load is 455μs,and the overshoot is 9%,which is significantly improved by 25.8% and 40%.The dynamic performance of the prototype is considerably enhanced.After adopting the efficiency optimization methods,the peak efficiency and average efficiency of the prototype under full load condition are 91.6% and 90.3%,and the peak efficiency under 10% load condition is 74.4%,which are 1%,2.4% and 25% improvements compared with previous experiment results.Therefore,the efficiency optimization has a measurable effect.