Research And Implementation Of A High Efficiency Soft-switching Active-clamp Flyback Converter

The current rapid development of small and medium power consumer charging adapters represented by PD fast charging requires higher power density,higher switching frequency,and higher conversion efficiency AC/DC power technology to meet the increasing power demand of charging adapters.The Active-Clamp Flyback（ACF）studied in this thesis is more suitable for the current development of AC/DC because it can realize zero voltage switching（ZVS）of power tubes and recover the leakage energy than the traditional flyback converter,but the control method of ACF is more complicated and needs to adapt to different application scenarios of flyback to improve the efficiency.In this thesis,the basic working principle and efficiency of ACF are briefly introduced,and then the working principle and implementation conditions of ACF peak current mode loop and adaptive low-side tube（Q_L）ZVS loop under dual-loop complementary control are theoretically derived and modeled,including the maximum value of negative excitation current I_m（-）required to achieve ZVS and the effect of dead-time between high-side tube off and low-side tube on（t_z）on ZVS.Based on the theoretical analysis of the ACF dual-loop control,the control architectures and corresponding transistor-level circuits for the peak-current-mode loop and the adaptive ZVS loop are designed and verified by simulation using a 0.18-μm BCD process.For the peak-current-mode loop,a secondary-side optocoupler feedback is used,and the peak-current-mode comparison threshold V_CST generation circuit and the corresponding high-speed comparator are designed.For the adaptive ZVS loop,based on the theoretical analysis of t_z,combined with consideration of the non-ideal factors in the actual dead time control,a set of dead time control modules is designed to sample the bus voltage(V_BULK)to adjust t_z;for the need to realize ZVS of Q_L,a set of sampling SW node voltage combined with Q_L tube turn-on timing is designed to determine whether ZVS is realized,based on which the adaptive The control circuit for adjusting I_m（-）is designed,and the excitation inductor voltage is sampled by the auxiliary winding to indirectly sample I_m so as to achieve accurate control of I_m（-）under different operating conditions;for the I_m（-）error caused by the drive delay,the I_m（-）maximum circuit is designed to prevent the ACF from working in CCM mode.Finally,based on the designed ACF control circuit,a system-level simulation is performed,and the simulation results show that the ZVS of the Q_L tube is realized adaptively in 1.4ms after startup under the application conditions of 380V bus voltage input and 20V/3A output;I_m（-）is obtained when the output power decreases so that ZVS is not affected;the measured system efficiency is 93.9%at full load,which meets the design expectation.