| The isolated AC-DC switching power supply applies transformer to achieve electrical isolation.Due to its high safety and high stability,it is widely used in small power applications such as mobile phone charger,tablet adapters,and LED lighting equipment.With the development and upgrade of smart devices,the consumed power keeps increasing.In order to maintain the portability of the adapters,high-frequency,high efficiency power converters with high performance should be developed.In the isolated AC-DC converter,soft-switching single-switched converter has several advantages such as simple circuit structure,simple control,and high stability.Thus,soft-switching single-switched converters are very suitable for design of high frequency AC-DC converter with low or middle power.But there are still some disadvantages which make the switching frequency difficult to increase such as low efficiency,high voltage stress and large switching noise.Thus,high frequency cannot be acquired in the current isolated single-switched converters.Thus,the research on isolated high frequency single-switched converter topology and control algorithm has high academic and practical value.This paper mainly studies a 1MHz isolated single-switched converter topology and its high-performance,low-cost control algorithm.The design difficulty of MHz isolated single-switched converter is analyzed.And a new converter topology is proposed to reduce the switching loss,and valley switching turn-on of the switch is realized based on resonance from parasitic parameter.The switching frequency is improved up to 1MHz.Based on the proposed converter,closed loop control algorithm with high efficiency low cost,output voltage control algorithm with high precision and low voltage ripple,and high dynamic control algorithm during load changes are proposed and studied in this paper.The main research contributions are as follows.1,This paper presents a MHz isolated single-switched quasi-resonant converter topology.Only one switch is used to achieve near fully soft switching technique,including valley switching turn-on of switch,zero-current switching turn-off of switch and zero-current switching turn-off of the output diode.Valley switching turn-on of switch is realized based on the resonance between the external resonant inductor and the equivalent drain capacitance of the switch.Compared to conventional quasi-resonant power converters,the switching loss is greatly reduced and the efficiency is improved a lot.Without synchronous rectification,the efficiency of the converter has an approximately 10% improvement after valley switching technique is realized.The open-loop peak efficiency is 90.4%.And the switching frequency is improved up to 1MHz.2,In the proposed new topology,a easily realized pulse frequency modulation(PFM)mode with time fluctuation is proposed to realize valley switching turn-on of switch in the closed-loop control.By zero-current detection,valley switching turn-on and zero-current turn-off of the switch are realized in continuous current mode(CCM)mode.The implementation cost in CCM mode is greatly reduced as no ADC is used to sample the varying current or voltage.By zero-voltage detection of secondary winding voltage,the switch is turned on at the peak voltage in discontinuous current conduction modulation(DCM)mode.The implementation cost in DCM mode is greatly reduced as no ADC is used to sample the varying voltage.Experiments show that only two comparators,an ADC device and a digital controller are needed in closed-loop control.3,In the proposed new topology,large output voltage ripple cannot be avoided in the closed loop control.A high precision low ripple digital control algorithm is proposed in this paper.By evenly distributing the switching period at different valley voltage of the switch drain,extremely low output voltage ripple is realized.In addition,a steady-state error compensation algorithm is proposed based on output voltage sampling.By adjusting the reference voltage,the output voltage error is eliminated which helps to improve the output voltage precision.In experiments,when the output voltage is 20 V,the output voltage precision is 0.4% of the standard value.The output voltage ripple is 240 mV,which is a quarter of the output ripple before optimization.The peak efficiency is 89.6%.4,In the proposed control,the dynamic performance is poor when load variation is large.A high dynamic digital control algorithm is proposed in this paper.By setting the upper and lower limits of the output voltage,two dynamic modes are introduced to reduce the voltage ripple and the recovery time.When the load changes from heavy to light,a proportional iterative method of switching period is presented to detect the stable control parameters of the changed load,which can be used to eliminate the voltage resonance during dynamic performance and reduce the recovery time.In experiments,when the load changes from 0.5% load to full load,the undershoot voltage is reduced from 11.6V to 1.04 V,and the recovery time is reduced from 654.0?s to 59.8?s.When the load changes from full load to 0.5% load,the overshoot voltage is reduced from 1.44 V to 0.88 V,and the recovery time is reduced from 47.4ms to 2.92 ms.The proposed flyback converter is suitable for AC-DC converter with small power or middle power.The frequency is improved to 1MHz.High efficiency,high precision and fast dynamic performance are acquired.Thus,the proposed converter has high academic and practical value. |
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