Research On Modification Of Switching Frequency Load-Dependence For Constant Output Voltage Resonant Converter

Nowadays,with the rapid development of power electronics technology,miniaturization,lightweight,high conversion efficiency and low electromagnetic interference(EMI)have become the main design requirements of modern Switching Mode Power Supply.On the other hand,improving switching frequency is the most effective way to realize miniaturization and lightweight of Switching Mode Power Supply.However,in traditional hard-switching DC/DC converters,power switches work in hard-switching state,simply improving switching frequency will result in serious switching-loss and EMI.Using soft-switching technology can well solve those problems.Among numerous soft-switching DC/DC converters,resonant converter,because of its natural soft-switching and low EMI characteristics,has a vast application prospect in the high-frequency field.Thus,DC/DC resonant conversion technology has become a research focus in recent years.Based on research various resonant converters widely,several classical resonant converters and their PFM characteristics have been analyzed in this paper.Beside,the load-dependence in PFM resonant converters has also been summarized.Aiming at the main drawbacks in conventional PFM series resonant converters(SRC),that in order to keep the output voltage constant,the switching frequency range is too wide duo to a wide load variation range and the switching frequency at light load condition is too low,a modified PFM SRC has been proposed in this paper.The modified SRC proposed in this paper is abtained by adding an auxiliary resonant circuit parallel to the primary side of transformer.Compared with conventional SRC,auxiliary resonant circuit and output circuit act as the load of the main series resonant tank together and shunt paths of the main resonant current in this modified converter.It can ensure that the switching frequency always stay above auxiliary resonant frequency with the aid of the auxiliary resonant circuit.Thus,this modified SRC can effectively reduce the switching frequency range within the entire load variation range and improve the switching frequency at light load condition.According to the equivalent circuits under different work modes,operation principle of the modified SRC with auxiliary resonant circuit is analyzed.Based on the Fundamental Harmonics Approximation(FHA),the DC voltage gain characteristics,no-load characteristics and load-dependence characteristics are analyzed detailedly in frequency domain.Besides,the influence of switching frequency to output voltage ripple and key parameters to steady-state performance are discussed.Finally,based on the theoretical analysis,the practical effectiveness of the proposed modified SRC is verified via a 300W Saber simulation model.