| LLC resonant converters are widely used due to their high efficiency,high power density,and wide voltage regulation range.At present,however,there is a wide range of frequency adjustment for the design of converter parameters with a certain voltage range.Widening of the frequency operating range will lead to a larger transformer and higher conduction losses,which is not conducive to the optimization of filters and magnetic components and efficiency.Promote.Therefore,the optimization of the LLC resonant converter needs to reduce the frequency variation range as much as possible while increasing the efficiency of the converter,while satisfying the desired gain.For the design of the converter with a certain voltage variation range,it is also necessary to consider the operating conditions under different loads,and there is a great difference between light-load operation and full-load operation.Because of the equivalent parasitic capacitance on the secondary side of the transformer and the intermittent effect of the load current at light load,there is a problem that the output voltage is pumped up under light load.The existing voltage modulation method can adjust the output voltage under light load to reduce,but there are disadvantages such as reduced efficiency,limited application,high hardware cost,and increased output voltage ripple.In the past,the influence of parasitic capacitance was not considered in the analysis of LLC.Under the different operating conditions of the converter,the influence of parasitic capacitance is quite different.First,this article considers the consideration of the parasitic capacitance in the circuit,studies its equivalent circuit model and compares it before and after.It is found that the parasitic capacitance has a negligible influence on the converter under heavy load or rated load,but under light load operating conditions Greater impact.This paper makes a detailed analysis of the frequency domain and time domain mode of LLC's light load operation mechanism,studies the influence of different parameters on the gain characteristics of the converter,and explains the mechanism of output voltage pumping under light load conditions.Secondly,based on the above analysis,this paper proposes an optimized LLC parameter design method that takes into account both the gain requirement and the high efficiency,ensures the ZVS from full load to no-load range at operating frequency,and can effectively reduce the required voltage gain.The frequency operating range of the converter ensures high efficiency of the converter.The proposed method is verified by simulation and experiment.The results show that the proposed optimization method can achieve approximately 1.6 times the normalized voltage gain in the frequency range of 18 kHz.The maximum efficiency of the prototype of 150 W can reach 96.3%.Finally,the existing light load control strategy of LLC is summarized and compared,and their advantages and disadvantages are pointed out.Based on this,the control strategy is improved and an improved three-pulse intermittent control based on energy feedback is proposed.Strategy,this control method can guarantee high efficiency and low ripple under light load conditions.When the load is further reduced,it will lead to an increase in the intermittent period,the introduction of low-frequency ripple,the output voltage ripple coefficient increases,and thus put forward higher requirements for the output filter capacitance,and may cause severe noise,can not meet the battery charging requirements.When LLC is used in battery charging,for an exhausted battery,it is necessary to supply the battery with a low voltage,constant current,and sufficient ripple voltage at the initial stage of charging.This process is conducive to extending the service life of the battery,and it can also be activated again.Lead-acid battery or lithium battery with poor performance.At the same time,because the power is very light,the efficiency is not the main performance factor.In this application,the output voltage ripple characteristic of the converter is more important for the stable operation of the converter.Based on this,this paper proposes a light-load voltage regulation strategy based on energy feedback and synovial control,which can eliminate the low-frequency ripple under extremely light-load operation conditions and prolong the service life of the battery.Control strategies were verified and compared.The proposed method realizes pressure regulation by improving the control strategy,without increasing the hardware cost,and improving the shortcomings of current research methods. |
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