基于能量模型的临界导通模式Boost变换器软开关方法

Soft Switching Method of Boundary Conduction Mode Boost Converter Based on Energy Model

在变换器高频化发展的趋势中,功率器件的软开关实现对变换效率的影响更加突出。无辅助电路的临界导通工作模式下,Boost变换器主开关管在特定增益下无法实现软开关。为此,该文提出一种能量模型及相应的软开关实现方法。首先,建立死区前后储能元件能量变化的数学模型。然后,结合死区起止时刻的能量平衡方程,研究软开关无法实现的电路机理。在此基础上,考虑开关管输出电容非线性特征,提出软开关实现方法,避免了复杂谐振过程的时域精确建模,提高软开关实现的准确性。最后,搭建500W实验样机进行实验,结果表明,相较于对谐振过程建模的传统时域模型,所提方法将实际开通电压降低47%,使峰值变换效率提升0.4%,进而验证了其有效性。

Soft switching is crucial for Boost converters,especially in the trend of high-frequency development.During boundary conduction mode(BCM),zero voltage switching(ZVS)of the main switch of the Boost converter is achieved without auxiliary circuits,which is favored in small and medium power applications.However,when the gain of the converter is greater than 2,ZVS based on BCM cannot be realized because of the positive resonance valley of its drain-source voltage vds in the dead zone.Therefore,a method extending the synchronized switch time is proposed based on time-domain dead zone analysis.A negative current is obtained on the inductor at the start of the dead zone,forcing the resonance valley of vds to get closer to 0.Due to the calculation of the resonance process,the complexity of the calculation process and result expression is greatly increased.However,in practice,the realization of soft switching only focuses on the start and end moment of the dead zone.Complex analysis of the resonance process is optional.Besides,nonlinear characteristics of the output capacitor Coss on power switches are ignored in the time-domain model,causing inaccurate inductor current at the start of the dead zone and is harmful to reducing switch losses.Thus,this paper proposes a ZVS method based on the energy model.Firstly,the energy model of passive power components is established during the dead zone.Instead of considering the entire dead zone,only energy change at the start and end of the dead zone is focused.Secondly,conditions with power variations are analyzed,and the requirement for ZVS realization is given from the energy balance perspective.Then,considering the nonlinear characteristics of Coss,an accurate expression of the negative inductance current needed is obtained.The ZVS realization method based on the power model avoids complex resonant processes of time-domain modeling.Accordingly,less calculation is needed,and the accuracy of soft switching implementation is improved.Finally,a 500 W experimental prototype is built to verify the proposed method.Experiments are conducted when the input voltage is 200 V and 250 V,respectively,and the output voltage is 300 V.With an input voltage of 200 V,the proposed method reduces the actual switching voltage by 86%compared with the critical conduction mode and 60%with the time-domain model.Under 250 V,the proposed method reduces the actual turn-on voltage by 70%and 47%compared with the critical conduction mode and the time-domain model,respectively.Since the switching current between different methods is almost the same,switching loss is proportional to the switching voltage.Therefore,the proposed method can effectively decrease switching loss.Conclusions below can be drawn from experimental results.(1)The proposed method is based on energy balancing the start and end of the dead zone,simplifying the analysis and calculation process of soft switching implementation.(2)The proposed model considers the dynamic characteristics of the output capacitor Coss accurately,which improves the accuracy of the negative inductance current.(3)Compared with the traditional time-domain model,the proposed model and ZVS implementation method can effectively reduce the actual switching loss and improve peak efficiency by 0.4%,which verifies the ZVS implementation method based on the energy model.