Modeling And Control Of Resonant Converters In Discontinuous Current Mode

The resonant converter has a significant advantage because it can use the distribution parameters of the transformer as its resonant components to realize zero-voltage switching or zero-current switching of switching devices,reducing loss and making use of the parasitic parameters of the transformer at the same time.Based on the number of resonant components and their arrangement,there are a large number of topologies for resonant converters and the methods for soft switching and the control methods are different.However,on the one hand,as the resonant converter has multiple operation modes and parameters between circuits are not clearly expressed,the design of resonant parameters is quite vague and there is a lack of clear design formulas.On the other hand,as most output gains of resonant circuits are non-linear and hard to describe,it is impossible to determine the relationship between the system switching frequency,the load and the resonant tank output gain.Therefore,there is no control method that can directly reflect the characteristic of the resonant tank output gain.Besides,based on the gain characteristic of the resonant converter in the discontinuous current mode,when the converter operates under a light load,the converter is hard to control as the switching frequency is relatively low and the magnetic component of the converter usually has trouble switching.Based on the above-mentioned problems,the main research points of this thesis are as follows.For the parameter design of the resonant component,a normalization resonant parameter model is built and a design project for the resonant component is proposed.Resonant converter modeling and analysis are performed through the state-space method.Relatively comprehensive mathematical descriptions are provided based on the different operation modes of series,parallel and LCC resonant converters.Based on the energy balance between the resonant tank and the load during converter steady state operation,a resonant parameter models for the three resonant converters are built and the design project for the corresponding resonant parameters is proposed through normalized analysis.For series and parallel converters,the normalized design models contain only one parameter,the resonant tank output gain,while for the LCC converter,the model contains two parameters,the resonant capacitance ratio and the output gain.Therefore,the built model can reflect the internal relationship between the resonant component and the resonant tank output gain in a straightforward and clear way,streamlining the design process of resonant parameters.For the non-linear characteristic of the resonant tank output gain,an output gain characteristic model based on the critical resonant mode is built and a corresponding trajectory control method is proposed.Based on the steady state operation model of the resonant converter working in discontinuous current mode,a model related to the mapping relation among the switching frequency,the load and the output gain is built by introducing two parameters,the critical resonant frequency and the resonant tank output gain in critical resonant mode,solving the problem of the resonant tank gain characteristic being non-linear and hard to describe.On this basis,a trajectory control method is proposed.This method can directly adjust the output gain without requiring a large amount of calculation based on the resonant tank gain characteristic,simplifying the control algorithm for the resonant converter.Besides,an optimization design model regarding the resonant component based on the resonant tank output gain characteristic model is built to reduce the electrical stress of the resonant tank and increase effective utilization of the resonant current,finally improving power efficiency.For the resonant converter in discontinuous current mode under a light load,a hybrid control method for such converter is proposed,and an analysis model for the resonant converter operating in pulse width modulation is built.Through analysis of each operation mode of the series,parallel and LCC resonant converters during pulse width modulation,mathematical descriptions of the three resonant converters under different operation modes are provided by using the state-space method,the expression formula of the resonant state quantity at each moment is deduced based on the energy conservation principle,and finally a resonant tank output gain model and switch tube turn-off loss model based the switch tube turn-on time regarding the three converters are built.These two models clearly reflect the internal connection among the switch tube turn-on time,the resonant tank output gain and the switch tube loss.On this basis,the mode switch process between the pulse width modulation and the pulse frequency modulation is deduced,and the parameter design for the resonant component is optimized.