Simplified Time Domain Analysis And Control Of LLC Resonant Converter Under Bidirectional Operation

Energy storage system is widely used in renewable energy power generation,electrified transportation and other fields.The bidirectional isolated DC/DC converter adapted to energy storage system is also developing towards higher efficiency and power density.Among the bidirectional isolated DC/DC converter topologies,LLC resonant converter is a simple converter with a wide gain range.It can achieve very efficient power conversion and higher power density with excellent soft switch characteristics.The asymmetry of the LLC resonant tank results in the difference of its characteristics in bidirectional operation,which limits its applicability in bidirectional operation.LLC converter achieves power transmission through a resonant tank in the form of AC.This process involves a large number of modal switching and will change with the change of control amount,load and other factors,which makes the analysis and modeling of LLC converter and parameter design more difficult.In this paper,based on time domain,LLC converter under bidirectional power transmission condition is studied.The wide voltage gain range control,simplified time domain analysis,parameter design,cross-series-parallel topology of LLC converter under bidirectional operation are analyzed and discussed.The correctness of the theoretical research results is verified by simulation and experiment.The main research work in this paper is as follows:A Boost-like backward step-up control method at resonance frequency is presented to solve the problem of control mode for LLC converter with bidirectional wide voltage gain range.The bidirectional wide voltage gain range of LLC converter can be achieved by combining backward step-up control with variable frequency control and phase shift control.By analyzing the equivalent circuit of the resonant tank,the time period for charging the resonant tank is reasonably selected,and the Boost-like backward step-up control mode is achieved by changing the driving signal pulse width of the primary side switch at the resonant frequency.The maximum backward normalized gain can be about 2.The principle of the backward step-up control and the mode under light and heavy load are analyzed in detail.This control method has the advantages of not changing the basic topology of LLC converter and realizing the soft switching characteristics of most switches.For the situation that only transient backward power is needed when the port voltage suddenly increases during the forward power transmission,the forward mode synchronous rectification control is used to realize the transient backward power transmission,which can control the transient backward power transmission process and improve the transient response speed.A simplified time domain analysis method for LLC converters under forward and backward operation conditions with different control modes is presented to solve the problem that LLC converters lack a universal approximate time domain analysis method which can be applied to different control modes such as backward step-up control,variable frequency control and phase shift control.First,the modals and equivalent circuits of the LLC converter are summarized,and the uniform equivalent circuit of the LLC converter is established.The time domain equation of the mode is simplified by triangular approximation and approximate linearization according to the mode time length.On this basis,by analyzing and simplifying the expression of the transmission power of the uniform equivalent circuit,the expression of the average transmission power of the resonant tank is obtained,which can characterize the factors affecting the transmission power of the resonant tank.The analysis method is based on the modals to the approximate time domain analysis with small error and can be applied to different control methods.By using the simplified time domain equation of the modal and the simplified expression of average transmission power,the forward step-up mode with variable frequency control,the forward and backward step-down mode with phase shift control and the backward step-up mode with Boost-like control of LLC converter are analyzed respectively,and the voltage gain and load characteristics of different modes are obtained.The voltage gain and load characteristics of different modes are verified by simulation and experiment,and the validity,higher accuracy and applicability of the simplified time domain analysis method are also verified..A design method of LLC converter resonance parameters and considering multiple constraints is presented to solve the problem of resonance parameters design without clear parameter relationships and less computation.Firstly,the requirements of the converter are analyzed,and the focus of the parameter design is the voltage gain range,soft switch characteristics and reducing the excitation current.Based on the simplified time domain analysis,the influence of the resonance parameters on the frequency range of the switch,the voltage gain range,the soft switch characteristics,the resonant capacitance withstanding voltage is analyzed.It is summarized that the frequency range of the switch and the voltage gain range mainly determine the inductance coefficient,the soft switch characteristics and the resonant capacitance withstanding voltage constrain the range of the resonant impedance.Based on the laws obtained from the simplified time-domain analysis above,a parameter design method with clear relationship between parameter variables and a smaller amount of calculation is obtained.Resonance parameters can be obtained according to frequency range,gain range,dead time,resonant capacitance withstanding voltage and the gain characteristics have higher accuracy.To solve the problem that LLC converter has poor backward mode characteristics when using module series-parallel topology expansion,a cross-series-parallel topology design based on LLC converter for bidirectional high efficiency is presented,and the control method for multi-state switching is given.By crossing two LLC converters in series and parallel,when power flows in both directions,the LLC converter with backward power is in constant voltage gain mode to achieve efficient power transformation,and the LLC converter with forward power is in high efficiency forward mode to regulate voltage and power.When the cross-series-parallel topology is running in two directions,LLC converter needs to adopt different control modes and converter modes.Combining the characteristics of different control modes and converter modes,the state machine is used to achieve smooth switching between different modes of the LLC converter,thus realizing the stable control of the cross-series-parallel topology.The introduction of voltage feed-forward control in state machine control improves the response speed of LLC converter under bidirectional operation.In the power direction change control of cross-series-parallel topology,the introduction of series-side current integral criteria,step-by-step power direction change control(power commutation,series-side capacitor voltage regulation and mode switching)make power direction change process more smooth and stable.