Study On LLC Half-bridge Resonant Converter And Its Application

Among many resonant circuit topologies,LLC half bridge resonant converter can realize zero voltage switching(ZVS)of transformer primary switching transistor and zero current switching(ZCS)of secondary rectifier diode,and has the ability to realize soft switching of switch transistor under wide input and output conditions.LLC half-bridge resonant converter has advantages in increasing switching frequency,reducing the volume of passive devices and increasing the power output density,so it has been paid attention by power electronics.In order to optimize the parameters of LLC half-bridge resonant converter,the transformer loss and fundamental component approximation are used to optimize the parameters of LLC resonant converter.The Fundamental Harmonic Approximation(FHA)model of LLC resonant converter is established.The voltage gain and input impedance of the converter are analyzed,which provides a basis for the design of the optimal switching frequency range of the converter.Finally,the experimental results show that the proposed parameter optimization method solves the power loss problem in the soft-switching process and avoids the loss of soft-switching in the full load range.Unlike conventional PWM converters,LLC resonant converters have nonlinear coupling between DC and AC,so their modeling is more complicated.In order to ensure the stability and rapidity of the converter feedback system,a small signal model of LLC half-bridge resonant converter is established on the basis of the analysis of its working principle and steady-state characteristics.On the basis of accurately establishing the small signal of-LLC half-bridge resonant converter,the transfer function from input to output and from control to output is deduced,which provides a theoretical basis for subsequent feedback circuit design.Electrolytic capacitance is one of the main factors affecting the life of LED driving power supply.In this paper,a LLC half-bridge resonant power supply with current doubling synchronous rectification is designed.The capacitor size is reduced by the energy storage of current doubling inductance.Therefore,a longer life film capacitor can be used instead of a shorter life electrolytic capacitor to solve the restriction of electrolytic capacitance on LED power supply problem.The use of LLC resonant circuit reduces the switching loss of the transformer's primary side,and the energy storage inductance in double current synchronous rectifier circuit can reduce the output voltage ripple.At the same time,the use of synchronous rectifier MOS transistor reduces the conduction loss of the secondary side of the transformer.Aiming at the problem that the traditional two-stage AC-DC LED driving power supply needs two control circuit units,this paper designs a kind of LED driving power supply which integrates Boost PFC circuit and LLC half-bridge resonant converter.Compared with the traditional two-stage circuit structure,the power supply only needs one control circuit unit and reduces the number of control circuit units.The reliability of the device is improved by the number of devices.A prototype is built and tested in the laboratory.The experimental results show that the theoretical analysis is correct.From the experimental results,it can be seen that the power factor of the power supply is high and the harmonic is low,which is suitable for small and medium power LED applications.