Research On High Frequency Full-bridge 2kW/24V LLC Resonant Converter

With the wide application of DC Distributed Power System in communication power supply, ships and planes back up power supply, as well in new energy development and other fields, LLC resonant DC / DC converter,who can meet the development direction like high power density, high efficiency and high frequency of transformation, has become the highlight research field in power electronics. The traditional DC / DC converter uses hard switching technology, and it will greatly limit the power density and the improvements in main power tube switching frequency. Phase shifted full bridge PWM ZVS converter has a mature control technology, but there are some inherent disadvantages, such as a unfull range of ZVS, disefficiency and low input voltage. LLC resonant converter can realize the full range of the main switch tube ZVS, rectifier diode can easily reach the ZCS and will not reverse recovery either. Therefore, the full bridge LLC resonant converter replacing the phase shifted full bridge PWM ZVS converter has gradually become the mainstream trend.However, the LLC resonant converter has two technical difficulties in designing: resonant parameter design, the control of small signal modeling.Designing resonant parameters is difficult because of its complexity working model, and too much resonant components, poor physical intuition.etc; Because of the difficulties in the design of resonant parameters, this paper puts forward a design idea of the DC gain characteristic curve combining with resonant parameter formula. The idea based on the DC gain characteristic curve diagram can easily tell frequency ranges which LLC resonant converter is suitable for, and it is simple and convenient to derivation the resonant parameter formula,and to describ the design flow chart, and finally, the prototype has designed. By useing this design ideas, design of resonant parameters will become more simple, more quickly and more likely to make people understand.Signal modeling control system is quite difficult to LLC resonant converter as the following reason, the switching frequency is the key control variables in the process of DC / DC converter and the harmonic component can not be ignored, so the common traditional PWM converter modeling method of is not suitable for LLC resonant converter in small signal modeling.In addition to the small signal modeling of control system, this paper uses the extended describing function method, which can accurately considering the effect of nonlinear state variable switching frequency of the LLC resonant converter and its harmonic components of LLC resonant converter. The modeling method use the extended describing function operator and the harmonic balance theory at first, according to the equation of nonlinear large signal model of the nonlinear system state variable frequency domain expression; then considering in the steady state conditions, ignoring the first-order time derivative of large signal model, and we will get the steady state equation; at the last of the small signal disturbance will be added to the steady state equation, with the ignor of the steady component of control system small signal model can achieve.Finally the simulation waveforms and experimental results verify the correctness of the theoretical analysis in this paper and the design ideas.