Research On The Serial-Parallel LLC Converter Suitable For Electric Vehicle Charger

Under the pressures of energy shortage and environmental pollution, many countries have begun to develop new energy vehicles. Among these, the plug-in Hybrid Electric Vehicles and pure Electric Vehicles are the main parts of the development. With the development of electric vehicles, the vehicle charger is attracting more and more attention. At the same time, high safety, high efficiency, high power density and the control method of electric vehicle charger have become hot research spot.This paper proposes a type of LLC converter charger based on serial-parallel split resonant capacitor and interleaved control. The primary side of the converter adopts series on the high voltage side, and adopts the parallel on the low voltage side. On the one hand, it is beneficial to restrain the large current ripple without the output filter inductance, so as to improve the service life of the battery; on the other hand, it is beneficial to choose the primary side switches of the converter. Compared with the converter with monomer resonant capacitors, the converter with split resonant capacitors can effectively reduce the current stress of resonant capacitors, prevent transformer saturation when starting, and help to reduce the input differential mode noise and the input capacitance current stress in DC side.To improve the efficiency and power density, variable frequency control is employed and higher resonant switching frequency is set on the LLC circuit. Digital-analog hybrid control method is proposed on the DC/DC converter of the charger in order to achieve higher dynamic response speed in this paper. Both PI voltage regulator and current regulator are implemented by analog circuit. The output of regulator is sent to the AD sampling of control chip and the switching frequency may change with these AD sampling values in digital control chip, so as to achieve the frequency conversion control of the LLC converter in this paper. In order to achieve multiple modules parallel of the charger, the LLC converter droop control strategy is studied, the different implementation methods of droop control is analyzed, and the specific droop control strategy is presented in this paper.To solve the problem of output voltage gain curve distortion in the light load or no load, the burst mode in the light load is studied. By setting the light load current threshold method and comparing the output voltage to the setting voltage, the fixed frequency and fixed duty ratio control in light load have been realized, so as to ensure the stability of output in light load. In this paper, the control parameter setting method based on the analysis of the small signal model is given, and over voltage, over current and over temperature protection are implemented by using digital-analog hybrid control, so as to realize the stable and reliable operation of the charger.Finally, a 5kW experimental prototype platform was established and the experiment was done under the digital-analog hybrid control. The experimental results validate the theoretical analysis. ZVS of primary switches can be gotten in full load range and ZCS in the secondary side rectifier can be achieved. So the output current ripple is small, and the highest efficiency can reach 96.42%.