| The non-renewable characteristic of oil and the pollution caused by combustion have aroused people's awareness of energy crisis and environmental protection.Electric vehicles(EV)can well alleviate this problem.The EV's working relies on the electrical energy stored in the battery.And the on-board charger(OBC)is one of the devices to charge the battery.The OBC must meet a wide battery voltage range and a wide input voltage range condition.And also,the OBC has strict requirements on power factor correction(PFC),efficiency and other mechanical structures such as volume and proof.A 3.3 k W on-board charger is developed,and a solution is proposed for the wide input and output voltage range.In order to satisfy the wide voltage gain range of the OBC,a two-stage structure is selected.The active power factor correction(APFC)converter is cascaded with the DC-DC converter.And they are decoupled through a bus capacitor.Converters cooperating with each other can achieve a high working efficiency and a wide voltage gain range.The APFC converter adopts a rectifier bridge + Boost structure.A closed-loop control of this converter is achieved by analyzing its working principle and establishing a small signal model.The DCDC converter adopts the LLC resonant converter.And its voltage gain curve is obtained by analyzing the working modes under the pulse frequency modulation(PFM)control with different load.For the problem that the trend of the voltage gain curve slow or even nonlinear in the switching frequency greater than the resonant frequency,a pulse width modulation(PWM)control strategy is proposed to solve it.The thesis analyzes the LLC resonant converter working modes in detail with changing the duty cycle of the driving signal.The non-monotonic problem brought by the PWM control is also researched.And a PWM + PFM hybrid control strategy is proposed to achieve effective regulation of the output voltage.Reasonably designing the bus voltage regulation strategy makes the two converters cooperate with each other and improves the voltage gain range of the charger,and also makes the LLC resonant converter work efficiently.Based on the research of the APFC converter,a simulation model is built in Saber.And a closed-loop simulation was performed to verify the power factor correction function of the APFC converter.The development of a PWM + PFM hybrid control strategy relies on analysis of the converter gain curve and circuit simulation.The thesis combines the simulation and analysis,and designs a PWM + PFM hybrid control scheme to achieve a wide voltage gain range working of this converter.Finally,the digital control method is researched.Based on the theoretical research and simulation results,the hardware circuit is designed.And a series of experiments are conducted to verify the effectiveness of the control strategy. |
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