Research On Single-Stage Full-Bridge Soft-Switching PFC Technology Based On Digital Control

The power electronic devices are traditionally fed by the diode rectifier with a capacitor filter, however, this mode of power supply will lead to the great harmonic pollution of the power grid. As an important means of reducing the harmonic pollution, Power Factor Correction (PFC) is concerned by more and more people and becoming the current research focus. In this thesis, both the topology and the control strategy have been studied.In the application occasions of high power, we usually use the full-bridge as the topology of the PFC converter, but the transformer leakage inductance of this topology will cause serious voltage spike and increase the voltage stress of the power switches greatly. To solve this problem, we added an active clamping circuit to the full-bridge converter. The clamping circuit can be used to suppress the voltage spike, furthermore, the energy stored in the clamping circuit will be transferred to the load at a certain time. With the help of the clamping circuit, the bridge switches can realize the soft-switching, as well as the clamping switch. At the basis of the operational modal analysis, this thesis did some research on the working principle of the converter, as well as the conditions of the soft-switching operation.The traditional PFC converters commonly adopt analog control scheme, however, it has many obvious flaws. Along with the development of the digital control chips and the digital PFC control arithmetic, importing the digital control to the PFC technology has already become an inevitable trend. The digital PFC control strategy of the average current mode is derived from the same analogy control strategy, it has the advantages of the average current mode and the digital control mode. Both the digital sine wave current reference and the input voltage feedforward segment have fully embodied the advantages of the digital control. This thesis gives the whole block diagram of the digital PFC control arithmetic and designs the PI regulators of the voltage and the current loop. This digital arithmetic has been proved to be able to realize the function of PFC very well by Pspice simulation and the hardware circuit experiment.As the digital PFC control arithmetic of the average current mode has the drawbacks of the complicated structure and the heavy computation, it can not fully play to the characteristics and advantages of the digital control. In order to solve this, a novel digital PFC control arithmetic named duty cycle prediction scheme has been studied, which has small calculating amount and short implementing time. According to constraint relations among the voltage, the current and the duty cycle, the thesis gives the theoretical derivation of the control arithmetic and also designs the software implementation process of the arithmetic, and then the arithmetic is analyzed by the Matlab simulation. In the end, the arithmetic has been verified on the hardware circuit experiment platform. The experimental results indicated that this arithmetic can possess more advantages than the arithmetic of the average current mode and can achieve an ideal effect of power factor correction.