Multi-level Passive Auxiliary Research In A Single-phase Current-fed Fullbridge Single-stage PFC Converter

The input side of traditional power electronics device is mostly composed of uncontrolled equipment, producing a large number of high-order harmonics and reducing the power factor. Active Power Factor Correction technology is an efficient method to suppress harmonic and improve the power factor. Single-stage full-bridge PFC technology has the advantages of simple structure, low cost and high efficiency, so it is suitable for medium and high power applications.However, there are some problems in the structure: due to the existence of the leakage inductor of the transformer, it will produce serious voltage spike when the state of the bridge-leg switches change, which will increase the voltage stress of the switches; the voltage of the output filter capacitor increases from zero when the converter starts up and the input current increases during a certain time and it will produce large impulse current. Therefore, it is necessary to study on these problems and solve them efficiently.The paper mainly researches a multi-level passive auxiliary circuit in a single-phase current-fed full-bridge single-stage PFC converter. A multi-level passive clamp technology is proposed. The circuit is composed of passive components-capacitors, inductors, diodes and so on, which is simple, reliable and don’t need control. A flyback start-up method with an extra flyback winding is proposed on the basis of the multi-level passive clamp technology.The multi-level passive clamp circuit in the single-phase current-fed fullbridge single-stage PFC converter uses a plurality of clamp capacitor in series by diodes to absorb the voltage spike across the bridge-arm switch, and the absorbed energy is transferred to the load by the inductors in the clamp circuit when the bridge diagonal-leg switches turn on. The paper analyzes the working process of the converter and the voltage spike suppression of clamp circuit in detailed, and the design method of related parameter is given.The resonance in the clamp circuit is not synchronized because of the existence of device error. To solve the problem, the coupled-inductor are used to suppress the unbalance of voltage and current and achieve synchronizationresonance. The equivalent model of the coupled-inductor is set up based on the resonance process of the multi-level passive clamp circuit and the structure of the coupled-inductor. The mechanism of coupled-inductor realizing synchronizationresonance is analyzed.There are large impulse current when the single-stage full-bridge PFCconverter starts up, which reduce the reliability of the system. The paper proposes a flyback start-up method on the basis of the proposed multi-level passive clamp technology. And the proposed circuit sets up initial voltage for the output filter capacitor by the flyback winding. The paper analyzes the working process of the start-up circuit in detailed and gives the design method of the key parameters such as the turn ratio of flyback link.On the basis of theoretical analysis and simulation, an experimental platform is built. The multi-level passive clamp technology and the flyback start-up method of the single-phase current-fed full-bridge single-stage PFC converter are verified by experiments. And the experimental results verify the validity of the theoretical analysis.