| As specifications become more stringent, switch-mode power supply design is facing more and more challenges. Among the requirements, the mandatory ones for Power Factor(PF) and harmonic content have become widespread as equipment’s power are increasing. Power Factor Correction(PFC) technique could effectively suppress current harmonic and improve PF, and improve current efficiency.Currently single stage PFC flyback convertor is widely used. The relevant architectures presented in the market could be divided into two categories. The first architecture is boundary conduction mode single stage PFC flyback convertor. This architecture utilizes simple peak current control mode. It has the advantages of simple circuit architecture and low cost, and is suitable for small power application. The second architecture is working under continuous conduction mode. Normally this architecture utilizes average current control mode and is suitable for mid-power(about 100W) applications. However, the control algorithm of average current control mode is complex, the circuit cost is high, and it needs dedicated control ICs.This thesis proposed a third architecture of single stage PFC flyback convertor, which combines the strengths of previous two ones. This architecture uses a fixed off time for switch, and allows the system utilizes the simple and low cost peak current mode under the pre-requisite of working in continuous mode. Compared to boundary mode, this architecture effectively reduced the peak current, thus reduced the high order harmonic content in EMI filters. As for continuous mode single stage PFC flyback convertor working in average current mode, this architecture eliminated the need for dedicated control chips, and has a simpler circuit, and is of lower cost.This thesis focuses on the principles of fixed off time single stage PFC flyback convertor. Besides, the author designed system circuits, completed the circuit simulation, and have a prototype fabricated and tested. |
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