Abstract:

The control strategy of constant on time (COT) is usually used in Boost power factor correction (PFC) circuits working in boundary conduction mode.The change of switching frequency in this control strategy is related to the input voltage,output power and the inductor.The conversion efficiency of the circuit will be affected by too high switching frequency under light load.Aiming at the above problems,a control strategy of valley count frequency flyback (VCFF) is proposed.By detecting the valley count of the MOS transistor voltage,the circuit works in discontinuous conduction mode during light load to reduce the switching frequency and improve light load efficiency.A prototype experiment is made.The experimental results show that compared with the COT control strategy,the proposed control strategy can effectively improve the light load efficiency.Under the input voltage of 220 Vrms,the 10%~50% load efficiency can be reached up to above 0.96.

Key words: LED driver, Boost power factor correction (PFC) circuit, boundary conduction mode (BCM), light load efficiency