Abstract
Constant frequency controllers designed for continuous conduction mode (CCM) give poor efficiency and power factor at light load in power factor correctors (PFCs) of computer and server power supplies. With the increasing efficiency and power factor requirements at light load demanded by energy saving programs, a multimode digital control scheme that provides higher efficiency and power factor at light load without additional and costly external components is proposed. The CCM-DCM control of the multimode control scheme reduces RMS current drawn from the AC mains by ensuring good input current shaping in both CCM and discontinuous conduction mode (DCM), leading to higher efficiency and power factor at light load. It is computationally simple as compared to other CCM-DCM control schemes. The no load control of the multimode control scheme reduces constant losses at very light load by turning the PFC on and off to regulate the output voltage within a hysteresis band. Compared to other on-off control schemes, a small load jump is sufficient to exit the proposed no load control scheme, allowing a smooth transition between the no load control and the CCM-DCM active mode control.
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