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Digital controller with integrated valley switching control for light load efficiency and THD improvements in PFC converter

In this paper a digital control technique for light load efficiency and THD improvement for a PFC converter is presented. This technique does not require any complex external logic or cycle intensive software algorithms. When a PFC converter runs in discontinuous conduction mode (DCM) and the PFC MOSFET is in off state, the energy in the MOSFET parasitic capacitance resonates between the PFC inductor and the parasitic capacitance. As a result the inductor current flows in reverse direction and allows MOSFET voltage to oscillate to a low voltage at the resonant frequency creating ideal condition for valley voltage switching. This valley voltage turn on of the MOSFET improves light load efficiency of the PFC converter. Also, at the valley of the voltage, the inductor current returns back from negative to zero. If the MOSFET is turned on exactly at this point, the effect of resonant current on the inductor switching current is minimized. This improves the light load THD of the input current. A Texas Instruments microcontroller with integrated programmable valley switching is used to implement the valley switching control on a 300W PFC converter. Test results are provided to validate the performance of this technique.