CUK converter operation in PFC 

The system detail consist of rectifier diode D1,D2,D3 & D4 which provide the dc input to the cuk converter, CF is the filter capacitor which smoothen the DC output of the rectifier & reduces the ripple in the voltage. The cuk converter consist of primary inductor L1, MOSFET Switch S1, fly-back capacitor Cc, Diode DS, Secondary inductor L2, output capacitor Co & this converter is driving 27W LED. The each component of cuck converter works as follow

1.       When S1 is turned ON, the inductor L1 start charging & in starting phase it opposes the flow of current, it act as the high resistance component but after sometimes, its impendence decreases & current IL1 starts rising.

2.      Now the S1 is turned off, due to sudden drop in current the polarity of the L1 reverses, it induces the flyback voltage VL1, the voltage in inductor is in same direction as of source, therefore added voltage, the flyback capacitor CC act as a short circuit because of decreasing current from the L1 & DS become forward bias & It completes its circuit.

3.      Now S1 is turned on again, the CC already charge, therefore it start discharging through S1 & it provides the power to the load. Simultaneously L2 stores the energy, provided by the CC.

Now the S1 is turned off again, as the L1 is already charged which tries to charge the CC again though the DS. Inductor L2 is also already charged, its polarity reverses & it also provides the power to the load through the DS.