1.2. Operating Modes for DCM
Discontinuous conduction mode (DCM) for this converter consists of five operating modes. These operating modes and typical waveforms during DCM are depicted in figures 5 and 6, respectively.
Mode1 [t0-t1]; Fig. 5(a): This mode starts by switching power MOSFET on. D2 and D4 are conducting but D1, D3 and D5 are reverse biased. Input inductor Lin is charged by input source through D2 and S. Magnetizing and leakage inductors are also charged by capacitor C1 and their currents start increasing from zero. Current of the secondary side inductor passes through D4 and C2 is discharged to charge C3. Output capacitor CO feeds the load. This mode continues till switch gets turned off.
Mode2 [t1-t2]; Fig. 5(b): This mode starts by switching power MOSFET off. D2 is reverse biased and C1 is charged by the energy of input inductor and input source through D1. D3 gets forward biased and the energy of leakage inductor is stored in capacitor C2. Current of the leakage inductor decreases in this short duration period. This mode is finished when ILm becomes equal to ILk.
Mode3 [t2-t3]; Fig. 5(c): In this mode, D5 becomes forward biased when direction of the coupled inductor’s current is changed. The energy of secondary side inductor and capacitor C3 charge the output capacitor CO and feed the load. D1 and D3 are still forward biased, but D2, D4 and S are not conducting.