Mode3 [t2-t3]; Fig. 3(c): By turning off the switch in this mode, D3 becomes forward biased and transfers the energy of leakage inductor to C2, therefore C2 is being charged. D2 and D1 are reverse and forward biased, respectively. Capacitor C1 is charged by input source and input inductor and the current of leakage inductor is decreasing. This short duration mode is finished when ILm becomes equal to ILk.
\(I_C2=-(1/n(I_Lk-I_Lm)-I_Lk)\) (4)
Mode4 [t3-t4]; Fig. 3(d): Diode D5 becomes forward biased when direction of the current in primary and secondary windings is changed. Energy stored in C3 and secondary side inductor feeds the load besides charging the output capacitor. D1 and D3 are forward biased in this mode, but D4 is not conducting.
Mode5 [t4-t5]; Fig. 3(e): Current of the leakage inductor starts to decrease from the end of mode 2 and mode 5 starts when this current becomes zero; thus diode D3 becomes reverse biased. Only D1 and D5 are conducting in this mode. CO and C1 are being charged, but C3 is discharged.
\(I_C1=I_Lin-1/nI_Lm\) (5)