L1, L2, Lm and Lk are primary side, secondary side, magnetizing and leakage inductors, respectively. Input side of the converter consists of Lin, diodes D1 and D2, capacitor C1 and primary side of the coupled inductor. C2 and D3 together form the passive clamp circuit. The intermediate capacitor C3, diode D4 and secondary side of the coupled inductor boost the voltage level and finally D5 is the output diode of the topology. High voltage gains are available with appropriate choices of n and D that lead to lower voltage stress on power switch and optimum sizing of coupled inductor, respectively. Following assumptions are considered to simplify the analysis of proposed converter:
· Coupling coefficient k is obtained by and its turn ratio (n) is where N2 and N1 are the numbers of turns for secondary side and primary side inductors, respectively.
· Capacitors C1, C2, C3 and CO are large enough, so their voltages are considered to be constant in a switching period.
1.1. Operating Modes for CCM
Continuous conduction mode (CCM) for this converter consists of five operating modes that are depicted in figures 3 and 4. These five operating modes are repeated in any switching period.
Mode1 [t0-t1]; Fig. 3(a): This mode starts by switching the power MOSFET on. D1 and D2 are reverse and forward biased, respectively. Input inductor Lin is charged by input source through D2 and S. C1 is discharged to charge magnetizing and leakage inductors. The energy stored in C3 and secondary side inductor charges output capacitor CO and feeds load through D5. Current of the leakage inductor starts increasing in this short duration to get equal to the current of magnetizing inductor. This mode is finished at the moment when currents of inductors Lk and Lm become equal.
\(I_D5=1/n(I_Lm-I_Lk)\) (1)
Mode2 [t1-t2]; Fig. 3(b): S is still on and direction of current in inductor windings is changed by increasing the value of ILk to more than ILm. Energy stored in inductor and C2 charges the capacitor C3 through D4. Diodes D1 and D5 are reverse biased. Input inductor Lin is charged by input source through D2 and S and also its current increases. Capacitors C1 and CO are discharged to charge magnetizing and leakage inductors and feed load, respectively. This mode continues until switch gets turned off.
\(I_D4=I_C3=-I_C2=1/n(I_Lk-I_Lm)\) (2)
From modes 1 and 2, following equations can be derived: