Operation of modified SEPIC converter.

As stated above a modified SEPIC is built by adding only two components diode D_M and capacitor C_M to the classical SEPIC circuit. The circuit diagram for modified SEPIC without magnetic coupling is in fig 1.3. Modified SEPIC operates in continuous conduction mode and discontinuous conduction mode. For continuous conduction mode modified SEPIC operates in two stages depending upon the on and off condition of switch.

Stage 1. When the switch is in off state.

At time t₀ the switch is turned off; the input energy stored in inductor L₁ is passed on to the output through C_M and output diode D₀, and also to C_M through diode D_M. Here the switch voltage is same as the voltage across C_M, the energy stored in inductor L₂ is passed on to output through diode D₀.

During this time period, the current in L₁ is equal to input current i.e. i_L1 = i_In. As the diode is in conducting state voltage across it is zero, while current through it is in decaying state. The waveforms to understand these operations are shown in fig.

Stage 2 – When switch is in on state.

At t₁, the switch is turned on and the circuit is as shown in fig. In this mode the inductors instead of transferring energy, they store it. The input energy is applied to L₁, i.e.   Voltage is applied to inductor L₂.The diodes D_M and D₀ are reverse biased

The current through inductor L₂ is equal to output current. Switch is in conducting state so voltage across it is zero, while current flowing through switch is equal to addition of current flowing through both the inductors. i.e.  .Voltage across diode D_O = V_CM, and . The waveforms to understand these operations are shown in fig.