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.

Modified SEPIC converter.

           In modified SEPIC converter, a voltage multiplier circuit is applied to the classical boost converter, for higher output voltage, improved efficiency and low switch voltage. This is done by adding diode D_M and Capacitor C_M to the classical SEPIC as shown in fig..

            Capacitor C_M of the modified SEPIC is charged with the output voltage, hence when the switch is conducting the voltage applied to inductor  is higher than that in traditional SEPIC. In case of modified SEPIC converter, the losses due to diode reverse recovery current are reduced and soft commutation is obtained with regenerative snubber circuit. The static gain of modified SEPIC is higher than classical boost converter for high values of duty cycle, and low input voltage.A gain up to 5 is considered as standard static gain, gain larger than 10 is high static gain and that larger than gain equal to 20 is considered as very high static gain. If compared with other converters for high gain, modified SEPIC utilizes lesser number of switches and less passive components than those converters.

           The main purpose of magnetic coupling in modified SEPIC is to increase the voltage gain leading to boost in the output voltage. In past various other techniques like voltage multiplier, interleaved boost converter, zero voltage switching boost integration technique, etc each of them have certain advantages and disadvantages and can be implemented as per application. If compared with these techniques for high gain, proposed modified SEPIC with magnetic coupling utilizes lesser number of switches and less passive components.

The theoretical explanation, working and simulation results of modified SEPIC without magnetic coupling and modified SEPIC with magnetic coupling are explained in following sections. For magnetic coupling an inductor operating as a flyback transformer is used.

SEPIC(Single Ended Primary Inductor Converter)

A SEPIC is a short form for Single Ended Primary Inductor Converter. SEPIC is a DC-DC buck- boost converter, whose output is controlled by controlling the duty cycle of the control transistor present in the circuit. SEPIC is different from the conventional buck-boost converter in terms of polarity of the output. The conventional buck boost converter has inverted output, while the SEPIC has advantage of non-inverted output. The circuit diagram for classical SEPIC converter is as shown in fig.

Fig.– Circuit diagram of classical SEPIC.

The classical SEPIC consists of two identical inductors, a control transistor (switch) two capacitors and a diode. The inductors L₁ and L₂ are coupled inductors. Coupled inductors are inductors that share the flux lines, i.e. voltage in second inductor is induced because of the voltage in first inductor. A coupled inductor is at a low cost than two separate inductors and also it takes less space on PCB. The inductors are selected on the basis of peak to peak ripple current, it should be approximately equal to 38%-40% of the maximum input current at maximum input voltage. C_S is the coupled capacitor, and isolates the input from output protecting against shorted load. C_S is such selected that RMS current though it must be very small. The control transistor can be FET or MOSFET. In case of power MOSFET selection, minimum threshold voltage, the ON resistance, gate-drain charge, and the maximum drain to source voltage are considered. The output diode should be able to handle peak current and reverse voltage, hence it should be selected accordingly. The output capacitor C_O, is charged with inductor current L₂, hence must sustain maximum RMS current.