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.