3.4.2.1 Basic Operation Principle of SEPIC Converter

In a single ended primary inductance converter (SEPIC) design, the output voltage can be higher or lower than the input voltage. The SEPIC converter shown in Figure 9 uses two inductors: L1 and L2. The two inductors can be wound on the same core, or uncoupled since the same voltages are applied to them throughout the switching cycle.

Figure 9. SEPIC Converter Topology

To understand the voltages at the various circuit nodes, it is important to analyze the circuit at DC when Q1 is off and not switching. During steady-state CCM, pulse-width modulation (PWM) operation, and neglecting ripple voltage, capacitor Cs is charged to the input voltage, Vin. When Q1 is off, the voltage across L2 must be Vout. Since Cin is charged to Vin, the voltage across Q1 when Q1 is off is Vin + Vout, so the voltage across L1 is Vout. When Q1 is on, capacitor C, charged to Vin, is connected in parallel with L2, so the voltage across L2 is –Vin. The currents flowing through various circuit components are shown in Figure 6. When Q1 is on, energy is being stored in L1 from the input and in L2 from Cs. When Q1 turns off, L1’s current continues to flow through Cs and D1, and into Cout and the load. Both Cout and Cs get recharged so that they can provide the load current and charge L2, respectively, when Q1 turns back on.