A SEPIC (Single-Ended Primary Inductor Converter) is a non-isolated switched-mode power supply topology that generates an output voltage that can be higher, equal to, or lower than the input voltage. Typical applications include battery-powered products and chargers, automotive power systems, photovoltaic inverters, off-grid LED lighting, and PFCs for power factor correction. This Application Note provides a detailed analysis of the SEPIC converter, with particular attention to inductors. Würth Elektronik places special emphasis on implementations with coupled inductors such as the WE-MCRI family, and analyzes both ripple current control and the critical role of leakage inductance in converter performance. The analysis utilizes SPICE simulations and measurements on a real prototype DC-DC SEPIC converter.
Coupled or uncoupled solution
Unlike single-inductor topologies such as Buck, Boost, or Buck-Boost, the SEPIC power stage requires two inductors. These can be implemented as independent, uncoupled inductors or, alternatively, as a coupled power inductor with two windings on a common core. This design not only reduces the number of components but also requires a smaller inductor to generate the same ripple current amplitude compared to a solution with uncoupled inductors. Furthermore, the magnetic coupling of the windings allows for the implementation of a technique called Ripple Current Steering. This technique involves controlling which winding carries more ripple current, thus reducing conducted EMI noise. Understanding the impact of leakage inductance on the performance of a SEPIC with coupled inductors is crucial. Contrary to what happens in most applications, a higher leakage inductance can bring advantages in this case,” explains Eleazar Falco, senior applications engineer at Würth Elektronik eiSos and author of the paper.
