The SEPIC (Single-Ended-Primary-Inductor-Converter) is a non-isolated, switching power supply topology generating an output voltage which can be higher or lower than the input voltage. This is a common requirement in applications like battery-powered products and chargers, automotive power systems, photovoltaic converters, offline LED lighting as well as power factor correction stages, amongst others.
Unlike single-inductor topologies like buck, boost or buck-boost, the power stage of the SEPIC requires two inductors. These can be implemented as uncoupled, separate power inductors, or alternatively, they can be built on the same magnetic core as the two windings of a coupled power inductor. This approach not only reduces the component count, but also requires lower inductance to set the same ripple current amplitude compared to a solution with uncoupled inductors. In addition to this, magnetically coupling the windings allows to implement ripple current steering, a technique in which the ripple current of the input winding is ‘steered’ to the output winding, reducing conducted EMI noise. However, it is important to understand the impact of the leakage inductance on the performance of a coupled-inductor SEPIC. Here, and contrary to the usual cases, a higher leakage inductance can actually be of advantage.
In this application note, the operation of the SEPIC in continuous as well as discontinuous conduction modes (CCM and DCM) is analyzed, and important design considerations and guidelines are provided. Special attention is given to the implementation with a coupled inductor, including an analysis of ripple current steering technique and the key role of leakage inductance in the converter performance. The analysis is supported by SPICE simulations and measurements on a real DC-DC SEPIC converter prototype.
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Download: ANP135
