Coupled-Inductor Filter: A Basic Filter Building Block

Coupled-magnetics and coupled-inductor filters provide smoothing in a power converter by steering ripple current away from the "quite port." While this concept is not new, topologies claiming to be "new" or "novel" continue to be reported in the literature, suggesting that the ripple-steering phenomenon may not be well understood to be a property of the filter block itself. This paper describes a general coupled-inductor filter block and presents a brief history of its application starting with the earliest known version of the circuit, which dates back to a 1923 patent application. The coupled-inductor circuit model is reviewed, starting with the basic topology and ideal circuit elements. Real circuit elements are then considered along with the effects of equivalent series resistance and other parasitic elements. A circuit-based filtering approach is used to emphasize the filtering aspect of the coupled inductor. It will be seen that the coupled-inductor filter can exhibit a low pass with notch frequency response. This provides the ability to achieve extra attenuation at a particular frequency, such as the converter switching frequency, which can reduce the overall filtering burden without adding extra inductive components. When the filter components are treated as a functional circuit group, instead of an interconnection of inductors and capacitors, the coupled-inductor filter can be analyzed as a drop-in replacement wherever a smoothing choke would normally be used and does not depend on converter voltage waveforms. A topology transformation of the Cuk converter shows that "zero ripple" in not a special property of the topology, but can be achieved using the coupled-inductor filter block.