Design Support Tools Use our suite of tools to help select, analyze and compare inductors Parametric Search View our list of inductors and filter results based on your exact parameters Looking for a datasheet? Search by product and download a PDF FREE Samples Get free samples quickly - most orders ship the same day! Looking for a datasheet? Search by product and download a PDF. Student Support Learn more about magnetics, request free samples or ask our engineers a question. A Guide to Coupled Inductors What is a coupled inductor? A coupled inductor has two or more windings on a common core. Coupled inductors function in dc-dc converters by transferring energy from one winding to the other through the common core. They are available in many sizes, inductance values, and current ratings and most are magnetically shielded for low electromagnetic interference (EMI). The windings may have equal (1:1) or unequal turns ratios (1:N). Due to widespread demand in a variety of circuits, many 1:1 and 1:N standard coupled inductors are readily available off-the-shelf. What are common applications for coupled inductors? Typical circuits using 1:1 COUPLED INDUCTORS Figure 1 Purpose-designed coupled inductors are used in a variety of power conversion circuits such as flyback, SEPIC, Fly-Buck™, Cuk, ZETA and multi-phase topologies. These voltage regulation circuits use coupled inductors to efficiently provide the required output voltages, which may be stepped up or down from the input. To do this, SEPIC and Zeta circuits use 1:1 coupled inductors, whereas flyback and other circuits use 1:N inductors. In multi-phase power converters, coupled inductors provide inherent ripple current cancelling that is not provided by single inductors. Typical circuits using 1:N COUPLED INDUCTORS Figure 2 You can connect coupled inductor windings in different configurations to serve different purposes. The windings may be connected to circuits separately for use as isolation transformers or as common mode chokes . You may also connect them in series to achieve a higher inductance or for use as a tapped inductor. As the use of standard coupled inductors has grown, Coilcraft datasheets now provide application-specific characterizations to help designers select the best-suited inductor. What is the difference between a coupled inductor and a transformer? There is often little or no physical difference between what one engineer calls a coupled inductor and another calls a transformer. Both may have either a 1:1 or 1:N turns ratio and have similar winding-core arrangements, and they may even "look the same." Actually, the most appropriate name depends on the intended application. Coupled inductors and flyback transformers both use cores to store energy received from a winding and then transfer that energy to the other winding. For both transformers and coupled inductors, the efficacy of the winding coupling, expressed as coupling coefficient k, depends on the core material properties as well as the physical arrangement of the windings and core. Common to transformers and coupled inductors is the fact that close coupling between the windings (k > 0.9) results in low leakage inductance and generally provides the most efficient energy transfer and widest usable frequency bandwidth. High-performance designs like Coilcraft's LPD and LPD-V families are able to provide the desired coupling factor while also providing a high measure of voltage isolation between windings. Close coupling may not be optimal for all applications. In fact, Coilcraft's MSC1278 Series of coupled inductors is designed with high leakage inductance for use in certain SEPIC applications. The loosely coupled windings (k ≈ 0.8) improve SEPIC efficiency by reducing circulating current and still provide twice the ripple current reduction of separate inductors. You can connect coupled inductor windings in different configurations to serve different purposes. The windings may be connected to circuits separately for use as isolation transformers or as common mode chokes. You may also connect them in series to achieve a higher inductance or for use as a tapped inductor. As the use of standard coupled inductors has grown, Coilcraft datasheets now provide application-specific characterizations to help designers select the best-suited inductor. How do I choose a coupled inductor? Application-specific data for off-the-shelf coupled inductors Figure 3 Selection of a coupled inductor is application-dependent. As shown in Figure 3, Coilcraft datasheets characterize coupled inductors for multiple circuit uses, and Coilcraft has developed online tools for selecting coupled inductors. For a particular inductance and current rating, our Coupled Inductor Finder Tool provides a list of off-the-shelf coupled inductors that meet those requirements. This document describes the particular SEPIC calculations for determining the required coupled inductance parameters. What types of coupled inductors does Coilcraft manufacture? Coilcraft designs and manufactures a variety of surface mount coupled inductors for many applications including, 1:1 low profile, low DCR, miniature, high temperature, and high isolation alternatives. Coilcraft also offers various 1:N coupled inductor alternatives in ratios from 1:1.5 up to 1:100. Designer Kit's Copyright 2026, Coilcraft, Inc. Taiwan +886 2-2264 3646 We value your privacy We use essential and non-essential cookies on our Site, including third party advertising cookies to provide tailored ads. For more details or to manage your preferences, please see our Privacy Policy