What Is a Coupled Inductor?
A coupled inductor is a magnetic component that contains two or more windings on a single core, magnetically linked so that energy can transfer between them while also storing energy. It is a core building block in modern power electronics — used in flyback converters, SEPIC and Cuk converters, interleaved boost converters, LLC resonant converters, and multi-phase VRM designs. The global coupled inductor market has grown steadily with the demand for higher power density in EV charging, telecom power, solar inverters, and data center power supplies.
When engineers search for “coupled inductor,” they are usually evaluating how a coupled magnetic component can improve efficiency, reduce ripple, or shrink converter size. This guide explains the working principle, key specifications, and design considerations for selecting the right coupled inductor.
Working Principle
A coupled inductor works on the same magnetic flux coupling as a transformer, but with an important difference: it stores energy in the air gap during the switching cycle instead of transferring energy continuously. The coupling coefficient k (usually 0.9 – 0.99 for practical designs) defines how much of the flux links both windings. The leakage inductance — the flux that does not couple — is often deliberately controlled, for example in LLC converters where leakage inductance acts as the resonant inductor.
In a coupled inductor, the current ripple in one winding can be reduced by the opposing magnetomotive force from the other winding. This “ripple cancellation” effect is why interleaved and coupled designs achieve lower output ripple with smaller inductance values.
Key Specifications
| Parameter | Typical Value / Consideration |
|---|---|
| Inductance per winding | µH to mH, depending on topology and frequency |
| Coupling coefficient (k) | 0.9 – 0.99 |
| Leakage inductance | Defined for resonant topologies (e.g., LLC) |
| Saturation current | Must exceed peak operating current with margin |
| DC resistance (DCR) | Low DCR for high efficiency |
| Operating frequency | Typically 100 kHz – 2 MHz |
| Core material | Ferrite, sendust, high flux, MPP, or nanocrystalline |
| Isolation voltage | 500 V – 4 kV between windings where isolation is required |
Design Steps
1. Define the topology and electrical targets. Identify whether the coupled inductor provides energy transfer, ripple cancellation, or both, and set the input/output voltages, current, and switching frequency.
2. Calculate inductance and turns ratio. Use the volt-second balance to determine the required inductance and the winding ratio between phases or primary/secondary.
3. Select the core material and size. Choose ferrite for high frequency low-loss operation, or powder cores (sendust, high flux, MPP) when high DC bias and soft saturation are needed.
4. Design the windings. Interleave windings to improve coupling, or separate them to increase leakage inductance as needed. Use litz wire or foil for high-frequency current to reduce skin effect.
5. Verify thermal and EMI performance. Calculate core loss (Steinmetz equation) and copper loss, check hotspot temperature, and evaluate common-mode noise coupling.
6. Prototype and characterize. Measure inductance vs. DC bias, leakage inductance, saturation current, and temperature rise with a network analyzer or impedance analyzer.
Applications
Coupled inductors are widely used in flyback converters, SEPIC/Cuk converters, interleaved PFC and boost converters, multi-phase VRM for CPUs and GPUs, LLC resonant converters, battery chargers for EVs, solar micro-inverters, and telecom power modules. In every case, a well-designed coupled inductor reduces component count and improves power density.
Why Choose Sirui-Tech for Coupled Inductors
Shaanxi SiRui Technology Co., Ltd. manufactures high-quality coupled inductors with ferrite and powder cores, covering power ratings from a few watts to several kilowatts. We are ISO 9001:2015 certified and our products meet RoHS and REACH requirements, verified by SGS and CTI. Our engineering team supports custom coupled inductor design, including leakage inductance control, so you get a component matched to your converter — not a compromise.
FAQ
Q1: What is the difference between a coupled inductor and a transformer?
A transformer transfers energy with little stored energy, while a coupled inductor stores energy in the core gap and uses magnetic coupling for ripple cancellation or multi-phase sharing.
Q2: Why is leakage inductance important in coupled inductors?
In resonant topologies such as LLC, leakage inductance is used as the resonant inductor; in other designs, excessive leakage causes voltage spikes, so it must be minimized.
Q3: What core material is best for a coupled inductor?
Ferrite for high-frequency low-loss designs; sendust or high-flux powder cores when high DC bias tolerance and soft saturation are required.
Q4: Can coupled inductors reduce output ripple?
Yes. In interleaved converters, coupled inductors provide ripple cancellation, which allows smaller output capacitance and lower switching noise.
Q5: Do you provide custom coupled inductors?
Yes. We design and manufacture custom coupled inductors with controlled coupling and leakage inductance for OEM and ODM projects.