A coupled inductor for an interleaved buck converter uses magnetic coupling between phase windings to shape ripple current and core flux. Compared with separate inductors, it can reduce output ripple, phase ripple, or magnetic volume in the right operating range. The result depends on coupling polarity, duty cycle, phase shift, leakage inductance, current balance, saturation, and control behavior.
Start with the converter ripple objective
Interleaving already cancels part of the output ripple when phases are evenly shifted. Coupling changes the impedance seen by common and differential phase-current components. The desired coupling depends on whether the priority is lower per-phase ripple, lower output ripple, transient response, size, or efficiency.
Provide input and output voltage ranges, phase count, switching frequency, phase relationship, duty-cycle range, average and peak phase current, current-balance tolerance, and transient requirements.
Coupling polarity is fundamental
The dot convention determines whether phase ripple flux reinforces or opposes in the shared core. An incorrect winding polarity can increase ripple dramatically or drive unexpected core flux. The drawing, schematic, pinout, and production polarity test must use the same convention.
Separate magnetizing and leakage behavior
The coupled structure has self-inductance, mutual inductance, and leakage inductance. Different measurement connections reveal different modes. Specify the relevant inductance matrix or clearly defined common-mode and differential-mode tests rather than one ambiguous inductance number.
Check current imbalance
Controller offset, current-sense tolerance, DCR variation, timing mismatch, and unequal PCB paths can create DC imbalance between phases. Coupling may oppose or redistribute that imbalance depending on topology. The core and each winding must tolerate the worst credible phase current, not only ideal sharing.
Core flux is duty-cycle dependent
Flux cancellation varies with duty cycle and phase relationship. An operating point with excellent cancellation at nominal voltage may not be the magnetic worst case. Startup, phase shedding, current limit, and a failed phase can impose very different volt-seconds.
Thermal and mechanical design
Each winding carries DC current plus ripple. Proximity effect between windings, gap fringing, termination resistance, and uneven airflow can create hot spots. Coupled magnetic forces can also produce audible noise. Measure winding, core, gap, and terminals under balanced and intentionally imbalanced conditions.
Prototype verification checklist
- Self, mutual, and leakage inductance with defined connections
- Turns ratio and polarity between phase windings
- Per-phase and output ripple across duty cycle
- Current balance during steady state and transients
- Inductance behavior at peak and fault current
- Core, winding, gap, and terminal temperature
- Phase shedding, startup, and current-limit behavior
BaoHui Tech can develop custom coupled inductors from phase waveforms, coupling targets, bias current, thermal limits, insulation, and mechanical constraints. Early discussion with the transformer manufacturer helps align magnetic tests with the converter control model.
Frequently asked questions
Can two separate inductors be replaced directly by one coupled inductor?
Not without redesign. Coupling changes phase dynamics, ripple, fault behavior, and control-loop interaction.
Is a higher coupling coefficient always better?
No. The optimum depends on topology and ripple objective. Very high coupling can create unwanted interactions or impractical winding capacitance and insulation.
What production tests are essential?
Polarity, defined inductance connections, winding resistance, dielectric strength, dimensions, and workmanship are typical. Coupling or leakage limits should be included when functionally critical.