Interleaved converters split power among phases and shift switching waveforms so ripple partly cancels. Coupling the phase inductors can further reduce external ripple or magnetic size. The result depends critically on coupled inductor polarity and current sharing. A reversed winding can turn intended cancellation into reinforcing flux and excessive current.
Separate phase ripple from terminal ripple
Low output ripple does not mean each winding sees low ripple. Phase currents can have large triangular components that cancel when summed. Copper and core losses are driven by individual currents and magnetic flux modes, not only the combined waveform.
Simulate every phase current and voltage across each winding. Phase shift, duty cycle, and operating mode determine cancellation. Ripple changes with duty cycle and may be least favorable away from nominal.
Polarity controls magnetic modes
The dot convention identifies voltage polarity for mutually coupled windings. Depending on connection, common phase-current components may produce reinforcing or opposing flux. Differential ripple may use a different magnetic path.
Verify polarity from a diagram and bench test, not color or physical direction alone. A low-voltage series-aiding test can confirm production units.
Coupling and leakage
Perfect coupling is not always desirable. Leakage gives each phase independent ripple impedance and affects transient sharing. Strong coupling can improve cancellation but transfers disturbances between phases.
Winding separation, interleaving, core geometry, and gaps set coupling. A concentrated gap creates fringing that can heat nearby conductors. Distributed-gap materials have different loss and bias behavior.
Current sharing
Unequal DCR, turns, leakage, gate timing, current-sense gain, PCB resistance, and temperature cause imbalance. Coupling can oppose or amplify it. Passive DCR matching helps, but active current balancing may still be needed.
Test from cold start through thermal equilibrium. Resistance temperature feedback is not a substitute for controlled sensing and layout.
Saturation and faults
The core sees combinations of common and differential magnetomotive force. A design safe under balanced phases may saturate during startup, phase shedding, current limit, a failed switch, or a disabled channel. Define whether the converter can run with fewer phases.
Specification
- Define self, mutual, and leakage inductance test methods.
- State polarity and terminal diagram.
- Specify inductance under common and differential current at temperature.
- Set DCR matching and imbalance limits.
- Measure each phase current with deskewed probes.
- Test startup, phase shedding, overload, short circuit, and sensor faults.
BaoHui Tech needs topology, phase count and shift, voltage waveforms, currents, duty range, ripple targets, coupling target, control, fault modes, cooling, insulation, and package.
Frequently asked questions
Can output ripple be near zero while the inductor runs hot?
Yes. Large phase ripple can cancel at the output but still create winding and core loss.
Does strong coupling guarantee equal current?
No. DCR, timing, sensors, control, layout, and thermal variation also matter.