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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →A SEPIC’s coupled inductor does not automatically need the tightest possible coupling. In continuous-conduction mode (CCM), the AC voltage across the coupling capacitor is impressed across the inductor’s leakage inductance. If that leakage is very small, the resulting circulating current can increase winding RMS current, reduce efficiency, and worsen EMI. The right leakage is a design choice to calculate or simulate for the actual converter—not a value to maximize blindly.
Why circulating current occurs in a SEPIC
A SEPIC (single-ended primary-inductor converter) can step its input voltage up or down. In the conventional topology, input current is continuous while output current is pulsed. Its series coupling capacitor separates the converter’s energy-transfer paths and carries an AC voltage.
In Robert Kollman’s CCM explanation, the AC voltage across that capacitor appears across the coupled inductor’s leakage inductance. The DC components cancel in the modeled winding structure, leaving the AC component to drive current through the leakage inductance. From the inductor voltage-current relationship, less inductance means a greater current change for the same applied voltage. This is why a tightly coupled part—with very low leakage—can produce substantial circulating current in a SEPIC coupled inductor.
That current adds to winding current without representing useful load current. As Kollman put it, “A large circulating current will degrade the efficiency and EMI performance of a converter, which is undesirable.” The extent depends on the converter’s operating conditions and component values.
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What the reported comparison shows—and does not show
Kollman’s 2011 EE Times comparison used a 47 μH MSD1260 with approximately 0.5 μH leakage and a 47 μH MSC1278 with 14 μH leakage. In the reported 8–36 V input, 12 V output example, the low-leakage case had peak current almost twice the DC input current, RMS current 50% higher, and AC input currents with an almost 5-to-1 ratio. In a described 12 V-to-12 V example, the loosely coupled inductor yielded 1–2 percentage points better efficiency over the tested load range.
These are results from Kollman’s particular tests, not expected ratios or guaranteed efficiency gains for other SEPIC designs. The part numbers are historical examples, not current purchasing recommendations; verify current specifications and availability before selecting a component.
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Choosing leakage: a design tradeoff
There is no single leakage value that suits every SEPIC. Increasing coupling capacitance can reduce circulating current, but a larger capacitor may cost more, take more space, and introduce reliability tradeoffs. Choosing an inductor with greater designed leakage is another possible approach. Either choice has consequences elsewhere in the converter.
| Design consideration | What to evaluate |
|---|---|
| Circulating and ripple current | Whether winding current and ripple stay within the converter’s targets across its operating range. |
| RMS current and losses | Winding RMS current, conduction loss, component temperature, and the efficiency impact. |
| EMI | Whether input-current behavior increases the burden on the EMI filter or complicates compliance. |
| Coupling capacitor | AC ripple, RMS current, voltage rating, size, and cost. Excess capacitor RMS current can cause overheating and damage. |
| Dynamic behavior | Transient response and the effects of the selected operating mode and component values. |
| Implementation | Suitable component availability, current ratings, layout, and the physical or cost tradeoff of coupled versus separate inductors. |
TI’s 2023 design note says a SEPIC may preferably have 10%–15% leakage. Treat that as application guidance from that note, not a universal requirement. A design that benefits from more leakage in one respect may still need to meet ripple-current, transient, capacitor, thermal, and EMI limits.
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How to establish a leakage target
- Define the operating envelope. Record the input-voltage range, output voltage and power, switching frequency, and intended conduction mode (CCM or DCM). TI treats conduction-mode selection as an early design decision.
- Set ripple and capacitor limits. Estimate the coupling capacitor’s AC voltage and define an allowable ripple-current target. Check its RMS current and voltage rating as well as the inductor winding currents and component temperature.
- Model the actual magnetic structure. Use a coupled-inductor model that represents leakage, magnetizing inductance, and the ideal transformer, or measure the relevant winding parameters. A perfect-coupling simulation that omits leakage can produce discontinuous or inaccurate waveforms.
- Sweep plausible leakage values. Simulate across the input, load, and operating-mode range, then compare circulating and ripple current, RMS losses, capacitor stress, efficiency, EMI-relevant current behavior, and transient response. Kollman described simulation as the simplest way to choose an acceptable leakage amount.
- Verify the selected component. Check its measured or specified inductance, leakage, current capability, temperature behavior, and fit with the layout. Confirm the design under the operating conditions that matter to the application.
When tight coupling is still appropriate
This mechanism is not a general argument against tight coupling. It is a warning against assuming that a SEPIC should follow the same coupling rule as a flyback converter. Choose leakage to manage this topology’s circulating current while satisfying the complete converter’s requirements. Depending on the application, a tightly coupled part may still be suitable; the decision should come from the operating envelope, component model, and verification results.
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