To reduce transformer interwinding capacitance, first limit the voltage swing and turns ratio where your topology allows; then consider sectional or split-bobbin winding, greater winding separation, or a correctly connected Faraday shield. These choices trade lower common-mode current against leakage inductance, size, loss, insulation requirements, and manufacturability, so verify them in the finished converter rather than optimizing capacitance alone.
Why interwinding capacitance causes trouble
Interwinding capacitance is the parasitic capacitive path between a transformer’s windings. In an isolated switching supply, fast voltage changes on the switched winding can drive high-frequency common-mode current across that path and contribute to emissions. Texas Instruments describes the feedthrough from a switched primary to the secondary as a source of common-mode noise; Analog Devices models the parasitic paths as CWA and CWB.
The current through a capacitance rises with both capacitance and the rate of voltage change. That means a modest capacitance can matter when the winding has a large, fast switching-voltage swing. The effect is not determined by a capacitance value alone: switching waveform, winding arrangement, circuit grounds, and the converter’s operating conditions all matter.
Why turns ratio matters
In a Texas Instruments example published by Robert Kollman in 2011, a 40:1 transformer with 20 pF of distributed secondary capacitance has an equivalent capacitance of 20 pF × 40² = 32 nF when reflected to the primary. Kollman calculated that, at 100 kHz with a 12 V input, this capacitance contributed almost 1 W of loss in a 4 W supply. These figures describe that example, not a universal loss estimate for transformers with the same nominal turns ratio.
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Which transformer changes can reduce it?
Reduce turns ratio or winding voltage swing when the topology permits
A lower turns ratio can reduce the capacitance reflected across the transformer, while reducing the voltage across the parasitic path reduces the current it carries. Kollman’s design guidance is to “minimize the transformer-turns ratio and minimize the voltage across it.” In practice, topology and required input, output, and isolation voltages constrain these choices; do not change the ratio without checking the converter’s operating requirements.
Use banked or sectional windings to control voltage gradients
Bank winding limits the voltage difference between adjacent turns. Sectional winding divides a winding into sections, reducing the effective coupling across the full voltage gradient. In Kollman’s cited example, two sections reduced effective capacitance by about half, and four sections reduced it by a factor of four. Those are construction-specific results, not guaranteed factors for every transformer.
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Separate primary and secondary windings where the design allows
Greater physical separation, including a split-bobbin construction with windings in separate cavities, can reduce interwinding capacitance and capacitive coupling. Bel Fuse identifies split-bobbin windings as an approach to reducing common-mode noise. The cost is that increased separation can raise leakage inductance and may require more package volume.
Consider a Faraday shield only with a deliberate connection
A Faraday shield is thin foil or metallized insulating film placed in the interwinding space to intercept capacitive current. Texas Instruments advises insulating the overlap so it cannot form a shorted turn, and making the foil thinner than the penetration depth to limit eddy-current loss. Its guidance is to connect the shield directly to the quiet side of the transformer primary with minimum lead inductance.
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Shield connection depends on the converter’s grounding and winding arrangement. Skyworks advises that, when one shield is used, the winding with the largest voltage swing should be shielded to the circuit ground on that side. These recommendations are not a substitute for analyzing the actual topology: follow the applicable safety and grounding architecture and the transformer manufacturer’s design guidance. A shield also adds capacitance and can increase eddy-current or switching losses if its construction or connection is poor.
Compare the trade-offs before choosing a construction
| Design choice | Potential benefit | Trade-off or design check |
|---|---|---|
| Lower turns ratio or voltage swing | Can reduce current through the parasitic capacitance. | Must remain compatible with the topology and required conversion and isolation voltages. |
| Banked or sectional winding | Limits voltage gradients or reduces effective capacitance; the cited TI example reports about half reduction with two sections and a factor of four with four. | Actual results depend on winding construction; evaluate winding loss, thermal performance, and manufacturability. |
| Split bobbin or increased winding separation | Reduces physical capacitive coupling. | Can increase leakage inductance, size, and cost. |
| Interleaved windings, broad windows, or close primary-secondary spacing | Can reduce leakage inductance and some winding losses. | Increases interwinding capacitance, so it may worsen common-mode coupling. |
| Faraday shield | Diverts capacitive common-mode current when correctly insulated and connected. | Adds shield capacitance and may cause eddy-current or switch-loss penalties if poorly designed. |
Compare the complete transformer and converter against the application: primary-secondary capacitance, leakage inductance, winding and core losses, isolation system, creepage and clearance, thermal performance, physical size, cost, and measured EMI. There is no single construction that minimizes all of these simultaneously.
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Reduce residual common-mode noise in the circuit
Split secondary rectification and filtering
Using split secondary sections with rectifiers and filters can reduce the effective capacitance seen by the switching circuit. Kollman’s two-section example reports about half the effective capacitance, while the four-section example reports a factor-of-four reduction. Apply those results only as guidance for comparable winding arrangements; assess the resulting circuit and transformer together.
Evaluate cancellation windings and common-mode chokes
Skyworks documents cancellation windings as another way to address residual emissions. In one design, increasing tape spacing from 1 to 10 turns reduced emissions by about 33%; that result is specific to the documented design, not a general performance guarantee.
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A common-mode choke can provide additional attenuation. In Skyworks’ case study, the input choke gave the greatest benefit among the approaches considered. A choke becomes less effective above its self-resonant frequency, so select and verify it for the relevant frequency range rather than assuming it attenuates all switching harmonics.
Quick Recap
Verify the finished transformer and converter
- Measure primary-secondary capacitance. Record the instrument, fixture, and measurement frequency so that results can be compared consistently. Measure the finished transformer, not just a winding estimate.
- Measure the other relevant transformer properties. Check leakage inductance and insulation parameters, and confirm that the construction meets the design’s isolation, creepage, and clearance requirements.
- Test emissions in the complete converter. Measure conducted and radiated emissions, including common-mode behavior, under the operating conditions and regulatory target that apply to the product.
- Inspect switching behavior and losses. Check for the failure modes Kollman describes: slowed drain-voltage transitions, false current-limit triggering, and excess switch loss.
- Set application-specific acceptance limits. Choose limits for the actual topology and compliance target; the cited design guidance does not establish one universal capacitance or emissions limit.
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