Winding arrangement is one of the strongest design levers in a high frequency transformer. Splitting and interleaving primary and secondary sections can improve magnetic coupling and reduce leakage inductance. The same increased overlap normally raises interwinding capacitance and common-mode current. A good design therefore does not aim for the lowest possible leakage value; it finds the combination that best supports switching performance, EMI, insulation, loss, temperature, and manufacturing.
Why interleaving reduces leakage inductance
Leakage flux is the portion of magnetic flux that links one winding but not the other. When primary and secondary ampere-turn regions are physically separated, more field energy is stored in the space between them. Alternating winding sections brings opposing ampere-turn distributions closer and reduces this stored leakage energy.
Common arrangements include primary-secondary, primary-secondary-primary, and more highly sectioned stacks. Foil, planar PCB, round wire, and litz constructions each create different field distributions. The optimum split also depends on turns ratio and current: equal section counts do not necessarily produce equal ampere-turn balance.
Benefits of lower leakage
In hard-switched converters, leakage energy contributes to voltage overshoot and clamp or snubber dissipation. Lower leakage can reduce switch stress and improve regulation. In LLC, phase-shifted full bridge, dual active bridge, and active-clamp topologies, leakage may form part of the resonant or commutation inductance. Removing too much can therefore change soft-switching range or require a separate inductor.
The leakage target should come from the circuit model and tolerance analysis. A transformer manufacturer needs to know whether leakage is an unwanted parasitic, a functional energy-storage element, or both.
Why capacitance rises
Primary-to-secondary capacitance grows with conductor overlap area and decreases with insulation thickness. Interleaving creates more facing surfaces and can place high dv/dt conductors closer to the isolated output. Displacement current then crosses the isolation barrier and returns through chassis, heatsinks, cables, load capacitance, or measurement equipment.
This current can worsen conducted and radiated EMI, disturb sensors, increase bearing current, or create touch-current concerns. Capacitance is also voltage dependent through geometry and dielectric behavior, and the effective common-mode source depends on winding polarity and switching-node placement.
AC copper loss may improve or worsen
Interleaving can reduce proximity field in some layers by improving ampere-turn balance. It can also increase the number of layers, lead length, parallel interfaces, and termination complexity. A split winding may force smaller conductors or longer mean turn length.
Dowell-style analysis, finite-element analysis, or validated field models can compare layer current distribution. The calculation should use the actual harmonic spectrum, conductor geometry, porosity or fill, and temperature. A winding with low DC resistance may still run hot from proximity loss.
Insulation and partial discharge
Every new primary-secondary interface requires the specified solid insulation, creepage, margin, and process control. More interfaces can increase opportunities for tape wrinkles, particles, voids, and misalignment. Triple-insulated wire or molded barriers can simplify some arrangements, but terminations and layer transitions still need review.
For high dv/dt and high-altitude applications, partial discharge inception may control the design. A thin dielectric chosen only to lower leakage can create excessive local electric field or insufficient production margin.
Electrostatic shields
A single-ended conductive shield can redirect capacitive current to a chosen reference. It adds winding-window usage and capacitance to that reference, and it can increase leakage. The shield must remain open around the magnetic path so it does not become a shorted turn. Lead inductance can reduce shield effectiveness at high frequency.
Production tolerance
Tape thickness, winding position, tension, margin width, foil alignment, bobbin dimensions, and lead routing affect both leakage and capacitance. BaoHui Tech specifies test frequency, terminal connections, shorted windings, and fixture method for leakage measurements. Capacitance should likewise state the exact winding groups and core or shield connections.
Design workflow
- Set acceptable leakage range from switching, resonant, and clamp behavior.
- Set primary-secondary capacitance or common-mode current target from the system EMI model.
- Compare winding stacks for AC copper loss, insulation, and thermal path.
- Prototype at least the leading alternatives with production-like materials.
- Measure leakage, capacitance, impedance, loss, temperature, switching stress, and emissions.
- Confirm tolerance through representative production samples and environmental testing.
Frequently asked questions
Is more interleaving always better?
No. It generally lowers leakage but can raise capacitance, insulation complexity, cost, and common-mode EMI.
What should an OEM send to BaoHui Tech?
Provide topology, winding waveforms, switching frequency, current spectrum, target leakage and tolerance, capacitance limit, isolation standard, altitude, thermal limits, package, and EMI constraints.