Wireless Power Transfer Lives and Dies by the Coil
In a wireless charging system the coil pair is not a passive accessory — it is the transformer. Coupling coefficient, AC resistance, self-resonance and thermal behaviour are all set by the winding and the ferrite behind it. Getting the electronics right and the magnetics wrong produces a system that works at 15 W and overheats at 60 W, or one that couples well at 1 mm alignment and fails at 5 mm.
This guide covers coil design for inductive (Qi-class) and resonant wireless power transfer, from strand selection to ferrite shielding and thermal management.
The Magnetics of a WPT Link
A WPT link is loosely coupled: k is typically 0.3–0.7 for a well-designed pad pair and falls quickly with lateral misalignment and gap. The transferred power scales with k and with the quality factor of both coils:
P ∝ k · √(Q1 · Q2)
That single relationship explains most WPT design decisions. If you cannot raise k (mechanical constraints, alignment tolerance), you must raise Q — which means lowering AC resistance and core loss at the operating frequency.
Winding: Where the Losses Are
Skin and proximity effect
At 100–250 kHz, the skin depth in copper is roughly 0.2–0.13 mm, and in a multi-turn coil the proximity effect dominates far beyond the skin effect. A solid 1.0 mm wire at 200 kHz behaves like a resistor several times larger than its DC value.
- Use Litz wire with strands of 0.05–0.08 mm for frequencies up to ~200 kHz; 0.1 mm strands are acceptable up to ~100 kHz.
- Keep the number of layers low. Two layers of Litz can cost 30–50 % more AC resistance than one layer of the same total copper, because the field between layers drives proximity loss.
- For pads above 200 kHz, consider printed spiral coils with wide traces: the loss is dominated by the copper thickness, and 2 oz copper is often better than thin Litz at those frequencies.
Geometry
| Parameter | Effect | Practical rule |
|---|---|---|
| Outer diameter | Sets coupling at a given gap | Pad diameter ≈ 3–4× the intended gap |
| Inner diameter | Little contribution to coupling, but adds resistance | Keep the inner hole as small as winding allows |
| Turn spacing | Too close → proximity loss; too wide → larger area, less coupling per turn | Spacing roughly equal to wire diameter |
| Number of turns | Raises L and voltage, raises resistance | Choose turns to hit the target L for resonance, then optimise strand count |
| Self-resonance | Inter-turn capacitance limits usable frequency | Keep SRF at least 3× the operating frequency |
Ferrite Shielding: Function, Not Decoration
The ferrite plate behind the coil does three jobs: it raises the coil’s self-inductance and coupling by providing a low-reluctance return path, it shields the field from the metal chassis behind it (which would otherwise absorb power and heat up), and it keeps the field out of the battery and electronics.
- Mn-Zn power ferrite tiles are the standard choice at 100–350 kHz; choose a grade with low loss at the operating frequency and high Bsat at 100 °C.
- Nanocrystalline ribbon offers higher permeability and can be thinner, but is conductive and must be laminated or insulated to prevent eddy-current heating.
- Tile thickness and gaps matter. A continuous plate is better than a broken one; small gaps between tiles act as air gaps and reduce the effective shielding. Use a compliant adhesive layer and keep tile gaps below 0.5 mm.
- Add a conductive shield (aluminium foil or copper) behind the ferrite only if you need to stop field leakage into the housing — and keep it far enough away that it does not absorb the working field.
Loss Budget and Thermal Design
A 15 W Qi coil pair typically dissipates 0.3–0.8 W; a 60 W pad can dissipate 3–6 W. Because the coil is buried under the housing and behind the ferrite, heat has nowhere to go. Design for a coil temperature rise of no more than 30–40 K above ambient, and verify with a thermal camera at maximum power and worst-case alignment.
Where the heat actually goes, typically:
| Element | Share of total loss |
|---|---|
| Coil copper (AC resistance) | 40–60 % |
| Ferrite core loss | 20–35 % |
| Resonant capacitors and semiconductors | 15–25 % |
| Nearby metal (eddy currents) | Up to 20 % if shielding is poor |
Alignment, Foreign Object Detection and Safety
- Tolerance: lateral misalignment of 20–30 % of the pad diameter usually keeps k above 0.4; beyond that, efficiency and thermal margin degrade quickly. Design the coil for the worst-case offset, not the nominal centre.
- Foreign object detection relies on a Q-factor or power-loss measurement; the coil design must keep the no-load Q stable over temperature, or the FOD threshold will drift.
- Field exposure: verify with an EMF probe at the specified distance. Ferrite shielding reduces leakage but does not eliminate it.
Ferrite and Coil Assemblies for WPT
Shaanxi SiRui Technology supplies Mn-Zn power ferrite tiles and nanocrystalline ribbon cores for wireless charging pads, with loss curves measured at 100–350 kHz and at operating temperature, plus custom Litz-wound coils matched to your target inductance and Q. If you are moving from a 15 W to a 60 W platform, the coil and ferrite are usually the first parts that have to be redesigned — we can quote both together, with the coupling and loss data your system simulation needs.
Related reading: Litz wire design · Litz wire vs round wire · Nanocrystalline core guide
About Shaanxi SiRui Technology Co., Ltd.
Mn-Zn ferrite tiles, nanocrystalline cores, Litz-wound coils and magnetic assemblies for wireless power transfer and EV charging.
Siri Yuan, Sales Manager · Email: sales@sirui-tech.com · Web: www.sirui-tech.com
No. 2555, Chanba Avenue, Chanba Ecological Zone, Xi’an, Shaanxi Province