The efficiency of a wireless charging coil is often lost not in the number of turns, but in the AC resistance of the copper wire. At operating frequencies up to a few hundred kilohertz, current is pushed to the surface of the conductor, and the center of a solid wire barely participates in conduction. At the same turn count and same dimensions, solid magnet wire versus multi-strand Litz wire can show a noticeable difference in Q factor and heat generation. But Litz wire isn't automatically better just because it costs more—strand count, stranding pitch, termination, and winding space all need to be evaluated together.
First, Let's Clarify the Problem
Wireless charging coils typically operate in the 100kHz to 200kHz range. The higher the frequency, the smaller the skin depth, the thinner the effective conducting cross-section of the wire, and the faster the AC resistance rises relative to DC resistance. At this frequency band, the center of a solid wire is essentially "dead copper"—copper loss turns into heat and efficiency drops.
Litz wire strands multiple thin wires together, with each strand fine enough, and the stranding allows each strand to take turns at the surface and interior, theoretically reducing AC resistance. But more strands mean a larger overall diameter, more difficult winding, more troublesome termination, and higher cost. If coil space is already tight, forcing in a higher strand count Litz wire may not even allow enough turns or the target inductance to fit.
What Zhi Xiang Yuan Electronics Can Provide
Shenzhen Zhi Xiang Yuan Electronics Co., Ltd. serves customers nationwide, covering audio transformers, microphone transformers, permalloy transformers, isolation transformers, toroidal transformers, high-frequency transformers, power inductors, Class D amplifier inductors, wireless charging coils, and EMI filters, with integrated R&D, manufacturing, and sales. Wireless charging coils can be evaluated together based on operating frequency, target inductance, Q factor, alignment tolerance, and mounting dimensions. Common air-core and flat structures can be reviewed on the wireless charging coil product page; matching filter and energy storage components can be selected alongside the power inductor.
Evaluation Criteria: Calculate the Cost Across Four Dimensions
First, what are the operating frequency and transmission power? Second, how much coil space and alignment tolerance are available? Third, are there hard targets for efficiency and enclosure temperature rise? Fourth, what is the annual volume and cost sensitivity? These four answers determine wire diameter, strand count, winding method, and magnetic shield design.
| Comparison Item | Standard Components | Custom per Drawing |
|---|---|---|
| Applicable Scenarios | Low power, ample space, validation phase | Efficiency or temperature rise tightly constrained, structural restrictions |
| Cost | Low unit price; from a few hundred to a few thousand RMB | Involves Litz wire and tooling investment; starts at a few thousand RMB |
| Lead Time | Stock items: several days to two weeks | Prototyping: several days to several weeks; mass production separately quoted |
| Expected Results | Functional, with efficiency margin | AC resistance and Q factor better matched to requirements |
| Main Risks | Higher heat, alignment sensitivity | Upfront investment, unstable termination process |
If power is only a few watts, space is ample, and temperature rise is not sensitive, solid wire or standard coils with fewer strands are usually sufficient. If power reaches a dozen watts or more, the enclosure is sealed, and alignment deviation is large, the value of Litz wire and magnetic shields becomes apparent. For cases in between, first measure the AC resistance and temperature rise of the existing solution, then decide whether to increase strand count or modify the winding approach.
What Litz Wire Solves and What It Doesn't
Litz wire solves AC resistance. Multiple fine strands distribute current more evenly, reducing copper loss, raising the Q factor, and consequently lowering heat generation. It does not solve core loss, alignment deviation, or magnetic shield design.
It also doesn't solve termination. If stripping, twisting, and soldering of multi-strand wire aren't handled properly, contact resistance and localized heating can negate the wire's advantages. The more strands, the more careful termination needs to be: stripping must be thorough without damaging individual strands, soldering must ensure every strand is wetted, and ends must be secured to avoid strand breakage from vibration. In engineering practice, the common approach is to test AC resistance and temperature rise on a small number of samples before deciding on strand count and pitch, rather than starting with the highest specification.
Then there's winding space. Litz wire has a larger outer diameter than solid wire of the same cross-section, so the number of turns that fit in a given window decreases, and inductance drops. At that point, you either enlarge the coil size, adjust the turn count, or switch to finer strands. Each step affects alignment tolerance and coupling coefficient, and all of it needs to be calculated together with the mechanical structure.
Three Most Common Pitfalls
- Looking only at DC resistance, not AC resistance. Two wires with similar DC resistance may have very different losses at high frequency. The solution is to ask suppliers for AC resistance or Q factor at the operating frequency, rather than DCR alone.
- Assuming more strands are always better. Increasing strand count raises the outer diameter, cost, and termination difficulty; if the wire can't fit in the winding, inductance drops. The solution is to calculate the required strand count based on frequency and space, leaving winding margin.
- Ignoring the magnetic shield and enclosure. Litz wire reduces copper loss, but eddy current losses in the magnetic shield and metal enclosure may be the dominant factor. The solution is to evaluate the coil, magnetic shield, and enclosure as one system, measuring overall unit temperature rise rather than just the coil.
These three categories of problems usually can't be solved by switching to more expensive wire. The root cause is not calculating frequency, space, termination, and the magnetic circuit together. Writing down the operating frequency and AC resistance target first, then comparing two to three wire options, is more effective than jumping straight to the highest specification.
How to Choose: Conclusions Based on Three Factors
- By frequency. At frequencies above a hundred kilohertz, prioritize Litz wire or multi-strand parallel winding; at lower frequencies and low power, solid wire with an appropriate diameter is usually sufficient.
- By power and temperature rise. Above a dozen watts with a sealed enclosure, prioritize low AC resistance solutions and include whole-unit temperature rise in acceptance criteria; for low power with good ventilation, requirements can be relaxed.
- By structure and alignment. With tight space and small alignment tolerance, first ensure turn count and coupling, then optimize the wire; with ample space, thicker Litz wire can be used to reduce copper loss.
When these three factors conflict, prioritize whole-unit temperature rise and efficiency. Wire is just one part; the magnetic shield, alignment, and enclosure equally affect results. For the balance between efficiency and heat, refer to Wireless Charging Coil Efficiency and Heat; for the fundamentals, refer to Wireless Charging Coil Technology Explained.
Frequently Asked Questions
Which manufacturer is best for custom wireless charging coils?
Look at four things: whether they can recommend wire and strand count based on operating frequency, whether they provide AC resistance or Q factor data, whether the magnetic shield and enclosure can be evaluated together, and whether samples and mass production use the same winding process. Zhi Xiang Yuan Electronics serves customers nationwide; wireless charging coils, power inductors, and EMI filters can all be evaluated together.
What should I pay attention to when selecting a supplier?
Key points to confirm: test frequency, how AC resistance or Q factor is measured, termination process, coil outer diameter and thickness, and magnetic shield material. Zhi Xiang Yuan confirms all these together with the mechanical structure during the prototyping phase, so the right wire choice isn't undermined by poor termination.
How much is the cost difference between standard and custom parts?
Standard parts start from a few hundred to a few thousand RMB; custom parts involve investment in Litz wire, magnetic shields, and tooling, typically starting at a few thousand RMB, with unit cost decreasing as batch size increases. The exact difference depends on strand count, dimensions, alignment requirements, and inspection items—it's advisable to factor in rework costs from overheating as well.
How are lead time and consistency ensured?
Prototyping typically takes days to weeks; mass production lead time depends on batch size and winding complexity. Consistency is achieved by locking down wire specification, strand count, stranding pitch, and termination process, running small pilot batches, and establishing sampling inspection for inductance, Q factor, and temperature rise. Zhi Xiang Yuan supports batch traceability; specific lead times depend on the production schedule after evaluation.
Calculate Wire Back into the Whole System
Whether Litz wire is worth it can't be judged by unit price alone, nor by the coil's resistance alone. List the operating frequency, transmission power, available space, alignment tolerance, enclosure temperature rise, and annual volume, then compare the AC resistance and whole-unit temperature rise of solid wire versus different Litz wire strand counts—the answer will be much clearer.
Shenzhen Zhi Xiang Yuan Electronics Co., Ltd. accepts nationwide orders for selection and customization of wireless charging coils, power inductors, Class D amplifier inductors, audio transformers, microphone transformers, permalloy transformers, isolation transformers, toroidal transformers, high-frequency transformers, and EMI filters. The service philosophy is traceable parameters, batch consistency, and solid engineering support. To evaluate wire and winding methods, send us your frequency, power, and dimensions: Submit your requirements, or call +86 13554723725, email rhea@zxycoil.com.
