A practical guide from the engineers at Moetion Technologies
Wireless charging sounds like it should be simple. No wires, no connectors, just set the device down and let it charge. And for a consumer buying a phone charger off the shelf, it usually is that simple.
For engineers designing a custom wireless charging solution, the experience is very different. The underlying physics are well understood, but the engineering decisions to make it real can be surprisingly complex.
But the challenges are not insurmountable. The key is to understand what questions to ask. This guide walks through how our team at Moetion Technologies approaches custom wireless power design in 6 sequential steps.
Why Wireless Charging Is Harder Than It Looks
The core physics of wireless power transfer are well established: an alternating current in a transmitter coil generates a time-varying magnetic field, which induces a voltage across a receiver coil placed within that field. That induced voltage drives current through a load, transferring power without physical contact.
The complications start the moment you move beyond consumer charging pads into custom products. A few reasons:
- Power and distance requirements vary enormously
- The surrounding environment matters more than most engineers expect
- Coil manufacturing is a specialized discipline with a shrinking domestic knowledge base
- The IP landscape is crowded
The good news is that a structured approach to requirements, technology selection, and design tolerancing can address all of these. The teams that run into trouble are almost always the ones that jump to hardware before the requirements are clear.
Step 1: Define Power, Distance, and Geometry
Every wireless charging design starts with three foundational constraints: how much power needs to be transferred, over what distance, and within what physical envelope. From there, the physics will start to guide us in what’s not just possible but practical.
[1] Image: SciTechDaily / Aalto University
Power
Wireless charging systems can span an enormous range, from milliwatts for small IoT sensors to kilowatts for electric vehicle charging. It’s critical to define not just how much power you need to deliver, but also how much power you can afford to lose (ie how inefficient your system can be). Lost energy generates heat and requires more power at the system input.
Distance
Coupling coefficient drops quickly as the gap between coils increases. For tightly coupled inductive systems (like most Qi chargers), effective transfer happens within a few millimeters. Resonant systems can extend this to several centimeters and beyond by tuning both sides to the same frequency, but the tradeoffs in efficiency and complexity are real. Be specific about your gap: a system designed for 3mm of separation will not behave the same way at 8mm, and the difference is not trivial to design around after the fact.
Geometry
Coil size and shape are heavily constrained by the product form factor. Flat spiral coils are the most common, but products with curved surfaces, tight corners, or very thin profiles may require custom geometries. Will the coil wrap around a surface, occupy a small corner of the PCB, or sit in a dedicated pocket? These questions drive mechanical and integration constraints within the entire product, not just the charging interface.
| Our Recommendation Before evaluating any wireless charging technology, write down your power requirement (at the load and the input), the minimum and maximum gap between coils in the final product, and the available footprint for each coil. These three constraints will eliminate most options and focus the conversation on what can actually work. |
Step 2: Check Whether an Existing Standard Fits
The easiest path, when it exists, is to build on an established standard. There are several off-the-shelf options available from vendors, two are worth evaluating seriously for most applications:
Qi (Wireless Power Consortium)
Qi is the dominant standard for consumer wireless charging, backed by over 700 member companies and certified across more than 9,000 products. It covers power levels up to 25W under the current Qi 2.1 specification (with Qi 2.2 expected to push that to 25W+ for compatible devices), and it operates at a nominal frequency of around 100–205 kHz. If your product needs to charge consumer devices, or if you want interoperability with the existing Qi ecosystem, this standard deserves serious consideration.
Certification is required to legally use the Qi brand and claim compliance. The certification process involves testing at a WPC-approved lab and is not trivial for a first-time applicant. Low-power receivers (under 5W) are royalty-free for WPC members. Higher-power products are subject to RAND licensing terms.
NFC (Near Field Communication)
NFC can carry up to approximately 1W of power at very short range (typically under 20mm). It is a good fit for applications that need to transfer both data and a small amount of power simultaneously, such as smart cards, RFID tags with active components, or medical wearables or implants. However, most of these applications still require custom designs of antennas and power components and must be certified with the NFC to use the logo.
When A Standard Doesn’t Fit
Consumer standards are designed around consumer form factors and power levels. If your product needs more than 25W, requires power transfer over distances of more than a few centimeters, has an unusual coil geometry, or needs to operate in an environment with specific regulatory constraints, a custom solution is usually the right answer. The rest of this guide assumes that path.
Step 3: Identify Environmental and Interference Constraints
Wireless power does not exist in a vacuum (figuratively, in reality it does exist in a vacuum!). The materials and electronics surrounding the coils have a real impact on system performance and identifying them early prevents costly redesigns.
Conductive and Magnetic Materials
Metal enclosures, structural steel, magnetic latches, and nearby PCB ground planes all interact with the magnetic field. Conductive materials induce eddy currents that generate heat and reduce transfer efficiency. Ferromagnetic materials (iron, nickel, certain steels) can distort the field pattern in ways that are difficult to predict without simulation.
Nearby Wireless Systems
If the product already includes Bluetooth, Wi-Fi, cellular, or other wireless radios, the wireless power system can interfere with them. The primary concern is harmonic emissions from the power electronics falling into frequency bands used by communications systems. This is a regulatory issue as well as a performance issue, and it needs to be addressed in the system architecture before layout begins.
Thermal Constraints
Wireless power systems are not 100% efficient. The losses show up as heat in the coils, the switching electronics, and any conductive materials nearby. In sealed enclosures, thermally constrained products, or medical devices with skin-contact requirements, thermal management needs to be part of the initial design, not an afterthought.
| Our Recommendation Before finalizing coil placement, document every conductive or magnetic material within 1 diameter of each coil, identify all wireless radios in the product, and define any thermal limits. These inputs define the operating environment your system needs to work within. |
Step 4: Select Your Frequency and Topology
Operating frequency is one of the most consequential decisions in a wireless power design. It affects coil size, efficiency, regulatory compliance, component availability, and compatibility with other electronics in the product.
Low Frequency (< 200 kHz)
This is the range used by most Qi chargers and many industrial inductive systems. At these frequencies, coils can be physically larger, eddy current losses in conductive materials are more manageable, and the power electronics are well understood. The tradeoff is that coil size must be larger to achieve the same coupling, which can be a constraint in compact products.
| Best for: High-power transfer, tightly coupled systems, products with significant surrounding metal. |
Mid Frequency (200 kHz – 10 MHz)
Increasing frequency allows smaller coil geometries for the same power level, which is valuable in space-constrained products. The tradeoff is higher losses from skin effect in the conductors (which is why Litz wire is commonly used in this range) and greater sensitivity to component tolerances. Resonant systems operating in this range can achieve good efficiency with some spatial freedom, making them attractive for applications where precise coil alignment is difficult to guarantee. This range also includes 6.78 MHz, which is the AirFuel resonant standard’s operating frequency.
| Best for: Space-constrained designs, systems that need some tolerance to misalignment, multi-device charging surfaces. |
High Frequency (> 10 MHz)
RF-based power transfer at ISM band frequencies (13.56 MHz and above) is used primarily for NFC power delivery and specialized applications requiring very long range at very low power levels. Not a common choice for most embedded product applications.
| Best for: Niche applications: RFID power harvesting, implantable and wearable devices, and low power sensors. |
Closed-Loop Power Control
Most wireless power systems need some form of communication between the transmitter and receiver. At minimum, the receiver needs to signal its power state to the transmitter so power delivery can be adjusted to avoid overcharging or overheating. In simple systems, this can be done through back-channel modulation of the power signal itself (as Qi does). More complex systems may require a dedicated communication link, particularly if the receiver needs to report state-of-charge, temperature, or fault conditions in real time. If your product already includes a wireless radio, using it for this communication is worth evaluating before adding dedicated wireless power control hardware.
Step 5: Understand the IP Landscape
The IP landscape for wireless charging is a thicket of various claims, major breakthroughs, partnerships, and at times suspect physics. Starting in the early 2000’s and continuing through almost 2020, dozens of companies claimed to have the secret sauce of wireless charging.
Many of those companies formed the Wireless Power Consortium and pooled their patent portfolios to create a monolith of IP. Concurrently, the Power Matters Alliance (started in 2012) and the Alliance for Wireless Power (A4WP) combined their efforts to create the AirFuel Alliance in 2015 as a competing standards body. Still others have focused on developing their own IP around NFC-based and RF based charging. It’s an arena that can feel constantly in flux.
The result is a difficult-to-navigate jungle of current, expired, and abandoned IP along with trade secrets. Few can claim that their solution is not dependent some other existing IP, and those that do often charge an ongoing royalty feel that can lock you into a vendor you may not care to work with.
The good news? Much of the foundational IP developed in the 1980’s – early 2000’s is starting to expire, providing “whitespace” in the field and giving opportunities for custom solutions to leverage. The key is to work with a partner that is familiar with the patent landscape and how to find the path of least resistance. You should also avoid committing to an IP license until a vendor can prove the solution meets all of your needs, including manufacturability.
Step 6: Think About Coil Manufacturability Early
Coil design and coil manufacturing are separate disciplines, and the gap between them causes significant problems for teams without experience in both.
Some example coils developed and used by Moetion Technologies
A coil that works perfectly in simulation may be difficult or impossible to manufacture at the tolerances required, particularly for resonant systems where inductance variation directly affects tuning. Key factors include:
Geometry
Flat spiral coils are the easiest to manufacture in volume. Custom geometries, such as 3D wound coils, toroidal designs, or coils that wrap around curved surfaces, require specialized winding equipment, flexible or complex ferrite materials, and typically more lead time and cost. Knowing what is manufacturable at your target volumes before designing in a complex geometry prevents a painful redesign.
Materials
Material tradeoffs for wire and ferrite materials can be challenging to understand even for seasoned electrical engineers. Wire types can include Litz wire, stranded or solid wire assemblies, copper-clad vs aluminum, and a host of other exotic material types. Shielding materials such as sintered ferrite, flexible polymer matrix ferrites, pressed-iron-powder, and a long list of emerging materials all have applications. But because the characteristics of these materials change with frequency, temperature, and magnetic field strength, it can be difficult to predict how they perform.
Tolerance
This is where many wireless charging programs fail quietly. Highly resonant systems require tight tolerances on inductance, and component-to-component variation in production coils can be significant. A system that works perfectly with hand-selected components from a prototype run may have a 20% failure rate with production-tolerance parts. The solution is to design for tolerance from the start, not to compensate for it at end-of-line.
| Our Recommendation End-of-line calibration is a red flag. If a vendor’s solution requires individual tuning of each unit coming off the line, that is a sign the system was not designed to tolerate production variation. The cost and reliability implications of this approach tend to become apparent only when you are close to shipping. |
At Moetion Technologies, our background in coil design combined with active manufacturing relationships across the US means we can get prototype coils in hand quickly and design for real-world production tolerances from the first revision.
Final Thoughts
Wireless charging design is one of those domains where the early decisions have outsized consequences. Power level, coil geometry, frequency selection, and tolerance budgets set the trajectory of the program. Getting them right requires experience across electromagnetics, power electronics, mechanical design, and manufacturing, which is part of why teams without that background so often end up with costly late-stage surprises.
The teams that do this well share a common trait: they treat wireless power as a system problem from day one, not a component selection exercise. Interface with the surrounding environment, coil manufacturability, production tolerances, and IP exposure all need to be in the picture before the first schematic is drawn.
If you need to incorporate wireless charging into your products, or are working through a wireless charging design and want a second opinion, our team at Moetion Technologies is happy to talk. Reach out at moetiontech.com/contact.
Image Citations
[1] Image: SciTechDaily / Aalto University — “Revolutionizing Wireless Power: Scientists Achieve Breakthrough in Long-Distance Charging Efficiency” (March 11, 2025)