Blog first published and contributed to Planet Analog
Recent innovations have enabled the creation of robust and reliable wireless power systems that can be tailored for use in a wide variety of industrial and consumer settings. There are two key design considerations when building such a system. One is frequency of operation, which we’ve explored before and will again more in-depth in our next piece. The other is coil geometry.
Coil geometry refers to the design of the transmitter coil (or transmitter coils) that create the electromagnetic field for the transfer of power to the receiver. There are two proven and reliable geometries that are available on the market today — coil array and perimeter coil. Each has features that make it well-suited to certain situations.
A coil array is as it sounds — a collection of small coils placed next to each other in a grid fashion across a flat surface. (Often some sort of charging pad.) The key benefit of a coil array system is that each coil can be individually turned on or off by a special detector circuit. When nothing is near a transmitter, all the coils are off. Placing a phone or other device on or near the transmitter creates a slight magnetic disturbance. That then triggers the coils to scan, determine that a valid device is present (not simply a foreign object like keys or coins), and activate the coil beneath the device.
A perimeter coil is made up of a single large transmitter coil that fills a large area with magnetic current or flux when turned on. Perimeter coils fill the air with flux, enabling 3D charging at very low power levels. However, it is less efficient than a coil array. It’s also not as effective for charging multiple devices simultaneously, or charging at high power levels (>2W).
There is generally a trade-off between coil size, performance, and cost. Coil size and cost are inversely related — smaller coils give a better performance, but at a higher cost. This means that a perimeter coil geometry costs less than a coil array for a given charging area.
Beyond cost, the selection of which geometry to use is based on a variety of considerations, including safety, Z-height, efficiency, and power level.
Safety & EMC interference
The key consideration from a safety perspective is the amount of current or flux that’s released or lost during power transfer. Radio frequency (RF) emissions can be dangerous to human health in excessive quantities, while electromagnetic compatibility (EMC) emissions can interfere with other devices. A coil array design allows individual coils to be turned on, which ensures that the receiver coil — the coil on the device being wirelessly charged — almost always covers most of the transmitter coil. The result is that minimal flux is leaked out. Thus it’s easier to achieve safety compliance as power is scaled up.
It is more difficult to control the direction of the magnetic flux with a perimeter coil. Since it is one continuous coil, flux is not contained to the receiver. Safety testing for EMC compliance in particular becomes difficult with restrictions for the amount of power output that is allowable.
Z-height refers to the distance of power transfer between the transmitter and receiver coils, measured in vertical height. It is particularly important for the integration of wireless power systems into infrastructure where Z-height enables charging through surfaces — think table tops, office desks, or car dashboards.
Both designs enable Z-height charging, but to slightly different degrees. A coil array presents some difficulty — making coils larger to enable greater distance of power transfer can introduce dead spots. (Think of coils like pixels of a TV screen: The more you have, the greater the resolution, and the finer the level of control.) A perimeter coil system achieves Z-height more easily with increased flux — but again, the safety considerations can restrict the amount of output power.
Efficiency equates to the percentage of power lost from the power source to the battery of the device. Higher efficiency means faster charging, responsible use of resources, and overall lower cost (lower power transmitter for a given receiver power requirement).
By energizing specific coils, a coil array provides a better coupling coefficient and thus efficiency. Selective energization of localized coils does not vary significantly based on the position of the receiver on the transmitter. Selective energization also prevents foreign objects — even metal in the receiver device — from getting hot.
A perimeter coil has a lower coupling coefficient that equates to higher power loss — more power is needed to ensure enough is sent to the receiver. Imagine a widespread magnetic field sending out power over the extent of the transmitter charging area. Without selective energization, foreign metal objects may get hot, further reducing efficiency.
Large variations in the induced field depend on the receiver position on the transmitter, which may also result in uneven efficiency across the transmitter, although newer perimeter-coil designs have improved flux-density somewhat. Overall system efficiency is improved if several receivers are placed in the charging area and share the charging energy. In this case, sophisticated communication and control techniques are required to allow the transmitter to recognize the unique receiver requirements and eliminate crosstalk.
One of the key considerations from a design perspective is ensuring that the charging speed of a wireless power system is better than, or at least equivalent to, that of a wired charger. This means different things for array and perimeter set-ups.
Perimeter coil systems are challenged to stay within ICNIRP (International Commission on Non-Ionizing Radiation Protection) limits as power is increased beyond 40W (although this shouldn’t be a factor when powering one or two cell phones). In terms of scalability, a larger pad area will act to lower the efficiency further, thus potentially reducing the power capacity in the center of the transmitter.
Coil-array type systems, because of their higher coupling coefficients, are not as power-constrained. It is easier to scale the power up (beyond 2k watts) without approaching ICNIRP EMI or RF safety limits. This also enables scalability of the transmitter to cover multiple devices of different power requirements and scalability of charging area without sacrificing power transfer efficiency (although at the cost of more coils).
Coil array is optimal is any environment where you want:
- High efficiency (70%+)
- Multiple simultaneous device charging (e.g. a laptop, tablet, cell phone, or wearable device)
- Compliance with safety regulations without limiting transmit output power
- The ability to provide high power levels for multiple devices
Perimeter coil is optimal in situations where:
- Power needs are lower
- Efficiency is not important, and the lower cost of the coil will not be outweighed by higher energy costs
- Cross-talk between transmitters is avoided due to proximity
- Multi-device charging and high power levels or charging speed are not important