Wireless Charging PCB: Coil Drive, Shielding and Thermal

A wireless charging board converts a DC supply into an alternating magnetic field and couples it into a coil in the device being charged. The electronics are a resonant converter with a coil as its load, and the layout has to handle the same switching currents as any power supply while also managing a magnetic field that extends beyond the board.

This article covers the blocks on a wireless charging PCB, the resonant tank and coil drive, and the shielding and thermal measures that make the design practical.

What Is on a Wireless Charging Board

The board contains a bridge that drives the coil, a resonant tank of capacitors, a controller that regulates the power and manages communication with the receiver, and the sensing circuits that detect a foreign object and confirm that a device is present.

The coil itself may be wound wire bonded to the board or a printed spiral on the board, depending on the power level. High-power designs use wound coils because the copper cross-section needed for the current is greater than a reasonable board trace can provide, while low-power designs print the coil directly.

The Resonant Tank and Coil Drive

The resonant coil drive consists of the coil and the tank capacitors, which together form a circuit driven by a bridge at a frequency near its resonance. Operating near resonance is what allows a modest drive voltage to produce a large coil current, and the circulating current in the tank can be many times the DC input current.

That circulating current is the dominant layout consideration. The loop formed by the tank capacitors and the coil has to be as small as the geometry permits, because its area determines how much field escapes and how much loss the loop resistance produces. The DC-DC layout and routing rules describe the same principle for a switching converter, and the situation here is more demanding because the current is larger and the frequency is chosen for power transfer rather than for efficiency alone.

Coil Geometry and Coupling

The coupling between the transmitter and receiver coils depends on their geometry, their spacing and their alignment. The board layout cannot change the spacing, but it does determine the coil position, the clearance around it and the amount of metal in the field.

Any conductive material inside the field absorbs energy and heats up, and it also detunes the resonant circuit. That is why the area above and below the coil has to be kept clear of copper, and why the shielding layer is placed behind the coil rather than in front of it. The trace width and current calculation determines the copper geometry for the drive path, and the coil area is defined by the coupling requirement rather than by the current.

Shielding and Field Containment

A ferrite shield behind the coil does two jobs. It increases the coupling by providing a low-reluctance path for the magnetic field, and it reduces the field that reaches the metal parts of the enclosure, where it would otherwise induce eddy currents and heat them.

The shield has to cover the coil area completely and extend slightly beyond it, and it has to be bonded so that it does not move. A shield that is too small allows stray field to reach the battery or the chassis, and one that is too thick adds height to an assembly where height is usually the critical dimension. Different shield materials trade thickness against loss, and the choice follows from the space available.

The shield also affects the electromagnetic signature of the product. A transmitter that radiates a large field outside its intended area will struggle with emissions testing, and the shield is the first line of defence. The drive waveform matters as well: a sinusoidal drive produces far less harmonic content than a square wave, which is one reason many designs use a resonant topology rather than a hard-switched bridge.

Thermal Design Under the Coil

Heat is generated in the coil, in the tank capacitors, in the bridge devices and in the shield. The coil and the shield are the parts closest to the device being charged, so their heat raises the temperature of the battery in the receiving device, which is the parameter that usually limits the charging rate.

Copper area is the primary thermal measure on the board. Wide, thick traces under the coil spread heat laterally, and thermal vias carry it to the opposite side where it can reach the enclosure. The bridge devices need their own copper areas, and the tank capacitors have to be chosen for the ripple current they carry, because a capacitor that overheats will change value and detune the circuit. Placing the tank capacitors close to the coil reduces the loop area, and spacing them apart so that each carries part of the ripple current spreads the losses across several parts rather than concentrating them in one.

Foreign Object Detection and Safety

A metal object placed on the charging surface absorbs energy and can become hot enough to be a hazard. Detecting it is a safety requirement, and it is implemented by measuring the power loss between what the transmitter delivers and what the receiver reports, or by monitoring the coil parameters for the change that a metal object produces.

The sensing circuit has to be accurate enough to distinguish a genuine foreign object from a slight misalignment of the receiving device, and that accuracy depends on the measurement chain rather than on the algorithm alone. The sense connections should be Kelvin-connected where they measure a voltage across a current-sense element, and the sense traces should be kept away from the drive loop so that the switching edges do not corrupt the measurement.

Communication and Control

The receiver communicates back to the transmitter by modulating the load it presents, which the transmitter detects as a variation in the coil current or voltage. That back-channel carries the control information that sets the power level, and it is decoded from a small amplitude modulation on a large drive signal.

Recovering that signal requires a clean measurement of the coil waveform, which means the sense path has to be filtered and referenced to a quiet ground. The emissions layout measures for switching regulators apply to the drive stage, and here they serve a second purpose: reducing the field that escapes the board also reduces the interference that reaches the communication detector.

FAQ

Why does the coil area have to be free of copper? Copper in the field absorbs energy, heats up and detunes the resonant circuit. The area in front of the coil and the layer immediately behind the shield should both be clear of conductive material.

Can the coil be printed on the board? For low power levels yes, with the coil patterned as a spiral on one or more layers. Above a few watts the required copper cross-section makes a wound coil more practical, and the board then carries the connection pads for it.

How is charging efficiency measured? By comparing the DC power delivered to the coil drive with the DC power the receiver reports at its output. The difference is the loss in the coil, the shield, the coupling and the receiver rectifier, and it is the figure that determines the temperature rise.