Wireless Charging Circuit Design for 15 W Systems
Inductive charging at fifteen watts is a mature technology with a well defined architecture, and the design effort goes into the details: the coil, the resonant network, the control loop and the thermal path, not into the principle.
How Inductive Charging Works
A transmitter drives a coil with an alternating current, which produces a magnetic field. The receiver coil intercepts part of that field, and the induced voltage is rectified and regulated to charge the battery.
The coupling is loose by design, because the two coils are separated by an air gap and by the housings of both products. That looseness is what makes the alignment and the resonant network important.
The Three Coupling Methods
Inductive coupling is the dominant method below about fifteen watts. It works over a gap of a few millimetres and tolerates a small lateral offset, and the standards for it are settled.
Magnetic resonance extends the distance to tens of millimetres at the cost of efficiency, and radio frequency harvesting is limited to very low power. The design decision is therefore usually made for the designer rather than by them.
Coil Design
The coil sets the inductance, the quality factor and the coupling. A multi strand construction reduces the losses at the operating frequency, and the number of turns is chosen with the resonant capacitor to place the resonance where the standard expects it.
Shielding on the back of the coil keeps the field out of the metal parts behind it, which would otherwise absorb energy and heat the product. The shield is a lossy ferrite rather than a conductor, and its thickness is a design variable.
The Resonant Network
The coil and a capacitor form a resonant tank. Its quality factor determines how much the current rises at resonance and how sharply the system responds to a change in coupling.
The capacitor has to carry the full resonant current, which is several times the load current, and it has to do so with a low loss and a stable value over temperature. A capacitor that drifts moves the resonance and reduces the delivered power.
The Power Stage
The transmitter drives the coil through a half bridge or a full bridge, switched at the resonant frequency. The switching devices have to carry the resonant current with a low conduction loss, which is why the on resistance matters more here than in a conventional supply.
Gallium nitride devices become attractive at the higher frequency band, because they switch with a lower loss and allow a smaller resonant network. The trade is cost and a stricter gate drive requirement.
Rectification on the Receiver
The receiver rectifies the induced alternating voltage, and a synchronous rectifier replaces the diode bridge to reduce the loss. The result is then regulated, either by a buck converter or by a linear stage, depending on the voltage range.
The rectified voltage varies with the coupling, so the regulation has to work over a wide input range. The control loop communicates with the transmitter rather than operating alone, because the transmitter has to adjust its drive to keep the receiver in range.
Communication and Control
The receiver sends its status to the transmitter by modulating the load, and the transmitter detects the change in coil current. The protocol defines the messages, including the power level requested and the error conditions.
The control loop has two jobs: deliver the power that is asked for and stop when the receiver asks it to. The second is a safety function, and the components around it should not be shared with anything that could fail silently.
Foreign Object Detection
A metal object in the field absorbs energy and heats up, which is why the standard requires a method to detect it. The detection may use a change in the quality factor, a change in the power balance between transmitter and receiver, or a separate sensing coil.
The threshold has to be set so that a legitimate receiver is not rejected. That is a calibration problem as much as a design one, and it is tested with the objects the standard specifies.
Thermal Design
The losses are distributed between the coils, the power stage and the battery. The receiver coil is usually the hottest component because it is thin and poorly cooled, and it sits against the battery it is charging.
The power has to be derated when the temperature rises. A charger that delivers full power in a laboratory at room temperature may have to reduce it in a product that is warm from use, and the thermal model has to include that case.
Efficiency and the Budget
End to end efficiency is the product of the inverter, the coupling, the receiver rectifier and the regulation stage. Each of them is around ninety percent in a good design, and the product of the four is what the user experiences as heat.
Improving the weakest stage gives the largest gain. That is usually the coupling, which is why alignment tolerance and coil geometry receive so much attention while the electronics are kept efficient but conventional.
Standards and Certification
Products are certified against the wireless power standard for the power class they operate in, together with the safety and electromagnetic compatibility requirements that apply to any mains powered product.
The certification covers the combination of transmitter and receiver behaviour, so a design that works with one phone may need re-testing with another. The test matrix is part of the project plan rather than an afterthought.
Layout and Interference
The switching stage is a source of emission, and the coil is an antenna. Both have to be laid out with short loops and a controlled return path, in the same way as any switching converter.
The receiver electronics also have to coexist with the radio in the same product, which means the emission from the switching stage has to be kept away from the antenna, and the conductors that carry the resonant current have to be dimensioned for it using the current capacity rules.
Alignment and Mechanical Design
The coils have to line up within the tolerance the design allows, which is a mechanical problem as much as an electrical one. Magnets, a shaped housing or a locating feature are used to guide the two products into position before the transfer begins.
The gap between the coils is set by the housing wall and by the thickness of the surfaces in contact. A small change in that dimension changes the coupling, which is why the mechanical drawing and the electrical budget have to be developed together.
What the Receiver Tells the Transmitter
The messages the receiver sends are a control input rather than an optional status report. The transmitter uses the requested power level, the rectified voltage and the fault flags to set its drive, and a message that is not received is treated as an error.
The protocol also defines the conditions under which the transfer ends. A receiver that reports an over temperature or a full battery expects the transmitter to reduce or stop, and the behaviour on both sides is verified as part of the certification.
Testing a Charging System
The tests cover efficiency, thermal behaviour, the alignment tolerance and the response to a foreign object. Each of them is performed over the range of gap and offset the product allows, because the worst case is usually at the extreme rather than in the centre.
Interoperability is tested with more than one receiver. A design that works with a single sample phone proves very little, and the certification path expects the behaviour to be repeatable across the products in the market.
FAQ
Why is the coil current higher than the load current? Because the resonant network circulates energy. The current in the tank is set by the quality factor and is several times the current delivered to the load.
What limits the air gap? The coupling falls quickly with distance, and below a certain coupling the system cannot deliver the required power within its thermal limit.
Why does foreign object detection matter? A metal object in the field heats up and can reach a temperature that damages the surface it rests on. Detection is a safety requirement rather than a convenience.
