By: Loek Janssen
Project Engineer, Sensor Systems

1st March 2017

Recently, I have been working on a project where we really wanted wireless charging for the device. Being only in the prototype phase, it was considered more a “nice to have” than a complete requirement, with several engineers putting it into the “requires a lot of work” category. However, after doing some research, I decided that the standards (such as Qi and PMA) were developed enough, with excellent support, to give it a try.

So how does it work?
Most wireless charging is based around the idea of inductive charging, i.e. inducing a current using a magnetic field. In many ways, the idea is similar to that used in transformer, current in one coil, induces current in a second coil. However, transformers share the same magnetic core and when the two coils are separated by air the method is horrendously inefficient with most of the power wasted instead.

When the two coils are, in combination with capacitors, used to form resonant circuits, the efficiency over air increases dramatically. The range is only a few centimetres but reasonable amounts of power can be safely and easily transferred.

As only alternating signals can be used in the inductive coupling, any receiver then needs to rectify the signal, filter and provide the output to power the system or charge a local battery. A control loop usually exists in the receiver to limit the overall current drawn. Additionally, the transmitter and receiver can also communicate by gently modulating the signal, while only a small amount of data can be transferred; it is more than enough to allow basic control messages between the two devices.

Standards
Several standards now exist using this idea of inductive charging, using different frequencies, voltages and modulations for power transfer and communication. While none have yet won out, multiple mature ICs, which handle much of the heavy lifting of the receiver (and transmitter) side, happily exist for the QI, PMA and Airfuel standards. The Qi standard, as an example, uses a frequency of 100-200 kHz and ASK (amplitude shift key) modulation for communication.

Getting it into a product
While the datasheets require some maths to determine the correct component values, it is fairly straightforward, and once a suitable charging coil has been chosen, I quickly got around to prototyping the design. With the right evaluation board, I was able to get a nice receiver working over short distances, allowing me to test various coils to see which would work best through different plastic materials. I had chosen a QI receiver IC, as the QI protocol seemed very mature, and looking ahead to the future, plans were in place for extensive improvements from the recently released V1.2 standard.

Now the prototyping was done, I designed the IC, components and charging coil into our system and once the final PCB was back, tested the wireless charging system. As expected, power could be drawn through the system, with the IC internally controlling the voltage to produce a nice 5V DC signal to power to the system.

Despite not having any personal experience in the area, the wealth of information and useful components allowed me to quickly prototype and design a circuit with wireless charging. It is a seriously useful component of the system, allowing devices to be sealed and protected, while still being easy to recharge.

Regularly, people are surprised by the addition of successful wireless charging to the system but I think this is a hangover from the past; when implementing such a design was difficult, complex and required a lot of effort to get working. Now we have the addition of excellent standards and mature ICs in mass production, it is definitely time to stop being afraid of wireless charging.

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