No longer anything to fear from wireless charging

No longer anything to fear from wireless charging

Loek Janssen - Project Engineer, Sensor Systems

By: Loek Janssen
Project Engineer, Sensor Systems

1st March 2017

Home » Technical

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.

wireless

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.

Save

50 Years in Engineering

Fifty Years in Engineering

Stewart Da'Silva - Senior Designer, Product Design

By: Stewart Da’Silva
Senior Designer, Product Design

22nd February 2017

Home » Technical

pcb_layouttoolsIn 1966 whilst serving my apprenticeship as a mechanical design draughtsman, I was assisting a senior draughtsman on a design that required a simple power supply. This product was going to be manufactured in medium volume and he suggested that maybe I would like to investigate the possibility of using a Printed Circuit Board to connect it up instead of using wire to make the connections. This was my first introduction to PCBs.

He had really set me a challenge as no-one in my company had used one before. Anyway, suffice to say that that very first layout of mine was constructed using an ink pen, rule and compass using Indian black ink on white Bristol board. To increase accuracy of the finished PCB, it was drawn 4:1 scale. As is probably obvious to the reader, mistakes whilst drawing the PCB usually necessitated starting from scratch again. The next stage was arranging for an industrial photographer to generate a 1:1 positive film from the 4:1 artwork that could be used by a printed circuit board manufacturer to fabricate the PCB.

I finished my apprenticeship in 1968 and not long afterwards started work as a mechanical designer in a ’Contract Office.’ This was essentially a design house offering design capabilities to companies that did not have the necessary skills or had to outsource projects due to a high volume of work.

taped-artwork_black-tapeIt was here that I learnt the basis of PCB design. Things had moved on, although designs were still only single or double sided. Instead of ink on Bristol board, the initial design was drawn, again at a scale of either 2:1 or 4:1, on a stable semi-transparent plastic film that was placed over a similar transparent film with a 0.1inch matrix printed on it that was fixed to an A0 drawing board. This grid was used as a guide for the PCB layout.

If the PCB design was double sided, the usual convention used was blue pencil for the component side and red pencil for the solder side. Once completed and checked this pencil layout was flipped over and secured over another grid that was in turn attached to the surface of an A0 size light box. A translucent film was positioned over.

lightbox1Using pre-cut adhesive backed-tapes and pads of various sizes and following the red colour of the layout that was under this sheet as a guide, the solder side of the PCB took form as the designer built up the artwork. When the solder side artwork was complete it was removed from the light box together with the pencil layout. The artwork was flipped over and secured once again to the light box, another plastic sheet was placed over this and again, using the pre-cut pads, the designer aligned these with the pads on the completed solder side artwork. Once all the pads were positioned, the solder side artwork was removed and this ‘pads only’ component side was again placed over the now turned over pencil layout and the blue colour followed to tape up the component side. The two sides of the finished and checked artworks were then sent to an industrial photographer who generated a 1:1 artwork from the originals.

red-blue-artworkThe next step that the industry took was to use only one piece of stable plastic sheet instead of two. The pre-cut black pads were still used but instead of black tapes, transparent blue and red tapes were used and were placed on opposite sides of the sheet. The industrial photographer would then attach filters such that only the red or the blue traces appeared as black when he created the 1:1 artworks. This may seem a small step but it did mean that alignment of both sides of the PCB artworks was guaranteed as exactly the same pads were used.

Read part 2 of Stuart’s blog here.

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save

Save