Introduction
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And he waved his magic wand…
Transferring power wirelessly can seem like magic. You have no doubt come across devices that use this technology. They now sell products to charge your phone wirelessly, just lay it on top and away you go. I have a toothbrush that does not plug in but sits on a little stand, see picture.
There are several reasons why you would want your product to have this technology. The most obvious is convenience. If I can just lay my cell phone on a pad and it is charged and ready to go, it is very convenient and there are no connectors to wear out. In the case of the toothbrush, it is a safety issue. You don’t want to have some touching electrical devices in a bathroom if you don’t have too.
How much does it cost?
I come across this question a lot in my line of business. If you ask me to design you a wireless transfer station I will likely quite $10-20,000. But it is just an air-core transformer, right? Well, no, not really. Production engineers will realize right away that just because you can purchase a device for $20 does not mean you can design one from scratch for that amount. A simple COTS wireless device spec’ed for 5V and 1A can cost $500-1000. That is usually a none starter for a commercial product. Others will say you can buy one for a cell phone for about $20. Yes, and it will charge your cell phone fine, but may or may not work for your project. If you are a hacker, then by all means go for it. But if you are a serious product developer, you want to design the device the right way, specified for your product.
Wireless Power Consortium
In 2008 the Wireless Power Consortium (WPC) was established to be of benefit to the designers and consumers. The WPC defines wireless transfer of 0 to 5W as low power. They also specify some characteristics of how a wireless device should be constructed. For example, the distance between a receiver coil and a transmitter coil (more about this below) should be only 5mm. Many coils are flat, those in your cell phone for example. Others have a ferrite core that is shared by both coils, as in the wireless electric toothbrush.
There are two coils involved and the one that is connected to the wall is called the transmitter coil. The one in the target device, connected to the battery ofttimes, is the receiver coil. I know you are still saying, yes but you have just described a transformer. That is true. But a rather complicated one.
Hello, is it me your looking for?
Low power is the name of the game, even when you are connected to a coal fired power plant. So, we don’t want to have the transmitter coil running and wasting power 24/7. To accomplish this, it goes to sleep and every once in a while, it wakes up, sends out a test pulse to see of the receiver coil is in place. If not, it goes back to sleep.
I’ve got 0.5, do I hear 0.6?
Once the transmitter establishes that there is a receiver in range, it begins negotiations. It establishes communications with the receiving station and they negotiate voltage and current limits. This is similar to the way power transfer is negotiated for USB devices. Devices that transfer less than 5W are considered low power, see above. Those that transfer more will have different regulations. Meeting these regulations is not just a good idea. If you want to get your product certified for sale to consumers it is essential.

Figure 1 Typical wireless-power functional diagram.

What is doing all that communication?
It does not sound so simple anymore. There is a communication channel, a transformer driver, there must be a current sense and feedback loop with a way to control the transformer driver output.
You don’t need to reinvent the wheel. There are resources that will help you with the design process. There are chips that will negotiate the communication and control the current according to the protocol. That does not mean there is not still a lot of design work to be done.
There is always a tradeoff in making a new product. You can spend very little on design by purchasing COTS products. The downside to this is that your manufacturing costs will not be competitive. Yes, there are similar products to yours that sell for a fraction of the cost. So, you go down the road of designing your own product rather than purchase the COTS product. That will certainly decrease the manufacturing cost. The downside is now you have to develop the circuity and hardware, in this case the coils and fixtures. That drives up development cost. If you are going to make 100 of these a year it might be better to pay more for the COTS products. But if you are going to make 50,000 per year then everything you can do to drive down production cost will increase your profit margin and it is worth spending extra money up front on development costs.
Final thoughts
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