Wireless power transfer is defined as a transmission of electrical energy from a primary (Tx) to a secondary side (Rx). The two sides take advantage of mutually coupled coils. This power transfer relies on magnetic induction between the primary and secondary coils. The two coils simply form two halves of a transformer and can be modeled as a loosely coupled transformer.
The primary side has a flat surface that allows the secondary coil to be placed on top of it. When a Tx and Rx are aligned and placed near one another, they form a mutually coupled-inductor relationship, or a simple transformer with air core. Appropriate shielding between the bottom side of the Tx coil and top side of the Rx coil is required. The shielding material on both sides serves as a magnetic flux short. This allows the magnetic field lines (flux) to be contained between the two coils and allows efficient power transfer. The direction of power is always going into the receiver that typically consists of portable devices.
Wireless power DC/DC system efficiency
As we just described, a wireless power system consists mainly of a primary and secondary side coupled with coils. System efficiency is defined as the ratio of final power transferred to the load with the DC input power applied to the transmitter (Equation 1):
Figure 1: Wireless power transfer system consisting of a primary side, Transmitter (Tx), and a secondary side, Receiver (Rx).
Figure 2: Power receiver block diagram -- Power losses within the receiver.
Figure 1 shows a schematic diagram of a wireless power transfer system, consisting of a wireless power transmitter coupled to a wireless power receiver.
Improving system efficiency requires reducing losses within the path from the input to output. This depends on how the specifics of the transmitter module (primary coil, AC/DC converter, driver), and the receiver (rectifier, voltage conditioner, secondary coil, battery charger), are combined and aligned during power transfer. Since the two modules are two separates pieces, the efficiency of each module is independent from the other. The focus of this blog is the Rx side.
Improving system efficiency requires reducing the losses within the path from the input to the output. This depends on the combination of specifics of the transmitter module (primary coil, AC/DC converter, driver), and the receiver (rectifier, voltage conditioner, secondary coil, battery charger), and their alignment during power transfer. The focus of our discussion in this article is on the receiver side. Detailed information about the receiver sub-circuitry is provided next.
Wireless power receiver
A wireless Rx typically is part of a portable device, such as a cellphone. The Rx itself consists of multiple key circuits as shown in figure 2. The secondary coil is responsible for receiving the transferred power from the transmitter as magnetic flux. The rectification circuit is used to convert the received AC-to-DC power. A voltage conditioning circuit buffers the unregulated received DC power to a regulated and clean DC output power ready for down system use. The communication circuit is responsible for all communications with the transmitter and unidirectional from receiver to the transmitter.
Taking Chuck's question one step further, besides the additional applications, what are the realistic distances that can be expected in the near future before the costs get prohbitive?
At least this article did define the expression for the system efficiency of the charging system, even if it did not give any numbers. I am sure that if the actual efficiency were stated that the whole concept would lose a whole lot of support. Ff course, it is a "really neat gimmick", there is no question about that, but the amount of wasted power plus the large magnetic fields should really be considered as part of the package. But thye promotors are quite aware of those numbers, which is why they are not typically displayed.
I agree with eafpres: It is indeed an important article for our readership. Aside from cell phones, Tahar, can you tell us some of the biggest applications for this technology going forward? Coincidently, Design News has an article coming up involving wireless chaging of a piece of sports equipment, which also involves TI.
I'm with Eafpres on the magnetic resonance coupling. Distance is not as big of an issue. TI should get on it. (see "witricity" for more)
When I first used wireless charging with the old Palm webOS phones, I loved it. I know that the charging base (Tx) is still tether to the wall, and you place the device (Rx) on it and so the device is still tetherd, I liked the idea of setting it down freely. It is silly to use a transformer coupler like this for using a device in bed/leasure. I would still use a plug. Any other time...it's the way to go.
My HP (webOS) Touchpad wirelessly charges to my left, right now.
If we could just focus low frequency energy as we do microwaves we could increase efficiency. So the answer may be in higher frequencies, but that also causes problems. And a dish large enough to handle 60 Hz would not fit in most cities, and thus be a bit impractical. But, it may be inefficient the way it is now, and surely can be improved, but hey, it works! And it sure is convenient!.
Hi Tahar--nice article, it seems to me that TI is really leading the world in integrated solutions for all kinds of front ends, energy harvesting, low power, and wireless applications, to name a few.
I was interested in the last part of your note; I have the impression the integrated IC you note saves board space, design time, and cost, but probably does not improve electrical efficiency. Can you clarify that?
Also, I have heard a little bit about various competing wireless charging standards; in particular I heard about a magnetic resonance technique, which I suppose is different in the mechanism of power transfer. Is TI working on any solutions for this or other wireless charging approaches?
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