Wireless charging for cellphones or e-book readers is appearing more regularly. We've all seen articles showing various methods to cut the annoying cord.
There are various mats and coils available for the task. These devices still have a ways to go. They need to increase their power capability, improve their efficiency, and, of course, lower their costs. But a paper presented at the IEEE International Solid-State Circuits Conference in February opens a path to improved efficiency.
The paper, “A Resonant Regulating Rectifier (3R) Operating at 6.78 MHz for a 6W Wireless Charger With 86% Efficiency” (registration required), describes a prototype resonant rectifier architecture that comes in two versions. Both architectures use a transmitting coil and a receiving coil. The first uses simple inductive coupling, so it functions as a broadband untuned transformer. The second version tunes both the transmitting and receiving coils with series connected capacitors. The resonant frequency with this version is either 6.78MHz or 13.56MHz.
The inductive version can provide higher efficiency as long as you keep the two coils close and properly aligned. The resonant version is more forgiving in alignment. Not surprisingly, it has lower efficiency. The obvious goal here is to increase efficiency and power capability.
So far, the circuitry seems like nothing special, and it certainly doesn't seem like a candidate for integration of analog functionality, but it is. On the receiving side, you will need voltage regulation circuitry. It could be an LDO regulator, but that wastes power. It could be a DC-to-DC switcher (a buck regulator), which, though quite power efficient, will need another inductor.
The paper's authors (Jun-Han Choi, Sung-Ku Yeo, Chang-Byong Park, Seho Park, Jeong-Seok Lee, and Gyu-Hyeong Cho) have devised a power-supply topology that is a switcher but needs no additional inductor. In fact, they neatly blend the needed functionality by combining the resonant tank circuit, the bridge rectifier, filter capacitors, and multiple power FETs to make a step-down regulator — a wonderful example of integrated analog functionality.
The FETs' on and off switching times are carefully controlled via circuitry that is similar to the control loop seen in most switchers.
As you can see, there is additional circuitry to monitor the applied AC waveform and to provide a feedback path to the transmitting coil circuitry. This drawing shows that as if it were a three-bit hard-wired connection. There are versions of this design where a data path is provided as a backchannel through the two coils. This allows transmitted power to be adjusted as needed.
The image below shows details for the circuit that monitors the received AC voltage on the left and the circuitry to generate a boost bias voltage for the FETs' gates on the right.
At POUT of 3.4W, the authors show an efficiency of 86 percent. For load regulation, VOUT shows a 3.3 percent variation with load current changing from 70mA to 700mA. Here's a comparison with some other wireless power supply versions.
Here's a look at the layout for the IC. It's fabed in a 0.35μm BCD (bipolar CMOS DMOS) process. The chip size is 5.52mm2 .
Are you working on any power projects like this? What do you think of the architecture? Any applications come to mind beyond wireless charging?