(Note: A more polished version of this posted has now been published as a PDF entitled NCK University Uses ADS to Create an Impedance Matching Method for Wireless Battery Charging)
In a previous Case Study I pointed out work that used a high Q quartz crystal resonator for wireless power transfer at 24 MHz. The load in that case was a switched-mode power converter. But what if you want to charge a battery directly? It's tricky because the resistance, reactance, and the cell voltage of a rechargeable battery all vary dramatically with its state-of-charge. What load do you optimize the matching network for when the load is dynamic? In their paper Direct current driving impedance matching method for rectenna using medical implant communication service band for wireless battery charging, H-W Cheng, Tsung-Chi Yu, and Ching-Hsing Luo of National Cheng Kung University address this problem with a simple yet effective method using Advanced Design System. They also used ADS EM-circuit co-simulation to design the printed circuit antenna.
Within a tiny 14 mm (0.55 inch) diameter disk, the wireless battery charger has to include a printed circuit antenna, matching network, two rectifier diodes, and an LC low pass filter. The received RF power was a mere 10 mW at 405 MHz. The challenge is to recharge the Li-ion cell, which had a 10mA.hour capacity, in 5 hours (a so-called "0.2 C rate"). The charging current was 2 mA but the cell voltage varies from 3.0V (empty) to 4.2V (full) and impedance varies wildly with state-of-charge.
The authors employed ADS harmonic balance to simulate the circuit and explore the design space. Using a sweeping method detailed in the referenced paper, they came up with a optimized circuit consisting of only three capacitors, one inductor, two diodes, and of course the PCB itself, which includes not only interconnect but also the antenna, designed using ADS EM-circuit co-simulation.
Figure 7 of their paper shows the finished implementation. They achieved 76% conversion efficiency of incident power to battery charging power averaged over a five hour charge cycle. Comparable prior work achieve only 50%.
Are you working on wireless power transfer? Do you face the same challenge? Or something else? Please log in and leave a comment and/or "like" this posting!
PS Here's the link to request an ADS evaluation license if you want to try it.