For most electrical engineers – even those familiar with the details of analog circuit design – magnetic components can still seem a bit mysterious. But the inductor is the core (pun intended) of any power-conversion circuit, and the starting point for any wireless power design. In the case of wireless charging, there are two inductors in the system: one for the transmitter (TX) and the other for the receiver (RX). The coupling factor between these coils will determine system performance, as will the specifications (inductance value, equivalent series resistance [ESR] and shielding) for each individual coil.
One key design consideration is how to optimize coil performance based on the operating power level of your application. For the low power levels used in wearable applications, product size must be very small; thus the wireless charger receiver coil will be very small as well. To maximize coupling, the transmitter coil (shown as the lower component in Figure 1) should be similar in size to the receiver coil (shown as the upper component in Figure 1).
Figure 2 gives an example of the difference in efficiency for well-aligned coils. The solid blue trace shows the overall system efficiency using a 30mm diameter TX coil, while the red dashed curve is the efficiency with a 43-mm diameter TX coil. Both examples use the same 17-mm round RX coil.
The same TX coils were used to test a second 20-mm x 10-mm oval RX coil. The efficiency results shown in Figure 3 indicate a clear benefit for the smaller TX coil.
These plots indicate that the size and shape of the RX coil have an impact, just as the relative size of the TX coil has a great influence on the resulting efficiency of a wireless power system. The round coil has a higher efficiency overall than the oval coil due to better coupling with either type of TX coil.
Traditionally, circuit designers had to start from scratch and develop custom magnetic components for their wireless power designs, or choose from a very limited set of options.
As wireless power has grown in popularity, many magnetic component suppliers have started adding wireless charging coils to their catalogs.
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