Close the Loop Between Wireless Charger Receiver and Transmitter Without Digital Controllers

Wireless charging is increasingly utilized for small size wearable devices, as it eliminates the need for cables or any exposed connectors on the device. For applications with charge current less than 10 mA, closed-loop control between the wireless charger receiver and the transmitter is not required as the power dissipation is low. However, for higher charge current, it is essential for the transmitter to actively adjust its output power based on the demands of its receiver and coupling coefficient between the two sides. Otherwise, the receiver may have to dissipate the extra power in the form of heat, impacting the user experience and posing a threat to the health of the battery. Digital communication from the receiver to the transmitter is usually used to close this loop, but digital control adds complexity in overall design and increases the size of the application.

This article introduces a method for closing the loop between the receiver and the transmitter without increasing the number of components (and precious overall footprint) on the receiver board. A closed-loop, controlled wireless charger prototype is built using an LTC4125 AutoResonant™ transmitter and an LTC4124 wireless Li-Ion charger receiver to demonstrate this concept.