This was my Master of Engineering final year project. The system transfers 75 W  across two circular coils. The air gap between the coils varies within the range of 10cm to 18cm. There is a push-pull converter in the primary side with switchable capacitors to switch in and out the capacitors from the resonant tank. To minimize the VA rating of the converter, the primary  controller measures the mistuning of the resonant tank (which happens as the result of coupling factor variations) to add or remove the required capacitance to tune back the system. A PI controller on the secondary side also controls the buck converter to keep the delivered power constant.