ZVS Tank Optimization for Class-D Amplifiers in High Frequency WPT Applications

Increasing the operation switching frequency of resonant wireless power transfer (WPT) systems could minimize the converter and coil size, for low and medium power applications, as for example in medical implants. Maintaining zero voltage switching (ZVS) is one of the main concerns of designing the inverter at multi-MHz switching frequencies (6.78 MHz and 13.56 MHz industrial scientific medical bands), especially when the coupling is very low. ZVS Class-D amplifiers appear as a good candidate due to their good behavior at high frequency and load-independent characteristic. However, the deadtime in high frequency will cause a big error to design the ZVS tank inductance when the GaN devices are used due to the high body diode forward voltage drop. This paper focuses on the ZVS Class-D amplifier analysis, presenting an accurate modeling of the switching node waveforms, proposing a phase correction method based on first harmonic approximation (FHA) to design the ZVS tank circuit. This model is then used to derive an optimization method for designing the ZVS network. To validate the model, an experimental prototype of a 2W battery charger for medical implant is designed and compared to the analytical solutions and simulations.