Providing high power levels of several watts, achieving high-efficiency in-vivo receivers, and reaching implantable size simultaneously within the specific absorption rate (SAR) and heat dissipation limits are challenging for wireless power transfer (WPT) in implantable medical devices (IMDs). To overcome these challenges, waveforms and receivers together with operating frequencies should be optimized. In this paper, we propose a solution that optimizes WPT waveforms with different peak-to-average power ratios for different loads, controlled by duty cycles, quality factors, and phase shifts between the transmitter and receiver. With a diameter of 65 mm, three-turn, four-layer PCB coils operating at 6.78 MHz, and a coupling coefficient of 0.255, the optimized system can achieve a maximum power of 5.9 W at a load of 20 Ω, and 6.4 W at a load of 25 Ω under SAR and heat dissipation limits, reaching the end-to-end efficiency of 76.4% and 77.4%, respectively. Compared to conventional operating conditions, the optimized system increases the power level by 19.6% and 7.0% at load resistances of 20 Ω and 25 Ω, respectively.
