A 13.56-MHz Primary Driver with Fractional Capacitance Auto-Tuning Loop for Wireless-Powered Implantable Medical Devices

A primary driver with a fractional capacitance (FC) auto-tuning loop (ATL) for wireless-powered biomedical devices is presented. The proposed ATL maintains the resonance of the primary LC tank against varying inductance over a wide range in real time. The analog ATL has a wide loop bandwidth and achieves phase locking within six cycles. The fractional capacitor is realized with a switch-controlled capacitor. By varying the turn-on time of the switch, the effective capacitance is changed, which tunes the resonant frequency of the primary LC tank. The primary driver is a full-bridge Class-D power amplifier (PA). High-side NMOS power switches are driven by bootstrap circuits. An adaptive offset controller is proposed to compensate for the delays of comparators to enhance tuning accuracy. The chip is fabricated using a 0.18-μm bipolar-CMOS-DMOS (BCD) process, and the active area is 0.66 mm². The system operates at 13.56 MHz and maintains both zero-voltage switching (ZVS) and zero-current switching (ZCS) in a steady state. The maximum PA output power is 200 mW, the measured tuning range is 31.5%, and the tuning error is 1.89 ns.