A 6.78MHz Semi-Synchronous Rectifier for Deep Biomedical Implant Applications

Inductive powering is a popular choice for powering of wireless implants, but at high implant depth the link efficiency and hence available power drop dramatically. To compensate for this the link frequency may be increased, leading to increased power consumption of the active rectifier. This work proposes a semi-synchronous rectifier which consumes from $9.1 \mu \mathrm{~W}$ down to $1.6 \mu \mathrm{~W}$ dynamic power, allowing consumption to scale with a number of resonant cycles used to store energy in the receiver resonant element, increasing efficiency and output power. The proposed circuit is implemented in TSMC 180 nm CMOS and measured, allowing operation from 20 mW transmitted power at up to 135 mm distance in phantom, using a 50 mm and 7.5 mm radius TX and RX coil respectively.