Minimizing MOSFET power losses in near-field electromagnetic energy-harnessing ICs

The value of distributed sensors, embedded biomedical implants, and other wireless microsystems is the information they collect, process, and transmit over time. Unfortunately, powering such tiny devices for extended periods is a major challenge because miniaturized batteries cannot store sufficient energy and connecting wires to recharge the batteries demands considerable space overhead. Electromagnetically coupling energy to recharge these devices wirelessly is possible, but practical only if power losses are low enough for sufficient energy to reach the batteries. Because small inductors capture little energy, minimizing the power that switches dissipate to energize and de-energize an inductor is critical. This paper presents how transmitted power changes with distance and, as a result, with inductive coupling factor k_C, and shows how to use that information to minimize the power lost in the interconnecting MOSFETs. This way, a 0.18-μm near-field electromagnetic energy-harnessing IC loses on average (in simulations) 3 μW across k_C´s ranging from 0.01 to 0.1, which roughly represents 3.9% of the total power transferred.