A 13.56 MHz reconfigurable step-up switched capacitor converter for wireless power transfer system in implantable medical devices

In this paper we present a reconfigurable, step-up switched-capacitor DC-DC converter conceived to handle without efficiency penalty the wide range and sudden input voltage variations that characterize implantable medical devices supplied by a wireless power transfer system (WPT). In particular, we target the specifications of an implantable neural stimulator for retinal prosthetics supplied by a WPT based on an inductive link. We propose an original design methodology based on a model to estimate a priori the input voltage range and thus the set of required conversion ratios, and a procedure that, taking into account the switches conduction and driving losses, allows to derive the absolute switch size that maximizes the reconfigurable SCC overall efficiency. The SCC is designed in a commercial CMOS technology using a reduced number of external components to implement the flying and load capacitors. A thorough set of post-layout simulations shows that the converter is capable of dynamic self-reconfiguration and self-regulation over the whole 1.3-3.3 V input voltage range, achieving the desired 5 V output voltage with an efficiency between 57 and 74%.