Design of DC-DC Converters for Rechargeable Hearing Aids

In this project switched capacitor converters (SCCs) for rechargeable hearing aids (RHAs) are studied. The power management system in an RHA should step down a Li-Ion battery voltage to the voltage levels needed by the system. To achieve long battery life, a high conversion efficiency is required. At the same time, the physical size should be low as it is desirable for the end-user to have small hearing aids. The small size, combined with a requirement of high quality audio and wireless connectivity, makes electromagnetic interference a challenge (EMI). The main results of the project is a low-EMI controller that regulates both switch conductance and switching frequency, a sizing methodology for multi-topology output stages, a switching technique for lowering radio disturbances, a reconfigurable converter output stage, and a systematic method for synthesizing switched capacitor topologies. The frequency-planned switch-conductance modulating controller yields a 84.8% reduction in worst-case lowload output ripple voltage and a 1.5% increase in peak efficiency. The low EMI switching technique shows a measured 45% reduction of near field magnetic induction (NFMI) radio disturbance from the (SCC) while resulting in a 0.28% efficiency penalty. The systematic topology synthesis method is employed to find all step-down topology implementations in output stages with one to four flying capacitors. The resulting number of unique topologies, in terms of switch placements, are 8.4 billion for the four flying capacitor case. Finally, a number of related subjects has been studied. Namely, efficiency limits of fully-integrated interleaved SCCs, a capacitor-free linear regulator for hearing aids applications, and a gearbox fully-integrated SCC is designed on schematic level.