A Resonant Class-E DC-DC converter for Wireless Power Transfer in implantable Stimulators

Recent research developments in electronics and progresses in biotechonlogies gave rise to a new class of devices and tecniques which lead to systems able to hugely improve the quality of healthcare. Implanted neurostimulators are always more often taking place of drugs in classical polytherapy and being used to treat a variety of illnesses such as Parkinson or Ephilepsy. In some other cases neurostimulation is used to restore sensory inputs, restoring hearing to people with hearing loss or view to people with retinal problems . All these active devices require the use of batteries to deliver power and per-cutaneous cables to link the external device to the implanted one. However both the limited capacity of a battery, which limits the system in terms of performance, and the use of percutaneous cables, which makes patients susceptible to infections, leads to Wireless Power Transfer (WPT) techniques. The physical and electrical requirements imposed by the design impose strong limitations in the choice of the circuits to be used. In this thesis work, the WPT system has been analysed as a DC-DC converter and in area of high efficient topologies the class E isolated-and-direct topology has been chosen. Said topology is a novelty in the biomedical field since, albeit promising, it has never been applied for biomedical implants. A new approach, based on the solution of the ODE, to design a resonant DC-DC converter for biomedical applications is proposed, achieving both a more accurate implementation and simpler architecture, by reducing the number of passive components required. This represents an important breakthrough with respect to the state of the art, where class-E circuit analysis is based on strong simplifying assumptions and does not take care of biomedical constraints. As a proof of concept a design example of a class E DC-DC converter to be used in cochlear implants is shown. The topology proposed, achieving its optimum condition only for a specific operating point, has been improved: the use of a shunt regulator to face load variations together with the exact measure of the coupling factor, allows not only to estimate the electric quantities at the secondary side of the transformer directly from the primary (external to the body) circuit, avoiding more complex back telemetry communication, but also to change the converter working point to reach the minimum working Power Point (mPP) and therefore the maximum achievable efficiency for the whole system.