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Session: Poster session I
Session starts: Thursday 24 January, 15:00

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Rico van Dongen ()
Senad Hiseni ()
Wouter Serdijn ()

Abstract:
Electrical stimulation of targets deep inside the brain has proven to be a powerful treatment for various movement disorders like Parkinson’s disease and essential tremor. Potentially even psychiatric disorders can be treated using electrical stimulation [1]. For electrical stimulation of the brain tissue, pulses are generated by an Implantable Pulse Generator (IPG). Due to the size of the IPGs used today, they need to be implanted inside the chest and connected to the stimulation electrode via an extension lead. To increase patient comfort and minimize the risk of lead failure the device should be miniaturised so that the complete system can be placed in the skull of the patient. Since battery replacement requires surgery, miniaturisation should be achieved without compromising the current lifetime of the device. Although rechargeable batteries have a lower volumetric energy density than their non-rechargeable counterparts they can be recharged hundreds of times. By switching from a non-rechargeable IPG to a system that can be recharged transcutaneously, the volume can be reduced without reducing the lifetime. Due to the sensitive nature of Lithium-Ion batteries they have to be recharged according to strict methods. If voltage or current limits are exceeded the battery can be damaged, reducing the capacity considerably. Traditionally Lithium batteries are charged in two phases via the Constant Current Constant Voltage (CCCV) method. In the first phase a Constant Current (CC) is pushed into the battery until the battery reaches its maximum voltage. After this a Constant Voltage (CV) is maintained across the battery and the current gradually decreases. Although this method ensures safe and complete charging it has some drawbacks. First of all, the control is rather complex and requires two separate control loops: One to control the charge current in the CC phase and one to control the charge voltage in the CV phase. Furthermore, instability of the charger near the switchover from phase one to phase two may occur resulting in an increased charging time. By utilizing the hyperbolic tangent transfer of a subthreshold transconductance amplifier the two charge phases of the CCCV method can be merged into a single control loop. The abrupt transition between the two charge phases is herby eliminated. The result is an inherently stable and simple control circuit [2]. A system that charges a battery through an inductive link has been designed. Switched mode operation has been used to further increase the power efficiency of the circuit. Chip area has been kept to a minimum by combining the charge current control switch with the AC to DC rectifier. Circuit simulations showed that the power transfer efficiency from the inductive link to the battery is, on average, 87.25% over a full charge cycle. A prototype using discrete components has been built to verify the correct operation of the circuit. Both simulations and measurements show that the charger efficiency is dominated by the loss in the rectifier circuit.