Development of an autotuned transcutaneous energy transfer system

A transcutaneous energy transfer (TET) system has been developed to power implanted devices such as heart assist devices, artificial hearts, defibrillators, and electrical stimulators. The TET system transfers power by electromagnetic induction without the need for percutaneous leads. For ease of implantation and patient comfort, it is desirable to use TET coils that are as small as possible. One problem encountered with TET designs that use small coils is a high sensitivity to coil electromagnetic coupling caused by changes in separation. These changes can result from variation in tissue thickness between subjects and from displacement of the coils that can occur during breathing and general body movement. Changes in coil coupling result in similar changes in the resonant frequency of the TET transformer, which can reduce power transfer and efficiency. The EVAD TET system was designed to address this problem by incorporating a technique for automatically tuning the power driver stage of the transformer. The system is able to deliver maximum output power of approximately 60 watts at a coil separation of 5 mm, falling to approximately 45 watts at a coil separation of 15 mm. The system can deliver a maximum efficiency of 75-80%; reducing to approximately 60% and 60 watts. The results presented demonstrate the system's ability to compensate for variations in coil separation by resonant frequency tracking. This has optimized power transfer throughout the required range of coil coupling conditions.