Determining optimal tuning capacitor values of TET system for achieving maximum power transfer

Abstract

Optimal tuning capacitor values corresponding to the maximum power transfer of a voltage-fed series-tuned transcutaneous energy transfer (TET) system are derived. The theoretical analysis is verified by experimental results. The maximum power transfer capacity can be three times greater compared to the traditional method, which tunes the capacitor at the nominal resonant frequency without considering the mutual inductance effect. These results are useful to enable an existing seriestuned TET system to meet short-term high-power demands where efficiency is not a key factor.Introduction: Transcutaneous energy transfer (TET) systems are designed to deliver power from a primary source to a secondary implantable device over relatively large airgaps via time-varying magnetic fields. It eliminates the serious infection risk associated with direct cable connections through the skin and these advantages were recognised decades ago [1]. The power transfer of a TET system relies on a resonant circuit at both the primary and secondary sides to compensate for low magnetic coupling between an external coil and an internal pickup coil [2]. Traditionally, the value of each resonant capacitor is selected based on the self inductance of the primary and secondary coils and the nominal resonant frequency [2,3]. However, owing to the mutual coupling between the circuits, such a practice does not necessarily lead to the maximum power transfer at a system level. Although various series and parallel resonant topologies have been studied with different tuning methods to improve the system performance, the optimal tuning capacitors to obtain the maximum potential power transfer capability remain an unsolved problem [4,5]. Choi and Lee reported work on adding a parallel tuning capacitor to a series-tuned TET system [6], but the purpose was to stabilise the output voltage and improve the system power efficiency. This Letter presents a new method for determining the optimal capacitor values of a voltage-fed series-tuned TET system for achieving maximum power transfer.

Optimal Tuning of Inductive Wireless Power Links: Limits of Performance

“…Using (2) and the accompanying equations for the SP circuit gives efficiency as (8) where (9) (10) (11) For the SS circuit, using (2) and accompanying equations, the load power is given by (12) while for the SP circuit, the load power is given by (13) where coefficients and are in Table III and IV. Finally, for the untuned-primary, parallel-tuned secondary (UP) circuit, the load power is given by (14) obtained by taking .…”

“…This all motivates a design trade-off between efficiency and delivered power. Such a trade-off has been demonstrated in [11] for the SS configuration and is a key aspect of [18]. This paper, as in [11] and similarly to [13] and [15], starts with given driving voltage, coupled inductors and resistive load, and then chooses the capacitors to optimize performance.…”

“…Such a trade-off has been demonstrated in [11] for the SS configuration and is a key aspect of [18]. This paper, as in [11] and similarly to [13] and [15], starts with given driving voltage, coupled inductors and resistive load, and then chooses the capacitors to optimize performance. One motivation for this approach is the relative ease of changing capacitors, which are standard components, compared with changing coupled inductors, which are usually specially built for an application.…”

“…Two notable performance criteria for designing wireless power systems are maximizing delivered power for a given input drive voltage [2], [11]- [14], [21], [24] and maximizing power transfer efficiency [3]- [6], [8], [15], [22], [23]. Although maximizing efficiency minimizes the required input power for a given delivered power, it can result in a higher driving voltage being required.…”

“…Results on capacitor selection for maximum power delivery for both configurations with and by making the driving impedance seen by real are in [13]. Capacitor selection giving power maximization for the SS configuration has been extended in [11] to allow non-zero coil resistances and to obtain improved results for high coupling where pole splitting [16] occurs. Corresponding results for the SP configuration, that is selecting and to maximize delivered power in the presence of high coupling and non-zero coil resistances are covered in the present paper.…”