Oliver Knecht

High-Efficiency Transcutaneous Energy Transfer for Implantable Mechanical Heart Support Systems

Inductive power transfer technology is a promising solution for powering implantable mechanical circulatory support systems, due to the elimination of the percutaneous driveline, which is still the major cause of severe infections. However, at the present time, no transcutaneous energy transfer (TET) system is commercially available and ready for long-term use. Specifically, the heating of the tissue due to power losses in the TET coils and the implanted electronic components are a major problem. The focus of this paper is, therefore, on the design and realization of a highly efficient TET system and the minimization of the power losses in the implanted circuits in particular. Parameter sweeps are performed in order to find the optimal energy transmission coil parameters. In addition, simple and meaningful design equations for optimal load matching are presented together with a detailed mathematical model of the power electronic stages. To achieve highest efficiencies, a high-frequency self-driven synchronous rectifier circuit with minimized volume is developed. Extensive measurements are carried out to validate the mathematical models and to characterize the performance of the prototype system. The optimized system is capable of transmitting 30 W of power with an efficiency greater than 95 %, even at a coil separation distance of 20 mm (0.79 in) and 70 mm (2.76 in) coil diameter.

Comprehensive evaluation of GaN GIT in low- and high-frequency bridge leg applications

In power electronics applications with power ratings around several kilowatts, wide band gap semiconductors are more and more replacing state-of-the-art Si MOSFET. SiC MOSFETs with blocking voltage rating up to 1200V and low-voltage GaN devices are already commercially available on the market since a couple of years. Now also 600V GaN devices are entering the market, which are a cost-effective solution in many 400V key applications in order to increase the system performance in terms of achievable efficiencies or power density. Besides the employed semiconductor devices also the design of the appropriate gate drive circuit is important. In this paper a simple and reliable gate drive circuit for driving GaN switches is presented. In addition, the proposed gate drive is used to evaluate the switching performance of a GaN Gate Injection Transistor (GIT) under soft- and hard-switching condition, which provides a basis for further optimization of totem-pole converter systems.

Optimization of Transcutaneous Energy Transfer coils for high power medical applications

Inductive Power Transfer (IPT) technology is a promising solution for powering medical implants with a continuous high power consumption, due to the elimination of the percutaneous driveline, which is still the major cause of severe infections. However, at the present time, no Transcutaneous Energy Transfer (TET) system is commercially available and ready for long-term use. Specifically the heating of the tissue due to power losses in the TET coils is a major problem. The focus of this paper therefore is on the minimization of the power losses in the energy transmission and receiver coils of a TET system. Extensive parameter sweeps were performed in order to find the optimal winding configuration with minimized parasitic resistances and optimal inductance value. A thermal model of the human skin is developed to estimate the thermal limits. Based on the results, a prototype TET system is built to validate the optimization process. The prototype system is capable of transmitting 30W of power with an efficiency greater than 93 %, even at a coil separation distance of 20mm (0.79 in) and 70mm (2.76 in) coil diameter.

iAssist : rapid deployment and maintenance of tiny sensing systems

Commercial, coin-sized iButton temperature logger devices are well-suited for densely instrumenting large outdoor areas. An efficient workflow for deploying and maintaining those devices is necessary when striving to deploy and operate several hundreds of data logger devices. Additionally, a sophisticated data management is required for handling the emerging, large amounts of meta and measurement data. Therefore, we developed iAssist, a solution that integrates the handling of iButton data logger devices together with a GPS receiver and a digital camera for gathering accurate location information. iAssist efficiently supports the whole workflow consisting of deploying, relocating and reading tiny sensing systems. iAssist is especially tailored for outdoor operation asking for as little user interaction as possible.

Comparative evaluation of a Triangular Current Mode (TCM) and Clamp-Switch TCM DC-DC boost converter

For the power management of a wireless power transfer system for implantable mechanical heart pumps, an additional boost DC-DC converter stage is needed in order to control the power delivered to the implant. Particularly, battery powered and implantable medical devices pose special demands on the efficiency and/or power density of the employed converters. Accordingly, soft-switching and/or high switching frequencies must be targeted. Modulation schemes that allow for Zero-Voltage-Switching (ZVS) such as Triangular Current Mode (TCM) offer a highly efficient operation, but suffer from a large operating frequency variation, which is mainly limited by the digital control. Therefore the Clamp-Switch TCM (CL-TCM) converter can be employed which allows also for the control of the switching frequency variation. In this paper, the CL-TCM and the TCM converter are compared regarding the power conversion efficiency and the power density of the converter. Since the CL-TCM converter is not well known in the literature, the converter is analysed in detail and a modulation scheme is explained that allows for ZVS for all switches in the entire range of operation. In addition, the requirements for ZVS and a control scheme (i.e. timing calculations) are provided for the converter in order to limit the maximum switching frequency. The modulation and control scheme are verified with a hardware prototype. Finally, the performance of the CL-TCM converter is measured and compared to the performance of the converter operated in TCM mode. The measurements show that the CL-TCM converter offers similar performance compared to the TCM operation at lower inductor power density, but has the advantage of a significantly reduced switching frequency variation. In applications, where a very high power density is needed, the TCM converter outperforms the CL-TCM converter in terms of efficiency.

Impact of Transcutaneous Energy Transfer on the electric field and specific absorption rate in the human tissue

Inductive power transfer technology has proven to be a promising solution for powering implantable heart pumps such as left ventricular assist devices, eliminating the need for a percutaneous driveline and reducing the risk of severe infections significantly. However, the required high power transfer capability of a Transcutaneous Energy Transfer (TET) system raises questions about human safety regarding the exposure to electric and magnetic fields. The focus of this paper is on the internal electric fields and the Specific Absorption Rate (SAR) caused by a prototype TET system designed to transfer 30 W across the skin at 800 kHz and 35 V output voltage. Numerical simulations show that the internal electric field and the SAR can locally attain high values within the fat tissue due to the large voltage potential at the implanted coil terminals. It is further shown that the parasitic capacitances of the energy transmission coils and the power electronic circuit of the implant can cause common-mode voltages at the energy receiving coil terminals, which increase the internal electric field strength additionally. Hence, the power electronic circuit and the grounding scheme of the system need to be adapted in order to eliminate common-mode voltages. As an additional countermeasure, an electric shielding based on carbon conductive compounds is presented in this paper, which is able to reduce the maximum internal electric field strength from 224 V/m to 77V/m and the maximum SAR and from 1.21 W/kg to 0.25 W/kg with only 1 % of additional power loss.

Inductive Power Transfer Efficiency Limit of a Flat Half-Filled Disc Coil Pair

The efficiency limit for an inductive power transfer between two flat half-filled disc coils is derived based on a model for the eddy current losses in the coils and the losses due to electromagnetic radiation. Analytic approximations for the coupling factor of the coils and eddy current losses are proposed and experimentally verified. It is shown that the approximative terms allow us to express the maximum efficiency of the coil pair analytically. If the strand diameter of the coil is sufficiently small, the efficiency depends only on the strand diameter, diameter of the coils, and the gap between the coils—but not on the operating frequency. Therefore, increasing the frequency does not result in higher efficiency but allows to reduce the coil thickness.

ZVS Modulation Scheme for Reduced Complexity Clamp-Switch TCM DC–DC Boost Converter

DC–DC boost converter zero-voltage-switching (ZVS) modulation schemes such as triangular current mode (TCM) offer a highly efficient operation but suffer from large switching frequency variations, which are complicating the EMI filter design and the digital control. As a solution, a tri-state boost converter operated in ZVS mode, referred to as clamp-switch TCM (CL-TCM) operation can be introduced, which allows to limit the switching frequency variation significantly. This paper presents two variations of the CL-TCM boost converter with reduced number of active switches in the circuit, which are suitable for high input-to-output voltage conversion ratios. In addition, the ZVS modulation schemes, its limitations, the converter design and the controller implementation are presented and analyzed in detail for both converter topologies. The timing calculations for the switching signals are provided for two operating modes, either offering a minimized switching frequency variation and minimized RMS inductor current or a constant switching frequency operation, which in turn comes at the expense of an increased RMS inductor current. The ZVS operation and the operating modes are experimentally verified using a hardware prototype.

Planar inverted-F antenna design for a fully implantable mechanical circulatory support system

Transcutaneous Energy Transfer (TET) is a promising solution to operate implantable mechanical blood pumps such as Left Ventricular Assist Devices (LVAD) without the need for a percutaneous driveline. In order to control the power flow of the wireless energy transfer system and in order to transmit sensor data, a wireless communication channel is needed. This paper describes the design of an implantable Planar Inverted-F Antenna (PIFA) for the use with a TET system, which operates in the MedRadio band at 403.5 MHz and which is suitable for implantation depths of more than 20 mm. The focus of this work is on the practical implementation of the antenna, and the critical design parameters as well as the influence of the human tissue on the antenna design are discussed in detail. Finally, the antenna is built in hardware and the electrical characteristics of the antenna are measured using a muscle tissue mimicking liquid solution. The implemented antenna achieves nearly perfect impedance matching at 403.5 MHz with a return loss of more than 25 dB and a high gain of −26.3 dBi, even for a placement as deep as 20 mm within the tissue mimicking liquid. Numerical simulations further show that a maximum spatial-average SAR of 0.27 W/kg is achieved in the muscle model with the maximum allowed antenna input power of 9.35 dBm, which is a factor of six below the most stringent SAR limit of 1.6 W/kg.