Design and Analysis of A Class-EFrequency-Controlled Transcutaneous Energy

Transfer SystemAhmad MizannojehdehiDepartment of Electronics

Carleton UniversityOttawa, Onterio K1S 5B6

Email: amizan@doe.carleton.ca

Maitham ShamsDepartment of Electronics

Carleton UniversityOttawa, Onterio K1S 5B6

Email: shams@doe.carleton.ca

Tofy MussivandOttawa Heart Institute

40 Ruskin St., Ottawa, ON K1Y 4W7Telephone: 1(613) 761–4545

Fax: 1(613) 761–4548

Abstract— This article introduces a frequency-controlled tran-scutaneous energy transfer system as well as a particular circuitstructure for it. The analysis method, design and realization ofthis system are studied and its performance evaluation based onthe simulations and experimental results of a prototype circuitis carried out. The simplicity of the required hardware as wellas the high performance and reliability are among the attractivefeatures of the proposed system.

I. INTRODUCTION

Implanted electronic devices provide an improved qualityof life for patients suffering from malfunctioning of an organand also provide the possibility of releasing patients fromhospital after implantation and recovery instead of keepingthem for a long time in hospital under intense clinical care. Thetranscutaneous energy transfer (TET) systems provide wirelessenergy for operating the implanted electronic devices and playa vital role in development of these devices by eliminating theneed for a percutaneous connections and the inherent risk ofinfectious complications.

In a TET energy is transferred through an inductive link,provided by a transcutaneous transformer. A transcutaneoustransformer is mainly composed of two pancake-shape coils,one of which, called the secondary coil, is implanted underthe skin and the other one, called the primary coil, placedover it outside of the body. The electromagnetic flux producedby the primary coil, is partly captured by the secondary coilwhen passed through the skin. The coupling coefficient ofthe two coils are usually small and around 0.1 to 0.4, whichsignificantly impacts the system efficiency. On the other handthe portion of flux not passed through the secondary coilcloses its path only in the primary coil, producing a leakageinductance at this side and likewise a leakage inductance atthe secondary side. A significant part of the voltage appliedto the primary coil drops across the leakage inductors, leadingto small overall system voltage gain and the requirement foran impractically large portable battery at the system input.Designing of a TET is a challenging task, mainly because ofthe low coupling coefficient and the existence of the leakageinductances. Further complications arise from the fact that the

coils separation-distance is constantly being varied by patient’smovements and that the load and input voltage variation mustalso be considered in the system design.

A number of TETs have been already proposed, amongthem a Class-E TET with a variable input voltage [1], afrequency-tracking TET [2] and a double-tuned series resonantfrequency-controlled TET [3] are the most important ones.Here a class-E TET with variable operating frequency is pro-posed. For this system, the operating frequency band, for thedifferent coils separation-distances, under which the proposedTET has maximum power transfer efficiency is identified. Atthe end the capability of this TET to compensate the coilsseparation-distance variations while providing a regular andfixed stream of desired amount of power for an implantedelectronic device is evaluated.

II. SYSTEM STRUCTURE

A simple block diagram of a TET is shown in Figure 1.Basically, a portion of the alternating magnetic flux producedby an alternating current supplied by the coil-driver to theprimary coil is captured by the secondary coil, inducing analternating voltage at this side according to Faraday’s law.This voltage after being rectified is regulated by employinga closed-loop voltage control. The Information of the outputvoltage level, transmitted by a wireless or infrared system tothe primary side, is detected and processed to determine theamount of current required to be driven to the primary side tokeep the output voltage fixed and regulated.

A schematic drawing of the power section of the proposedsystem is shown in Figure 2, where a Class-E amplifier [4]is used for the coil-driver block, the rectifier is a Class-Etype [5], and the transcutaneous transformer is replaced byits equivalent circuit [6]. In the transcutaneous transformerequivalent circuit, L2 and L3 represent the primary andsecondary leakage inductances while L5, a mutual inductance,indicates the coupling value of the transcutaneous transformer[6]. Note that the leakage inductance of the primary sideis incorporated into the Class-E amplifier structure. Thiseliminates the negative effect of this inductance on the overall

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Fig. 1. A TET block diagram

Fig. 2. The Class-E TET

system voltage gain. Capacitor C3 is also used to eliminatethe effect of the secondary side leakage inductor, L3, at aparticular resonant frequency. The interaction of this resonantfrequency with the operating frequency of the system providessituations which are discussed in the next section.

III. SYSTEM ANALYSIS

The Class-E rectifier at the secondary side is replaced byits equivalent circuit which is a capacitor in series with aresistor [5]. This capacitor in series with C3 and secondaryleakage inductance constitute the secondary tank filter whichis represented by jX in Figure 3.

Fig. 3. The Class-E TET with equivalent secondary side circuit

When the operating frequency is lower than the resonantfrequency of the tuned filter, the jX impedance would changeto a capacitive impedance while otherwise to an inductiveimpedance. To simplify analysis, the series combination ofRL and jX is transformed to a parallel combination with thefollowing equations and transferred to the primary side( Figure4).

Rp = RL(1 + Q2) (1)

Xp = X(1 + Q2

Q2) (2)

Q =X

RL(3)

Fig. 4. The Class-E TET with equivalent parallel circuit of secondary sidewhen transfered to primary side

The parallel combination of Rp and the inductive or capac-itive impedance, resulted from paralleling of jX and jωL5, istransformed back to a series combination with the followingequations ( Figure 5)

Rs = Rp1

(1 + Q2)(4)

Xs = Xp(Q2

1 + Q2) (5)

Q =Rp

Xp(6)

Fig. 5. The Class-E TET when transformed to Class-E amplifier

The resulted circuit, which is reduced to a Class-E amplifiercould be analyzed by analysis method proposed by Raab [8].

IV. THE CLASS-E TET FREQUENCY RESPONSE

The Class-E TET of Figure 2 is mainly composed of aClass-E amplifier and a Class-E rectifier. In this section afterthe Class-E amplifier and its frequency response is brieflyintroduced the effect of the Class-E rectifier on overall systemfrequency response is studied.

A. Frequency Response of the Class-E Amplifier

A Class-E amplifier consists of a tuned load network (C2,L2, RL), a switching transistor, M1, that is switching atthe carrier frequency of output signal and a capacitor, C1,in parallel with the transistor (Figure 6). This amplifier hastheoretical efficiency of 100%, capability of working at highfrequency and tolerant to load short circuit [4].

Assuming a very high Q for the tuned load network con-sisting of capacitor C2, inductor L2 and the load, the currentflowing through this network can be considered pure sinu-soidal. This current flows through the switch when it is on andkeeps flowing when the switch turns off, however, now throughthe capacitor C1, rising the voltage across it and the switch.When the direction of the current is reversed, the capacitorC1 start supplying this current, reducing the voltage across it

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Fig. 6. A Class-E amplifier

[9]. The values for the C1 capacitor and the series resonantload network elements determine the voltage across the switchat its off state. By choosing proper values for C1, C2, L2and a particular load resistance, which is called optimum loadresistance, this voltage not only reaches zero but also hasa zero slope at this instant. For any load resistance smallerthan the optimum value, only the zero-voltage switching ismaintained and otherwise none of the zero-voltage and zero-slope are maintained. Zero-voltage switching guarantees zeroswitching losses and must be maintained during all workingphases of a TET to ensure the maximum efficiency of thesystem. Inductor L1 acts as a current source supplying theenergy dissipated during each cycle.

The frequency response of the Class-E amplifier of Figure6, obtained by Raab equations [8], is shown in Figure 7. Itis very important to note that only for a specific band offrequencies, indicated in this figure, the zero-voltage switchingfor the amplifier is maintained.

Fig. 7. Frequency response of Class-E amplifier

B. Frequency Response of the Class-E TET

By including the effects of the secondary tuned filter asdescribed in section III, depending on the relative situationsof the resonant frequency of secondary tuned filter, fres, andthe center frequency fc in Figure 7, different overall frequencyresponses for the TET may be resulted which are depicted inthe graphs of Figure 8. The areas in these graphs where zero-voltage switching are maintained could be explored by eitheranalysis or simulation methods. For example in the graph‘A’ which fc is very close to fres, part of right hand slopemaintains zero-voltage switching condition while in the graph‘B’ where fres is smaller than fc, it is likely to happen ateither slopes of the rightmost peak and in the graph ‘C’ wherefres is larger than fc, it is likely to happen at either of thenegative slope of the leftmost peak or the positive slope of the

Fig. 8. Frequency response of Class-E TET when A) fres is very close tofc, B) fres smaller than fc and C) fres larger than fc respectively from topto bottom

rightmost peak. Depending on the control strategy dictated bya particular circuit either of the above options could be adoptedfor designing a frequency-controlled output regulated TET.

V. SIMULTION AND EXPERIMENTAL RESULTS

The design procedure outlined in the previous sections areapplied for designing a TET with an input voltage of 14V ,output voltage of 12V and maximum transferred power of15W which could find applications in some areas like HeartAssist Devices [7]. Along with the construction of the powertransfer section, an analog control circuit according to Figure9 is designed. Here temporarily hardwire feedback is utilizedwhich could be replaced with a high speed wireless datatransmission system and A/D and D/A circuits in the future. Aclosed-loop stability analysis is performed by aid of MATLABto determine the compensation circuit required to stabilize thewhole system.

Fig. 9. Block diagram of the circuit used for experimental tests

The coils are wound using Litz wire to reduce resistivelosses of the coils at high working frequency. The elementsof the equivalent circuit for the transcutaneous transformer arecomputed for an average coils separation-distance of 15mm.By experience it is found out that the system designed inthis way can more easily accommodate itself for the extremeseparation-distances of 10mm and 24mm by varying theoperating frequency.

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Fig. 10. Output and drain voltage waveforms of the designed TET when atthe frequencies of 344, 355, 372, 385, 403, and 424Khz from top-left tobottom-right

For this particular case fres is chosen to be smaller thanfc, thus the transfer function of the system is similar to graph‘B’ of Figure 8. The waveforms of the drain and the outputvoltages of the system, chosen among those experimentallyobtained in the frequency band of 344Khz to 424Khz,are shown in Figure 10. The zero-voltage switching is onlymaintained in the frequency band of 385Khz to 408Khz,thus chosen as the working frequency band of the this proto-type. Figure 11 represent the efficiency of the system versusoperating frequency which are experimentally obtained fordifferent coils separation-distances. Obviously the areas ofhigh efficiency correspond to frequency band in which thezero-voltage switching is maintained.

Fig. 11. Efficiency versus operating frequency for different coils separation-distances

In vitro tests indicate that the system can keep supplyingregulated voltage to maximum load by varying frequency overthe designed frequency band even though the coils separation-

distance is being varied from 10mm to 24mm. For coilsseparation-distance of 15mm the overall efficiency of thissystem is 73%.

VI. CONCLUSION

In this work a novel frequency-controlled Class-E TET isproposed. Its design, operation analysis, and test results arealso reported. The simulations and experimental results indi-cate that the outlined method is very promising for controllingthe output voltage of a TET. This control involves compen-sating for the effects of coils separation-distance changes, theeffects of exposing the coils to a metal object and the load andinput voltage variations. The relatively high efficiency of thismethod is guaranteed provided that the operating frequency ofthe system is adjusted for zero-voltage switching area of thetransfer function. This area can be determined analytically orthrough simulations. The interaction of the secondary tunedfilter with the Class-E coil-driver provides more flexibility inchoosing the desired frequency band for the system operationas well as choosing desired control methodology accordingto selected area of the transfer function. This method doesnot require a bulky dc-dc converter at the front side asopposed to voltage controlled Class-E method [1] or a complexcircuitry for compound frequency and duty cycle control [2]and since employs Class-E coil-driver, it is superior over themethods employing series resonant converter [9]. The selftuning capability along with the small component count ofthe proposed system are the indications of the reliability, highefficiency and the small size of the TET designed with thisapproach. The process of perfecting of the system is continuedto make it ready for in vivo tests.

ACKNOWLEDGMENT

This research is partly supported by NSERC and Cardio-vascular Devices Center of Ottawa Heart Institute.

REFERENCES

[1] Guoxing Wang et. al., A closed-loop transcutaneous power transfer sys-tem for implantable devices with enhanced stability Circuits and Systems,2004. ISCAS ’04. Proceedings of the 2004 International Symposium on,Volume 4, 23-26 May 2004 Page(s):IV - 17-20 Vol.4.

[2] Miller JA, Belanger G, Mussivand T Development of an autotunedtranscutaneous energy transfer system, ASAIO J 39:M706-M710

[3] Gyu Bum Joung and Bo. H. Cho, An Energy Transmission systemfor an Artificial Heart Using Leakage Inductance Compensation ofTranscutaneous Transformer, 1998 IEEE.

[4] Nathan O.Sokal and Alan D. Sokal, Class-E - a new class of high-efficiency tuned single-ended switching power amplifiers, IEEE Journalof Solid-State circuits, Vol. SC-10, No. 3, June 1975.

[5] Marian K. Kazimerczuk and Krzysztof Puczko, Exact analysis of Class-Etuned power amplifier at any switch duty cycle , IEEE Transactions oncircuits and system, Vol. CAS-34, No. 2 February 1987

[6] Ali Ghahari, Bo. H. Cho Design of a Transcutaneous Energy TransmissionSystem Using a Series Resonant Converter. IEEE Transactions on powerElectronics, Vol. 7, No. 2, April, 1992.

[7] Tofy Mussivand, Mechanical circulatiry devices for treatment of heartfailure, Journal of cardiac surjury, 1999, May-June 14(3):218-28

[8] Fredrick H. Raab, Idealized operation of the Class-E tuned poweramplifier , IEEE Transactions on circuits and system, Vol. CAS-24, No.12 December 1977

[9] Philip R. Troyk, Martin A. K. Schwan Closed-Loop Class-E transcuta-neous power and data link for micro-implants, IEEE trans. Biomed. Eng.,Vol. 39, no. 6, June 1992.

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