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Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 1923, 2012 445

Wireless Power System for Implantable Heart Pumps Based on

Energy Injection Control

H. Y. Leung1, D. M. Budgett1, D. McCormick1, and A. P. Hu2

1Auckland Bioengineering Institute, University of AucklandAuckland 1142, New Zealand

2Department of Electrical and Computer EngineeringUniversity of Auckland, Auckland 1142, New Zealand

Abstract Inductive power transfer (IPT) for powering high power implantable devices, suchas total artificial hearts and heart assist devices, greatly reduces the risk of infection by elim-inating the driveline cable which otherwise needs to puncture the skin to provide power. Theoperating conditions are demanding in terms of the power level, a wide range of coupling vari-ations, restrictions on heat generated and resultant temperature rise in the surrounding tissue.This paper presents a wireless power transfer system which satisfies the requirements for power-ing a high power implant. The system consists of a half-bridge energy injection circuit which isfully soft-switched. No extra switching or power components are required to regulate the power

flow. This is achieved by injecting energy into the tank when required, and allowing the resonanttank to free oscillate when power is sufficient. Feedback from the implanted device is providedvia a radio link completing the feedback control loop. The external and internal power transfercoils are air-cored and have a maximum diameter of 75 mm and thickness of 7 mm including thebiocompatible encapsulation, making a light and compact transcutaneous energy transfer (TET)system. The presented system is capable of delivering over 15 W to the implanted load over awide range of coupling variation (k = 0.15 to 0.3) which corresponds to 20 mm to 10 mm coilseparation. The system has achieved an end to end power efficiency of 78.7% to 82.2%.

1. INTRODUCTION

This paper presents a standalone Transcutaneous Energy Transfer (TET) system for powering highpower implantable medical devices such as artificial heart pumps. Current state-of-the-art heartpumps require a level of power consumption that ranges from 5 to 15 W [1]. Such power levels

are too demanding for implantable batteries for a heart pump which must run continuously. Thuselectric power must be externally provided either with a percutaneous cable (punctures throughthe skin) or via a transcutaneous (through unbroken skin) method. Inductive power transfer (IPT)is a well known technology and is also very promising for delivering power to implantable devices.

In this paper, a TET system based on a half-bridge energy injection resonant converter wasdesigned for powering an implantable heart pump. A TET system, when used by a patient, mustbe able to adapt to the prevailing coupling conditions as the separation of the coils depends on thetissue thickness of the patient and surgical placement. In addition to this the system should beable to tolerate slight changes in coupling, due to movement from physical activity and repetitiveactivity such as respiration. Size and weight of the power transfer coils is another important factor,especially for the implanted coil which has to be thin enough in order for it to be placed just underthe skin. Finally the heat generated by the power transfer coils must not cause damage to thesurrounding tissue.

2. SYSTEM ARCHITECTURE

A high level block diagram of the TET system is shown in Figure 1. It consists of an externalprimary power converter with its power delivery coil L1 and an internal secondary power receivingcoil L2 with its power conditioning circuit. Resistor R2 represents the load of the implantabledevice. Power regulation is performed on the primary side with an energy injection converterwhich drives the power delivery coil which is part of a series resonant tank.

2.1. Resonant System and Power Transfer Coils

The transfer of power is achieved through the use of two LC resonant circuits, where the coils aremutually coupled together via the interaction of oscillating magnetic fields. The strength of thecoupling is determined by a coupling co-efficient k. In this particular application the coupling kmay vary quite significantly during practical usage.

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446 PIERS Proceedings, Moscow, Russia, August 1923, 2012

MOSFET

Gate Drive

L1

S1

S2

Vin

R2

C.T.

L2

C1C2 Rectifier

Wireless

TX

Wireless

RX

Current

limit

ZCS

logic

Figure 1: High level block diagram of presented TET system.

(a) (b)

Figure 2: (a) Primary and (b) secondary power transfer coils.

Since the energy injection converter is essentially a voltage-fed half bridge converter the primaryLC resonant tank topology must be a series tuned tank, as a parallel tuned tank has voltage sourcetype characteristics and must be driven by a current-fed converter [2]. The configuration of thesecondary resonant tank is also series tuned; this was chosen as this makes the secondary resonantcurrent the same as the load current.

The most ideal shape of the power transfer coils is as flat and small as possible. The requirementof it being flat is because the secondary coil has to be implanted inside the body and near the surfaceof the skin, a flat coil allows the coil to be implanted just under the skin secured by nearby bonemass (such as on top of the rib cage), without creating a large extrusion. The coils used in thesystem are shown in Figure 2, they were made by winding 2mm diameter Litz wire in a twolayer pancake configuration creating a 4 mm thick coil, encapsulating the coils with biocompatiblesilicon brought the thickness to 6 mm. The diameter of the implanted coil including encapsulation

is 55 mm and the external coil is 65 mm. The weight of the coil is also important, as a heavycoil would induce more discomfort, thus no ferrite core was used in our coils and the primary andsecondary coils weighed 60 g and 45 g respectively. The size of the coils were designed so a maximumdisplacement of 20 mm of the coils either vertical or horizontally could be tolerated without theircoupling co-efficient dropping below k = 0.15. The inductances of the primary and secondary coilswere 14.2 H and 11.45 H respectively.

The tuning capacitors of the primary and secondary capacitors were not selected to make theindividual primary and secondary tanks resonate at a nominal resonant frequency. As the couplingvariation during operation is large (k = 0.15 to 0.3) it is important to analyze how the impedance ofthe system changes across all coupling conditions and select capacitor values which will minimizeloss and avoids any frequency bifurcation behavior within the operating coupling range. Thecapacitor values were selected using an optimisation process previously described [3], the primaryand secondary capacitances used in this setup are 12 nF and 16 nF respectively.

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Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 1923, 2012 447

2.2. Energy Injection Converter

A half-bridge energy injection converter [4] was used in our TET system and is depicted in Figure 3.This converter was chosen due to a low component count, ability to implement soft-switching and itprovides complete power flow control without additional power stage components. The half-bridgeenergy injection converter consists of two operating states; injection state and free oscillation state.During the injection state the high side switch is on while the low side switch is off, connecting theresonant tank to the DC input source, thus injecting energy into the tank. During the free oscillationstate the low side switch is on while the high side switch is off, which connects the resonant tanktogether allowing it to free oscillate, damped by its own or any reflected impedance. In order forthe system to be soft-switched the converter must change states during the zero crossings of theresonant current, this is achieved using a current transformer on the primary resonant track. Theinjection state can only occur during the positive phase of the resonant current as the power sourceis DC and can only provide power in one direction, but free oscillation state can occur in bothphases of the resonant current.

2.3. Feedback and Control

A current limit controller was used to control power flow. The peak of the resonant current duringthe negative current phase is compared against a current limit. If the peak is below the limit, thenan injection state will occur during the next positive current phase; else if the peak is above thelimit, the converter will stay in free oscillation state during the next positive current phase. This

current limit is the output of a proportional-integral controller which operates on the error of theoutput voltage with the desired output voltage. The output voltage at the secondary is receivedthrough a low power 2.4 GHz radio link. Control of the system is achieved completely onboard withdiscrete analog components and no external controllers were used, hence it is a standalone system.

3. RESULTS

The performance of the TET system was characterized at a load of 10 ohms. The control systemwas set to regulate the output voltage to 12.5 V corresponding to an output power of 15 W beingavailable to drive the heart pump and charge any internal battery. Power measurements wererecorded at different separations between the power transfer coils, this separation refers to thedisplacement of the coils with their centers concentric and radii parallel to each other. For this setof coils a 5 mm separation corresponds to a coupling co-efficient ofk = 0.41 and a 20 mm separation

corresponds to a coupling co-efficient ofk

= 0.15. Note that coupling is not linearly proportionalto separation.

We have chosen 10 mm as a minimum coil separation to accommodate encapsulation, implanta-tion, tissue, coil holder and clothing contributions. The output power vs. separation relationship isshown in Figure 4, it can be seen that the system is only able to reach desired output voltage whenthe separation is greater than 10 mm (or k < 0.3). It is unintuitive that output power decreasesat close coupling. Figure 5 plots the injection percentage (power flow percentage) of the energyinjection converter vs. vertical separation to help explain the effect of close coupling. Figure 5shows that more energy injection is required when coupling is good than when coupling low, this

C1

L1 L1

C1

VIN

I1

Positive

or Negative

current

phase

Positive

current

phase

only

Injection

State

Free oscillation

State

Figure 3: States of a half-bridge energy injection converter.

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448 PIERS Proceedings, Moscow, Russia, August 1923, 2012

10

11

12

13

14

15

5 10 15 20 25 30

Voltage(V)

Separation (mm)

Figure 4: Output voltage vs. separation.

0

20

40

60

80

100

Injection%

5 10 15 20 25 30

Separation (mm)

Figure 5: Injection percentage vs. separation.

65

70

75

80

85

Efficiency%

5 10 15 20 25 30

Separation (mm)

Figure 6: Efficiency vs. separation.

80

85

90

95

100

Efficiency%

5 10 15 20 25 30

Separation (mm)

Figure 7: Theoretical maximum efficiency vs. sepa-ration.

is because the reflected impedance seen by the converter is greater when coupling is good. Thus ittakes more energy injections to sustain the same resonant current. If operation at higher couplinglevels is desired, different tuning capacitors would be selected to allow the desired output voltageto be reached at the cost of inferior efficiency at the low coupling end. A higher input voltage canalso improve operation at high coupling, however high DC voltages come with safety issues andcause greater stress on the power components.

The end to end power efficiency vs. vertical separation is shown in Figure 6. Also a simulationof the resonant system was performed in order to find the theoretical maximum efficiency of thesystem when only the losses of the coils and resonant capacitors are taken into account; the result ofthis is shown in Figure 7. In Figure 6, it can be seen that for the designed separation range of 10 mmto 20 mm, the efficiency steadily decreases from 82.2% to 78.7%. This 3.5% decrease in efficiencyacross the coupling change of k = 0.3 to 0.15, is reflective of the theoretical maximum efficiencydrop of 3.3% across the same coupling range. Thus the energy injection converter maintains almostconstant efficiency throughout its control range.

4. CONCLUSION

This standalone TET system accommodates the power conditions applicable for an implantableheart pump. A half-bridge resonant converter was used to drive the power transfer coils, and powerflow control was performed on the external primary side via feedback through wireless communi-cation. The system was able to provide 15 W of power across a coil separation of 10 mm to 20 mmcorresponding to a coupling change of k = 0.3 to 0.15; with a maximum end to end efficiency of82.2% and a minimum efficiency of 78.7%. It has also been demonstrated that the energy injectionconverter retains it efficiency across a control range of 100% to 25% energy injection, making itideal for TET systems which need to operate across a wide range of coupling. The presented powertransfer coils and circuitry are also sufficiently compact for implantation.

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Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 1923, 2012 449

REFERENCES

1. Struber, M., A. L. Meyer, D. Malehsa, C. Kugler, A. R. Simon, and A. Haverich, The currentstatus of heart transplantation and the development of artificial heart systems, Dtsch ArzteblInt., Vol. 106, 471478, 2009.

2. Hu, A. P., Selected resonant converters for IPT power supplies, Ph.D. Dissertation, Depart-ment of Electrical and Electronic Engineeing, University of Auckland, Auckland, New Zealand,2001.

3. Leung, H. Y., D. M. Budgett, and A. P. Hu, Minimizing power loss in air-cored coils forTET heart pump systems, IEEE Journal on Emerging and Selected Topics in Circuits andSystems, Vol. 1, 8, 2011.

4. Li, H. L., A. Hu, and G. A. Covic, Development of a discrete energy injection inverter forcontactless power transfer, 3rd IEEE Conference on Industrial Electronics and Applications,ICIEA 2008, Singapore, 2008.