GB2239802A

GB2239802A - Transcutaneous energy transfer device

Info

Publication number: GB2239802A
Application number: GB9009713A
Authority: GB
Inventors: John A Miller
Original assignee: University of Ottawa
Current assignee: University of Ottawa
Priority date: 1990-01-09
Filing date: 1990-05-01
Publication date: 1991-07-17
Anticipated expiration: 2010-05-01
Also published as: CA2007439A1; GB9009713D0; CA2007439C; GB2239802B

Abstract

A transcutaneous energy transfer device comprises a primary winding L1 for placement on or near a skin surface, and a secondary winding L2 for implantation under said skin surface. A field effect transistor 10 (FET) is arranged to switch said primary coil across an external DC power supply. A tuning capacitor 11 is linked to said primary coil whereby said primary coil, when said FET is turned off, will resonate thereby compensating for drift in component values and reducing power transfer sensitivity to component drift. <IMAGE>

Description

TRANSCUTANEOUS ENERGY TRANSFER -DEVICE The present invention relates to the field of medical devices.

In particular, the present invention relates to power supply systems for transcutaneous energy transfer (TET) devices. Even more particularly, the present invention relates to an improvement in TET devices which simplifies such devices and improves their energy transfer efficiency.

A TET device is a device for providing electrical power to an implanted mechanical or electrical medical device, such as prosthetic hearts and ventricular assist devices, without having to breach the skin to lead conducting wires therethrough.

An example of a TET device is shown in U.S. Patent No.

4,665,896 (LaForge et al) dated May 19, 1987. That patent shows a blood pump system powered by a TET device having an external primary winding and an implanted secondary winding. It is designed to be regulated to a precise degree, the power delivered to an implanted medical device. However, it is not concerned with power transfer efficiency across the skin.

U.S. Patent No. 4,408,607 (Maurier) dated October 11, 1983, on the other hand, describes a TET device which charges an implanted capacitor. Power is then drawn by an implanted medical device from the capacitor. Maurier does not require particularly efficient TET efficiency, it will be understood, because it utilizes TET technology to provide an induced voltage to charge a capacitor. An efficient capacitor is, under Maurier's proposal, much more crucial than efficient TET.

In U.S. Patent No. 4,741,339 of May 3, 1988, Harrison et al describe a TET with improved coupling between internal and external inductive coils. The means for achieving such improved coupling proposed by Harrison includes a circuit electrically coupled to the primary coil, tuned to increase the quality factor of the primary transmitter circuit which includes the primary coil.

The object of the present invention is to provide a simple means of increasing power transmission efficiency levels in a TET device to over 80% - higher than in previous TET devices. The present invention accomplishes this result without the need for complex and expensive additional circuitry.

In a broad aspect, the present invention relates to an improved transcutaneous energy transfer (TET) device including: a primary winding for placement on or near a skin surface; a secondary winding for implantation under said skin surface; a field effect transistor (FET) arranged to switch said primary coils across an external DC power supply; and a tuning capacitor linked to said primary coil whereby said primary coil, when said FET is turned off, will resonate at its natural frequency thereby compensating for drift in component values and reducing power transfer sensitivity to component drift.

In another broad aspect, the present invention relates to a transformer having a primary coil and a secondary coil, said primary coil being substantially bell-shaped, and said secondary coil being suitably shaped and dimensioned whereby to allow for a substantial amount of variation in the relative positions of said primary and secondary coils while minimising the variation in coupling between said coils.

In drawings which illustrate the present invention by way of example: Figure 1 is a schematic of a DC to AC converter utilizing the improvement of the present invention; Figure 2 is a detail of the circuit of the secondary winding shown in Figure 1; Figure 3 is a cross sectional schematic of the configuration of the primary and secondary coils according to the present invention.

Referring now to Figures 1 and 2, it will first be appreciated that the present invention is designed to induce A.C. current in a subcutaneous winding, for transformation to DC to power of a medical device. AC current is induced in L2, the secondary winding which may be, for instance, a torus wound with a core of Litzendraht (Litz) wire implanted just under the skin S with electrical leads connected to a medical device requiring electrical power. A similar primary winding L1 is located in alignment with the secondary winding, on the skin surface.

Primary winding L1 is connected to a capacitor 11 that is connected to the negative of a DC input bus. Winding L1 is also connected to a field effect transistor (FET) 10, as indicated in Figure 1.

Power transfer takes place in two phases, a storage phase and a resonant phase. During the storage phase, energy is stored in the primary coil using a field effect transistor (FET) to switch the coil directly across the DC input supply. The FET is selected for its very low "on" resistance to minimize the conduction losses and operates as a "single-ended" power switch.

The ratio of duty time/cycle time for the system of the present invention is about 75%. Accordingly, the subcutaneous secondary circuit is, in the present invention, tuned to half the frequency of the primary, forming a dual resonant design. This causes the secondary circuit to uncouple from the primary during the primary resonant phase, thereby reducing waveform distortion resulting from wide variations in load.

During the resonant phase, the FET is turned off allowing the TET transformer primary to resonate with a tuning capacitor 11, thus transferring energy into the secondary coil. Allowing the transformer to resonate at its natural frequency enables automatic compensation for any drift in the component values in the primary circuit, thus reducing the power transfer sensitivity to component drift.

The resonant phase is terminated when the voltage across the FET reaches zero. At this point, the FET is again turned on to begin a new energy storage phase. Since the FET is only turned on close to a zero voltage crossing, switching losses in the FET are minimized. This enables the TET operating frequency to be increased over previous designs. Operating at higher frequencies permits smaller capacitors to be used for energy storage and smaller magnetic components for the transformer.

In addition, the use of a single ended quasi-resonant drive for the primary coil enables this circuit to tolerate variations in the transformer coupling due to coil separation. In previous designs, the primary transformer current increased as coupling was reduced, theoretically approaching infinity as the coupling reached zero. Thus it was necessary to include special circuitry to turn off the primary coil driver under such conditions. This additional circuitry is not required in the present design since a constant maximum stored energy operating mode is employed.

This mode of operation also allows the TET to tolerate induction losses due to adjacent conducting masses. In previous designs, the TET would shut down under such conditions, ceasing power transfer. The present design copes with this situation by reducing power transfer efficiency, shutting down only in extreme situations.

The use of the Litz wire contributes to the overall efficiency of the TET, which is over 80% for a wide range of load conditions.

The Litz wire is composed of many individually insulated strands which are bunched in a particular way to reduce eddy current losses. There are five bunches of five bunches of three bunches of 23 strands in the Litz wire giving a total of (5x5x3x23=) 1,725 strands. The increased surface area of the Litz wire contributes to the reduction in the losses in the coils.

As can be seen in Figure 2, the AC current induced in secondary winding L2 which resonates with capacitor 12. The AC is converted to DC by means of a simple circuit including a complimentary resonant capacitor 14 to further enhance the transmission efficiency of the TET systems. Resonant capacitor 12 is split into two capacitors 13 and 14. Under heavy load conditions, L2 resonates with 13. Under light or no load conditions, L2 resonates with 13 and 14.

The inclusion of this load sensitive tuning tends to stabilize the voltage transfer ratio of the TET against load variations.

This is achieved by modifying the resonant frequency of the secondary circuit as the load varies. This improves load regulation, and permits operation of the secondary circuit without complex feedback regulation.

Turning to Figure 3, the configuration of the primary and secondary coils is illustrated. It will be understood in previous TET designs, the implanted secondary coil is substantially encircled by the torus-like primary coil which sits on the skin surface. This arrangement permits fairly accurate emplacement of the primary coil over the secondary, and means that there is very little change in coupling co-efficient if the primary and secondary coils are moved slightly, as can easily happen in normal use.. The problem with this type of arrangement is that it is very sensitive to inductive influences, and the proximity of a large metal object will result in a complete shutdown of energy transfer.

The present invention however, provides a coil configuration that is relatively insensitive (about 12% power loss) to the presence of metallic objects. As can be seen from Figure 3, the present transformer employs a primary coil having a shallow bell shaped profile which covers the secondary coil. This results in a design which is relatively insensitive to inductive interference by adjacent conducting objects. The present method of electronic power transfer is also more tolerant to inductive interference and thus the overall TET system enables the energy transfer to tolerate close contact with a metallic surface.When a large metallic plate is brought into close contact with the TET primary coil, (limited only by the insulation thickness of said primary) energy transfer efficiency falls by only about 128. A similar situation applied to the prior systems would result in a complete shutdown of energy transfer.

The dome shaped construction of the secondary coil assists in coupling stabilisation and also mechanical alignment of the primary coil. The internal space that this affords is utilised to house the internal AC-DC converter 13, which results in a number of significant advantages: (1) Power dissipation in the AC-DC converter is better distributed by the large copper mass of the secondary coil. (2) This power no longer contributes to the increased temperature of the internal electronic controller. (3) High frequency, high voltage AC is kept within the secondary coil and away from other sensitive electronics. (4) The interconnecting wires from the secondary coil to the electronics and pump module carry DC and are not part of the tuned secondary circuit. This reduces the effective resistance and thereby increases the efficiency of the tuned circuit and enables conventional smaller gauge stranded wire (not Litz) to be used to carry the DC from the coil to the electronics.

In a typical embodiment, the primary coil will be about 90mm in diameter, with a depth of 23mm, and the secondary coil will be 66mm in diameter, with a depth of 24mm.

It is to be understood that the examples described above are not meant to limit the scope of the present invention. It is expected that numerous variants will be obvious to the person skilled in the TET art, without any departure from the spirit of the present invention. The appended claims, properly construed, form the only limitation upon the scope of the present invention.

Claims

CLAIMS:

1. An improved transcutaneous energy transfer (TET) device comprising: i) a primary winding for placement on or near a skin surface; ii) a secondary winding for implantation under said skin surface; iii) a field effect transistor (FET) arranged to switch said primary coil across an external DC power supply; and iv) a tuning capacitor linked to said primary coil whereby said primary coil, when said FET is turned off, will resonate at its natural frequency thereby compensating for drift in component values and reducing power transfer sensitivity to component drift.

2. A device as claimed in Claim 1, wherein said windings are made from Litzendraht (Litz) wire.

3. A device as claimed in Claim 2, wherein said FET is turned on when voltage across it reaches zero and said primary winding ceases resonating, whereby switching losses in the FET are minimised.

4. A device as claimed in Claim 1, 2 or 3, wherein a second tuning capacitor is provided, linked to said secondary coil, to stabilize the voltage transfer ratio from the primary to the secondary coil permitting operation without feedback regulation to control for output variations with load.

5. A device as claimed in Claim 1, 2 or 3, wherein said secondary coil is tuned to half the frequency of the first, resulting in dual resonance, and a duty cycle ratio of on time/cycle time of about .75.

6. A transformer having a primary coil and a secondary coil, said primary coil being substantially bell-shaped, and said secondary coil being suitably shaped and dimensioned to allow for a substantial amount of variation in the relative positions of said primary and secondary coils while minimising the variation in coupling between said coils.

7. A transformer as described in Claim 6, wherein said transformer is for use in a TET system, said primary coil is for placement on a skin surface, and said secondary coil is for subcutaneous implantation.

8. A transformer as described in Claim 7, wherein said secondary coil is provided with sufficient space inside its windings for inclusion of circuitry associated with said TET internal AC to DC converter.

9. A transformer as described in Claim 8, wherein said primary and secondary coils are made of Litz wire.

10. A transcutaneous energy transfer device substantially as herein described with reference to the accompanying drawings.

11. A transformer substantially as herein described with reference to Figure 3 of the accompanying drawings.