Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relate~ to the f ield of medical device~: .
In particular, the present invention relates to power supply
systems for transcutalleous energy transfer (TET) devices. Even
more particularly, the present invention relates to an improvement
in TET devices which simplifies such devices and; ov~:s their
5 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 design5~1
to be regulated to a precise deqree, the power delivered to zm
15 implanted medical device. ~owever, 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
20 device from the capacitor. Maurier does not retluire particularly
efficient TET efficiency, it will be understood, because it
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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
5 describe a TET with i ~ ~ v~d 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
secr~ ry winding for implantation under said skin surface; a field
effect transistor ~FET~ arranged to switch said primary coils
20 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
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c~ - - ting 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
5 primary coil being substantially bell-shaped, and said secondary
coil being 6uitably shaped and dimensioned whereby to allow for a
substantial amount of variation in the relative positions of said
primary and ~ n~9~ry 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 u~ i 1 i 7 i n~ the
pL~V~ ~ of the present invention;
Figure 2 is a detail of the circuit of the secondary winding
15 shown in Figure 1;
Figure 3 is a cross sectional schematic of the conf iguration
of the primary and secondary coils according to the present
invention .
Referring now to Figures 1 and 2, it will first be appreciated
20 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 in;uced in L2, the secon~ary winding
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which may be, for instance, a toruæ 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 Ll is located in alignment with
5 the secondary winding, on the skin surf ace .
Primary winding Ll is connected to a capacitor 11 that is
conn~t~fl to the negative of a DC input bus. Winding Ill 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
15 and operates as a "single-ended" power switch.
The ratio of duty time/cycle time for the system of the
present invention is about 759~. Accordingly, the subcutaneous
secondary circuit is, in the present invention, tuned to half the
~requency of the primary, forming a dual resonant design. This
20 causes the s~nnA~ry circuit to uncouple from the primary during
the primary resonant phase, thereby reducing waveform distortion
resulting from wide variations i load.
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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
5 compensation for any drift in the comron~nt 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
10 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
15 smaller magnetic r ~ ~nts 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
20 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
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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
5 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
10 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 fi~e bunches of five bunches of three bunches
of 23 strands i~ the Litz wire giving a total of (5x5x3x23=) 1,725
15 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
20 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
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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 transf er ratio of the TET against load variations .
5 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 conf iguration of the primary and
lO 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
15 little change in coupling co-efficient if the primary and ~econ~lAry
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 ob jects. As can be seen from Figure 3, the
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present transformer e~ploys ~ 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 c~n~llle~;n~ objects. The present method of electronic
5 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
lO efficiency falls by only about 12%. A similar situation applied
to the prior systems would result in a complete shutdown of energy
trans f er .
The dome shaped construction of the secondary coil assists in
coupling st~h; 1 i~4Ation and also mechanical alignment of the primary
15 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
20 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 th- tune~ secondary circuit. This
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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.
