Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-10-27
2001-05-29
Getzow, Scott M. (Department: 3737)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06240318
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to transcutaneous energy transmissions systems. In particular, the invention relates to the use of two coils to inductively transmit radio frequency power across an intact skin boundary for use by a device implanted under the skin, or otherwise within the body of, a living being.
2. State of the Art
Transcutaneous energy transfer systems (TETS) have been used to provide power for a number of implanted devices such as: low power prosthetic devices; cochlear implants, muscle stimulators, bone growth stimulators and stimulators of the visual cortex, and higher power devices; total artificial hearts, and ventricular assist devices. The inductively coupled coils of a TETS, one implanted under the skin, the other resting on the surface of the skin, permit electrical interaction between the implanted device and external circuits through intact skin, and bring about the transfer of power from the external circuit to the internal circuit avoiding penetration of the skin by electrical conductors.
FIG. 1
illustrates a vertical cross section through an exemplary prior art TETS as implanted and in position for use.
The interaction between the coils requires alternating current; usually a frequency between 100 kHz and 1 MHz is chosen. In order to improve the efficiency with which power is delivered, the coils are incorporated into either series or parallel resonant circuits by connecting them to capacitors. The resonant circuits can be tuned with a natural resonant frequency less than, equal to, or greater than the operating frequency, as described by the U.S. Pat. Nos. 4,441,210 and 5,070,535 to Hochmair et al. and U.S. Pat. No. 4,679,560 to Galbraith. In the following discussion, the efficiency with which power is transferred to the internal coil will be referred to as link efficiency. This quantity is related to the overall efficiency of the TETS by the expression:
&eegr;
overall
=(&eegr;
DC to RF
)(&eegr;
link
)(&eegr;
RF to DC
) (1)
Thus, the overall efficiency is equal to the product of three sequential process efficiencies.
DC to RF Conversion
LaForge in U.S. Pat. No. 4,665,896 and Miller in U.S. Pat. No. 5,350,413 describe the need to have an active control circuit to adjust the frequency of the circuitry driving the external resonant circuit such that a constant phase relationship is maintained between the drive voltage and the resonant current. With this “auto-tuning” circuit, as described by LaForge and Miller, the frequency of operation is maintained at, or near, the resonant frequency. Because of variable coupling between the transmitter and receiver coils, the resonant frequency is a function of relative coil position.
Link Efficiency
In U.S. Pat. No. 4,441,210 to Hochmair et al., resonant frequencies of both the external and the internal circuits were implicitly tuned to the operating frequency. Hochlmair et al. explicitly clarified this point in his U.S. Pat. No. 5,070,535. Hochmair et al. adjusted the quality factor, Q, of his circuits in order to achieve critical coupling and an output that was not sensitive to the relative position of the coils. The degree of coupling is indicated by the coupling coefficient, k, which is equal to unity for an ideal transformer and equal to zero for isolated coils. For given values of Q
R
and Q
T
in the external and internal circuits, there is a value of the coupling coefficient at which the coupling is critical, that is, the output is then a maximum. Critical coupling is achieved when
k
2
=
1
Q
R
Q
T
(
2
)
By adjusting the Q values, the output can be made to have its maximum at the value of k corresponding to the coil separation expected when the system is actually used. A by-product of the Q tuning approach is that the link efficiency of a TETS would be a maximum of 50% when operated at critical tuning.
Galbraith, U.S. Pat. No. 4,679,560, explored the operation of a TETS with resonant frequencies varying from the operating frequency. Galbraith used frequency modulation to communicate digital information to a cochlear implant to which a TETS was connected. Setting the resonant frequencies above and below the center frequency, in what he called “stagger tuning,” increased the range of signal transmission frequency over which the system could operate. Galbraith studied the variation in the voltage gain of a TETS with coupling coefficient, k, and found that stagger tuning permitted the gain to be substantially unchanged within a range of frequencies and coupling coefficient. Galbraith also pointed out that, by designing high values of Q and tuning the external circuit to resonate below the operating frequency, an overall efficiency of greater than 50% was possible while maintaining insensitivity to variation in relative coil position.
Hochmair et al., in U.S. Pat. No. 5,070,535, discloses improved efficiency while maintaining insensitivity to relative coil position brought about by detuning the transmitter resonant frequency. In other words, Hochmair et al. set the resonant frequency of the transmitter different from the operating frequency, while the receiver resonant frequency was equal to the operating frequency.
RF to DC Conversion
Current produced in an internal coil of a TETS alternates polarity at the frequency of operation. However, implanted devices require a supply of direct current. A rectifier is typically used in a TETS to convert alternating current to direct current. There are many types of rectifiers. The most common rectifier used in a TETS is a bridge type full wave rectifier. Other conventional TETS use a center tapped rectifier. Exemplary bridge type full wave rectifiers may be found in U.S. Pat. Nos. 3,867,950, 3,942,535, 4,187,854, 5,350,413, 5,702,431, 5,713,939, 5,735,887 and 5,733,313. Exemplary center tapped rectifiers may be found in U.S. Patent Nos. 3,454,012, 4,082,097, 4,096,866 and 4,665,896. Rectifiers differ in efficiency. The ratio of DC current output by the rectifier to the AC current input into the rectifier is characterized by the conversion coefficient K
i
. Table 1 below, lists values of K
i
for a number of current driven rectifier circuits.
TABLE 1
AC to DC conversion ratios for Class D (bridge) rectifiers.
Current Driven Rectifier Type
AC to DC Conversion Coefficient, K
i
Class D half wave
0.45
Class D transformer center tapped
0.90
Class D full wave bridge
0.90
A rectifier may be considered current driven if the resonant circuit to which it is connected has a quality factor, Q, greater than three, where the load is a resistor.
However, Class D rectifiers have an undesirable ringing problem as described by Bowman et al. in U.S. Pat. No. 4,685,041. In bridge rectifiers, the diodes which are not conducting at some instant are contributing reverse bias capacitance to the circuit which, in combination with parasitic inductance in the connections among the components, promotes ringing. Bowman et al. suggested that these parasitic circuit elements be considered and utilized, if possible, in rectifiers for use at high frequencies. R. J. Gutmann,
Application of RF Circuit Design Principles to Distributed Power Converters,
IEEE Transactions Industrial Electronics, Control, and Instrumentation, Vol. IECI-27, pp. 156-64, (1980), discloses a rectifier design using LC filters to control the ringing problem.
Miller, in U.S. Pat. No. 5,350,413, discloses a full bridge rectifier with a capacitor across the input to the rectifier for the purpose of maintaining a high quality factor, Q, of the receiver resonant circuit when the implant presented a high resistance at light load. As disclosed in Miller, AC current flows through the capacitor during all load conditions and increased the RF current flowing in the receiver coil, but did not contribute to the DC output current.
FIG. 2
is a circuit diagram of a Class E half wave low dv/dt rectifier with a parallel capacitor as disclosed in the prior art. As shown in
FIG. 2
, the Class E half wave low dv/dt rectifier includes a diode, D, a shunt capacitor, C, a fi
Getzow Scott M.
TraskBritt
LandOfFree
Transcutaneous energy transmission system with full wave... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Transcutaneous energy transmission system with full wave..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Transcutaneous energy transmission system with full wave... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2470008