TET system for implanted medical device
A TET system is operable to vary an amount of power transmitted from an external power supply to an implantable power unit in accordance with a monitored condition of the implantable power unit. The amount of power supplied to the implantable power unit for operating a pump, for example, can be varied in accordance with a cardiac cycle, so as to maintain the monitored condition in the power circuit within a desired range throughout the cardiac cycle.
1. A system, comprising:
(a) an implantable electrical device adapted for mounting within a body of a patient;
(b) an implantable power unit adapted for mounting within the body of the patient, the power unit having a receiving coil, and a power circuit connected to the receiving coil so that power applied to the receiving coil can be transmitted to the implantable device;
(c) an external power supply having a transmitting coil adapted for transcutaneous inductive coupling with the receiving coil to define a system having a resonant frequency varying in relation to a distance between the transmitting and receiving coils, a drive circuit operable to apply an alternating current having a variable transmission frequency to the transmitting coil at a variable drive pulse width, and a control circuit operable to (i) monitor a parameter related to mutual inductance between the transmitting and receiving coils; and (ii) adjust the transmission frequency to a value having a predetermined difference from the resonant frequency, such that the control circuit is operable, using the monitored parameter, to maintain the transmission frequency with the predetermined difference from the resonant frequency.
2. A system as claimed in claim 1 , wherein the control circuit is operable to maintain the transmission frequency below the resonant frequency.
3. A system as claimed in claim 2 , wherein the predetermined difference is a percentage difference from the resonant frequency.
4. A system as claimed in claim 1 , wherein the control circuit is operable to adjust the variable transmission frequency from a current value to a new value different from the current value when the difference between the new value and the current value is greater than a threshold.
5. A system as claimed in claim 4 , wherein the control circuit includes a table storing a plurality of frequency values corresponding to different values of the monitored parameter, wherein the control circuit is operable to use the monitored parameter to look up the new value from the table.
6. A system as claimed in claim 1 , wherein the monitored parameter includes an estimate of the distance between the transmitting and receiving coils.
7. A system as claimed in claim 6 , wherein the control circuit is operable to estimate the distance using at least a measurement of a voltage across the transmitting coil.
8. A system as claimed in claim 7 , wherein the control circuit is operable to estimate the distance by also using a measurement of the current supplied to the transmitting coil.
9. A system as claimed in claim 1 , wherein the implantable device is a circulatory assist pump having an electric motor with a time-varying power demand.