The present invention relates to a system, comprising:(a) animplantable electrical device adapted for mounting within abody of a patient；(b) an implantable power unit adapted formounting within the body of the patient, the power unit havinga receiving coil, a power circuit connected to the receivingcoil so that power applied to the receiving coil can betransmitted to the implantable device；(c) an external powersupply having a transmitting coil adapted for transcutaneousinductive coupling with the receiving coil to define a systemhaving a resonant frequency varying in relation to a distancebetween the transmitting and receiving coils, a drive circuitoperable to apply an alternating current to the transmittingcoil at a variable drive pulse width, and a control circuitoperable to (i) monitor a parameter related to mutualinductance between the transmitting and receiving coils； and(ii) adjust a variable transmission frequency to a valuehaving a predetermined difference from the resonantfrequency, so as to maintain a transmission frequency withthe predetermined difference from the resonant frequency.
