Energy transfer control adapted to a medical device system

An apparatus adapted to transmit wireless energy from an external energy transmitting device placed externally to a human body to an internal energy receiver placed internally in the human body is disclosed. The apparatus comprises a first electric circuit to supply electrical pulses to the external transmitting device, said electrical pulses having leading and trailing edges. The electrical circuit is adapted to perform at least one of the following: vary the lengths of first time intervals between successive leading and trailing edges of the electrical pulses and/or the lengths of second time intervals between successive trailing and leading edges of the electrical pulses, and vary the first and second time intervals to have lengths in the range of, that are in the magnitude of order of or are shorter than, a time constant of the first electric circuit, and a relationship or proportion between the lengths of the first and second time intervals. The transmitting device is adapted to supply wireless energy to the internal energy receiver, wherein the wireless energy is generated from the electrical pulses having a varied power, the power depending on the lengths of the first and/or second time intervals.