An external transmitter apparatus for a transcutaneous energy transfer system for supplying power for use in energising an implantable medical device in use, the external transmitter apparatus comprising; a plurality of transmitter coils (17, 19), each transmitter coil (17; 19) being capable of transmitting power transcutaneously to a respective one of a plurality of receiver coils (21, 23) of an implantable receiver apparatus of the TET system when located in proximity thereto in use for use in energising an implantable medical device connected to the implantable receiver apparatus in use, the external transmitter apparatus of the TET system further comprising power supply means for providing a power supply to each one of the transmitter coils (17, 19) of the external transmitter apparatus in use for transmission transcutaneously by the transmitter coil (17; 19) to a respective receiver coil (21; 23) of the implantable receiver apparatus located in proximity thereto, wherein each transmitter coil (17; 19) is associated with a respective power supply channel (13; 15), and the power supply means is arranged to deliver the power supply to each transmitter coil (17; 19) over the respective power supply channel (13; 15) associated therewith, characterised in that the power supply means is arranged to deliver a pulsed power supply to each transmitter coil (17; 19) over the respective power supply channel (13; 15) associated therewith, wherein the power supply means of the external transmitter apparatus comprises control means for controlling the supply of power over the different channels (13, 15) of the transmitter apparatus, the control means being arranged to control a timing of the pulses of the pulsed power supply provided on different ones of the plurality of channels (13, 15), and wherein the control means is arranged to control a timing of the pulses of the pulsed power supply over different ones of the plurality of channels (13, 15) such that each channel has a different respective timeslot in which pulses are provided over the channels (13, 15). a plurality of transmitter coils (17, 19), each transmitter coil (17; 19) being capable of transmitting power transcutaneously to a respective one of a plurality of receiver coils (21, 23) of an implantable receiver apparatus of the TET system when located in proximity thereto in use for use in energising an implantable medical device connected to the implantable receiver apparatus in use, the external transmitter apparatus of the TET system further comprising power supply means for providing a power supply to each one of the transmitter coils (17, 19) of the external transmitter apparatus in use for transmission transcutaneously by the transmitter coil (17; 19) to a respective receiver coil (21; 23) of the implantable receiver apparatus located in proximity thereto, wherein each transmitter coil (17; 19) is associated with a respective power supply channel (13; 15), and the power supply means is arranged to deliver the power supply to each transmitter coil (17; 19) over the respective power supply channel (13; 15) associated therewith, characterised in that the power supply means is arranged to deliver a pulsed power supply to each transmitter coil (17; 19) over the respective power supply channel (13; 15) associated therewith, wherein the power supply means of the external transmitter apparatus comprises control means for controlling the supply of power over the different channels (13, 15) of the transmitter apparatus, the control means being arranged to control a timing of the pulses of the pulsed power supply provided on different ones of the plurality of channels (13, 15), and wherein the control means is arranged to control a timing of the pulses of the pulsed power supply over different ones of the plurality of channels (13, 15) such that each channel has a different respective timeslot in which pulses are provided over the channels (13, 15).