A transcutaneous energy transfer system for transferring energy across a boundary having dual power supply switching circuitry, comprising: an implantable medical device disposed interior of the boundary; an internal power source for powering the implantable medical device; an external device separated from the implantable medical device by and disposed exterior to the boundary, the external device being in telemetric communication with the implantable medical device and generating an external RF energy source during telemetric communication with the implantable medical device; power switching circuitry for switching from the internal power source to the external RF energy source during communication from the external device to the implantable medical device when power supplied by the external RF energy source exceeds that required for powering the implantable medical device; characterised in that: the power switching circuitry is a passive power switching circuit, wherein the passive power switching circuit comprises: a first diode electrically connected to draw power from the external RF energy source; a second diode electrically connected to draw powerfrom the internal power source; and wherein the power switching circuit further comprises a leakage current path via a switch connected in series to a resistor whose resistance is lower than that of the internal power source, wherein in presence of an external RF voltage, the switch is closed so that the leakage current flows through the resistor rather than the internal power source. an implantable medical device disposed interior of the boundary; an internal power source for powering the implantable medical device; an external device separated from the implantable medical device by and disposed exterior to the boundary, the external device being in telemetric communication with the implantable medical device and generating an external RF energy source during telemetric communication with the implantable medical device; power switching circuitry for switching from the internal power source to the external RF energy source during communication from the external device to the implantable medical device when power supplied by the external RF energy source exceeds that required for powering the implantable medical device; characterised in that: the power switching circuitry is a passive power switching circuit, wherein the passive power switching circuit comprises: a first diode electrically connected to draw power from the external RF energy source; a second diode electrically connected to draw powerfrom the internal power source; and wherein the power switching circuit further comprises a leakage current path via a switch connected in series to a resistor whose resistance is lower than that of the internal power source, wherein in presence of an external RF voltage, the switch is closed so that the leakage current flows through the resistor rather than the internal power source. the power switching circuitry is a passive power switching circuit, wherein the passive power switching circuit comprises: a first diode electrically connected to draw power from the external RF energy source; a second diode electrically connected to draw powerfrom the internal power source; and wherein the power switching circuit further comprises a leakage current path via a switch connected in series to a resistor whose resistance is lower than that of the internal power source, wherein in presence of an external RF voltage, the switch is closed so that the leakage current flows through the resistor rather than the internal power source. a first diode electrically connected to draw power from the external RF energy source; a second diode electrically connected to draw powerfrom the internal power source; and wherein the power switching circuit further comprises a leakage current path via a switch connected in series to a resistor whose resistance is lower than that of the internal power source, wherein in presence of an external RF voltage, the switch is closed so that the leakage current flows through the resistor rather than the internal power source.