Inductive power transfer system

An inductive power transfer system for coupling a power source to a load across an air gap, the system comprising a primary unit associated with a host platform and a secondary unit arranged to receive power transmitted inductively from the primary unit,

• wherein the primary unit includes:
• a phase detection circuit configured to detect phase changes in a signal in the primary unit indicative of changes in an operating condition within the secondary unit;
• a drive;
• circuit
• configured to adjust the power level transmitted to the secondary unit depending on the detected phase,
• wherein the drive circuit comprises a primary inductor coil and a series resonant capacitor forming a resonant circuitry;
• and
• a primary switching frequency circuit comprising an internal clock and configured to alter a switching frequency of the internal clock;
• and
• wherein the phase changes detected by the phase detection circuit are phase changes in a resonant signal of the resonant circuitry relative to a clock signal phase;
• and wherein the phase detection circuit is configured to measure the resonant signal phase by detecting the phase of a voltage waveform on the junction of the series resonant capacitor and primary inductor coil.