Reference - WPT Application Note
One of the pitfalls of this circuit is that the power consumption can be high when the secondary coil is absent.
Is there a way to design an auxiliary circuit to detect when the receiver is not present and to reduce the idle power consumption (possibly, by introducing a high series impedance and decrease resonant current)? However, it should also be able to detect when the receiver is coupled and activate the normal operation. Can we use something like tickler coil for this purpose?
I want to exclude obvious approaches which require the installation of additional sensing layer, for example, proximity sensors to detect receiver or additional communication link.
Next, as some of the answers suggested, using a simple micro to detect the changes in primary phase or define a current threshold to cut off the supply can be implemented. However, such approaches are highly dependent on the circuit parameters (frequency, power, etc) and most importantly, these sensing become challenging if we want coupling to be weak. When coupling is small, reflected impedance may be very small to make significant changes in primary side.