A power transmitter includes a control and communications unit, an inverter circuit, a coil, and a shielding. The control and communications unit is configured to provide power control signals to control a power level of a power signal configured for transmission to a power receiver, provide a power timing request to an external power supply, and determine if the timing of the external power supply is compliant if the power timing request is met within a timing interval. The coil is configured to transmit the power signal to a power receiver, the coil formed of wound Litz wire and including at least one layer, the coil defining, at least, a top face. The shielding comprises a ferrite core and defining a cavity, the cavity configured such that the ferrite core substantially surrounds all but the top face of the coil.