The total power output can be increased (or decreased) by varying the switching frequency on the primary side. To allow the lowest possible inductance, the system operates above the resonant frequency. At the resonant point, the output voltage reaches its maximum value. As the frequency is increased above the resonance point, the amplitude will decrease. Typically operating range is 100 ~ 200 KHz. So what happens in an actual application is that when the receiver device has a high load and requires more power out from the transmitter, it sends a communication packet asking for more power. The transmit controller adjusts the switching frequency closer to the resonant peak until the receiver device gets the power level it is asking for. When the receiver device does not need as much power anymore (for example when the battery charge is completed), the transmitter will get a packet asking to decrease its output and the switching frequency can be shifted further away from the resonant peak to reduce power output.