Recently, many applications have emerged that use wireless power delivery technology. A new standard "Qi"  has been established for battery operated portable devices. The rechargeable battery acts as a power buffer and shields the rapid load variation. Therefore, the requirement of the response to load variation is not so stringent and time constant of the power control loop is longer than several tens of milliseconds. Several hundred kHz band is chosen for switching frequency to reduce the power loss and frequency modulation (FM)  or pulse-width modulation (PWM)  are used for power control because of their relaxed in-band spurious emission regulation.