When thinking about the type of low-power wireless charging that has been used in devices like electric toothbrushes for many years, they are basically “open loop” and not well regulated systems. The charging source (transmitter – or stand for the toothbrush) simply generates a continuous AC excitation on the primary coil, and if the toothbrush is placed on it, power is applied to the load to charge the internal battery. This may be acceptable for output power levels less than one watt, but at higher power levels this becomes both wasteful and inefficient, generating a lot of heat. Like any inductively-coupled application, current flowing through the transmit coil generates a magnetic field. When the receiver coil is placed in proximity and aligned with the transmit coil, that magnetic field will cause a current to flow in the receiver coil. However in the case of the Qi system, there is a shielding material below the transmit coil, and above the receive coil. This keeps the field contained in a fixed area. However, unlike the “open loop” approach used in simple wireless power applications, the Qi system has intelligence built into the system (in the form of microcontrollers on both the receive and transmit side circuits). Firstly, the transmitter will not randomly generate output power unless it knows that a valid receiver is placed on it. So, if a transmit pad is plugged in with nothing sitting on it, very little power will be wasted. Secondly, if a device is placed on the charging pad (or table), the transmitter will only provide the level of output required, as requested by the receive controller based on the needed load current level. Feedback is provided from the receiver back to the transmitter back through the same inductive coupling as the forward power transfer, as will be shown in later pages.