Impedance transformation network for improved driver circuit performance

A method of adjusting an impedance of a driver circuit characterized by an efficiency, the driver circuit electrically connected to a transmit circuit having an impedance, the impedance of the transmit circuit within a complex impedance range including resistive and reactive variations, the complex impedance range defined by a minimum real impedance value and a maximum real impedance, a ratio between the minimum and maximum real impedance value being at least two to one, the method comprising:

• modifying the impedance to maintain the efficiency of the driver circuit at a level that is within 20% of a maximum efficiency of the driver circuit when the impedance is within the complex impedance range, and to maintain a substantially constant power output level irrespective of the reactive variations within the complex impedance range.

A transmitter device, comprising:

• means for driving characterized by an efficiency, the means for driving electrically connected to a transmit circuit having an impedance, the impedance of the transmit circuit within a complex impedance range including resistive and reactive variations, the complex impedance range defined by a minimum real impedance value, and a maximum real impedance, a ratio between the minimum and maximum real impedance value being at least two to one;
• and
• means for filtering electrically connected to the means for driving and configured to modify the impedance to maintain the efficiency of the driving means at a level that is within 20% of a maximum efficiency of the driving means when the impedance is within the complex impedance range, the means for filtering further configured to maintain a substantially constant power output level irrespective of the reactive variations within the complex impedance range.

An apparatus for wirelessly transmitting power, comprising:

• a transmit circuit characterized by a first impedance and configured to generate a magnetic field for wirelessly transferring power to one or more receiver devices to power or charge respective loads of the one or more receiver devices positioned within the magnetic field, the first impedance varying over a first range in response at least in part to changes in a number of the one or more receiver devices positioned within the magnetic field;
• a driver circuit configured to drive the transmit circuit with a time-varying signal to cause the transmit circuit to generate the magnetic field;
• a passive filter circuit comprising one or more reactive components having respective reactance values selected to transform the first impedance presented to the passive filter circuit by the transmit circuit within the first range to a second impedance presented to the driver circuit by the passive filter circuit that has a complex impedance value that is within a second range of complex impedance values corresponding to a range of complex impedance values that maintains an efficiency level of the driver circuit above a threshold efficiency value, the passive filter circuit further configured to maintain a substantially constant power output level irrespective of the reactive variations within the complex impedance range.