Balanced coil for multi-filar windings

A base power-transfer apparatus for a wireless electric vehicle charging system, the base power-transfer apparatus comprising:

• a power converter for converting power from a power supply into a power signal;
• and
• a coil structure configured to be driven by the power signal to generate a magnetic field, the coil structure comprising:
• a layer of magnetically-permeable material;
• and
• a plurality of conductors that follow respective convoluted paths relative to the permeable layer to form respective inductors, the plurality of conductors having respective overall proximities to the permeable layer along respective lengths of the plurality of conductors that are substantially equal, the plurality of conductors having substantially equalized inductances based on the convoluted paths and interaction with the magnetically-permeable material.

A vehicle power-transfer apparatus for a wireless electric vehicle charging system, the vehicle power-transfer apparatus comprising:

• a coil structure configured to receive power from a magnetic field, the coil structure comprising:
• a layer of magnetically-permeable material;
• a plurality of conductors that follow respective convoluted paths relative to the permeable layer to form respective inductors, a first conductor of the plurality of conductors having a first overall proximity to the permeable layer along a length of the first conductor that is substantially equal to a second overall proximity of a second conductor of the plurality of conductors to the permeable layer along a length of the second conductor, the plurality of conductors having substantially equalized inductances based on the convoluted paths and interaction with the magnetically-permeable material;
• and
• control circuitry for controlling delivery of power from the coil to a battery.