Wireless power transfer

A wireless charging receiver, comprising:

• a receiver configured to receive wireless power transfer from a charger having a charger coil, a substantially planar surface, and a thin magnetic planar layer, wherein the receiver is coupled to a mobile device to provide power to charge a battery in the mobile device, wherein the receiver comprises a receiver coil coupled to one or more resonant capacitors, a rectifier circuit, and smoothing capacitors, wherein the receiver is configured to operate at an operating frequency;
• wherein the receiver coil is a solenoid comprising a ferrite core with a relative magnetic permeability exceeding 1 and with copper Litz wire wrapped around the ferrite core along a winding axis that is located at a center of the ferrite core and that is substantially parallel to the planar surface to form a wound wire section, wherein the ferrite core extends beyond the wound wire section in the direction of the winding axis and terminates in a first end and a second end with a distance between the first end and the second end defining the length of the solenoid, and wherein the first end and the second end each have a smaller dimension in either axis of the end that is smaller than the length of the solenoid;
• wherein when the receiver coil is placed in a charging position relative to the planar surface, magnetic flux from the charger coil is guided through the ferrite core of the receiver coil and returns to close a magnetic loop through the thin magnetic planar layer to transfer power to the receiver to charge the battery;
• and
• wherein the receiver is configured such that, in the charging position, a system quality factor of the receiver and the charger is sufficiently high to guide the magnetic flux to the receiver coil from the charger coil when the winding axis of the solenoid is located at an operating distance from more than 3 mm to 2 cm from the planar surface.

A wireless charger comprising:

• a charger coil comprising copper material, having a substantially planar surface;
• a thin magnetic planar layer located under the charger coil;
• and
• a charger drive circuit including resonant capacitors to supply an alternating current to the charger coil at an operating frequency;
• wherein the charger coil is patterned in a substantially spiral shape defining a charger coil area;
• and
• wherein when the charger coil is driven by the charger drive circuit with the alternating current, an inductive magnetic field in a direction substantially perpendicular to the planar surface, at the planar surface at a center of the charger coil area is generated;
• and
• wherein when a receiver that has a receiver coil comprising a solenoid with a ferrite core and is coupled to a mobile device is placed in a charging position relative to the planar surface, magnetic flux from the charger coil is guided through the ferrite core of the receiver coil and returns to close a magnetic loop through the thin magnetic planar layer to transfer power to the receiver to charge a battery in the mobile device;
• and
• wherein the charger is configured such that, in the charging position, a system quality factor of the charger and the receiver is sufficiently high to guide the magnetic flux from the charger coil to the receiver coil when a winding axis that is located at a center of the ferrite core and that is substantially parallel to the planar surface is located at an operating distance from more than 3 mm to 2 cm from the planar surface.