This publication describes methods to accommodate a decrease in the vertical distance between a power transmitter (PTx) of a wireless power-transfer (WPT) system (e.g., wireless-charging pad, Qi charger) and a power receiver (PRx) of a user equipment (UE), such as a smartphone, earphones, a smartwatch, and the like. Original design manufacturers (ODMs) design and build increasingly thinner WPT systems and thinner UE. A decrease in the vertical distance between the PTx and the PRx causes an increase in the PTx-to-PRx inductive coupling coefficient (Kc), where 0c<1. Subsequently, the input-to-output voltage gain (AV) is lower, which is not a desirable design. In addition, the range of AV grows tighter, which translates to a poorer response of the PRx output voltage (VOUT) to a PTx switching frequency (fSW). To maintain the same resonant frequency and to accommodate for a decrease in the vertical distance between the PTx and PRx, an ODM can add an external inductance (LExt) that is coupled in series with a leakage inductance (LLeak) of the PRx. In addition, by adding an LExt to the PRx, the ODM of the WPT system can shrink the absolute vertical distance needed between the PRx and PTx and extend the range of the vertical distance needed between the PRx and PTx to maintain the desired output voltage. Therefore, adding an external inductance to the PTx of a WPT system, enables a WPT system ODM and a UE ODM to design and build thinner devices and still maintain, or even increase, the desired output voltage.
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"Method to Decrease the Vertical Distance in Wireless Charging", Technical Disclosure Commons, (August 20, 2019)