Mathematical Analysis of Duty Cycle Variation based Wireless Power Control Technique

Class-D power amplifier (PA) is used to drive the primary coil of a wireless power transfer (WPT) system. The PA is driven by a PWM controller, and the inductor current is computed. The analysis is verified by SPICE simulations and measurement results using a commercial EPC9507 wireless PA demonstration board, with accuracy better than 98% and 90%, respectively. Harmonic analysis of the inductor current shows that total harmonic distortion increases as the duty ratio D changes from 0.5 to other values. For a small change in D, the sensing voltage does not change significantly; but for a large change in D, the sensing voltage decreases exponentially. Therefore, output power vs D is a non-linear function, and both fine and coarse power modulation is possible.