One of the difficulties in wireless power transfer systems (WPTs) at 6.78 MHz is a large variation in the impedance (Zvar) of the transmitter (Tx) coil due to effect of metal in receivers (Rxs) such as smartphones, which mainly causes the variation in the imaginary part (i.e., Im{Zvar}). This paper presents a dual harmonic/negative peak-detector (HNPD) in class EF2 inverters to sense Im{Zvar} while avoiding the use of bulky current transformers and expensive phase-detector circuits. Through a state-space model of the inverter with a LCC-based WPT, an algorithm using two linear gain curves of the HNPD is also developed.
