Impedance-Sensing Technique Using a dual Harmonic/Negative Peak-Detector in Class EF2 Inverters for LCC-based WPTs

One of the difficulties in wireless power transfer systems (WPTs) at 6.78 MHz is a large variation of the transmitter (Tx) coil impedance due to effect of metal near receivers (Rxs) such as smartphones. To compensate the variations in the reflected reactance
${(X_{var}})$
on the Tx coil, impedance-sensing is necessary for the WPTs. However, the conventional methods should use a costly gain/phase detector, and a bulky current transformer or a directional coupler. In this paper, a novel impedance-sensing technique using a dual harmonic/negative peak-detector (HNPD) in a class EF2 inverter is proposed. Through the state-space model including the inverter and the LCC-based WPT, the impedance-sensing technique is designed in terms of two linear curve-fits in the gains of HNPD. Prototypes of the WPT, the HNPD, and a sliding-ferrite modulator (SFM) were fabricated. The distance
$(D)$
between the Tx coil and Samsung Galaxy S10 and the ferrite distance
$(D_{f})$
of the SFM were adjusted to manipulate
$X_{var}$
. Then,
$X_{var}$
were measured with a network analyzer as from
$-\mathrm{j}0.69\Omega$
to
$\mathrm{j}0.71\Omega$
. The two curve-fits of the HNPD over the impedance variations were experimentally validated and compared to the measured results. Sensitivities of the measured results were −1.44 and −2.38 in each dynamic range, while those of the estimation were −1.44 and −2.18, respectively. The residuals were maintained within 0.16 under load variations from 1
$\mathrm{m}\Omega$
to 100
$\mathrm{m}\Omega$
.