  More and more applications are adopting the charging topology of wireless
power transmission. However, most wireless charging systems can not charge
mobile devices which are moving in position while charging. Currently, many
commercialized wireless charging systems adopt an inductive coupling method,
which has very short charging distances. In addition, the frequency of the two
coupled coils that produce maximum power transfer keeps varying, depending on
the coupling coefficient that relies on the separation between coils, and this
tendency becomes more severe when the coupling is strengthened at a close
charging distance by the phenomenon called frequency splitting. Therefore, the
existing wireless power transmission system using a fixed operating frequency
can not optimize power transmission for a fluctuating charging environment as
the coupling between coils changes, and charging efficiency is greatly reduced
by frequency splitting when charging at a very short distance. To solve this
problem, we proposed the method of estimating the RX side power and mutual
inductance using the information from the TX side such as input impedance
rather than using a direct communication link which adds more cost and
complexity. Also, we derived a mathematical model for the above estimation
method. To prove this mathematical model, the proposed wireless power
transmission system was implemented in a SIMULINK environment, and the system
model was validated through simulation. Also comparison between the adaptive
frequency tracking method and static impedance matching circuit is made by
analyzing simulation results.
