  Dynamical models of wireless power transfer (WPT) systems are of primary
importance for the dynamical behavior studies and controller design. However,
the existing dynamical models usually suffer from high orders and complicated
forms due to the complex nature of the coupled resonances and switched-mode
power converters in WPT systems. This letter finds that a well-tuned WPT system
can be accurately described by a much simpler dynamical model. Specifically, at
the tuned condition, the existing dynamical model can be decomposed into two
parts. One is controllable and the other one is uncontrollable. The former
should be considered in the modeling while the latter can be ignored because it
always exponentially converges to zero. For illustration, the recently proposed
zero-voltage-switching full-bridge pulse-density modulation WPT system is
modeled as an example since such a system can efficiently operate at the tuned
condition with soft switching and control capabilities. The derived model was
verified in experiments by time-domain and frequency-domain responses.
