WPT systems with almost constant output voltage – method design including voltage/current distortions

The concept of wireless power transfer (WPT) systems with almost constant output voltage despite load changes and its design method including voltage/current distortions are presented in the paper. The analysis is carried out for two-coils inductive power transfer circuits with series-series compensation. Both inductive and capacitive components are used for compensation. Such circuits (assuming lossless analysis for fundamental harmonic approximation) have an ability to keep a constant output to input voltage ratio at variable load. This voltage ratio can be divided into a transformer ratio and an additional compensation ratio. The presented efficiency analysis explains how to calculate the optimal secondary coil reactance and how to select the additional compensation ratio. At the same time, it is showed that the voltage independency is not ideal and depends mainly on quality factors of the magnetically coupled coils. The concept is verified for a DC/DC system containing a bridge inverter at the input side and a bridge rectifier with capacitive filter at the output side. First, the converters are approximated with fundamental harmonic waveforms and then as rectangular voltage waveforms. Both cases are compared to show the influence of voltage/current distortions on WPT system properties. The experimental verification is included.