Class EF inverter are very suitable for high frequency operations and are widely used in power converter. However the design of such converter is still a problem since it is very sensitive to load variation. This paper provides analytical study of the overall behaviour of the class EF / Φ inverter whether it is loaded with its optimal impedance or not. Because obtained design equations are highly non-linear, they were gathered in a single design chart that enable quick and easy design of class EF inverter for any loading conditions. This work stands as a strong contribution to the mathematical modeling of the class EF inverter and provides an in-depth understanding of its overall behaviour.