Abstract
This paper presents a detailed analysis of power transfer efficiency for inductive battery charging systems designed for sub-resonant frequency control during large variations in coupling conditions. It is first demonstrated how the design for power control by sub-resonant operation allows for almost constant current amplitude and thereby constant conduction losses independently of the variations in the coupling coefficient. Since the design also can ensure slightly inductive current in the full operating range, zero voltage switching (ZVS) with very low switching losses is also ensured. Thus, the studied charging system can maintain almost constant efficiency for large variations in the magnetic coupling coefficient. A comparison with a conventionally designed inductive charging system relying on voltage control at the resonant frequency is also presented. The comparison highlights the advantages of the sub-resonant frequency control in terms of limiting component stress and maintaining a high efficiency in a wide range of coupling conditions. The efficiency characteristics that can be obtained by design for off-resonant frequency control are also demonstrated by experimental results from a small-scale laboratory prototype.