Design for continuous-current-mode operation of inductive-power-transfer converters with load-independent output

The control design of inductive power transfer (IPT) converters can be greatly simplified by exploring the property of load-independent output. However, IPT converters can easily enter a discontinuous current mode (DCM) operation due to the non-linearity of the output diode rectification circuit at some loading conditions. The switching between operations of continuous current mode (CCM) and DCM within a switching cycle is highly non-linear, making the converter behaviour difficult to predict and analyse. The well-known first harmonic approximation (FHA) analysis method and the load-independent output property are no longer applicable when the converter enters DCM operation. This paper presents a simple and yet effective harmonic analysis method to reveal the main reason for the converter to enter DCM operation. Subsequently, the load boundary between CCM and DCM operations is derived in detail as a design criterion. A solution by increasing the input impedances at some higher order harmonic frequencies is also suggested to extend the load range against DCM operation. Finally, IPT prototypes using two higher order compensation circuits, having load-independent voltage output and capacitor filter, are demonstrated to verify the theoretical analysis.