Analysis and Mitigation of Oscillations in Inductive Power Transfer Systems with Constant Voltage Load and Pulse Density Modulation

This paper analyses how constant voltage load (CVL) characteristics impacts the dynamics of Inductive Power Transfer (IPT) systems. Specifically, it is demonstrated how modulation strategies relying on elimination of voltage pulses, such as Pulse Density Modulation (PDM), can behave completely different when applied to systems with a CVL instead of a constant resistance load (CRL). The critical oscillation frequency caused by the CVL characteristics is identified from a linearized state-space model of the system, showing that large current/power oscillations can occur due to low damping when this oscillation frequency is excited by the PDM pattern. To solve these issues, an enhanced PDM based on delta-sigma modulator is applied to reduce the excitation of the oscillation mode, which limits the current/power ripple to a certain extent. Moreover, a sending current feedback control method is proposed in this paper, which adds phase shift modulation with limited phase shift angle to the PDM when oscillations are excited. Effectiveness and feasibility of the proposed method are validated by simulations and experimental results from a small-scale laboratory prototype.