Control method for Inductive Power Transfer with high partial-load efficiency and resonance tracking

Frequency controlled Inductive Power Transfer (IPT) systems for Electric Vehicle (EV) battery charging applications often suffer from high power losses in partial-load, because the transmitter coil current is not significantly reduced at low output power. Therefore, in this paper a novel control method is presented that exhibits a substantially higher partial-load efficiency, while it also enables full control of the power semiconductor switching conditions. The power flow control is based on the dynamic regulation of the dc-link voltages on both sides of the resonant system with dc-dc-converters. Additionally, a tracking of the resonance with a current zero crossing detection circuit and a PLL makes the switched current an additional degree of freedom, that can be used, e.g., for the minimization of IGBT soft-switching losses due to stored-charge. All calculated results are supported by experimental measurements on an existing 5 kW/52mm air gap/210mm coil diameter prototype system with an efficiency of more than 96.5% at maximum power and above 96% down to 20% rated power.