US Patent 9577529 Energy harvesting circuit and method

An energy harvesting circuit is based on a switch mode inductive DC-DC converter circuit. The inductor current is sensed and a duration of an on-time is controlled in dependence on the sensed inductor current. A duration of an overall switching period of the converter circuit is controlled in dependence on an on-time set by a first timing control circuit and input and output voltages. This converter circuit enables independent control of the on-time and a full period of a converter cycle. Very rapid switching can be avoided which can give rise to very high energy consumption. The full cycle period can be set to achieve a desired constant value of an input resistance of the DC-DC converter, and thereby maximize power transfer.