An energy harvesting device includes: a rectifier circuit rectifying an AC supply voltage received between first and second input terminals thereof to generate a DC rectified voltage between first and second output terminals thereof; a converter circuit performing DC-to-DC conversion upon the DC rectified voltage based on a control signal to generate a DC output voltage; and a control circuit comparing a first to-be-compared voltage (correlated to the DC rectified voltage) and a second to-be-compared voltage (correlated to a difference voltage between one of the first and second input terminals of the rectifier circuit and one of the first and second output terminals of the rectifier circuit) to generate the control signal.