A power converter with bootstrap circuit, the power converter has a high side switch, a low side switch, a bootstrap circuit and a bootstrap capacitor for providing a bootstrap voltage to supply a high side driver of the high side switch. The power converter receives an input voltage and provides an output voltage based on driving the high side switch and the low side switch to switch on and off. The bootstrap circuit has a first comparing circuit, a first comparing circuit, a boost circuit and a second charging circuit. The second charging circuit charges the bootstrap capacitor when a voltage difference between the input voltage and the output voltage is smaller than a voltage threshold.