A buck converter includes an input terminal, two MOSFETs, a PWM module and a control module. The input terminal is coupled to a power source to provide power to the buck converter. The PWM module is coupled to the gates of the two MOSFETs to alternately turn on and turn off the two MOSFETs. The control module is configured to shut down the power source if a drain-source resistance or a gate-source resistance in either one of the two MOSFETs is not within normal values when the two MOSFETs are turned off.