A method of detecting a fault of a switching electricity source having a control stage responsive to a setpoint signal to deliver a control signal that is modulated with a predetermined duty ratio (α) to a power stage that is to deliver electricity to a load. The method includes the steps of: determining at least one nominal value (αmin, αmax) for the duty ratio for normal operation, and at least one fault threshold (αSC, αOC) as a function of the nominal value; determining at least one instantaneous value (α) of the duty ratio of the signal issued by the control stage; and comparing the instantaneous value with the fault threshold. A detection circuit implementing the method is also provided.