A method of controlling a direct current (“DC”) chopper controller having a switch and a storage choke in which the choke has a current dependent on the switching of the switch on and off includes the following. A current variable derived from one of the time averaged value of the current of the choke and the peak value of the current of the choke is determined. A switching frequency for synchronizing the turning on and off the switch is calculated as a function of the current variable and on whether the current variable exceeds a current threshold. The switching frequency is determined to be a predetermined low frequency if the switching frequency is lower than the low frequency and is determined to be a predetermined high frequency if the switching frequency is greater than the high frequency. The switch is switched on and off in accordance with the switching frequency.