A controller for a power converter to convert electrical power at an input voltage into electrical power at an output voltage and a method of operating such controller is presented. The controller for controlling a power converter has an input port to receive a voltage representative of the input voltage; an input voltage measuring unit to sample a measuring voltage and to determine a measurement value that is representative of the input voltage; a switch; and a diode connectable with a storage unit to provide a supply voltage for the controller during operation of the controller. The switch controls the charging of the storage unit from the voltage at the input port.