A controller used to control a flyback switching mode power supply. The flyback switching mode power supply is constructed to charge a capacitor, and includes a rectifying device, a series arrangement of a switch and a primary winding of a transformer for receiving an input voltage, and a secondary winding of the transformer for charging the capacitor via the rectifying device to an output voltage. The controller is configured to sense the output voltage and to turn on the switch when the change of the output voltage over time becomes smaller than a predetermined threshold. By using the controller to sense and use the output voltage across the capacitor to turn on the switch, a controlled flyback switching mode power supply that makes use of voltage control is realized.