An apparatus and a method for converting power from a power input to an DC output voltage or current, which apparatus has a serial resonance converter, where a first feedback circuit is connected from the output terminal to an error amplifier, where the apparatus further has a second feedback circuit with at least one first resistor that is connected to a coil and to ground, which second feed back circuit connects the line between the first resistor and the coil and towards an inverting integrator, the output of which is connected through a second capacitor to a second input at a control circuit. As a result, the oscillating frequency is under influence of a signal that depends on the voltage generated in the resistor connected in serial to the coil or transformer.