The present invention relates to a power unit with a self-oscillating series resonance converter for feeding of a load. The series resonance converter comprises: two transistors TR1, TR2 coupled together, which are arranged to individually be affected by a control transformer T1, a series connection of an inductor L1 and a capacitor C4, and means to change the oscillating frequency of the transistors TR1, TR2 and thereby the output voltage of the power unit. Each transistor TR1, TR2 together with the series coupled discharging capacitor C6, C7 or additional transistor TR3, TR4 belonging thereto is connected in parallel with said inductor L1, capacitor C4 and control transformer T1. The means to change the oscillating frequency of the transistors comprises a control oscillator OSC that in an alternating manner sends pulses that turns off the transistor TR1, TR2 that conducts current and thereby the oscillating frequency of the transistors is controlled by the pulse frequency of the control oscillator OSC.