In the case of known switched-mode power supplies with integrated preconditioner, the control curves are largely congruent, but deviate from one another in low-load operation, whereby the unregulated intermediate circuit voltage increases. To improve efficiency in low-load operation, closed-loop control of the burst cycle is effected on the primary side of the voltage transformer. The intermediate circuit voltage is limited to a permissible limit value. The closed-loop control device in the switching stage taps the intermediate circuit voltage at a voltage divider which is arranged between the preconditioner functional unit and the switching stage. An assembly additionally monitors the output voltage, for example by means of an optocoupler. Burst mode comprises one or more burst cycles. A burst cycle is started when the intermediate circuit voltage reaches its limit value. At this time, the voltage transformer is switched off. A burst cycle ends when the output voltage reaches a minimum value. At this time at the latest, the voltage transformer is switched on again. Burst mode is terminated again either when the switched-mode power supply is again operated under normal load or when a maximum time has elapsed, measured from the start of burst mode.