A switch-mode power supply includes a control element in a primary circuit for controlling a transformer for transmitting electric energy from the primary circuit to a secondary circuit, a first regulating element in the secondary circuit for regulating an electric output variable of the secondary circuit, and a second regulating element in the primary circuit for regulating an electric controlled variable of the control element as a function of a temperature of the primary circuit, the second regulating element being thermally coupled to an element of the primary circuit whose temperature is to be ascertained.