Switching power supply controlled by the primary side with a control circuit for controlling the output voltage and/or the output current, the switching power supply comprising: a transformer with a primary-sided and a secondary-sided main winding, wherein electrical energy pulses are transmitted from the primary-sided to the secondary-sided main winding, a primary-sided switch for switching on and off the primary current through the primary-sided main winding in response to a control signal of the control circuit, a primary-sided auxiliary winding, in which a voltage pulse is induced after switching off the primary-sided switch, wherein the level of the voltage pulse depends on a load at the secondary side, a sample and holding device (S&H) for sampling and storing a level of the voltage pulse for generating a control variable, wherein the control signal is generated on the basis of the control variable, wherein the control circuit comprises a primary current comparator, which is operable to determine a switch-off point in time of the primary-sided switch depending on the primary current, and wherein the control circuit is operable to feed back the control variable to the primary current comparator such that the switch-off point in time of the primary-sided switch can be influenced additionally depending on the control variable such that the duration during which the primary current flows through the primary-sided main winding is reduced differently, wherein the control circuit is operable to determine a switch-on point in time of the primary-sided switch depending on the control variable and thereby to determine the duration during which no primary current flows through the main winding, wherein the control circuit is operable to determine the switch-on time of the primary-sided switch by a comparison of the control variable with a comparator voltage, and is further designed that the comparator voltage is determined depending on the control variable so that at low load the energy per pulse is reduced. a transformer with a primary-sided and a secondary-sided main winding, wherein electrical energy pulses are transmitted from the primary-sided to the secondary-sided main winding, a primary-sided switch for switching on and off the primary current through the primary-sided main winding in response to a control signal of the control circuit, a primary-sided auxiliary winding, in which a voltage pulse is induced after switching off the primary-sided switch, wherein the level of the voltage pulse depends on a load at the secondary side, a sample and holding device (S&H) for sampling and storing a level of the voltage pulse for generating a control variable, wherein the control signal is generated on the basis of the control variable, wherein the control circuit comprises a primary current comparator, which is operable to determine a switch-off point in time of the primary-sided switch depending on the primary current, and wherein the control circuit is operable to feed back the control variable to the primary current comparator such that the switch-off point in time of the primary-sided switch can be influenced additionally depending on the control variable such that the duration during which the primary current flows through the primary-sided main winding is reduced differently, wherein the control circuit is operable to determine a switch-on point in time of the primary-sided switch depending on the control variable and thereby to determine the duration during which no primary current flows through the main winding, wherein the control circuit is operable to determine the switch-on time of the primary-sided switch by a comparison of the control variable with a comparator voltage, and is further designed that the comparator voltage is determined depending on the control variable so that at low load the energy per pulse is reduced.