The invention under consideration refers to a controller for a primary-side regulated control power supply unit for the regulation of the output of the primary regulated control power supply unit. The invention also concerns a method for the operation of a control power supply unit of the generic type and a pertinent control power supply unit. The controller comprises a control output (B), which can be connected with the control input of a primary-side switch (Q1) of the control power supply unit; a voltage measurement input (U), which can be connected with a primary-side auxiliary winding (L4) of a transformer (200) for the sensing of an auxiliary voltage, which is induced as a function of the secondary-side output voltage; a sample-and-hold element (SuH, SuH1) for the sensing of the auxiliary voltage; and an error amplifier (202) for a comparison of the sensed auxiliary voltage with a reference value (Vref2) and for the amplification of the deviation between the sensed auxiliary voltage and the reference value. The controller (IC2) also has a return branch for the return of the output signal of the error amplifier (202) to the voltage measurement input (U).