A
control circuit for a switch unit of a clocked power supply circuit, the switch unit being designed to effect input-side excitation of a resonant
transformer arrangement, comprises an input for receiving an auxiliary
signal from the resonant
transformer arrangement. The auxiliary
signal exhibits an essentially fixed
phase relation to a load
alternating current flowing through a resonant circuit of the
transformer arrangement. The
control circuit further comprises a
phase detector designed to detect reference crossing moments when the auxiliary
signal crosses a predefined reference value, a driver controllable to switch the switch unit, and a
synchronizer designed to synchronize a turn-on of the switch unit by the driver with regard to a phase position with the auxiliary signal so as to achieve a turn-on of the switch unit within a predetermined time interval around a
zero crossing of a
voltage present across the switch unit, or of a current flowing through the switch unit. The
synchronizer is further designed to receive information about the reference crossing moments from the
phase detector, and to provide a turn-on signal to the driver with a fixed phase
delay at the reference crossing moments, so as to define turn-on moments at which the driver is to turn on the switch unit. The
control circuit further comprises a
detector designed to determine an amplitude information which depends on an amplitude or a mean value of the auxiliary signal, and a
regulator designed to change an
operating frequency in dependence on the amplitude information supplied by the
detector, and to determine a
period duration between turn-off moments at which the driver is to turn off the switch unit as a reciprocal of the
operating frequency. An inventive
resonance converter enables independent control of frequency and turn-on moments, or
duty cycle, and thus enables a particularly efficient operation of the
resonance converter, and a particularly precise regulation.