switches between the open and the closed conditions more particularly according to this mode the driving signal vdr is generated based on the rising edges of the clock signal osc every time the clock signal osc exhibits a rising edge the driving signal vdr is brought to a high value in such a way to close or turn on the mosfet 150 the driving signal vdr remains at the high value for a period tclosed that depends on the current flowing through the primary winding 130 of the transformer 125 in particular as far as the mosfet 150 is in the closed condition the current flowing through the primary winding 130 of the transformer 125 increases so that the open voltage vop accordingly increases when the open voltage vop reaches a predetermined value the driving signal vdr is brought to a low value in such a way to open or turn off the mosfet 150 the mosfet 150 then remains open for a period topen until the next rising edge of the clock signal osc consequently by varying the frequency of the clock signal osc it is possible to vary the switching frequency at which the mosfet 150 switches and consequently the transfer of electric power from the primary winding 130 to the secondary winding 135 fig3 a and 3b show two different examples of how the control circuit 105 operates in the qr mode referring in particular to fig3 a the clock signal osc oscillates between a high value and a low value with a variable duty cycle which depends on the switching voltage vsw more particularly the period at which the clock signal osc is at the high value u2014 referred to as high period th u2014 is kept constant and can.