Abstract:
The invention relates to a method for provision of a drive signal for a switch which controls the current drawn by an inductive energy storage element in a power factor correction circuit, in which a control signal which controls the power consumption is available. One drive cycle of the switch comprises:
       detection of a predetermined storage state of the inductive energy storage element;   when the predetermined storage state of the storage element is detected, production of a switching-on level for the drive signal for a regular switched-on duration which is dependent on the control signal or for a maximum switched-on duration which is dependent on an input voltage, when the regular switched-on duration is greater than the maximum switched-on duration; and   production of a switching-off level for the drive signal for a switched-off duration until the next detection of the predetermined storage state.

Description:
BACKGROUND 
   The present invention relates to a method for driving a switch, which controls the current drawn by an inductive energy storage element, in a switched-mode converter which is in the form of a step-up converter, in particular in a switched-mode converter which is used as a power factor correction circuit (Power Factor Controller, PFC), and to a drive circuit for a switch such as this in a switched-mode converter. 
   A switched-mode converter that is used in a PFC circuit is described, by way of example, in DE 100 40 411 A1. A drive circuit for a switch to control the power consumption in a PFC circuit is the integrated module of the TDA4863 type from Infineon Technologies AG, Munich, which is described in “Boost Controller TDA 4683, Power Factor Controller IC for High Power and Low THD”, Data Sheet, V 1.0, Infineon Technologies AG, May 2003. The use of the integrated module in a power factor correction circuit is described in “TDA—Technical Description AN-PFC-TDA 4863-1”, Application Note, V1.2, Infineon Technologies AG, October 2003. 
   The basic design of a switched-mode converter such as this will be explained in the following text, with reference to  FIG. 1 , in order to assist understanding of the problem on which the invention is based. 
   The object of a switched-mode converter that is used as a PFC is to provide a DC voltage Vn for a load from an AC voltage Vn, in particular a power supply AC voltage, in which case the mean current drawn by the PFC should be at least approximately proportional to the profile of the input voltage Un in order to receive mainly real power. 
   The switched-mode converter that is illustrated in  FIG. 1  has connecting terminals K 1 , K 2  for application of an input voltage Vn, for example a sinusoidal power supply voltage, and a rectified GL which is connected downstream from the input terminals and produces a rectified voltage Vin from the input voltage Vn and the terminals K 3 , K 4 . These terminals K 3 , K 4  are also referred to in the following text as input terminals of the switched mode converter. A converter stage with a step-up converter topography is arranged between these input terminals K 3 , K 4  and output terminals KS, K 6 . In parallel with the input terminals K 3 , K 4 , this converter stage has a series circuit comprising an inductive energy storage element L 1 , for example a storage conductor, and a switch T which, for example, is in the form of a power transistor. A second rectify arrangement, which in the example comprises a diode D and a capacitor C, is connected in parallel with the switch T, and, when the switch T is open, in series with the inductive energy storage element L 1 . The capacitor C is connected between the output terminals KS, K 6 , at which an output voltage Vout is available. 
   In this switched-mode converter, which is in the form of a step-up converter, the inductive energy storage element Ll receives energy when the switch T is closed, and emits this energy to the output capacitor C and to the output terminals KS, K 6  when the switch is subsequently opened. 
   A control signal, which is dependent on the output voltage Vout and is provided by a regulator  10 , is available in the switched-mode converter. The regulator  10  forms the difference between this output signal Sout (which is produced by a voltage divider R 3 , R 4  from the output voltage Vout) and a reference value Vref, and produces the control signal S 10  as a function of this difference. In the simplest case, the regulator comprises an operational amplifier  11 , which is also referred to as an error amplifier and is connected externally to an impedance Z in order to adjust the control response. 
   In order to produce a drive signal S 20  for the switch T, the control signal S 10  is multiplied by an input signal Sin, which is dependent on the rectified input voltage Vin and is produced by means of a voltage divider R 1 , R 2 , C 1  from the input voltage Vin, in order to produce a comparison signal S 21  which is supplied to a drive signal production circuit  20 . 
   This signal production circuit  20  produces a pulse-width-modulated drive signal S 20  in order to drive the switch T and is designed to always produce a switching-on level for the drive signal S 20 , in order to switch on the switch T, as soon as the storage inductor is free of energy after the switch has been switched off, that is to say when the drive signal S 20  is at a switching-off level. An auxiliary winding is used to determine the storage states in which the storage inductor is free of energy, and is inductively coupled to the inductor L 1  and supplies a magnetization signal S 22  to the signal production circuit  20 , with this magnetization signal S 22  indicating the magnetization state of the storage inductor L 1 . 
   In order to adjust the switched-on duration, the signal production circuit  20  compares the comparison signal S 21  (which depends on the input voltage Vin and the control signal S 10 ) with a current measurement signal S 23  which is dependent on the current through the switch T. The current through the switch T, and thus the current measurement signal S 23  rise, when the switch T is closed, in proportion to the input voltage Vin. A switching-off level for the drive signal S 20  is produced by the signal production circuit  20  once the current measurement signal S 23  has risen to the value of the comparison signal. 
     FIG. 2   a  illustrates the waveform of the current measurement signal S 23  for two successive drive cycles. 
   The profile of the input current Iin is also shown, by dashed lines, with this input current Iin corresponding to the current through the switch T during the period in which the drive signal S 20  is switched on and falling to zero during the period in which it is switched off, which is equivalent to demagnetization of the inductor L 1 . For the illustration in  FIG. 3 , the value of the measurement signal S 23  corresponds to the input Iin, whose peak value is limited by the comparison signal S 21 . 
     FIG. 2   b  shows the profile of the drive signal S 20 , which is formed as a function of the magnetization state of the inductor and as a function of a comparison between the current measurement signal S 23  and a comparison signal. Ton in this case denotes the switched-on duration, during which the drive signal S 20  assumes a switching-on level for the switch T, and Toff denotes a switched-off duration, during which the drive signal S 20  assumes a switching-off level. 
     FIG. 3  shows the waveform of the input voltage Vin for one period of a rectified input voltage Vin in the form of the magnitude of a sine wave, the profile which results from this of the comparison signal S 21  in the presence of a control signal S 10  that is assumed to be constant for this period, and the profile of the input current. The relationship between the comparison signal S 21  and the input voltage Vin results in the comparison signal S 21  likewise rising when the input voltage Vin rises. Since the current through the switch T likewise rises as the input voltage Vin rises, constant switched-on durations Ton ideally result when the control signal S 10  remains the same, that is to say when the load conditions at the output remain the same, while the switched-off durations Toff vary. The mean value of the input current Iin is in this case proportional to the input voltage. 
   It can be shown that, for the instantaneous value of the power consumption of a power factor correction circuit such as this:
 
 P= 0.5· Vin   2   ·Ton/L   1   (1a)
 
   Furthermore, the power consumption can also be indicated using the relative switched-on duration d=Ton/T:
 
 P= 0.5 ·Vin   2   ·d·T/L   1 =0.5· Vin   2   ·d /( L   1 · f )  (1b).
 
   In this case, P denotes the instantaneous value of the power consumption, Vin the input voltage, Ton the switched-on duration, L 1  the inductance value of the inductor, and f=1/T the switching frequency. The above relationships for the instantaneous value of the power consumption P are also valid when the overall period duration T is not constant. 
   From (1a), the switched-on duration Ton is obtained as a function of the input current Iin as follows:
 
 Ton=Îin·L/Vin   (2a),
 
in a corresponding manner, the relative switched-on duration d is:
 
 d=Îin·L /( Vin·T )  (2b)
 
where Îin denotes the peak value of the input current Iin reached in each drive cycle. This peak value is proportional to the comparison signal S 21 , so that:
 
 Ton=k·S   21 · L/Vin   (3)
 
where k denotes a proportionality factor. Substitution of (3) in (1) gives:
 
 P= 0.5 ·k·S   21 · Vin   (4)
 
   As is evident from (1), the switched-on duration Ton for a given power consumption is inversely proportional to the square of the input voltage Vin. For so-called wide-range power supply units which have to be designed to produce a constant output voltage Vout for input voltages Vin with peak values between 90V and 270V, this means that the switched-on duration for an input voltage of 90V (=⅓·270V) must be 9 times the switched-on duration for a voltage of 270V. The comparison signal S 21  for a given power consumption is inversely proportional to the input voltage Vin. During one period of the input voltage, the power consumption is in each case a maximum when the input voltage Vin reaches its maximum value. The comparison signal S 21  is also maximized at this time. If one considers the range over which peak values of the input voltage Vin can fluctuate, then the comparison signal assumes its maximum value at the peak value of the smallest possible input voltage. 
   In the case of a wide-range power supply unit, it is assumed that the rated power consumption is reached when the input voltage Vin assumes a peak value 90V and the comparison signal S 21  assumes a maximum value S 21 max. If the input voltage changes such that peak values of 270V occur, then the maximum value of the comparison signal S 21  is reduced to S 21 max/3. This comparison signal S 21  is matched to different input voltage conditions by means of the control signal S 10 . 
   If an overload now occurs at the output of the converter when the input voltage is high, resulting in the output voltage Vout falling, then the comparison signal S 21  can be regulated up to its maximum value S 21 max by means of the regulator  10 , thus resulting in a power consumption which corresponds to 9 times the rated power. This can lead to instabilities in the power consumption control process. 
   One aim of the present invention is to provide a method for driving a switch which controls the power consumption in a power factor correction circuit, which method ensures stable control of the power consumption, and to provide a drive circuit for driving a switch in a power factor correction circuit. 
   SUMMARY 
   A method is disclosed for provision of a drive signal for a switch which controls the current draw by an inductive energy storage element in a power factor correction circuit, which has a rectifier arrangement which is coupled to the inductive energy storage element and has output terminals for provision of an output voltage and in which a control signal which controls the power consumption is available, with the method having the following method steps:
         detection of a predetermined storage state of the inductive energy storage element,   when the predetermined storage state of the storage element is detected: production of a switching-on level for the drive signal for a regular switched-on duration which is dependent on the control signal or for a maximum switched-on duration which is dependent on the input voltage, when the regular switched-on duration is greater than the maximum switched-on duration, in order in this way to limit the power consumption,   production of a switching-off level for the drive signal for a switched-off duration until the next detection of the predetermined storage state.       

   Instabilities in the control of the power consumption are avoided in the case of the method in that a maximum switched-on duration is defined as a function of the input voltage, in order to define a maximum power consumption which is not exceeded. 
   In addition to the definition of a maximum switched-on duration for power emitting, it is also possible to limit the power consumption by defining a maximum current draw. The method for producing the drive signal in this context provides the following method steps:
         detection of a predetermined storage state of the inductive energy storage element,   provision of a current measurement signal which is dependent on an input current,   when the predetermined storage state of the storage element is detected: Production of a switching-on level for the drive signal for a regular switched-on duration which is dependent on the control signal, or until a peak value of the switch current which is dependent on the input voltage is reached, if this peak value is reached within the regular switched-on duration,   production of a switching-off level for the drive signal for a switched-off duration until the next detection of the predetermined storage state.       

   The predetermined storage state of the inductive energy storage element is preferably that state in which a first complete demagnetization of the storage element occurs after the switched-on duration has elapsed. 
   One exemplary embodiment provides for the input voltage to be determined from the switched-on duration, the switched-off duration and the output voltage or a signal which is dependent on the output voltage. This makes use of the fact that, in a power factor correction circuit in which the switch is always switched on again on reaching demagnetization of the storage element, the input voltage Vin during one drive period of the switch is related in accordance with the following equation to the switched-on duration, the switched-off duration and the inductance value of the storage element:
 
 Vin=Toff·Vout /( Ton+Toff )  (5a)
 
where Vin denotes the input voltage, Ton the switched-on duration, Toff the switched-off duration and Vout the output voltage.
 
   In a corresponding manner, the input voltage Vin can also be determined from the relative switched-on duration d=Ton/T=Ton/(Ton+Toff), in which case:
 
 Vin=Vout ·(1 ·d )  (5b)
 
   With reference to equation ( 1   a ), the switched-on duration Ton of a switch in a power factor correction circuit for a given power consumption P as a function of the input voltage Vin and the inductance value L 1  of the inductive energy storage element is:
 
 Ton= 2 ·P·L   1 / Vin   2   (6a)
 
and the relative switched-on duration is:
 
 d= 2 ·P·L   1 /( Vin   2   ·T)   (6b)
 
   The maximum switched-on duration Vin in the case of the method according to the invention is preferably determined on the basis of the following relationship:
 
 Ton max=2 ·P max· L   1 / Vin   2   =klim 1/ Vin   2   (7a)
 
where Tonmax represents the maximum switched-on duration, Vin the input voltage and klim1 a predetermined limit value which takes account of the maximum permissible power consumption Pmax. The limit value klim1 also takes account of the inductance value of the inductive energy storage element.
 
   In a corresponding manner, the maximum relative switched-on duration donmax can be defined as follows:
 
 don max=2 ·P max· L   1 /( Vin   2   ·T )= klim 1/ Vin   2   ·T)   (7b)
 
   The switched-on duration is preferably determined using a current measurement signal that is dependent on the input current, a first comparison signal that is dependent on the control signal and a second comparison signal that is dependent on the input voltage, with a switching-off level for the drive signal being produced when the current measurement signal reaches the lower of the two comparison signals. The second comparison signal is in this case chosen such that the current measurement signal reaches this comparison signal after the maximum switched-on duration. 
   The peak current value on reaching which the switch is in each case switched off in the second alternative of the method, when this peak current value is reached within the regular switched-on duration, is preferably determined on the basis of the following relationship:
 
 I max= klim 2 /Vin   (8)
 
where Imax represents the peak current, Vin the input voltage and klim2 a predetermined limit value which takes account of the maximum power consumption and the inductance value of the inductive energy storage element.
 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in the following text using exemplary embodiments and with reference to figures, in which: 
       FIG. 1  shows a power factor correction circuit according to the prior art. 
       FIG. 2  shows waveforms of the input current and of the current through the switch in a power factor correction circuit ( FIG. 2   a ) and of a drive signal for the switch ( FIG. 2   b ). 
       FIG. 3  shows waveforms of the input voltage, of the input current and of the mean input current for a power factor correction circuit. 
       FIG. 4  shows a power factor correction circuit which has a drive circuit for a switch based on a first exemplary embodiment of the invention. 
       FIG. 5  shows a detailed illustration of a unit in the drive circuit illustrated in  FIG. 4 . 
       FIG. 6  shows a power factor correction circuit which has a drive circuit for a switch according to a second exemplary embodiment of the invention. 
       FIG. 7  shows a detailed illustration of a unit in the drive circuit illustrated in  FIG. 4 . 
       FIG. 8  shows a power factor correction circuit which has a drive circuit for a switch according to a further exemplary embodiment of the invention. 
       FIG. 9  shows the waveform of a current measurement signal during one switched-on duration. 
   

   Unless stated to the contrary, identical reference symbols in the figures denote identical parts, with the same meaning. 
   DESCRIPTION 
   Although the subject matter-of the present invention involves a method for driving a switch in a power factor correction circuit and a drive circuit for a switch such as this,  FIGS. 4 ,  6 , and  8  illustrate a complete power factor correction circuit, in order to assist understanding. The circuit components which correspond to the known power factor correction circuit shown in  FIG. 1  are in this case provided with corresponding reference symbols. Reference shall be made to the description relating to  FIG. 1  for the circuitry and method of operation of these circuit components. 
     FIG. 4  shows a power factor correction circuit with a drive circuit according to the invention, which produces a drive signal S 30  for a switch T which controls the power consumption of the power factor correction circuit. In the exemplary embodiment, the switch T is in the form of a power MOSFET, and is connected in series with an inductive energy storage element L 1  between input terminals K 3 , K 4 , at which a rectified input voltage Vin is available. A rectify arrangement D, C with a diode D and a capacitor C is connected in parallel with the switch T, or, when the switch T is open, in series with the energy storage element L 1 , with an output voltage Vout that is produced from the rectified input voltage Vin being available across the capacitor C. The power factor correction circuit also has a control arrangement  10  with a control amplifier  11 , which compares the output signal Sout (which is derived from the output voltage Vout by means of a voltage divider R 3 , R 4 ) with a reference value Vref that is produced by a reference voltage source  12 , in order to produce a control signal S 10 . The regulator  10  is, for example, a proportional integral regulator (PI regulator), an integral regulator (I-regulator) or a proportional regulator (P regulator). The regulator  10  comprises a control amplifier  11  which is connected externally to an impedance Z which determines the control response of the control amplifier  11 . The impedance Z represents a passive network which may, in particular, comprise capacitors in order to achieve a control arrangement  10  with an integrated control response. 
   The control signal S 10  that is produced by the regulator arrangement  10  is supplied to the drive circuit  30 . The drive circuit  30  is also supplied with an input signal Sin (which is dependent on the rectified input voltage Vin and, in the exemplary embodiment, is produced using a voltage divider R 1 , R 2 ), a magnetization signal S 22  and a current measurement signal S 23 . The magnetization signal S 22  in the exemplary embodiment is produced by an auxiliary winding L 2 , which is inductively coupled to the inductive energy storage element L 1  and one of whose connections is connected to a reference ground potential GND, to which the input voltage Vin is also related. Another connection of the auxiliary winding is connected to the drive circuit  30 . The current measurement signal S 23  is a voltage signal which is likewise related to the reference ground potential GND, corresponds to the voltage across a current measurement resistor Rs (which is connected in series with the switch T) and is proportional to the input current Iin when the switch T is closed. 
   The drive circuit comprises a logic storage element  31  which, in the exemplary embodiment, is in the form of an RS flipflop, and whose set input S is supplied with a switching-on signal S 33 , while its reset input R is supplied with a switching-off signal S 34 . The non-inverting output  O  of this flipflop  31  is followed by a driver circuit  32  which converts a logic signal S 31 , which is produced at the output of the flip-flop, to a level that is suitable for driving the power transistor T. The flipflop  31  is set by the switching-on signal S 33  in order to produce a switching-on level of the drive signal S 30  at the output of the driver circuit  32 , and the flipflop is reset as a function of the switching-off signal S 34  in order to produce a switching-off level of the drive signal S 30  at the output of the driver circuit  32 . The transistor T is switched on, or starts to conduct, at a switching-on level of the drive signal S 30 , and is switched off at a switching-off level of the drive signal S 30 . 
   The switching-on signal S 33  is produced by a detector circuit  33  which is supplied with the magnetization signal S 22  and detects the zero crossings of the magnetization signal S 22  in order to set the flipflop  31  via the switching-on signal S 33  on detection of such a zero crossing. In this case, a zero crossing of the magnetization signal indicates complete demagnetization of the storage inductor L 1 . 
   In order to produce the switching-off signal S 34 , the current measurement signal S 23  is compared by means of a comparator  34  with a comparison signal S 35  that is produced by a comparison signal production circuit  35 . The flipflop  31  is in this case reset in order to switch off the transistor T in each case when the current measurement signal S 23  exceeds the value of the comparison signal S 35  while the switch T is switched on. 
   The comparison signal S 35  is produced by the comparison signal production circuit  35  as a function of the control signal S 10  and the input signal Sin, which is dependent on the input voltage Vin, and will be explained in more detail in the following text with reference to  FIG. 5 . 
     FIG. 5  schematically illustrates one exemplary embodiment of the comparison signal production circuit  35 , to which the input signal Sin and the control signal S 10  are supplied. This signal production circuit  35  comprises a multiplier  354 , which multiplies the control signal S 10  by the input signal Sin in order to produce a first comparison signal S 354 . The signal production circuit also has a maximum value determination unit  351 , which produces a second comparison signal Smax as a function of the input signal Sin, which is dependent on the input voltage Vin. The two comparison signals S 354 , Smax are supplied to a multiplexer  353 , which in each case passes on the lower of the two comparison signals as the comparison signal S 35  to its output. The multiplexer is driven by a comparator  352  whose inputs are supplied with the comparison signals Smax, S 354 , and whose output drives the multiplexer  353 . 
   With reference to  FIG. 9 , the comparison signal S 35  determines the switched-on duration Ton of the power transistor. Once the transistor T has been switched on, the input current Iin, and thus the current measurement signal S 23 , lies linearly, with the end of the switched-on duration Ton being reached when the current measurement signal S 23  has risen to the value of the comparison signal S 35 . This switched-on duration Ton is given, as a function of the comparison signal S 35 , of the inductance value L 1  of the storage inductor, of the input voltage Vin and of the value of the measurement resistor Rs as:
 
 Ton =( S   35 · L   1 )/( Vin·Rs )  (9).
 
   The object of the second comparison signal Smax is to limit the switched-on duration Ton to a maximum value Tonmax on the basis of equation (7). The maximum value production circuit is for this purpose designed to determine a second comparison value Smax on the basis of the following relationship:
 
 S max=( Ton max ·Vin·Rs )/ L   1   (10)
 
Substitution of equation (6) or (7) gives:
 
   
     
       
         
           
             
               
                 
                   
                     
                       
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   If it is remembered that Sin=Vin/k3 then the following relationship becomes valid, on the basis of which the unit  351  produces the second comparison signal Smax from the input signal Sin:
 
 S max=2 ·P max· Rs·k 3/Sin= klim 3/Sin  (12)
 
   Where Pmax denotes the maximum permissible power consumption, which can be predetermined to be fixed. The limit value klim3 takes account of this maximum power consumption and, in the present case, of the proportionality factor between the input current Iin and the current measurement signal S 23 , as well as the proportionality factor between the input voltage Vin and the input signal. The second comparison signal Smax defines a maximum value for the input current Iin, in which case, as explained, Smax may be defined either on the basis of the maximum permissible switched-on duration Tonmax or the maximum permissible input current. 
   In this context, it should be noted that this current measurement signal S 23  can be produced by means of any desired conventional current measurement arrangement, in particular using a current measurement arrangement which operates on the basis of the so-called “current sense method”, and which does not have any current measurement resistor connected in the load circuit. 
   In the circuit arrangement shown in  FIG. 4 , the drive circuit  30  is supplied with an input signal Sin, which is dependent on the input voltage Vin, in order to produce the first and second comparison signals S 354 , Smax. 
     FIG. 6  shows a modification of the drive circuit illustrated in  FIG. 4 , in which the comparison signal S 35  is produced by means of a comparison signal production circuit  36 , which produces this comparison signal S 35  from the control signal S 10 , the output signal Sout, as well as the switched-on duration Ton and the switched-off duration Toff of the drive signal S 30 . The information about the switched-on duration and the switched-off duration of the drive signal S 30  is supplied to the comparison signal production circuit  36  in the exemplary embodiment via the output signal S 31  from the flipflop  31 . 
     FIG. 7  shows the design of this comparator circuit  36 , which differs from that illustrated in  FIG. 5  by having an input signal production circuit which uses the output signal Sout and the flipflop output signal S 31  to produce the input signal Sin which is required to produce the first and second comparison signals S 354 , Smax. This input signal production circuit  361  produces the input signal Sin on the basis of the equation (5), with the input signal Sin being set instead of the input voltage Vin, and the output signal Sout being set instead of the output voltage Vout. 
   The advantage of the drive circuit  30  shown in  FIG. 6  is that there is no need for a voltage divider to derive the input signal Sin from the input voltage Vin. 
     FIG. 8  shows a power factor correction circuit with a drive circuit according to a further exemplary embodiment of the present invention. In a corresponding manner to the drive circuits that have already been explained, this drive circuit  50  comprises a logic storage element  31  which is in the form of a flipflop and which is followed by a driver circuit  32 , at whose output the drive signal S 30  for the power transistor T is produced. The drive circuit  50  is also supplied with a magnetization signal S 22 , which is produced by an auxiliary coil L 2  and from which a detector circuit  33  produces a switching-on signal S 33 , which is supplied to the set input of the flipflop. In contrast to the circuit components in the already explained drive circuits  30 , the other circuit components in this drive circuit  50  are in the form of digital circuit components. 
   A digital counter  54  is used to produce a switching-off signal S 54 , which is supplied to the reset input R of the flipflop  31 , which digital counter  54  has a clock input for supplying a clock signal CLK, a loading input for supplying a switched-on duration value S 56 , and a drive input for starting the counter. The counter drive input is supplied with the switching-on signal S 33 , in order to allow the counter either to count up or to count down in time with the clock signal CLK, depending on the embodiment of the counter, when the power transistor T is switched on. Depending on the embodiment of the counter  54 , the switching-off signal S 54  for resetting the flipflop  31 , and thus for switching off the power transistor T, is produced when the counter reaches the switched-on duration value S 56  starting from a count of zero, or when the counter has counted down to zero starting from the switched-on duration value  56 . The switched-on duration value  56  thus directly represents a measure of the switched-on duration Ton of the power transistor T, with the switched-on duration in this case corresponding to the product of one period duration of the clock signal and the switched-on duration value S 56 . 
   The switched-on duration value S 56  is obtained from a first or second switched-on duration value by means of a digital comparator  56 . The first switched-on duration value S 42  in this case represents a regular switched-on duration Ton, and the second digital switched-on duration value S 55  represents a maximum switched-on duration Tonmax for the power transistor. This second switched-on duration value S 55 , which represents the maximum switched-on duration, is produced in a maximum value determination unit  55 , which is supplied with the output signal from the flipflop S 31  and with a digitized output signal S 41 . 
   The digitized output signal S 41  is in this case produced by means of an analog/digital converter  41 , which is supplied with the output signal Sout produced by the voltage divider R 3 , R 4 . The maximum value determination unit  55  is designed to analyze the output signal from the flipflop S 31  in order to determine the instantaneous switched-on duration Ton and the instantaneous switched-off duration Toff. For this purpose, the flipflop output signal is, for example, sampled in time with a clock signal and is compared with a reference value which, for example, is between the two output levels of the flipflop. The number of successive sample values above the reference value are in this case counted in order to determine the switched-on duration Ton or a value which is directly related to the switched-on duration. The number of successive sample values below the reference value are counted in a corresponding manner in order to determine the switched-off duration Toff or a value which is directly related to the switched-off duration. 
   The maximum value determination unit  55  uses the switched-on duration Ton and the switched-off duration Toff as well as the digital output signal S 41  to determine the second switched-on duration value S 55 , which represents the maximum switched-on duration, on the basis of the equation (7), using the equation (5) to determine the input voltage Vin from the digitized output signal S 41 . 
   The first digital switched-on duration value S 42 , which defines a regular switched-on duration for the switch T, is determined from the digital output signal S 41  by means of a digital regulator  42 . This regulator  42  is designed in particular to compare the digitized output signal S 41  with a reference value, in order to produce a digital control signal from the difference between the reference value and the digital output signal S 41 , in a manner which will not be described in any more detail, in which case, for example using a look-up table, each digital control signal that is produced in this way is assigned a digital signal S 42  which represents the switched-on duration. With this arrangement, there is no need to use a current measurement signal to produce a signal that defines the switched-on duration of the switch T. 
   LIST OF REFERENCE SYMBOLS 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               10 
               Regulator 
             
             
                 
               11 
               Control amplifier, operational amplifier 
             
             
                 
               12 
               Reference voltage source 
             
             
                 
               20 
               Drive circuit 
             
             
                 
               30 
               Drive circuit 
             
             
                 
               31 
               Logic storage element, RS-flipflop 
             
             
                 
               32 
               Driver circuit 
             
             
                 
               33 
               Detector arrangement 
             
             
                 
               34 
               Comparator 
             
             
                 
               35 
               Comparison signal production circuit 
             
             
                 
               36 
               Comparison signal production circuit 
             
             
                 
               41 
               Analog/digital converter 
             
             
                 
               42 
               Digital regulator 
             
             
                 
               54 
               Digital counter 
             
             
                 
               55 
               Maximum value production circuit 
             
             
                 
               56 
               Digital comparator 
             
             
                 
               C 
               Capacitor 
             
             
                 
               C1 
               Capacitor 
             
             
                 
               D 
               Diode 
             
             
                 
               Iin 
               Input current 
             
             
                 
               K1, K2 
               Power supply connecting terminals 
             
             
                 
               K3, K4 
               Input terminals 
             
             
                 
               K5, K6 
               Output terminals 
             
             
                 
               L1 
               Inductive storage element, storage inductor 
             
             
                 
               L2 
               Auxiliary winding 
             
             
                 
               M 
               Multiplier 
             
             
                 
               R1, R2 
               Voltage divider 
             
             
                 
               R3, R4 
               Voltage divider 
             
             
                 
               Rs 
               Current measurement resistor 
             
             
                 
               S10 
               Control signal 
             
             
                 
               S21 
               Comparison signal 
             
             
                 
               S22 
               Magnetization signal 
             
             
                 
               S23 
               Current measurement signal 
             
             
                 
               S30 
               Drive signal 
             
             
                 
               S31 
               Flipflop output signal 
             
             
                 
               S33 
               Switching-on signal 
             
             
                 
               S34 
               Switching-off signal 
             
             
                 
               S35 
               Comparison signal 
             
             
                 
               S36 
               Comparison signal 
             
             
                 
               S41 
               Digitized output signal 
             
             
                 
               S42 
               First digital comparison signal 
             
             
                 
               S55 
               Second digital comparison signal 
             
             
                 
               Sin 
               Input signal 
             
             
                 
               T 
               Switch, power transistor 
             
             
                 
               Vin 
               Input voltage 
             
             
                 
               Vn 
               Power supply voltage 
             
             
                 
               Vout 
               Output voltage 
             
             
                 
               Vref 
               Reference voltage 
             
             
                 
               Z 
               Impedance