Abstract:
For the purpose of power factor correction, an inductance ( 21 ) is supplied with an input voltage (Vin), wherein a controllable switching means ( 24 ) that is coupled to the inductance ( 21 ) is actuated in order to selectively charge and discharge the inductance ( 21 ). A control device ( 14 ) for actuating the switching means ( 24 ) is designed such that it actuates the switching means ( 24 ) selectively on the basis of one of a plurality of modes of operation. In a first mode of operation, a switch-on time is stipulated for the switching means ( 24 ) on the basis of a minimum waiting time and on the basis of a voltage that drops across the switching means ( 24 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method and a circuit for power factor correction. In particular, the invention relates to the technical field of power factor correction for use in operating devices or electronic ballasts for illuminants. 
       BACKGROUND 
       [0002]    Power factor correction (PFC), is used to eliminate or at least to reduce harmonic currents in an input current. Harmonic currents can occur particularly in the case of nonlinear loads, such as, for example, rectifiers with subsequent smoothing in power supply units, since in the case of such loads, despite the sinusoidal input voltage, the input current is phase-shifted and distorted non-sinusoidally. The higher-frequency harmonics that occur in this case can be counteracted by an active or clocked power factor correction circuit connected upstream of the respective device. 
         [0003]    Power factor correction circuits are also used in operating devices for illuminants, for example in electronic ballasts for fluorescent illuminants or in LED converters. The use of such circuits in devices for operating illuminants is expedient since standards restrict the permissible return of harmonics into the supply system. 
         [0004]    A circuit topology based on a boost converter, also referred to as step-up converter or up-converter, is often used for power factor correction circuits. In this case, an inductance or coil supplied with a rectified AC voltage is charged with an input current or discharged by a controllable switch being switched on or being switched off. The discharge current of the inductance flows via a diode to the output of the converter, said output being coupled to an output capacitance, such that a DC voltage increased relative to the input voltage can be tapped off at the output. Other types of converter can likewise be used. 
         [0005]    Power factor correction circuits can be operated in different operating modes. In particular, operation with a continuous current through the abovementioned inductance (so-called “Continuous Conduction Mode”, CCM), operation with a discontinuous inductance current or coil current (“Discontinuous Conduction Mode”, DCM) or operation in the borderline or boundary range between continuous and discontinuous current through the inductance (“Borderline Conduction Mode” or “Boundary Conduction Mode”, BCM) is known. In BCM operation a decrease in the coil current to zero during the discharge phase of the coil can be taken as a reason to start a new switching cycle and to switch the switch on again in order to charge the coil anew. The power factor correction circuit can be controlled or regulated by means of the time duration during which the switch is switched on in each case. In DCM operation, by contrast, after a zero crossing of the coil current during the discharge phase firstly there is a wait during a predefined additional waiting time until the switch is closed anew. 
         [0006]    DE 10 2004 025 597 A1 describes a power factor correction circuit in which an output DC voltage is derived during the switched-off time duration of the switch. 
         [0007]    When a power factor correction circuit is operated in the DCM mode, the waiting time before renewed switching-on of the converter can be chosen depending on a load, i.e. depending on an output power of the power factor correction circuit, in order to maintain a predefined bus voltage. If the switch is switched on again directly after this time has elapsed, this can lead to irregularities in the coil current. If the switch-on instant is chosen depending only on the predefined waiting time and independently of the behavior of the power factor correction circuit, an increased dissipation and thus heating of the switching means can also occur. 
         [0008]    It is an object to specify methods and devices which offer improvements with regard to the problems mentioned. It is an object to specify methods and devices for power factor correction in which operation over a larger range of loads is possible. It is also an object to specify methods and devices in which the dynamic behavior of the power factor correction circuit during the period in which the switch is in the off state can be taken into account when determining the switch-on instant. 
       SUMMARY 
       [0009]    A method, a power factor correction circuit and an operating device for an illuminant comprising the features specified in the independent claims are specified according to the invention. The dependent claims define advantageous and preferred embodiments of the invention. 
         [0010]    In methods and devices according to exemplary embodiments, a power factor correction circuit for an illuminant can be operated in a plurality of operating modes. Depending on a load, which can be detected by means of the output power, for example, it is possible to select an operating mode from a plurality of operating modes. In a first operating mode, which can be a DCM operating mode, a minimum waiting time between the switching-off of the switching means and renewed switching-on of the switching means is determined. A switch-on instant for the switching means is defined not only depending on the minimum waiting time but also depending on a voltage dropped across the switching means. This makes it possible to take account of the dynamic behavior of the power factor correction circuit during the off state of the switching means for determining the switch-on instant. In a power factor correction circuit according to one exemplary embodiment, a control device is configured to implement the corresponding method. 
         [0011]    The switching means can be a power switch, in particular a FET or MOSFET, and the voltage dropped across the switching means can be the drain-source voltage of the power switch while the power switch is switched into the off state. 
         [0012]    The control device of the power factor correction circuit can define the switch-on instant depending on the time-dependent behavior of the voltage dropped across the drain-source path of the power switch. The control device of the power factor correction circuit can define time windows which correspond to permissible switch-on times and which are in each case at the times at which the voltage dropped across the drain-source path of the power switch approaches a local minimum or passes through the latter. In other words, the switch is switched on only if it is the case not just that the minimum waiting time has elapsed but that the voltage at the switching means is in a “valley”. Such a procedure is also referred to as “valley switching”. 
         [0013]    The control device of the power factor correction circuit can obtain information about the voltage at the drain-source path of the switching means or the change thereof in various ways. In one configuration, the current flowing through the inductance of the power factor correction circuit can be detected and the instant at which the voltage at the drain-source path of the switching means has an extremum can be determined depending on a comparison of the current through the coil with a reference value. A corresponding measurement signal indicating the current in the coil or the zero crossings thereof can be fed to the control device. The measurement signal can be generated such that it indicates zero crossings of the coil current and the direction thereof. A local minimum or valley of the voltage at the switching means can be identified depending on a zero crossing of the coil current in a specific direction. 
         [0014]    For detecting the zero crossing of the current, a corresponding detection circuit can be provided, with which, by means of a winding, for example, the coil current is detected and compared with a reference value. 
         [0015]    The control device of the power factor correction circuit can perform a transition between the first operating mode and a second operating mode depending on the load or output power of the power factor correction circuit. The second operating mode can be CCM operation or BCM operation. In the second operating mode the power factor correction circuit can be controlled or regulated by means of the setting of the on time during which the switching means is switched on in each case. In the first operating mode the power factor correction circuit can be controlled or regulated by means of the setting of the minimum waiting time. In the first operating mode the on time during which the switching means is switched on in each case can be chosen to be equal to the value corresponding to the minimum value of the on times permissible in the second operating mode. The minimum waiting time can be defined and used only in the first operating mode for the control of the power factor correction circuit. 
         [0016]    If the control device of the power factor correction circuit recognizes that the load or output power falls below a threshold value, the control device can automatically change from the second operating mode to the first operating mode. 
         [0017]    The power factor correction circuit according to exemplary embodiments can be used in particular for power factor correction for an AC voltage/DC voltage converter, such that in this case the input voltage is a rectified AC voltage and the output voltage is a DC voltage. The power factor correction circuit according to exemplary embodiments can be constructed in accordance with the topology of a boost converter, such that the discharge current of the inductance is fed via a diode to the output terminal coupled to an output capacitance. The methods and configurations described are also applicable to other converter topologies. 
         [0018]    In each of the exemplary embodiments, in the first operating mode, for example in the DCM operating mode, a switch-on instant can be chosen such that cumulatively the following three conditions are fulfilled: the minimum waiting time has elapsed; the current through the inductance is at a zero crossing; and the drain-source voltage of the switching means has fallen to a local minimum. 
         [0019]    The control device can be configured in the form of an integrated circuit, in particular an Application Specific Integrated Circuit (ASIC). The control device can have a common measurement input for detecting a measurement variable which corresponds to the coil current or to a zero crossing of said current and which is also used for determining the time windows corresponding to a local minimum or the voltage dropped across the switching means. 
         [0020]    Method and power factor correction circuit can be used in an operating device for an illuminant, for example for an electronic ballast for a fluorescent illuminant or for an LED converter. In this application exemplary embodiments of the invention make it possible that an adaptation over a wide range of different power levels or components of the operating device used in each case is possible and in this case energy-efficient switching is achieved even in a DCM operating mode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention is explained below on the basis of preferred embodiments with reference to the drawings. 
           [0022]      FIG. 1  shows a lighting system comprising a power factor correction circuit according to one exemplary embodiment. 
           [0023]      FIG. 2  shows a circuit diagram of a power factor correction circuit according to one exemplary embodiment. 
           [0024]      FIG. 3  shows currents and voltages for explaining the functioning of the power factor correction circuit according to one exemplary embodiment in a second operating mode, which can be BCM operation. 
           [0025]      FIG. 4  shows currents and voltages for explaining the functioning of the power factor correction circuit according to one exemplary embodiment in a first operating mode, which is DCM operation. 
           [0026]      FIG. 5  shows currents and voltages for explaining the functioning of the power factor correction circuit according to one exemplary embodiment in the first operating mode. 
           [0027]      FIG. 6  schematically shows a current through a coil if a switching means is switched on again directly after a fixed waiting time has elapsed. 
           [0028]      FIG. 7  illustrates the functioning of the power factor correction circuit according to one exemplary embodiment in the first operating mode. 
           [0029]      FIG. 8  illustrates the functioning of the power factor correction circuit according to one exemplary embodiment in the first operating mode. 
           [0030]      FIG. 9  and  FIG. 10  illustrate a parameter adaptation by a control device of the power factor correction circuit according to one exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]      FIG. 1  shows a block diagram illustration of a lighting system  1  comprising an operating device  2  for an illuminant  3 , for example for LEDs. The operating device  2  can be connected to a bus  4  or a wireless communication system in order to receive dimming commands and/or to output status messages. 
         [0032]    The operating device  2  can be configured for example as an electronic ballast (EB), for a gas discharge lamp, fluorescent lamp or some other fluorescent illuminant or as an LED converter. The operating device  2  has a rectifier  10  for rectifying a supply voltage, for example the power supply system voltage. The operating device  2  has a circuit for power factor correction  11  with an assigned control device  14 . The power factor correction circuit provides an output voltage for components of the operating device  2  that are connected downstream, said output voltage also being designated as bus voltage Vbus. A further voltage conversion and/or dimming functions can be achieved for example by means of a DC/DC converter  12 , which can be configured as an LLC resonance converter, and/or an output driver  13 . 
         [0033]    The functioning of the power factor correction circuit and of its control device  14  is described in greater detail with reference to  FIGS. 2-10 . Generally, the control device  14  can control the power factor correction circuit in a plurality of operating modes comprising at least one DCM (“Discontinuous Conduction Mode”) operating mode as first operating mode and a second operating mode. The second operating mode can be selected from a CCM (“Continuous Conduction Mode”) and a BCM (“Borderline Conduction Mode” or “Boundary Conduction Mode”) operating mode. In the first operating mode an adaptation to a different load or different output power can be achieved by the setting of a minimum waiting time that has to elapse between the switching-off of a switching means and the renewed switching-on. In this case, the control device  14  controls the switching means such that not just the elapsing of the minimum waiting time influences the criterion for switching on the switching means, but in addition the time-dependent behavior of the power factor correction circuit is taken into account. The switching means can be switched on again depending on whether the minimum waiting time has elapsed and a drain-source voltage of the switching means fulfils a specific criterion. The criterion can include the fact that the drain-source voltage of the switching means attains a local minimum value as a function of time. This criterion can be checked by the control device  14  being fed a variable which includes information about zero crossings of the current in a coil of the power factor correction circuit. 
         [0034]      FIG. 2  is a circuit diagram of the power factor correction circuit  20  according to one exemplary embodiment. A supply DC voltage, for example the power supply system voltage, is converted by a rectifier (not illustrated in  FIG. 2 ) into a rectified AC voltage, which is present as input AC voltage Vin between an input terminal of the power factor correction circuit  20  and ground. The input AC voltage Vin is fed to an inductance or coil  21 . The inductance  21  is connected in series with a diode  22  between the input terminal and an output terminal  27  of the power factor correction circuit  20 . An output DC voltage Vout is provided at the output terminal  27 , which is coupled to a charging capacitor  23 . A further capacitor  25  can be connected between the inductance  21  and ground, said further capacitor being connected in parallel with a series circuit comprising a switch  24  and resistor  26 . The capacitor  25  can be connected to the same terminal of the diode  22  to which the inductance  21  is connected as well. 
         [0035]    The output DC voltage Vout serves for supplying a load upstream of which the power factor correction circuit  20  is connected. The load can be for example components of an operating device for an illuminant such as, for example, a fluorescent lamp, a halogen lamp, a light emitting diode arrangement, etc. The load can comprise an LLC resonance converter with potential isolation. 
         [0036]    A controllable electronic switch  24 , which is a power switch and which can be embodied for example as a field effect transistor (FET), in particular as a MOSFET, is connected to the connection between the inductance  21  and the diode  22 . The switch  24  can be connected to ground via a resistor  26 , wherein the resistor  26  can serve as a shunt resistor. The switch  24  is switched into the on state and the off state by the control device  14  of the power factor correction circuit  20 . The control device  14  has a corresponding output  41  for modulating a control signal with which, for example, the gate voltage of the switch  24  can be controlled. 
         [0037]    In the switched-on state of the switch  24 , the inductance  21  is connected to ground via the switch  24 , the diode  8  being in the off state, such that the inductance  21  is charged and energy is stored in the inductance  21 . By contrast, if the switch  24  is switched off, i.e. open, the diode  22  is in the on state, such that the inductance  21  can be discharged into the charging capacitor  23  via the diode  22  and the energy stored in the inductance  21  is transferred into the charging capacitor  23 . 
         [0038]    The switch  24  is driven by a control device  14 , which can be configured in the form of an integrated circuit, in particular as an ASIC. The power factor correction is achieved by the switch  24  being repeatedly switched on and off, wherein the switching frequency for the switch  24  is much greater than the frequency of the rectified input AC voltage Vin. The power factor correction circuit  20  can operate as a boost converter. 
         [0039]    Various measurement variables can be fed to the control device  14 , which measurement variables can be evaluated for controlling or regulating the power factor correction circuit  20  or other components of the operating device. By way of example, the control device  14  can detect the output voltage by means of a voltage divider comprising resistors  36 ,  37 . 
         [0040]    The control device  14  can also be fed a measurement variable which indicates when a current I L  in the inductance  21  has a zero crossing or the sign with which the zero crossing takes place. The corresponding detection circuit can have a winding  31  or small coil  31 , which is inductively coupled to the inductance  21 . The winding  31  is connected to a node via a diode  32  and a resistor  33 , which node is connected via a further resistor  34  to a node between the switch  24  and the resistor  26 . The signal at the input  42  of the control device  14  indicates zero crossings of the current I L  in the inductance  21  in particular in the time periods in which the switch  24  is switched into the off state. 
         [0041]    The control device  14  generates a control signal in order to switch the switch  24  into the on state or the off state. This can be done in various ways, in particular depending on a load or output power. In the case of relatively high loads or output powers, a second operating mode is used, which can be BCM operation or CCM operation. The time duration during which the switch  24  is switched on in each case can be set here in order to keep the output voltage at a desired value. The time duration during which the switch  24  is switched on in each case can be chosen depending on a load or output power at the output  27 . As long as the load or output power is greater than a threshold value, an adaptation of operation can be performed by reduction of the on time, i.e. time duration during which the switch  24  is switched on in each case. If the load or output power attains the threshold value, a first operating mode can be activated, which is DCM operation. In this case, after the switch  24  has been switched into the off state, the switch  24  is not immediately switched on again if the current I L  in the inductance  21  has fallen to zero, rather a specific minimum waiting time is provided. In DCM operation, the on time can be kept at a predefined fixed value, which can be the smallest value that can be chosen for the on time in BCM operation. An adaptation to different loads or output powers can be carried out in the first operating mode, i.e. in DCM operation, through adaptation of the minimum waiting time. 
         [0042]    As described in even greater detail with reference to  FIGS. 4 ,  5 ,  7  and  8 , in the first operating mode, i.e. in DCM operation, a switch-on instant for the switch is defined not only in accordance with the minimum waiting time, but also depending on the time-dependent behavior of the current I L  through the inductance  21  and depending on the time-dependent behavior of the voltage dropped between drain terminal and source terminal of the switch  24 . 
         [0043]      FIG. 3  illustrates the control of the power factor correction circuit  20  by the control device  14  in the second operating mode, which is illustrated by way of example as BCM operation. The switch is switched into the on state and into the off state by means of the gate voltage Vg at the switch  24 . If the switch is switched into the off state, the inductance  21  is discharged and the coil current  51  decreases. In BCM operation, a new switch-on process can be initiated as a result of the current  51  falling to zero or having a zero crossing at  54 . The switch  24  is then switched on again by means of the corresponding control signal  52  in order to charge the inductance  21  anew. In BCM operation, the on time  55  can be adapted in order to keep the bus voltage stable for different loads and/or output powers.  FIG. 3  likewise illustrates the voltage  53  dropped between drain terminal and source terminal of the switch  24  in BCM operation. 
         [0044]    While BCM operation is illustrated by way of example in  FIG. 3 , the second operating mode, which can be activated in the case of relatively high loads and/or relatively high output powers, can also be CCM operation. In CCM operation, the switching-on of the switch  24  can be initiated if the current I L  through the inductance  21  attains a reference value different than zero. 
         [0045]      FIG. 4  illustrates the transition from the second operating mode to the first operating mode, i.e. to DCM operation. By means of a suitable choice of the switch-on instant  68  at which the switch  24  is switched into the on state again, irregularities in the coil current I L  can be reduced or eliminated and the dissipation in the switch  24  and thus the heating of the switch  24  can be kept reduced. 
         [0046]    In the first operating mode, i.e. in DCM operation, the control device  14  can define a minimum waiting time  69  before the renewed switching of the switch  24  into the on state. The switch-on instant  68  at which the switch  24  is switched into the on state again does not necessarily coincide directly with the end of the minimum waiting time  69 . The switch-on instant  68  is defined such that the minimum waiting time  69  has elapsed and additional criteria have been fulfilled which are dependent on the time-dependent behavior of the power factor correction circuit  20 . The additional criteria used for defining the switch-on instant  68  can include the fact that the current  61  through the inductance has a zero crossing and the fact that the voltage  66  dropped between drain terminal and source terminal of the switch  24  in the first operating state attains a local minimum  67 . 
         [0047]    In the case of the power factor correction circuit  20  from  FIG. 2 , the presence of the criteria relating to the current  61  through the inductance  21  and the voltage  66  at the switch  24  can be checked on the basis of the signal at the input  42  of the control device. Said signal provides information about the presence of a zero crossing of the current  61  through the inductance  21  and the sign of the zero crossing, such that it is possible to ascertain whether the drain-source voltage at the switch  24  is presently at a local maximum or a local minimum. 
         [0048]    Applying these criteria has the effect that after the switch has been switched off, the switch is not switched on again upon a first zero crossing  62  of the current  61 . The inductance  21  and capacitance  25  of the power factor correction circuit  20  form a resonant circuit, such that after the decrease in the current  61 , the coil current  61  exhibits oscillations as long as the switch  24  remains in the off state. In the example illustrated, the renewed switching of the switch  24  into the on state also does not take place upon the second zero crossing of the current  61 , since the minimum waiting time  69  has not yet elapsed. In the example illustrated, the renewed switching of the switch  24  into the on state also does not take place upon the third zero crossing of the current  61 , since, although the minimum waiting time  69  has elapsed, the direction of the zero crossing is such that it corresponds to a local maximum of the voltage at the switch. The switch-on instant  68  is determined by the zero crossing  63  at which the voltage dropped between drain terminal and source terminal of the switch  24  has a local minimum. 
         [0049]    The criteria mentioned result in a number of effects being achieved. By setting the minimum waiting time  69 , it is possible to carry out an adaptation to different loads and/or output powers even if an adaptation by further reduction of the on time  55  or  65  is no longer possible or is possible only with difficulties. Overshooting of a desired peak value for the current I L  through the inductance  21  can be reduced, and the peak value of the current I L  through the inductance  21  that is attained upon each occasion of the switch being switched on can be kept at a consistent desired value. By switching at the local minimum or valley of the voltage dropped between drain terminal and source terminal of the switch  24 , it is possible to reduce the dissipation and thus the heating of the switch  24  in comparison with operation in which switching is always effected immediately when a fixed waiting time has elapsed. 
         [0050]      FIG. 5  illustrates these effects. The inductance  21  and capacitance  25  of the power factor correction circuit  20  form a resonant circuit, such that after the switch has been switched off, the coil current  69  through the inductance  21  and the voltage  70  at the switch  24  exhibit oscillations having a phase shift with respect to one another. 
         [0051]    If the switch  24  were switched on again at an instant at which the drain-source voltage  70  at the switch  24  is not an extremum and corresponds to the input voltage  79 , for example, the current I L  through the inductance  21 , upon the switch being switched on, would have a value shifted by a specific value with respect to the zero line. Switching the switch  24  into the on state at this instant would lead, upon the subsequent charging of the inductance  21 , to the result of a peak value  74  or a peak value  75  of the current I L  which does not attain or overshoots a desired value  77  for the peak current. 
         [0052]    If the switch  24  were switched on again at an instant at which the drain-source voltage  70  at the switch  24  has a local maximum  71  at which the voltage is equal to the bus voltage  78 , for example, a consistent peak value of the current through the inductance  21  would still be attained. However, the local voltage maximum, upon the switch  24  being switched on, would lead to an increased dissipation and thus to increased heating. 
         [0053]    What is achieved by the switching at the local minimum or “valley”  67  of the drain-source voltage  70  at the switch  24  is that the peak value of the current through the inductance attains the desired value  77  and the dissipation during switching remains smaller than during switching at one of the points  71 - 73 . 
         [0054]    For further elucidation of the effects of the power factor correction circuit,  FIG. 6  illustrates for comparison the current I L  through the inductance which would result if, for example, after a change in the waiting time in DCM operation independently of the time-dependent behavior of the power factor correction circuit, the switch were immediately switched into the on state again at the end of the waiting time. In the case illustrated, the current exhibits a peak value  75  that exceeds a desired value  77 . Such an irregular behavior of the current can be reduced or prevented in exemplary embodiments in which criteria for the switch-on instant which depend on the dynamic behavior of the power factor correction circuit  20  are also taken into account in addition to the minimum waiting time. 
         [0055]    In the case of the power factor correction circuit  20 , at the input  42  of the control device a signal is provided which gives information about whether the current I L  through the coil has a zero crossing and whether the drain-source voltage at the switch  24  presently has a local maximum or a local minimum or valley. This signal is evaluated by the control device  14 . The control device  14  can generate time windows for switching-on the switch  24  depending on the signal at the input  42 . The control device  42  can logically combine said time windows with a check as to whether the minimum waiting time  69  has already elapsed, in order to ascertain when the switch  24  is to be switched into the on state. The time windows can be generated in each case such that they are initiated by a zero crossing of the current I L  in a specific direction. The duration of the time windows can have a predefined value. The latter can be equal, for example, to a minimum on time of the switch  24  that can be set in the second operating mode. 
         [0056]      FIG. 7  and  FIG. 8  illustrate the determination of the instant at which the switch is switched into the on state again in the first operating mode. While the switch  24  is in the off state, the current I L  through the inductance  21  effects oscillations about a zero line that are caused by the resonance circuit formed from inductance  21  and capacitance  25 . These oscillations are correspondingly discernible in the signal  82  at the input  42  of the control device  14 . Instances at which the current I L  through the inductance  21  has a zero crossing in each case are discernible by comparing the signal  82  with a reference value  81 . The control device  14  can comprise a comparator, for example, to which the signal  82  and the reference value  81  are fed on the input side. Jumps in the output signal of the comparator indicate instant and direction of the zero crossing of the current I L  through the inductance  21 . 
         [0057]    On the basis of the identified zero crossings of the current I L  through the inductance  21  which have a sign change in a predefined direction, time windows are generated in each case, of which only time windows  84 - 86  are illustrated. These time windows correspond to the times at which, on the basis of the time-dependent behavior of the current through the inductance  21  and the drain-source voltage at the switch  24 , the switch  24  should be switched on. These time windows are chosen depending on the fact that the drain-source voltage at the switch  24  is in any case close to a local minimum and the fact that the current through the inductance  21  is in any case in the vicinity of a zero crossing. 
         [0058]    An additional criterion taken into account is that the minimum waiting time  69  must have elapsed. Since the time windows  84 ,  85  precede an end  89  of the minimum waiting time  69 , the switch  24  is not yet switched into the on state anew. The switch  24  can be switched into the on state again, however, for example in the first time window  86  which succeeds the end  89  of the minimum waiting time. The switch-on instant is defined depending on whether the end  89  of the minimum waiting time has already elapsed, if the signal at the input  42  of the control device  14  attains the reference value  81  at an instant  87 . 
         [0059]    In the case of the power factor correction circuit and method according to exemplary embodiments, in the first operating mode the control device can adapt a waiting time such that it is at least equal to a predefined minimum waiting time and furthermore depends on the time-dependent drain-source voltage at the switch  24 , which is detected by means of an input of the control device to which a signal indicating zero crossings of the coil current is fed. 
         [0060]    Depending on whether the control device  14  operates in the first operating mode or in the second operating mode, the control device  14  can automatically implement different measures for controlling the behavior of the power factor correction circuit  20 . Such an adaptation can be carried out, for example, in order to readjust the output voltage Vout to a desired value. An adaptation can also be carried out in order, depending on the load or output power, to adapt the control of the power factor correction circuit  20  such that harmonics are suppressed well. If a load-based adaptation is carried out, the control device  14  can identify the load for example on the basis of a ripple, i.e. on the basis of the voltage ripples of the output voltage Vout. For this purpose, the output voltage Vout can be detected and fed to the control device  14 . 
         [0000]    a. The second operating mode, which can correspond to BCM operation or CCM operation, for example, can be activated in the case of loads or output powers greater than a threshold value. In BCM operation or CCM operation, an adaptation to different loads or different output powers can be carried out by means of the on time of the switch, i.e. the time duration during which the switch  24  is switched into the on state in each case. For a smaller load or smaller output power, the on time can be correspondingly reduced until it attains a permissible minimum value. If a further reduction of the on time is no longer possible, a transition to DCM operation can take place. 
         [0061]    In the first operating mode, for example in DCM operation, the on time can be kept at a fixed value. The latter can correspond to the permissible minimum value for the on time which can be set in the second operating mode. In the first operating mode, it is possible to carry out an adaptation of the operation of the power factor correction circuit by changing the minimum waiting time. 
         [0062]      FIG. 9  and  FIG. 10  illustrate the adaptation of the operation of the power factor correction circuit by changing parameters that influence the driving of the switch  24 . The adaptation can be carried out for example as a function of the output power of the power factor correction circuit or load. 
         [0063]      FIG. 9  shows the on time of the switch, i.e. the time duration during which the switch  24  is switched into the on state in each case. If the output power is decreased proceeding from a higher value at which the power factor correction circuit is operated in the BCM or CCM operating mode, this corresponds to a corresponding reduction of the on time of the switch to a permissible minimum value. In the event of a further reduction of the power, the transition to the DCM operating mode can take place. In this case, the on time can be kept at a constant value  91  corresponding to the minimum value for the on time that can be set by the control device for the second operating mode. A load-dependent setting of the minimum waiting time can be carried out in the second operating mode.  FIG. 10  shows by way of example the profile of a characteristic curve which can be used for adapting the minimum waiting time in DCM operation. The characteristic curves as illustrated in  FIG. 9  and  FIG. 10  can be stored in the form of a corresponding table in the control device  14  for example in the case of a digital configuration of the control device  14 . 
         [0064]    While exemplary embodiments have been described with reference to the figures, modifications can be realized in further exemplary embodiments. While a transition from BCM operation to DCM operation has been described, for example, in further exemplary embodiments the control device can be configured for driving in the CCM operating mode. A load-dependent transition from CCM operation to DCM operation can correspondingly take place. 
         [0065]    While a description has been given of exemplary embodiments in which the current through the inductance is detected inductively using a coil or winding, other circuits can be provided in order to identify zero crossings of the current and/or local extrema of the drain-source voltage of the controllable switch. While a description has been given of exemplary embodiments in which a local minimum or valley of the drain-source voltage of the controllable switch is detected on the basis of an input signal of the control device which indicates zero crossings of the current in the inductance, it is also possible to use other arrangements that allow the control device to identify a local extremum of the drain-source voltage of the controllable switch. 
         [0066]    Methods and devices according to exemplary embodiments can be used in operating devices for illuminants, for example in an electronic ballast or in an LED converter.