Abstract:
An operating circuit for light-emitting means ( 18 ), in particular an LED string, contains an actively clocked power factor correction circuit ( 11 ), which produces a regulated output voltage (U BUS ), by means of which the light-emitting means ( 18 ) are supplied directly or indirectly via at least one further converter stage ( 17 ). The regulation is performed by means of a control unit ( 16 ), which, as manipulated variable (t ON ), actuates clocking of a switch of the power factor correction circuit ( 11 ). The operating circuit is designed, in the event of the presence of a predefined event, for example a rapid or sudden change in the energy demand of the light-emitting means ( 18 ), selectively:—to change suddenly the value of the manipulated variable (t ON ) using feedforward control, and/or—to vary the time constant of the control loop of the output voltage (U BUS ).

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method and a circuit arrangement for operating light-emitting means, in particular a load, which perform rapid sudden power variations. 
       BACKGROUND 
       [0002]    In order to operate LED luminaires, generally power-factor-corrected switched mode power supplies are used. Since an LED string, in particular a dimmable LED string, does not represent a constant load, these switched mode power supplies are generally subjected to closed-loop control. For this, monitoring of the output voltage of the switched mode power supply is generally carried out. This output voltage is used as controlled variable. In order to achieve a favorable power factor and a low level of feedback onto the electrical grid, this closed-loop control generally has a very high time constant. Thus, the controller generally operates at &lt;20 Hz, for example. Closed-loop control which implements changes in the power factor during an entire oscillation of the line voltage would result in a less favorable power factor and thus in an increased level of feedback onto the electrical grid. 
         [0003]    In particular in the case of rapid changes in load, for example in the case of sudden connection, disconnection or dimming of an LED string, the conventional slow closed-loop control cannot follow. This results in a marked fluctuation in the output voltage of the switched mode power supply. 
         [0004]    For example, WO 2011/045372 A1 discloses an operating circuit for operating LED modules. A DC voltage is generated from an AC voltage by means of power factor correction in said document. In particular in the case of rapid changes in load, however, a fluctuation in the output voltage results. 
         [0005]    The present invention is based on the object of providing a method and a circuit arrangement which ensure safe and fault-free operation of a rapidly changing load. 
       SUMMARY 
       [0006]    An operating circuit according to the invention for light-emitting means, in particular an LED string, contains an actively clocked power factor correction circuit, which generates a controlled output voltage, by means of which the light-emitting means are supplied directly or indirectly via at least one further converter stage. The closed-loop control takes place by means of a control unit, which, as manipulated variable, drives the clocking of a switch of the power factor correction circuit. The operating circuit is configured, in the event of the presence of a predefined event, for example a rapid or sudden change in the energy requirement of the light-emitting means, selectively:
       to vary the value of the manipulated variable suddenly with feed-forward control, and/or   to vary the time constant of the control loop of the output voltage. It is thus possible to keep the operating voltage constant and thus to ensure safe operation.       
 
         [0009]    Preferably, the control unit contains a subtractor, which forms a difference between a power consumed at present by the light-emitting means and a power consumed by the light-emitting means after a change in power. The control unit preferably detects the predefined event depending on the difference. Thus, it is possible to determine in a very simple and flexible manner when intervention in the conventional closed-loop control is necessary. 
         [0010]    Advantageously, the control unit contains a comparator, which compares the previously determined difference with at least one threshold value and detects the predefined event depending on the comparison. Thus, it is possible to determine in a yet simpler manner when intervention in the conventional closed-loop control is necessary. 
         [0011]    Preferably, the operating circuit is configured to increase the value of the manipulated variable suddenly with feed-forward control in the event of the presence of a rapid or sudden increase in the energy requirement of the light-emitting means, and to reduce the value of the manipulated variable suddenly with feed-forward control in the event of the presence of a rapid or sudden reduction in the energy requirement of the light-emitting means. It is thus possible to compensate for a sudden load variation safely. 
         [0012]    Alternatively, the operating circuit is configured to reduce the time constant of the control loop of the output voltage in the event of the presence of a rapid or sudden increase in the energy requirement of the light-emitting means, and to reduce the time constant of the control loop of the output voltage in the event of the presence of a rapid or sudden reduction in the energy requirement of the light-emitting means. It is thus likewise possible to compensate for a sudden load variation safely. 
         [0013]    Furthermore, the control unit is preferably configured to determine the value of the manipulated variable or the time constant of the control loop by means of a calculation specification or by reading from a table in the event of the presence of the predefined event. Thus, intervention in the closed-loop control can be determined with a low memory space requirement or with little computation load. 
         [0014]    A method according to the invention is used for operating light-emitting means, in particular LED strings. A controlled output voltage is generated by means of actively clocked power factor correction. By means of said output voltage, the light-emitting means are supplied directly or indirectly. The closed-loop control uses, as manipulated variable, the clocking of the power factor correction. In the event of the presence of a predefined event, for example a rapid or sudden change in the energy requirement of the light-emitting means ( 18 ), selectively:
       the value of the manipulated variable (t ON ) is changed suddenly with feed-forward control, and/or   the time constant of the control loop of the output voltage (U BUS ) is changed. It is thus possible to keep the operating voltage constant and thus to ensure safe operation.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The invention will be described below by way of example with reference to the drawing, which illustrates an advantageous exemplary embodiment of the invention. In the drawing: 
           [0018]      FIG. 1  shows a first exemplary embodiment of the circuit arrangement according to the invention; 
           [0019]      FIG. 2  shows a first exemplary embodiment of the method according to the invention; and 
           [0020]      FIG. 3  shows a second exemplary embodiment of the method according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    First, details will be given, with reference to  FIG. 1 , of the general design and the general mode of operation of the circuit arrangement according to the invention on the basis of an exemplary embodiment. Then, the detailed operation of a controller within a further exemplary embodiment of the present invention will be illustrated by means of  FIG. 2 . Finally, the mode of operation of exemplary embodiments of the method according to the invention will be illustrated with reference to  FIG. 2  and  FIG. 3 . Similar features have not been illustrated and described repeatedly in similar figures. 
         [0022]      FIG. 1  shows a first exemplary embodiment of the circuit arrangement according to the invention. An operating circuit  1  comprises a power factor correction circuit  11 , which is connected to a mains connection  10 . A smoothing capacitor  12  is connected to an output of the power factor correction circuit  11 . The output of the power factor correction circuit  11  is furthermore connected to a voltage divider  13 . The voltage divider  13  in this case comprises ohmic resistors  14  and  15 . The ohmic resistor  14  is in this case connected to the output of the power factor correction circuit  11  and to the ohmic resistor  15 . The ohmic resistor  15  is furthermore connected at its other end to ground. The output of the power factor correction circuit  11  is furthermore connected to a pulse width modulator  17 . An output of the pulse width modulator  17  is connected to an LED module  18 . 
         [0023]    The center point of the voltage divider  13  is furthermore connected to a control unit  16 . In turn, the control unit  16  is connected to the power factor correction circuit  11 . Furthermore, the operating circuit  1  comprises a microcontroller  21 . The microcontroller  21  is connected to the control unit  16  and to the pulse width modulator  17 . Optionally, the microcontroller  21  is additionally connected to the power factor correction circuit  11 . 
         [0024]    The power factor correction circuit  11  implements an actively clocked power factor correction of the line signal, which is present at the mains connection  10 . That is to say that the power factor correction circuit  11  selects individual segments of each oscillation of the line signal and outputs these at its output. The smoothing capacitor  12 , which is additionally connected to ground, smooths the signal output by the power factor correction circuit  11 . Thus, a DC voltage U BUS  is produced. The DC voltage U BUS  is supplied to the pulse width modulator  17 . Said pulse width modulator performs a pulse width modulation of the DC voltage U BUS  and thus generates a pulse-width-modulated DC voltage U PWM . The output signal U PWM  of the pulse width modulator  17  is output to the LED module  18 . The pulse width modulator and the LED module are not critical for the present invention. A load which is different than the LED module can also be used. Pulse width modulation is not absolutely necessary. Alternatively, amplitude modulation or else another form of pulse modulation can also take place. Preferably, the pulse width modulator  17  is in the form of a switching controller with radiofrequency clocking. In this case, it may be a single-stage or multi-stage converter, for example a flyback converter (isolated flyback converter), buck converter (step-down converter), resonant half-bridge converter (preferably with electrical isolation) or a series circuit comprising such or similar converter topologies. The DC voltage signal U BUS  can also be used directly for operating the load, wherein a linear controller, for example, can then be provided for matching or a switch can be provided for interrupting the LED current. 
         [0025]    The voltage divider  13  divides the output voltage U BUS  of the power factor correction circuit  11  according to the ratio of the values of the ohmic resistors  14  and  15 . The resultant signal U M  is supplied as controlled variable to the control unit  16 . Depending on the controlled variable U M , the control unit generates a manipulated variable t ON  and transmits it to the power factor correction circuit  11 . The control unit  16  therefore controls the output voltage U BUS . In order to achieve a power factor which is as good as possible, the closed-loop control  16  in this case operates at a frequency which is much lower than the line frequency at the mains connection  10 . Preferably, the closed-loop control operates at a frequency of &lt;50 Hz, particularly preferably &lt;20 Hz. 
         [0026]    The microcontroller  21  receives signals which initiate a change in the power consumed at present by the LED string  18  and transmits a corresponding dimming signal  19  to the pulse width modulator  17 . Instead of a pulse width modulator  17  and an LED module  18 , it is also possible for a different adjustable light-emitting means to be used. It is merely important that the microcontroller  21  sets the power consumed at present. In this case, the microcontroller  21  processes, for example, signals in accordance with the DALI standard. A connection of a plurality of independent loads to the power factor correction circuit is also conceivable. The microcontroller  21  then controls at least one, preferably the majority of, particularly preferably all of the connected loads. 
         [0027]    The microcontroller  21  additionally communicates the pending sudden load variation via a control signal  20  to the control unit  16 . Preferably, the control signal  20  contains the level and the direction of the pending sudden load variation. Alternatively, it always communicates the present power requirement. As soon as the microcontroller  21  notifies the control unit  16  of such a sudden load variation, the closed-loop control is influenced. This can take place in two different ways. 
         [0028]    A first option is to accelerate the closed-loop control. In this case, the control unit  16  reduces the time constant of the closed-loop control in order to match the manipulated variable signal t ON  to the changed power requirement of the LED string  18 . Otherwise, the closed-loop control by the controller  16  remains unchanged. Owing to the acceleration of the closed-loop control, however, an unfavorable power factor is caused for a short period of time, and this is accepted. The unfavorable power factor is there as long as the closed-loop control is proceeding in the accelerated state. The closed-loop control then operates at &gt;20 Hz, preferably at &gt;50 Hz, particularly preferably at &gt;200 Hz. 
         [0029]    A second possibility is temporary bypassing of the conventional closed-loop control. In this case, feed-forward control of the closed-loop control by the control unit  16  takes place. More details will be given with reference to  FIG. 2  of the detailed configuration of this option. 
         [0030]    With this second option too, an unfavorable power factor is accepted for a short period of time in order to keep the output voltage of the power factor correction circuit stable. The power factor in this second case is in an unfavorable range for a much shorter period of time than is the case for the first option. During this very short period of time, the power factor is much less favorable than in the first option, however. 
         [0031]    As already explained, the closed-loop control operating mode may, however, be subject to excessive demands in the case of rapid load changes. The control unit  16  therefore determines a feed-forward coupling selection signal on the basis of the information in respect of the power requirement which is transmitted by the microcontroller  21  and transmits this feed-forward coupling selection signal to a selection device. Said feed-forward coupling selection signal is used for selecting between the just-illustrated conventional closed-loop control operating mode and feed-forward coupling. In addition, the control unit  16  determines a feed-forward coupling signal and supplies this to the selection device. The feed-forward coupling signal corresponds to a manipulated variable, which is applied to the closed-loop control. If the feed-forward coupling selection signal indicates a feed-forward coupling operating mode, the selection device feeds the feed-forward coupling signal back via the delay element. The control unit  16  therefore no longer feeds back its own output signal. The operating mode illustrated here corresponds to the second option, which has already been represented with reference to  FIG. 1 . 
         [0032]    The feed-forward coupling signal is in this case either calculated by a computation specification or read from a stored table. The feed-forward coupling signal is in this case dependent on the variable of the sudden load variation and on its direction. 
         [0033]    In particular when sudden load variations are intended to be compensated for by pulse width modulation, a constant value can be set for the feed-forward coupling. The sudden load variations in the case of pulse width modulation always take place over the same level. 
         [0034]      FIG. 2  shows a first exemplary embodiment of the method according to the invention for operating a load. In a first step  50 , a present power requirement and a future power requirement are determined. This takes place in the case of the circuit arrangement according to the invention by the control unit  16 . In this case, only the respective power requirement is communicated by the microcontroller  21 . 
         [0035]    In a second step  51 , the present power requirement and the future power requirement are compared with one another. If the difference is greater than a threshold value, this is detected as a sudden load variation. In this case, the method proceeds with a third step  52 . A suitable manipulated variable for compensating for the sudden load variation is determined in this step. This can take place, as already illustrated, by a calculation specification or by reading from a table. This step is performed in the circuit arrangement by the control unit  1 . 
         [0036]    In a fourth step  53 , the just-determined manipulated variable is processed in the controller. That is to say that the feed-forward control replaces the manipulated variable value determined conventionally by the closed-loop control. In a fifth step  54 , the power factor correction circuit is controlled. In this case, the manipulated variable from the fourth step  53  is used. 
         [0037]    If, in the second step  51 , the difference is less than the threshold value, no sudden load variation is detected. In this case, the method proceeds directly with the fifth step  54 . After the fifth step  54 , the method is begun again with the first step  50 . The method illustrated here is repeated as often as desired. 
         [0038]      FIG. 3  shows a second exemplary embodiment of the method according to the invention. This largely corresponds to the method illustrated in  FIG. 3 . However, if, in the second step  51 , the difference is greater than the threshold value, the closed-loop control is merely accelerated in a third step  60 . That is to say that the time constant of the closed-loop control is markedly reduced. After this step  60 , the method proceeds with the fifth step  54 . 
         [0039]    Alternatively, instead of the hard comparison, the method may proceed differently also depending on the difference. Thus, for example, the use of two threshold values is conceivable. The controller operates beneath a first threshold value without any intervention. The closed-loop control is accelerated between the two threshold values. This corresponds to the option illustrated in  FIG. 3 . Above the upper threshold value, a feed-forward control is implemented, as illustrated in  FIG. 2 . 
         [0040]    The invention is not restricted to the exemplary embodiment illustrated. As already mentioned, very different loads or else a plurality of loads can be used. Also, the use of a switched mode power supply which is different than a power factor correction circuit is also conceivable. All of the above-described features or features shown in the figures can be combined with one another advantageously as desired within the scope of the invention.