Abstract:
Techniques for controlled dimming of an illuminant such as, for example, a light-emitting diode (LED), an organic light-emitting diode (OLED) or gas discharge lamp are described herein. In one example, a control difference formed by subtracting actual values from desired values (dimming values) is smaller than actual values fed back by the lamp. Thus, without directly digitizing an analog feedback variable, the example digitizes a control difference (control deviation) determined in the analog domain, in order to process the latter in a digital control algorithm that determines a digital manipulated variable that influences power to the illuminant.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application is a National Stage of International Application No. PCT/EP2010/53522, filed Mar. 18, 2010, which claims foreign priority to DE Application No. 10 2009 013 897.8, filed Mar. 19, 2009, both of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for the controlled dimming of an illuminant such as, for example, an LED, OLEDs or gas discharge lamps in accordance with digital dimming values that form desired values. 
     Such a method is known and is, for example, embodied with the circuit arrangement according to the prior art as illustrated in  FIG. 1 . In the case of the known circuit arrangement, the illuminant is a gas discharge lamp  1 . The latter is operated with a ballast  2  that is embodied in a known way, and for this reason only the important components are illustrated in schematic form. The ballast  2  contains two series-connected electronic switches S 1 , S 2  that are supplied with a normal DC voltage. The two switches S 1 , S 2  are controlled by a digital switching unit  3  that can change the switching frequency and/or the duty ratio. A series resonant circuit formed from an inductor L and a resonant capacitor C 1  is located above the lower switch S 2 . The voltage drop across the resonant capacitor C 1  is fed to the lamp  1  via a coupling capacitor C 2 . There is a resistor in the circuit of the lamp. The voltage drop across the resistor R 1  is a measure of the luminous intensity produced by the lamp  1 , and can therefore be used to form analog actual values I a . The lamp  1  is dimmed via a digital control loop. Digital dimming values D d  are made available via a DALI bus, for example, by a dimming value transmitter  4 , which can be arranged in a remote control center. Digital dimming values can, for example, be formed by 12 or 13 bits in order to ensure as fine resolution of the dimming stages as possible. The abovementioned analog actual values I a  are converted into digital actual values I d  in an A/D converter  6 . The digital dimming values D d  and the digital actual values I d  are compared with one another in a digital comparator  5 . For the comparison, the A/D converter  6  should have as far as possible the same bit number as the digital dimming values D d . A consequence of this in the case of a relatively high bit number of—as previously mentioned— 12  or  13 , for example, is that the A/D converter  6  operates relatively slowly. By means of the comparison of the digital dimming values D d  and the digital actual values I d , the digital comparator  5  produces a digital control deviation X d  that is fed to a digital controller  7 . The digital controller  7  produces therefrom a digital manipulated variable X d  that is then fed to the digital switch unit  3 . 
     Since, as previously mentioned, the A/D converter  6  is relatively slow, it operates in the range of milliseconds. Consequently, the entire control loop is relatively slow. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the object of the invention to modify the method specified at the beginning to the effect that the requirements placed on the components used are reduced. 
     In particular, the requirements placed on the A/D converter are to be reduced. 
     The essential idea of the invention here is not directly to digitize an analog feedback variable, but to digitize a control difference (control deviation) determined in the analog domain, in order then to process the latter in a digital control algorithm that determines a digital manipulated variable that influences the power of the illuminant. 
     In accordance with the characterizing part of an embodiment, the object is achieved by virtue of the fact that desired dimming values (termed “dimming values”) digitally prescribed for an operating device are firstly converted into analog dimming values, the analog dimming values are compared with corresponding analog actual values, and that an analog control deviation is determined therefrom, and in that the analog control deviation is then converted into a digital control deviation in order to carry out the digital control. 
     The invention is based on the finding that the control difference formed by subtracting the actual values from the desired values (dimming values) is smaller than the actual values led back by the lamp. When use is made of a PI controller, the control difference is even reduced to zero after settling. The relatively slight analog control deviation must admittedly subsequently be converted into a digital control deviation; the A/D converter required therefor can, however, have a reduced bit number. It therefore operates more quickly than the previous A/D converter with which the analog actual values have been converted into digital actual values. The entire control loop therefore also operates more quickly. 
     It is true that the inventive method requires the digital desired dimming values firstly to be converted into analog desired dimming values; the D/A converter required therefor is, however, not part of the control loop, and therefore does not influence the control rate thereof. According to the inventive embodiment, said control rate can lie in the range of microseconds. 
     Expedient developments of the inventive method are the subject matter of additional embodiments. 
     The invention further relates to a circuit arrangement for dimming an illuminant in accordance with digital dimming values—that form desired values—by digital control, having a comparator in which the dimming values and actual values corresponding to the brightness of the illuminant are fed, and that determines a control deviation by comparison, and having a digital controller to which the control deviation is fed, and that produces therefrom the analog manipulated variable for controlling the illuminant. 
     The abovedescribed circuit arrangement has already been mentioned as known at the beginning, and explained in conjunction with  FIG. 1 . 
     The tasks set for the circuit arrangement correspond to those of the inventive method. 
     Circuitry for performing the tasks set consists in the fact that connected upstream of the comparator is a D/A converter that converts the digital dimming values into analog dimming values, that the comparator operates in analog fashion and determines an analog control deviation, and that connected downstream of the comparator is an A/D converter that converts the analog control deviation into a digital control deviation that is then fed to the digital controller. 
     Expedient refinements of the inventive circuit arrangement are the subject matter of additional embodiments. 
     It may be remarked at this juncture that the content of the previously mentioned embodiments (not quoted) are to constitute part of the disclosure of the description. 
     Finally, the invention also relates to a lighting system. 
     Exemplary embodiments of the invention are described below with the aid of the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a circuit arrangement according to the prior art; 
         FIG. 2  shows an embodiment of the inventive circuit arrangement; 
         FIG. 3  shows the logarithmic dependence of the subjective perceived brightness on the physically measurable luminous intensity of the illuminant; 
         FIG. 4  shows the linear dependence of the luminous intensity on linearly digitized dimming values; and 
         FIG. 5  shows the dependence of the analog dimming values on the digital dimming values with a targeted exponential distortion for the purpose of compensating the logarithmic curve in accordance with  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The known circuit arrangement in accordance with  FIG. 1  has already been explained at the beginning in conjunction with the description of the prior art. 
     Components in  FIG. 2  which are the same as those in  FIG. 1  are denoted by the same reference symbols. Newly added components and blocks are presented in bold lines. The circuit arrangement according to  FIG. 2  deviates from that of  FIG. 1  initially in that the digital dimming values D d  are converted into analog dimming values D a  in a D/A converter  11 . The particular properties of the D/A converter  11  will be explained later further in conjunction with  FIGS. 3-5 . 
     The analog dimming values D a  are fed to the negative input of an analog comparator  13  formed by an operational amplifier. The analog actual values I a  are fed to the positive input of an operational amplifier  12 , which has a variable gain V. The negative input of the operational amplifier  12  is connected to frame via a resistor R 4 ; however, it is connected to the output of the operational amplifier  12  via a feedback resistor R 5 . The analog actual value signals V*I a  amplified by V are present at the output of the operational amplifier  12 . They are fed to the positive input of the comparator  13 . The comparator  13  forms the difference of its two input signals, and therefore produces the analog control deviation X a . The latter is fed to an A/D converter  14 . The A/D converter  14  produces from the analog control deviation X a  a digital control deviation X d  that is fed to the digital controller  7 . The processing of the digital control deviation X d  is then performed as in the case of the circuit arrangement according to  FIG. 1 . 
     It is important in the case of the circuit arrangement according to  FIG. 2  that the digital dimming values D d  are converted into analog dimming values D a , and that the amplified analog actual values V*I a  are compared with the analog dimming values D a  in a comparator  13  operating in analog fashion, in order to produce the analog control deviation X a , the latter then again being converted into a digital control deviation X d  with the A/D converter. By comparison with  FIG. 1 , in the case of the circuit arrangement according to  FIG. 2 , the A/D converter  6  is thus omitted; in return, however, the D/A converter  11  and the A/D converter  14  are added. Despite this apparent complication, the circuit arrangement according to  FIG. 2  has a decisive advantage, specifically that the control loop can operate more quickly. The A/D converter  14 , which is part of the control loop, need only convert the relatively small analog control deviation X a  into a digital control deviation X d , and therefore manages with relatively few bits, for example 8 bits. This means that the control loop of the circuit arrangement in accordance with  FIG. 2  operates more quickly than that of the circuit arrangement according to  FIG. 1 , in the case of which the A/D converter  6  must process a high bit number, specifically that which is prescribed, as a rule, with the digital dimming value D d . 
     Reference is now made to  FIGS. 3 to 5  in order to explain an additional function of the D/A converter  11 . 
       FIG. 3  shows the known dependence of the subjectively perceived brightness of the luminous intensity—which can be measured physically in candelas—of an illuminant. It is to be seen that, in the case of relatively high luminous intensities, equidistant jumps in luminous intensity are still perceived only as small jumps in luminous intensity. In the case of low luminance intensities, by contrast, equidistant jumps in luminous intensity are perceived as correspondingly high jumps in brightness. 
       FIG. 4  shows the normal linear relationship between the luminous intensity produced by an illuminant, in particular by a gas discharge lamp, and linearly digitized dimming values D d . 
     When the illuminant is controlled with equidistantly linearized dimming values D d  in accordance with  FIG. 4 , the observer has the sensation of brightness in accordance with  FIG. 3 . The jumps in brightness differ in size in dependence on the luminous intensity. In order, nevertheless, to attain a brightness resolution that is satisfactory to a certain extent over the entire range of luminous intensity, the D/A converter  11  must have a relatively high bit number, for example 13 or 14 bits. This is certainly not a problem with regard to the reduced rate during dimming; all that is to be desired is rapid control. However, such a D/A converter is more expensive than a D/A converter with smaller bit number. 
     Use may be made of a D/A converter with a small bit number when one is chosen that additionally distorts exponentially, as is illustrated in  FIG. 5 . The distortion signifies that, for linearized digital dimming values D d , analog dimming values D a  are produced that are larger in the case of relatively high dimming values, and are smaller in the case of relatively low dimming values. In this way, the logarithmic curve in accordance with  FIG. 3  is compensated by the exponential curve in accordance with  FIG. 5 . 
     Such an exponentially distorting D/A converter  11  is used in the case of the circuit arrangement in accordance with  FIG. 2 . It manages with 8 bits. Consequently, it also has 8 bit inputs. Such a distorting D/A converter is not mandatory, but advantageous for the reasons described. It is also possible to make use instead of a linearly converting D/A converter with a relatively high bit number which then, however, is—as mentioned—unfavorable in terms of cost. 
     As described above, the gain V of the operational amplifier  12  is variable. The gain V can be reduced when the A/D converter  14  reaches its extreme value, that is to say reaches its limit. In this case, the gain V of the operational amplifier  12  is reduced to half, for example. It is then necessary for the purpose of compensation to multiply the digital range in the A/D converter  14  by a factor of 2. The indication of reaching the extreme value EW is shown by the thick dotted line between the A/D converter  14  and the operational amplifier  12 . The reduction in the gain V of the operational amplifier  12  is performed in practical terms by a factor that results from a binary combination, that is to say 2, 4, 8, etc. In order for the last mentioned function to be triggered, very high control deviations X a  must occur, and it is necessary in this case to accept that the multiplication in a digital range entails a worsening of the resolution. 
     The reduction in the gain V of the operational amplifier  12  can also be triggered in the case of another operating state, for example whenever lamp  1  has been started, or shortly thereafter, when the control system has settled specifically. It is only then that a fine resolution is desirable. The possibility that the gain V is reduced when the lamp  1  is started is indicated in  FIG. 2  by the thick dashed line, which leads to the A/D converter  14  from the node of a voltage divider R 2 /R 3  situated over the lamp  1  via the path Z-Z. The signal voltage Z tapped from the voltage divider R 1 /R 2  corresponds to the lamp voltage, which changes after the starting. 
     There is, furthermore, the possibility of transmitting to the operational amplifier  12  advanced information relating to an approaching relatively large jump in dimming. Furthermore, it is possible to modify the electronic ballast  2  such that it executes a large jump in dimming only when the operational amplifier  12  reduces its gain V, and the reduction by multiplication in the digital range in the A/D converter  14  has been compensated.