Patent Publication Number: US-11044793-B2

Title: LED lighting circuit

Description:
FIELD OF THE INVENTION 
     The invention describes an LED lighting circuit; a method of manufacturing such an LED lighting circuit; and a method of controlling such an LED lighting circuit. 
     BACKGROUND OF THE INVENTION 
     The ability to increase or decrease the colour temperature of white light is useful, with lower colour temperatures providing “warm” lighting, and higher colour temperatures providing a “cooler” light better suited for workplace lighting. The colour temperature of a conventional light source such as an incandescent lamp or a halogen lamp can be described by a black body locus in a chromaticity diagram of a colour space, and the colour temperature is generally expressed in degrees Kelvin. 
     Light-emitting diodes (LEDs) are being used to replace conventional light sources because of their low power consumption, long lifetime, and low cost. An LED light source generally comprises an array of LEDs, for example a string of LEDs or several strings connected in parallel, and a driver to supply the array with current. The driver current can be supplied as a constant DC current or—to reduce power consumption further—using a technique of pulse-width modulation. A single array is associated with a specific colour point or colour temperature. The light intensity of an array can be adjusted by increasing or decreasing the driver current as desired and/or by adjusting PWM (pulse-width modulation) parameters of the driver current. 
     An LED lamp that can output light of more than one colour requires at least two arrays, each with a different colour point. By regulating the current of each driver, it is possible to mix the colours and the intensities. For example, using three drivers for three LED arrays of different colour points, it is possible to obtain any colour within the colour gamut of that lighting circuit. However, while LED chips have become relatively cheap in recent years, the driver remains a significant cost factor for an LED lighting circuit. Therefore, it is still quite expensive to manufacture an LED lamp that mimics the dimming behaviour of an incandescent lamp. An LED lighting circuit that uses only two arrays—and therefore only two drivers—can only approximate the classic dimming behaviour of an incandescent lamp, since the transition from one colour temperature to the other must follow a straight line in the colour space, instead of a curved line like that of the black body locus. The dimming behaviour of such a prior art LED lighting circuit may therefore be perceived as “unnatural” by a consumer. 
     Therefore, it is an object of the invention to provide an alternative LED lighting circuit that overcomes the problems described above. 
     SUMMARY OF THE INVENTION 
     The object of the invention is achieved by the lighting circuit of claim  1 ; by the lighting unit of claim  6 ; by the method of claim  7  of manufacturing such a lighting circuit; and by the method of claim  12  of controlling such a lighting circuit. 
     According to the invention, the lighting circuit comprises a first array of semiconductor light sources and a separate second array of semiconductor light sources; a shared array of semiconductor light sources; a first driver arranged to drive the first array and the shared array; and a second driver arranged to drive the shared array and the second array. 
     An advantage of the inventive lighting circuit is that it can be controlled to behave as a lighting circuit that has three drivers, even though it only requires two drivers. This configuration of drivers and LED arrays makes it possible for the colour point of the light generated by the lighting circuit to follow any path—even a curved path—through a two-dimensional xy colour space, and at any level of luminous intensity. In contrast, a two-array lighting circuit with a separate driver for each array can only achieve a “straight line” locus through a colour space, and can only approximate a curved locus by a series of straight-line segments. 
     The inventive lighting unit or luminaire comprises such a lighting circuit. Depending on the choice of LEDs in each of the arrays, the inventive luminaire can precisely mimic the colour characteristics of a conventional light source such as an incandescent bulb. 
     According to the invention, the method of manufacturing such a lighting circuit comprises the steps of choosing a colour triangle within a colour space; determining colour points associated with the vertices of the colour triangle; selecting semiconductor light sources of the arrays on the basis of the colour points; arranging a first driver to drive the first array and the shared array; and arranging a second driver to drive the shared array and the second array. 
     According to the invention, the method of controlling such an LED lighting circuit comprises operating a driver according to a repeated control pattern, which control pattern specifies at least the amplitude and duration of the driver current during each period of the control pattern. 
     The dependent claims and the following description disclose particularly advantageous embodiments and features of the invention. Features of the embodiments may be combined as appropriate. Features described in the context of one claim category can apply equally to another claim category. 
     A semiconductor light source array can comprise any number of semiconductor light sources. A semiconductor light source of the inventive lighting circuit can be a light-emitting diode (LED) or laser diode (LD), or any other suitable semiconductor light source. In the following, but without restricting the invention in any way, it may be assumed that a semiconductor light source is an LED. Since the inventive lighting circuit may be used to mimic the light quality of an incandescent lamp or similar, in a preferred embodiment of the invention, one array comprises white LEDs and the other arrays comprise non-white LEDs that may be used to adjust the colour point of the total light output. Preferably, the LED colours for the three arrays are chosen by identifying a colour triangle in the colour space, so that the colour triangle at least partially encloses the black body locus. For example, the first LED array may comprise a set of white LEDs; the second LED array may comprise a set of orange LEDs, and the shared array may comprise a set of green LEDs. The LEDs of each array can be essentially identical LEDs, each with the same specific colour; alternatively, in a more economical approach, the LEDs of an array may be chosen to achieve—in combination—the desired colour. These can be controlled together, as will be explained in the following, to achieve essentially any shade of white along a black body locus in a colour space. 
     The first driver “feeds” the first LED array and the shared LED array, while the second driver “feeds” the shared LED array and the second LED array. To ensure that the current from a specific driver only drives its two arrays, the shared array preferably comprises two rectifying diode arrangements. A rectifying diode arrangement can comprise a single rectifying diode arranged between a driver and the light-emitting diodes of the shared array. Equally, such a rectifying diode arrangement can comprise two or more series-connected rectifying diodes, or two or more parallel-connected rectifying diodes. In other words, the cathode(s) of a rectifying diode arrangement are connected to the first anode of the LED string of the shared array. Each rectifying diode arrangement defines the direction of a current path from a driver through the LEDs of the shared array. In an alternative embodiment, a rectifying diode arrangement can be arranged between the last cathode of an LED array and the last cathode of the shared array. A rectifying diode arrangement can utilize LEDs to act as rectifying diodes. This may be preferred in the case that the LEDs are cheaper than comparable rectifying diodes. 
     Since the current provided by a driver is split between two arrays, in a preferred embodiment of the invention these are assembled to present matched arrays to their respective drivers. In other words, the diodes of each array are selected so that the sum of the forward voltages is the same for each array. This can be achieved in a number of ways. For example, the LED arrays can be matched by using the same number of diodes in each string, each with the same forward voltage. In an embodiment that uses rectifying diodes in the shared array, for example, the LEDs of the first array can be selected to arrive at the same total forward voltage as that of the shared array. The same applies to the second array. 
     Alternatively, the first array can incorporate a rectifying diode which serves no purpose other than to match the forward voltages of first array and the shared array. The rectifying diode can precede the string of LEDs, for example. The same applies to the second array, which can also include such a rectifying diode. 
     In the inventive method, the first driver is operated to inject a first current into the circuit portion comprising the first array and the shared array; the second driver is operated to inject a second current into the circuit portion comprising the shared array and the second array. Following this principle, the inventive lighting circuit allows a wide variety of control sequences. Since each driver drives the shared array, it is possible to operate the lighting circuit so that it behaves as if there were a “virtual” third driver present. When only the first driver is “on”, the first array will receive approximately half of the first driver current, and the shared array will also receive approximately half of the first driver current. The two active arrays receive essentially the same current, while the LEDs of the second array receive no current. When only the second driver is “on”, the second array will receive approximately half of the second driver current, and the shared array will also receive approximately half of the second driver current. The two active arrays receive essentially the same current, while the LEDs of the first array receive no current. A third effect can be achieved by operating both drivers simultaneously. During such an “overlap”, the first array will receive approximately two thirds of the first driver current, the second array will receive approximately two thirds of the second driver current, and the shared array will receive approximately one-third of the first driver current as well as one-third of the second driver current. 
     Clearly, the colour contribution from the shared array can be adjusted in many ways. In a preferred embodiment of the invention, a control pattern is defined such that the first driver current overlaps the second driver current for an overlap duration. The length of the overlap duration and the non-overlap durations (when only one of the drivers is “on”), and the amplitudes of the first and second driver currents can be chosen for each part of a control pattern to achieve a specific desired colour and a specific luminous flux for the overall lighting circuit. A driver can be controlled to provide a constant current value for a set “on-time” duration, or it can be controlled using pulse-width modulation to rapidly switch between on and off states during an “on-time” duration. 
     A control sequence can apply a series of slightly different transitioning control patterns in order to achieve a gradual “motion” through the colour space, for example a motion that smoothly follows a locus such as a black body locus. In this way, a specific illumination behaviour can be achieved, for example to mimic the dimming behaviour of an incandescent lamp. 
     Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a simplified CIE 1931 chromaticity space; 
         FIG. 2  shows a first embodiment of the inventive lighting circuit; 
         FIG. 3  shows a colour triangle determined by the inventive method; 
         FIG. 4  shows an exemplary control pattern for the inventive lighting circuit; 
         FIG. 5  shows a second embodiment of the inventive lighting circuit; 
         FIG. 6  shows a third embodiment of the inventive lighting circuit; 
         FIG. 7  and  FIG. 8  show prior art lighting circuits. 
     
    
    
     In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows—in a simplified manner—a chromaticity diagram or “slice” through a three-dimensional CIE 1931 colour space  2 . The outer curved boundary represents the spectral locus. A black body locus BB or Planckian locus is shown, indicating some reference colour temperatures. This curve extends from a warm reddish colour like sunrise (1800 K) through a yellowish white like that of an incandescent lamp (2848 K) and a daylight white (5400 K) to blue-white (infinity). When a white light source such as a dimmable incandescent lamp is controlled to increase or decrease its brightness, the colour of its light output will essentially follow the black body locus BB. A prior art LED lamp can achieve an approximation of this behaviour by using two LED strings, each string having a different colour point, whereby the two colour points are chosen to correspond to the end points of the straight line  2 D indicated in the diagram. The colour locus of such a lighting circuit is defined by the straight line  2 D. However, the difference between this straight line and the curved black body locus BB can be perceived by an observer, and may be considered irritating or unpleasant, since the light source is not behaving in an “expected” manner. 
       FIG. 2  shows a first embodiment of the inventive lighting circuit  1 . Here, there are three arrays S 1 , SH, S 2  of LEDs L 1 , LH, L 2  arranged so that a first driver  11  drives a first array S 1  and also a shared array SH, and a second driver  12  drives a second array S 2  and also the shared array SH. 
     In this embodiment, the first LED array S 1  comprises a string of series-connected light-emitting diodes L 1 , and the second LED array S 2  comprises the same number of series-connected light-emitting diodes L 2 . The arrays S 1 , S 2  are matched, i.e. the sum of the forward voltages of the LEDs L 1 , L 2  of each array S 1 , S 2  is essentially the same. 
     The shared array SH has two rectifying LEDs LH 0  preceding the string of series-connected LEDs LH. Each rectifying LED LH 0  is connected between one of the drivers  11 ,  12  and the shared array SH. The series-connected string of the shared array SH has (at least) one less LED than each of the first or second strings S 1 , S 2 . The two rectifying LEDs LH 0  are matched, i.e. the forward voltages of these two rectifying LEDs LH 0  are essentially identical. Furthermore, the rectifying LEDs LH, LH 0  are chosen so that the sum of the forward voltages in a string comprising one of the rectifying LEDs LH 0  and the series-connected LEDs LH is the same as the sum of the forward voltages of the LEDs of the first string S 1  (and therefore also the same as the sum of the forward voltages of the LEDs of the second string S 2 ). 
     The lighting circuit can generate a specific colour that lies on the black body locus BB described in  FIG. 1  above. This is achieved by a specific choice of colour points of the LEDs L 1 , L 2 , LH, LH 0  of the strings S 1 , S 2 , SH, and by operating each driver  11 ,  12  to generate a specific current level. The colour points (or colour temperatures) of the LEDs L 1 , L 2 , LH, LH 0  of the strings S 1 , S 2 , SH are chosen to define a bounding “colour triangle”  3  as shown in  FIG. 3 . This diagram shows a part of the colour space of  FIG. 1  along with the corresponding section of the black body locus BB. The bounding triangle  3  is defined by three vertices  31 ,  32 ,  33  and represents the gamut of that lighting circuit. The coordinates of a vertex correspond to the colour point of an LED array S 1 , S 2 , SH. By appropriate choice of colour point for each array S 1 , S 2 , SH, it is possible to define a specific triangle  3  that encloses a desired portion of the black body locus BB. 
     The first driver  11  provides a driver current I 11  that is divided between the first array S 1  and the shared array SH, while the second driver  12  provides a driver current I 12  that is divided between the shared array SH and the second array S 2 . When both drivers are “on”, the current I S1  through the first array S 1  is two thirds of the first driver current I 11 ; the current I S2  through the second array S 2  is two thirds of the second driver current I 12 ; and the current I SH  through the shared array SH is one third of the first driver current I 11  plus one third of the second driver current I 12 . 
     When only one of the two drivers is “on”, the current from that driver is shared equally between two strings. For example, when the first driver  11  is “on” and the second driver  12  is “off”, the current I S1  through the first array S 1  is one half of the first driver current I 11 ; the current I S2  through the second array S 2  is 0; and the current I SH  through the shared array SH is also one half of the first driver current I 11 . Due to the non-linear behaviour of a diode, as will be known to the skilled person, the currents I S1 , I S2 , I SH  drawn by the LED strings S 1 , S 2 , SH will not be exactly one-third, one half etc. of the driver current I 11 , I 12 . 
     By appropriately operating the drivers  11 ,  12  to generate a specific combination of first current I 11  and second current I 12 , the light output by the lighting circuit can follow the black body locus BB while the lamp is being dimmed or when its brightness is being increased. Possible “colours” of an exemplary lighting circuit are shown as dots lying close to or on the black body locus BB. Any colour within the colour triangle  3  is possible. 
       FIG. 4  is a simplified schematic of current I (in mA) against time (in ms) showing how the strings S 1 , S 2 , SH may be activated and deactivated according to successive periods P 1 , P 2 , P both , P off  of an exemplary specific control pattern P. The upper part of the diagram shows the current I SH  through the shared string SH of  FIG. 2 , the middle part of the diagram shows the current I S1  through the first string, and the lower part of the diagram shows the current I S2  through the second string. The first driver delivers a first current I 11  from time t 0  to time tb, and the second driver delivers a second current I 12  from time ta to time tc. From time t a  to time t b , the shared string SH is being fed with current from both the first and second drivers. 
     When only the first driver is “on” in period P 1 , the current I S1  through the first array is approximately 50% of the first driver current I 11 , and the current I SH  through the shared array is also approximately 50% of the first driver current I 11 . In this period P 1 , the LEDs of the second array receive no current. 
     When both drivers are “on” in period P both , the current I S1  through the first array is approximately 66% of the first driver current I 11 , the current I S2  through the second array is approximately 66% of the second driver current I 12 , and the current I SH  through the shared array is given by the sum of approximately 33% of the first driver current I 11  and approximately 33% of the second driver current I 12  When only the second driver is “on” in period P 2 , the current I S2  through the second array is approximately 50% of the second driver current I 12 , and the current I SH  through the shared array is also approximately 50% of the second driver current I 12 . In this period P 2 , the LEDs of the first array receive no current. 
     The control pattern P can persist for a desired length of time and may be preceded by and followed by other suitable control patterns of a dimming sequence, a colour adjustment sequence, etc. A control pattern P can include an “off” period P off  in which both drivers are off, for example. The current levels I 11 , I 12  of the drivers and the duration of periods P 1 , P 2 , P both , P off  of each control sequence can be carefully chosen to achieve the desired colour as well as the desired intensity. Of course, the control sequence shown in  FIG. 4  is only exemplary, and it will be understood that any sequence of active driver currents and on/off times is possible. 
       FIG. 5  shows a second embodiment of the inventive lighting circuit  1 . It is similar to that of  FIG. 2 , and only the difference is explained here: Instead of the rectifying LEDs LH 0  of  FIG. 2 , the shared array SH has two rectifying diodes RH at the beginning of the string of series-connected LEDs LH. The rectifying diodes RH are matched, i.e. the forward voltages of these two diodes RH are essentially identical. Furthermore, the LEDs L 1 , L 2 , LH and diodes RH are chosen so that the total forward voltage is essentially the same for each array S 1 , S 2 , SH. If the rectifying diodes RH are near-ideal, i.e. with near-zero forward voltage, the shared string SH can comprise an additional LED as indicated in the diagram. 
     In this embodiment also, the colour points of the LEDs L 1 , L 2 , LH can be chosen to define a colour triangle as explained in  FIG. 3  above, and the drivers  11 ,  12  can be operated to drive the LED arrays S 1 , S 2 , SH to generate a specific colour within the colour triangle, or to make the colour follow the black body locus BB. 
       FIG. 6  shows a third embodiment of the inventive lighting circuit  1 . It is similar to that of  FIG. 5 , and only the difference is explained here: Each of the first and second strings S 1 , S 2  includes a rectifying diode R 1 , R 2  at the beginning of the string of series-connected LEDs L 1 , L 2 . This makes it easier to match the forward voltages of the strings S 1 , S 2 , SH, and is a more economical realisation since rectifying diodes are generally very cheap components. Here also, the colour points of the LEDs L 1 , L 2 , LH can be chosen to define a colour triangle as explained in  FIG. 3  above, and the drivers  11 ,  12  can be operated to drive the LED arrays S 1 , S 2 , SH to generate a specific colour inside the colour triangle, or to make the colour follow the black body locus BB as the lamp is being dimmed or brightened. 
       FIG. 7  shows a prior art lighting circuit with two LED arrays. Here, two separate circuits  70 ,  71  are required. A first circuit  70  has a first driver  700  and a string of LEDs  7 A of a first colour. A second circuit  71  has a second driver  710  and a string of LEDs  7 B of a second colour. A driver  700 ,  710  can only adjust the light output of its own LED string by increasing or decreasing the driver current amplitude, by adjusting PWM parameters, etc. The colour space locus achievable using such a circuit will follow a straight line  2 D as shown in  FIG. 1 . This prior art realisation is therefore unsuitable for mimicking the colour behaviour of an incandescent lamp. 
       FIG. 8  shows another prior art lighting circuit. Here, three separate circuits  80 ,  81 ,  82  are required. A first circuit  80  has a first driver  800  and a string of LEDs  8 A of a first colour. A second circuit  81  has a second driver  810  and a string of LEDs  8 B of a second colour. A third circuit  82  has a third driver  820  and a string of LEDs  8 C of a third colour. The colour space locus achievable using such a circuit can follow a black body locus, but at the cost of an additional third driver. This prior art realisation is therefore unfavourably expensive. 
     Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
     For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. 
     REFERENCE SIGNS 
     
         
         lighting circuit  1   
         driver  11 ,  12   
         colour space  2   
         colour triangle  3   
         vertex  31 ,  32 ,  33   
         LED array S 1 , S 2 , SH 
         driver current I 11 , I 12    
         array current I S1 , I S2 , I SH    
         light-emitting diode L 1 , L 2 , LH, LH 0   
         rectifying diode RH, R 1 , R 2   
         control pattern P 
         duration P 1 , P 2 , P both , P off    
         black-body locus BB 
         straight locus  2 D 
         time t 0 , ta, tb 
         prior art circuit  70 ,  71   
         prior art driver  700 ,  710   
         prior art circuit  80 ,  81 ,  82   
         prior art driver  800 ,  810 ,  820