Abstract:
An LED lighting device may include a first constant current source, a switched mode power supply, a plurality of LEDs between the switched mode power supply and the first constant current source, and powered by the switched mode power supply, and a voltage divider between the switched mode power supply and the plurality of LEDs. In various embodiments, the voltage divider may include a plurality of switches. Each switch may be configured to transition between open and closed based on a lighting state of an LED of the plurality of LEDs to vary a divided feedback voltage provided to the switched mode power supply. In various embodiments, the switched mode power supply may be configured to supply different output voltages based on the divided feedback voltage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 13/212,537, filed Aug. 18, 2011, entitled “LED LIGHTING DEVICE,” which is a continuation of U.S. Pat. No. 8,004,211, entitled “LED LIGHTING DEVICE,” for which the National Phase was entered on Jun. 11, 2008 based on PCT/IB2006/054779, which was filed on Dec. 12, 2006 and claimed priority to EP05112040, filed on Dec. 13, 2005. The entireties of these disclosures are hereby incorporated by reference. 
    
    
     FIELD OF TECHNOLOGY AND BACKGROUND 
     The present invention relates to a LED (light-emitting diode) lighting device comprising at least a first and a second LED powered by a switched mode power supply. 
     Such a device is disclosed e.g. in EP 0716485 A1. 
     In such a device, a switched mode power supply (SMPS) is controlled to supply a constant current to one or more LEDs. The LED light output may then be controlled through pulse width modulation by varying the duty cycle of a bypass switch which is connected in parallel with the LED/LEDs. 
     The SMPS is superior to a linear power supply in terms of energy efficiency. However, the precision of the supplied current may not be sufficient for all applications, because the load varies, which induces transients in the used current feedback system. 
     SUMMARY 
     It is therefore an object of the present invention to provide a lighting device of the type mentioned in the opening paragraph, wherein the current precision is improved while maintaining a reasonably low power consumption. 
     This object is achieved by means of a lighting device as defined in claim  1 . 
     More specifically, in the above-mentioned device, the current through the first LED is controlled by a first bypass switch, connected in parallel with the first LED, the current through the second LED is controlled by a second bypass switch, connected in parallel with the second LED, the first and second LEDs being connected in series between the switched mode power supply and a first constant current source, and the switched mode power supply is arranged to supply a number of different output voltages depending on the state of the first and second bypass switches, such that the voltage depends on the number of LEDs emitting light. 
     Due to the constant current source, a very well defined current is drawn from the power supply. However, since the output voltage is varied in dependence on the states of the LEDs, the dissipation in the constant current sources can be kept at a low level. No feedback of the actual current is needed, which prevents instability and transient problems. 
     In a preferred embodiment, the first and second LEDs are arranged to be controlled by pulse width modulation, such that the first and second LEDs are switched on simultaneously at the beginning of a pulse width modulation period and are switched off during the pulse width modulation period at instants determined by their respective duty cycles. Then the voltage of the switched mode power supply may be arranged to rise towards a maximum voltage before the beginning of the pulse width modulation period. This ensures that a sufficient voltage is provided at the start of the period. 
     The device may comprise means for driving the output voltage to zero or close to zero when all LEDs are switched off. This saves additional power. 
     The first and second LEDs may emit light of different colors. 
     The first and second LEDs may emit light having a green and a blue color and may be connected to the first constant current source via a third LED, which is controlled by a third bypass switch. The first and second LEDs may further be connected to a second constant current source via a fourth LED, which is controlled by a fourth bypass switch, the third and fourth LEDs emitting red or amber colored light. This allows a complete, controllable RGBA (red-green-blue-amber) light source realized with a single SMPS. 
     These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates schematically the bypass control of a LED driven by a constant current source of the prior art. 
         FIG. 2  illustrates schematically the available color gamut for an RGB LED arrangement. 
         FIG. 3   a  illustrates schematically a LED lighting device in accordance with an embodiment of the invention. 
         FIG. 3   b  shows in more detail an example of the constant current source used in  FIG. 3   a.    
         FIG. 4  illustrates an embodiment of the invention, in which an RGBA lighting arrangement is realized. 
         FIG. 5  illustrates a time diagram for the arrangement of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates schematically the bypass control of a LED driven by a constant current source of the prior art. A constant current source (CCS)  1  is arranged to feed a constant current I const  to a LED  3  (Light Emitting Diode). For example, if the LED is a blue light-emitting LED, the constant current is typically I consT=700  mA. A bypass switch  5 , typically a MOSFET, is connected in parallel with the LED  3 . The bypass switch is controlled by PWM (Pulse Width Modulation) to either be fully conducting or fully blocking, using a PWM circuit  7 . When the bypass switch  5  is fully conducting, it bypasses the LED  3 , such that the LED stops emitting light. It is thus possible to control the light flow from the LED by varying the duty cycle of the bypass switch  5 . This is done at a switching frequency that is high enough to prevent any visible flicker, e.g. 150 Hz or higher. 
     It is possible to have two LEDs, each having a bypass switch, share a common current source. Then the two LED/bypass switch combinations are connected in series with each other. The requirement is that the LEDs use the same driving current (e.g. red and amber light-emitting LEDs (350 mA) or blue and green light-emitting LEDs (700 mA)). 
       FIG. 2  illustrates schematically, in a chromaticity diagram, the available color gamut for an RGB LED arrangement wherein the LEDs are controlled with bypass switches. By using a red R, a green G, and a blue B light-emitting LED in combination, a color triangle  9  covering a large part of the total color gamut  11  can be achieved. To emit light of a desired color  13 , the PWM circuit of each LED is given a predetermined duty cycle, such that the correct amount of light is emitted from each LED to produce the desired color. 
       FIG. 3   a  illustrates schematically a LED lighting device in accordance with an embodiment of the invention. It should be noted that the use of a switched mode power supply (SMPS) is considered to be advantageous, because an SMPS usually has a much better energy efficiency than a linear power supply. 
     It is possible to use an SMPS as a constant current source by placing a shunt resistor in series with the supplied LEDs and regulate the SMPS, based on the voltage across the shunt resistor, so as to supply the correct current. This approach is described e.g. in EP 0716485 A1. However, in practice, this may be quite complicated due to the high current precision requirements. 
     In an embodiment of the present invention, a different scheme is therefore used to control an SMPS. The embodiment shown in  FIG. 3   a  has a first LED  15  and a second LED  17  which are connected in series and require the same constant current (e.g. a red and an amber light-emitting LED). The first LED  15  is controlled by a first bypass switch  19 , which receives a first control signal sw 1 . The second LED  17  is controlled by a second bypass switch  21 , which receives a second control signal sw 2 . The bypass switches are connected in parallel with the respective LED to PWM-control the current therethrough as described hereinbefore. The LEDs  15 ,  17  are powered by a switched mode power supply (SMPS)  8  having an output voltage V out . The SMPS is also PWM-controlled but at a much higher frequency, e.g. a few hundred kHz. However, the output voltage V out  is not controlled by measuring the current through the LEDs. Instead, the LEDs are connected in series between the SMPS  8  and a constant current source (CCS)  25 . 
     As long as V out  is high enough, the constant current source  25  ensures that a constant and predetermined current is drawn through the LEDs  15 ,  17  or the bypass switches  19 ,  21 , if turned on. 
       FIG. 3   b  shows in more detail an example of the constant current source  25  used in  FIG. 3   a . This constant current source is known per se and comprises a first bipolar transistor T 1  and a second bipolar transistor T 2 , connected base to base. The first transistor T 1  is diode-coupled (collector-base) and its collector is connected to a reference voltage V ref  via a first resistor R 1 . The emitter of T 1  is connected to ground via a second resistor R 2 . The emitter of T 2  is connected to ground via a third resistor R 3 . This circuit will draw the constant current I const  at the collector of T 2 . This constant current is determined by R 1 , R 2 , R 3 , V ref  and the base-emitter voltage of T 1  and T 2 , all of which are constant. 
     It is evident from  FIG. 3   a  that the voltage drop across R 3  in  FIG. 3   b  will increase whenever a bypass switch  19 ,  21  is activated. Thus, if V out  is kept constant, the power dissipation in the constant current source will be quite high. There are of course constant current source topologies other than the one illustrated in  FIG. 3   b , but this problem remains. 
     The SMPS  8  in this embodiment of the invention is therefore adapted to supply a number of different voltages depending on the states of the bypass switches  19 ,  21 . This means that the output voltage V out  varies in dependence on the number of activated LEDs. Typically, the SMPS  8  receives the control signals sw 1  and sw 2  of the bypass switches  19 ,  21  as input signals. Thus, if none of the bypass switches  19 ,  21  is activated and both LEDs  15 ,  17  emit light, V out  has a first, high voltage. If one of the bypass switches  19 ,  21  is activated, the output voltage is forced down to a second, lower value. If both bypass switches are activated, V out  can become 0 V, or close to 0 V, constituting a third value. The power dissipation in the constant current source  25  can thus be kept at a low level. The SMPS  8  may be preferably any type of step-down or buck-converter. 
       FIG. 4  illustrates an embodiment of the invention, in which an RGBA lighting arrangement is realized. In this embodiment, four LEDs (Red  29 , Green  31 , Blue  33 , Amber  35 ) are used, each having a bypass switch  37 ,  39 ,  41  and  43 , respectively. Each bypass switch  37 ,  39 ,  41 ,  43  receives a PWM control signal R, Gr, Bl, A to control the light flow of the corresponding LED. 
     The red light-emitting LED  29  is connected to a first constant current source  45  comprising a resistor R 4  in series with a transistor T 3 . The amber light-emitting LED  35  is connected to a second constant current source  47  comprising a resistor R 5  in series with a transistor T 4 . The bases of the transistors T 3  and T 4  may be connected to a common voltage reference V ref . The first and second constant current sources  45 ,  47  are preferably identical, such that they draw the same current, which may be 350 mA for red and amber LEDs. 
     The red light-emitting LED  29 , with its bypass switch  37  and series-connected constant current source  45 , is connected in parallel with the amber light-emitting LED  35  with its bypass switch  43  and series-connected constant current source  47 . These circuits may thus draw a total current of 700 mA, which is a suitable driving current for the green and blue light-emitting LEDs  31 ,  33 . Together with their bypass switches  39 ,  41 , the green and blue light-emitting LEDs  31 ,  33  may therefore be connected in series with the parallel arrangement of the red and amber light-emitting LEDs  29 ,  35 . It is thus possible to supply all the LEDs from one common SMPS  49 . In this embodiment, the red and amber light-emitting LEDs should be controlled in a synchronized manner. This requirement is, however, compatible with most color control schemes. 
     In order to minimize the dissipation in the constant current sources  45 ,  47 , the SMPS  49  should be able to output four different output voltages depending on the number of turned-on LEDs ( 0 ,  1 ,  2  or  3 , red and amber light-emitting LEDs being counted as one, as they are switched in synchronism). The feedback network  51  of the SMPS  49  is therefore adapted to receive the PWM control signals Gr, Bl, A/R of the bypass switches  37 ,  39 ,  42 ,  43 . The feedback network  51  receives the output voltage V out , which is filtered by an output capacitor C 1 . A voltage divider comprising two resistors R 6  and R 7  is connected between the SMPS output and ground and generates a divided feedback voltage V f . In addition to R 6 , three resistors R 8 , R 9  and R 10  are connected between the SMPS output and R 7 . R 8  is connected via a switch  53 , which is controlled by the PWM control signal Gr of the bypass switch  39  of the green light-emitting LED  31 . The switch  53  is thus switched on if the green LED  31  is switched off. Similarly, R 9  is connected via a switch  55 , which is controlled by the PWM control signal Bl of the bypass switch  41  of the blue LED  33 . R 10  is connected via a switch  57 , which is controlled by the PWM control signal A/R of the bypass switches  37  and  43  of the red and amber light-emitting LEDs  29  and  35 . The voltage-dividing function of the divider network will thus vary in dependence on the number of switched-on LEDs. For example, if all LEDs are switched on: 
     
       
         
           
             
               V 
               f 
             
             = 
             
               
                 V 
                 out 
               
               × 
               
                 
                   R 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   7 
                 
                 
                   
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   + 
                   
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                 
               
             
           
         
       
     
     If the blue light-emitting LED  33  is then switched off, V f  increases to 
     
       
         
           
             
               V 
               f 
             
             = 
             
               
                 V 
                 out 
               
               × 
               
                 
                   R 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   7 
                 
                 
                   
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   + 
                   
                     
                       R 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       6 
                       × 
                       R 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       9 
                     
                     
                       
                         R 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         6 
                       
                       + 
                       
                         R 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         9 
                       
                     
                   
                 
               
             
           
         
       
     
     In the SMPS  49 , V f  is compared with an internal reference, and the output voltage is increased or decreased in conformity with this comparison. Thus, if V f  increases when the blue LED is switched off as above, the output voltage V out  decreases. This keeps the voltage across the constant current sources  45 ,  47  at a low level, thus preventing increased power dissipation therein. This does not only mean that energy is saved. The constant current sources  45 ,  47  can also have a lower heat-dissipating performance than in the case in which the SMPS output voltage is not regulated in this way. 
     This circuit may of course be adapted to other combinations of LEDs, e.g. RRGB (two red light-emitting LEDs, one green and one blue light-emitting LED), RGB or CMY (Cyan, Magenta and Yellow light-emitting LEDs). 
       FIG. 5  illustrates a time diagram for the arrangement of  FIG. 4 . In the upper part,  FIG. 5  illustrates V out  during a PWM period  61 , which may be e.g. 2 ms long. The lower part shows the inverse of the LEDs PWM control signals Gr, Bl, R, A (R and A being switched in synchronism as mentioned before). When these signals are at level  1 , the corresponding LED thus emits light. In the illustrated example, light of a predetermined color should be emitted. To obtain this color, in each PWM period, the green light-emitting LED emits light during a first time period  63 , the blue light-emitting LED emits light during a second, longer time period  65 , and the red and amber light-emitting LEDs emit light during a third, even longer, time period  67 . The emission of light starts simultaneously for all LEDs but ends at different points in time. The output voltage V out  starts the PWM period  61  at a first initial maximum voltage V max . When the green light-emitting LED is turned off at the end of the first time period  63 , the voltage drops to a second, lower voltage. Similarly, when the blue light-emitting LED is turned off, the voltage further drops to a third level. Finally, when the red and amber light-emitting LEDs are turned off, the voltage drops to a fourth minimal voltage. At the beginning of the subsequent PWM period, all LEDs are switched on and the SMPS output voltage again recovers to V max , i.e. the voltage used when all LEDs are turned on. 
     With reference to  FIGS. 4 and 5 , the circuit shown in  FIG. 4  can be modified in two ways in order to improve its function. 
     First, optional means can be provided to ensure that the output voltage is zero or close to zero when all LEDs are switched off. This may be, e.g., a circuit  69  with a logic AND gate, the output of which goes high when Gr, Bl, and A/R all go high. This gate can then be used for driving a switch  71  short-circuiting R 6 , thus driving the output voltage to a very low value. This saves some energy consumption. 
     Secondly, means can be provided to ensure that the output voltage rises already before the start of each PWM period, such that V out  has already reached V max  when the LEDs are turned on. This can be realized by adding an optional switch  73  that is arranged to disconnect resistors R 8 , R 9 , and R 10  during a short time period before the PWM period begins. This ensures that the constant current sources can draw the correct currents already from the start, thus precluding potential color errors. If the optional switch  73  is not used, R 8 , R 9 , and R 10  are instead connected directly to the voltage divider of R 6  and R 7 . 
     In summary, the invention relates to a multiple LED driver circuit in which each LED is controlled by a bypass switch. The LEDs are supplied by a switched mode power supply and are connected to a constant current source to draw a predetermined current through the LEDs. The switched mode power supply is arranged to output different voltages depending on the number of switched-on LEDs. This is carried out by supplying the control signals of the bypass switches to the switched mode power supply. In this way, the power dissipation of the constant current source can be kept at a low level. 
     The invention is not limited to the embodiments described hereinbefore. It can be altered in different ways within the scope of the appended claims.