Abstract:
The invention provides a LED arrangement including a LED string of a series arrangement of LED segments. A LED segment includes a single LED or a series arrangement of LEDs. A switching element ( 12, 22 ) is arranged in parallel with each corresponding LED segment ( 10, 20 ) of the LED string, for controlling a current ( 52, 62 ) through the LED segment ( 10, 20 ). A capacitor ( 13, 23 ) is arranged in parallel with each corresponding LED segment ( 10, 20 ) in order to prevent the occurrence of possibly harmful current spikes while switching one or more LED segments. The LED arrangement may also include a switched-mode power supply ( 2001 ). The invention further provides a LED assembly. A plurality of such LED assemblies assembles easily into a LED arrangement according to the invention.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a light emitting diode (LED) arrangement. The invention further relates to a LED assembly. The invention further relates to an illumination system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Such a LED arrangement is known from U.S. Pat. No. 5,959,413. U.S. Pat. No. 5,959,413 discloses a driving circuit in which each LED has a controllable logic switch in parallel across it and the switches are further in series circuit with each other to form a ladder network. Any selected LED may be switched off by closing its corresponding switch. The current continues to flow then through the shunting switch into the remaining LEDs in the series circuit that are on. A plurality of such ladder networks may be coupled in parallel with each other and each ladder network may be controlled by a switching gate which selectively couples it to the constant current source so that the LED ladder networks are operated at a predetermined duty cycle. Current spikes are avoided across the voltage supply by driving the connecting control gates of the parallel strings in an overlapping relationship so that the constant current source is never disconnected from the voltage supply. 
         [0003]    The known circuit has the disadvantage that it is required to be controlled in such a way that always a LED is driven to prevent current spikes in the power supply line. Hence it is needed to use an overlapping driving scheme for the parallel strings and it is needed to distribute all LEDs over a plurality of strings if a low duty cycle is required. This adversely limits the range of duty cycles that can be used when operating the LEDs. 
         [0004]    An alternative arrangement is known from US patent application US2005/0243022 A1. An efficient power supply in the form of a switched-mode power supply is provided in FIGS. 6 and 7 of US2005/0243022 A1. The switched-mode power supply uses a switch, a coil and a diode, where the switch is operated to charge the coil, which is discharged via the diode. In such an arrangement, the current shows a large ripple, i.e., it fluctuates with a large amplitude around an average level. A known solution to limit this ripple to a relatively small amplitude is to place a filter capacitor over the output of switch mode supply. A disadvantage of this approach is that current spikes occur when the load on the switch mode is changing, as a result of switching LEDs on and off in the series arrangement. The current spikes can damage the LEDs as well as the power supply. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention aims to provide a LED arrangement comprising a LED string and a driver circuit arrangement which can accommodate a wide range of duty cycles for driving each individual LED or each individual segment of several LEDs with bypass switches without the occurrence of current spikes, which could damage the LEDs. The invention further aims to provide a LED assembly to be applied in such a LED arrangement. 
         [0006]    Hereto the LED arrangement according to the present invention comprises a LED string and a driver circuit arrangement. The LED string comprises at least two LED segments, the at least two LED segments being arranged electrically in series. Each LED segment comprises at least one LED. The driver circuit arrangement comprises a segment driver unit for each of the at least two LED segments. Each segment driver unit comprises a first switching element arranged electrically parallel with a corresponding LED segment for controlling, during use, of a current through the LED segment. Each segment driver unit further comprises a first capacitor, the first capacitor being arranged electrically in parallel with at least one of the LEDs of the corresponding LED segment. 
         [0007]    These segmented capacitors prevent the occurrence of high transient current peaks, which could otherwise occur in the LED string when switching one of the LED segments, in particular in the LED segments that are not switched while another segment of the LED string is switched. These high transient current peaks could severely damage the LEDs. By placing a capacitor in parallel to at least one of the LEDs of each LED segment instead of placing a single capacitor parallel to the supply, these high transient current peaks are prevented. The lifetime of the LEDs is thus significantly improved. 
         [0008]    Usually, the capacitor is placed in parallel to the complete LED segment. This is however not necessary. It is not excluded that also the power to the driver of the bypass-switch is provided along the LED string, and thus via the first capacitor. The voltage over the series-connected LEDs in the LED segment may be too high in order to power the driver. This problem is then solved in that the power is then drawn from a node between two LEDs within the LED segment. As a consequence, the first capacitor will be placed in parallel only to some of the LEDs instead of all LEDs in the LED segment. Drawing the power for the driver from the LED string is considered advantageous in order to simplify the overall architecture: additional power source lines and voltage regulators are not required. Moreover, the resulting driver arrangement can therewith be split into segments corresponding to the LED segments. Such a modular construction of the arrangement allows flexibility in applications. That is often beneficial in lighting applications, which include more often than not a large area. The power can for instance be drawn from the LED string with a gating element between the node and the first capacitor. Such a gating element is for instance a diode or a sample switch with a sample driver coupled thereto. It is observed for clarity that this modular architecture of the driving arrangement does not require that the power is drawn between a first and a second LED in the LED segment. 
         [0009]    In one embodiment of the invention, the driver circuit arrangement comprises a segment controller. The segment controller is arranged for generating a first control signal for each segment driver unit, in order to drive the first switching element of the corresponding segment driver unit. The segment controller is arranged for executing a drive period, and repeating the drive period periodically. The drive period comprises at least three subsequent phases. The segment controller is further arranged for: in the first phase, closing the first switching element such that the current through the LED segment stops and the LED segment is switched off; in the second phase, keeping the first switching element closed for a specific duration of time for each individual drive period; in the third phase, opening the first switching element such that the current flows through the LED segment and the LED segment is switched on. 
         [0010]    The segment controller thus operates the segment driver units as to generate a required amount of light, by adapting the duty cycle of the LEDs to achieve a required amount of light averaged over the drive period. 
         [0011]    In a further embodiment of the invention, the segment controller is arranged for applying a timing compensation to the specific duration for each individual drive period, the timing compensation compensating for the switching delay of the corresponding segment driver unit. 
         [0012]    This provides a method to compensate for the switch-on delay that may occur especially when the segment driver unit does not comprise the sample-and-hold switch in series with the first capacitor (as in an embodiment described below). 
         [0013]    In a further embodiment of the invention, each segment driver unit comprises a second switching element, the second switching element being arranged electrically in series with the first capacitor. 
         [0014]    The series arrangement of the first capacitor and the second switching element is thus electrically parallel with the LED segment. This second switching element is used as a sample-and-hold switch, and is operated so as to set (sample) and keep (hold) the LED operating voltage on the first capacitor while the LED is not operated, i.e., when the bypass switch is closed. As a result, there is no need to first load the capacitor when switching on of the LED upon closing the bypass switch, and the switching on of the LED can occur without any switch-on delay. Moreover, the capacitive losses that would be associated with charging and discharging the first capacitor are prevented. As a result, an efficient operation can be achieved. 
         [0015]    In another further embodiment, the segment controller described above is further arranged for generating a second control signal for each segment driver unit, in order to drive the second switching element of the corresponding segment driver unit. The drive period comprises the at least three phases and a further first auxiliary phase prior to the first phase and a second auxiliary phase after the third phase. The segment controller is further arranged for: in the first auxiliary phase, opening the second switching element such that the voltage over the corresponding LED segment is held by the first capacitor; in the first phase, closing the first switching element such that the current through the LED segment stops and the LED segment is switched off; in the second phase, keeping the first switching element closed for a specific duration of time for each individual drive unit; in the third phase, opening the first switching element such that the current flows through the LED segment and the LED segment is switched on, and in the second auxiliary phase, closing the second switching element. 
         [0016]    The segment controller thus operates the segment driver units so as to generate a required amount of light, by adapting the duty cycle of the LEDs to achieve a required amount of light averaged over the drive period. The second switching element and the first capacitor are operated such as to hold the voltage across the LED for a next switching-on phase after the LED has been switched off. As a result, the switching on delay is reduced to essentially zero and a fast rise-time results when switching on the LED. Moreover, the timing of the activation and deactivation of the second switching elements is executed so as to prevent a short-circuit of the first capacitor and second switching element by this so-called non-overlapping clocking scheme. 
         [0017]    In a further embodiment of the invention, the segment driver unit comprises a second capacitor, the second capacitor being arranged electrically in parallel with the corresponding LED segment. 
         [0018]    This arrangement prevents possible problems while the first capacitor is disconnected and the LED current is only filtered by the parasitic capacitance of the LED itself, and thus relaxes the timing tolerances of the segment driver. 
         [0019]    In an embodiment, the LED arrangement further comprises a power supply arranged for energizing the LED string. 
         [0020]    During use, the power supply is arranged for supplying a supply current to the LED string which is substantially independent of the number of LEDs that are on and off at any moment in time. This way, the LEDs are always driven with a well-defined current, such that a stable output is achieved. 
         [0021]    In a preferred embodiment, the power supply comprises a switched-mode controller, a third switching element, an inductive element and a component selected from the group of a diode and a fourth switching element, wherein the switched-mode controller is arranged for operating the third switching element in order to charge and discharge the inductive element, wherein the inductive element is discharged via the component selected from the group of a diode and a fourth switching element. 
         [0022]    With these components, a so-called switch-mode DC/DC converter may be constructed which adjusts the effective voltage at its output terminal to the exact voltage needed by the driven system. This results in a very effective power conversion from a wide range of input voltages. 
         [0023]    In a preferred embodiment, the power supply is one selected from the group of a so-called Buck converter and a so-called Buck-boost converter. A Buck converter is a converter topology which can adjust its output voltage to any voltage below the input voltage. A Buck-boost converter is a converter topology which can adjust its output voltage below the input voltage as well as above the input voltage. When the LED string comprises a large number of LED segments, the voltage across the LED string can vary strongly depending on the number of LED segments that are switched on and the number of LED segments that are switched off because their bypass switches are closed. With an input voltage corresponding to the voltage over the LEDs when all LEDs would be on, the Buck converter topology adapts its output voltage to provide the required supply voltage to the LED string. The Buck-boost topology provides the required high supply voltage when all LEDs are on with, e.g., a voltage above the input voltage, and will also supply the required low supply voltage when all LEDs are off and a voltage below the input voltage is required. 
         [0024]    A LED assembly according to the present invention comprises at least one LED die and a first capacitor, the first capacitor being arranged electrically in parallel to the at least one LED die. 
         [0025]    A multiplicity of such LED assemblies can easily be assembled into a LED arrangement of any of the embodiments described above. It reduces the number of components, and moreover allows easy scalability of the LED arrangement when one or more LED segments need to be added or removed. 
         [0026]    Alternatively, a plurality of these assemblies can be put together to form a ladder network of LEDs and capacitors. This ladder network may then be connected to a plurality of external switches to create a LED arrangement according to the invention. Preferably, the light emitting diode (LED) assembly further comprises a carrier to carry the at least one LED die and the first capacitor. 
         [0027]    The scalability can be achieved with very small units, by having the capacitor and the LED die carried by a submount. The submount can be a silicon or a ceramic carrier, and the capacitor can be mounted on one of its surfaces or integrated in the submount itself. Alternatively, the carrier can be a printed circuit board (PCB) of, e.g., a larger size. Such a PCB may be a LED module of several LED segments with their associated segment unit drivers, such that arrangements of a large size can be made with easy-to-handle modules. In a further embodiment, the LED assembly comprises also a sample-and-hold switching element, wherein the carrier carries the sample-and-hold switching element, the sample-and-hold switching element being arranged electrically in series with the first capacitor. 
         [0028]    This allows easy assembly of further embodiments of the LED arrangement as described above. 
         [0029]    Alternatively or additionally, the LED assembly may comprise a second capacitor, wherein the carrier carries the second capacitor, and the second capacitor is arranged electrically in parallel to the at least one LED die. 
         [0030]    This capacitor prevents possible problems while the first capacitor is disconnected and the LED current is only filtered by the capacitance of the LED itself, and thus relaxes the timing tolerances of the segment driver. 
         [0031]    Alternatively or additionally, the LED assembly may comprise a bypass switching element, 
         [0000]    wherein the carrier carries the bypass switching element, and the bypass switching element is arranged electrically in parallel to the at least one LED die. 
         [0032]    This allows integrating also the bypass switching element itself in the LED assembly, thus providing a highly integrated and self-contained segment module containing the LED segment as well as its associated segmented capacitor and its associated bypass switch and associated bypass switch driver electronics. 
         [0033]    In a further embodiment, a LED arrangement as described above may be constructed from at least two LED assemblies as described above. The LED arrangement may comprise a power supply. 
         [0034]    A further embodiment of the invention relates to an illumination system comprising one of the LED assemblies described above. 
         [0035]    This may be a brightness controlled LED-lamp, a color-variable LED lamp, a LED matrix light source, a LED matrix display, a large-sized LED information display for advertisement or moving images, a LED-backlight for a LCD-TV, a LED-backlight for a LCD-monitor, or any other lighting system with at least two LED segments operated with bypass switches. 
         [0036]    A further embodiment of the invention relates to a method for controlling a 
         [0037]    LED arrangement according to the invention. Preferably the method comprises:
   generating a first control for each segment driver unit, the first control signal driving the first switching element of the corresponding segment driver unit,   executing a drive period, and   repeating the drive period periodically, each drive period comprising at least three subsequent phases; the method comprising:   in the first phase, closing the first switching element such that the current through the LED segment stops and the LED segment is switched off,   in the second phase, keeping the first switching element closed for a specific duration of time for each individual drive period,   in the third phase, opening the first switching element such that the current flows through the LED segment and the LED segment is switched on.   
 
         [0044]    The method thus operates the LED arrangement as to generate a required amount of light, by adapting the duty cycle of the LEDs to achieve a required amount of light averaged over the drive period. 
         [0045]    In a further embodiment, the method further comprises:
   applying a compensation to the specific duration time for each individual drive period, the compensation compensating for the switching delay of the corresponding segment driver unit.   
 
         [0047]    This provides a method to compensate for the switch-on delay that may occur especially when the segment driver unit does not comprise the sample-and-hold switch in series with the first capacitor. 
         [0048]    In an alternative further embodiment, the method further comprises:
   generating a second control signal for each segment driver unit, the second control signal driving a second switching element of the corresponding segment driver unit,   the drive period comprising a first auxiliary phase prior to the first phase and a second auxiliary phase after the third phase,   in the first auxiliary phase, opening the second switching element such that the voltage over the corresponding LED segment is held by the first capacitor,   in the first phase, closing the first switching element such that the current through the LED segment stops and the LED segment is switched off,   in the second phase, keeping the first switching element closed for a specific duration of time,   in the third phase, opening the first switching element such that the current flows through the LED segment and the LED segment is switched on,   in the second auxiliary phase, closing the second switching element.   
 
         [0056]    The method thus operates the LED arrangement as to generate a required amount of light, by adapting the duty cycle of the LEDs with the first switching elements to achieve a required amount of light averaged over the drive period. The second switching element and the first capacitor are operated such as to hold the voltage across the LED for a next switching-on phase after the LED has been switched off. As a result, the switch-on delay is reduced to essentially zero and a fast rise-time results when switching on the LED. 
         [0057]    Moreover, the timing of the activation and deactivation of the second switching elements is executed so as to prevent a short-circuit of the first capacitor and second switching element by this so-called non-overlapping clocking scheme. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0058]    The above and other aspects of the invention will be further elucidated and described in detail with reference to the drawings, in which corresponding reference symbols indicate corresponding parts: 
           [0059]      FIG. 1  a shows a LED arrangement comprising a LED string and a driver circuit arrangement according to the prior art; 
           [0060]      FIG. 1   b  shows again the LED arrangement comprising a LED string and a driver circuit arrangement according to the prior art; 
           [0061]      FIG. 2  shows another LED arrangement comprising a LED string and a driver circuit arrangement according to the prior art; 
           [0062]      FIG. 3   a  shows a LED arrangement comprising a LED string and a driver circuit arrangement with a Buck converter according to the prior art; 
           [0063]      FIG. 3   b  shows a simulation of the current waveforms when the LED arrangement of  FIG. 3   a  is operated; 
           [0064]      FIG. 3   c  shows an alternative arrangement to  FIG. 3   a;    
           [0065]      FIG. 4   a  shows a LED arrangement comprising a LED string and a driver circuit arrangement with a Buck converter with an output filter capacitor according to the prior art; 
           [0066]      FIG. 4   b  shows a simulation of the control and current waveforms when the LED arrangement of  FIG. 4   a  is operated; 
           [0067]      FIG. 5   a  shows a LED arrangement comprising a LED string and a driver circuit arrangement with a Buck converter according to a first embodiment of the present invention; 
           [0068]      FIG. 5   b  shows a simulation of the control and current waveforms when the LED arrangement of  FIG. 5   a  is operated; 
           [0069]      FIG. 6   a  shows a LED arrangement comprising a LED string and a driver circuit arrangement with a Buck-boost converter without an output filter capacitor according to the prior art; 
           [0070]      FIG. 6   b  shows a simulation of the current waveforms when the LED arrangement of  FIG. 6   a  is operated; 
           [0071]      FIG. 7   a  shows a LED arrangement comprising a LED string and a driver circuit arrangement with a Buck-boost converter with an output filter capacitor according to the prior art; 
           [0072]      FIG. 7   b  shows a simulation of the control and current waveforms when the LED arrangement of  FIG. 7   a  is operated; 
           [0073]      FIG. 8   a  shows a LED arrangement comprising a LED string and a driver circuit arrangement with a Buck-boost converter according to a second embodiment of the present invention; 
           [0074]      FIG. 8   b  shows a simulation of the control and current waveforms when the LED arrangement of  FIG. 8   a  is operated; 
           [0075]      FIG. 9   a  shows a LED arrangement comprising a LED string and a driver circuit arrangement with a Buck-boost converter according to a third embodiment of the present invention; 
           [0076]      FIG. 9   b  shows a simulation of the control and current waveforms when the LED arrangement of  FIG. 9   a  is operated; 
           [0077]      FIG. 9   c  shows another LED arrangement according to an embodiment of the present invention; 
           [0078]      FIG. 10  shows a LED arrangement comprising a LED string and a driver circuit arrangement with a Buck-boost converter according to a fourth embodiment of the present invention; 
           [0079]      FIG. 11   a - 11   i  show LED assemblies according to the invention; 
           [0080]      FIG. 12  shows an illumination system according to the invention; 
           [0081]      FIG. 13  shows a method according to the invention; 
           [0082]      FIG. 14  shows a further method according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0083]      FIG. 1   a  shows a number of LEDs  10 ,  20  arranged electrically in series forming a LED string  1000 . The LED string is equipped with a driver circuit  2000 . The driver circuit comprises a current source  30  which supplies a current  31 , electrical switches  11 ,  21  and nodes  10 T,  10 B,  20 T and  20 B. The switches  11 ,  21  are each arranged electrically parallel with a LED  10 ,  20 . The switch  11  connects between node  10 T and  10 B on either side of LED  10 . Likewise, the switch  21  connects between node  20 T and  20 B on either side of LED  20 . When the switches  11 ,  21  are open, the current  31  flows through the LEDs  10 ,  20 , causing the LEDs to emit light, as shown in FIG  1   a.    FIG. 1   b  shows the same arrangement, but with the top switch  11  closed. This gives a lower-resistive current path through the top switch  11  as through the top LED  10 , causing the current to flow through the top switch  11  instead of the top LED  10 , and thus causing the top LED  10  to switch off. The current is thus bypassing the LED  10 . In  FIG. 1   b,  the lower switch  21  is still open, such that the lower LED  20  is still on. By operating the switches  11 ,  21 , the duty cycle at which the corresponding LEDs  10 ,  20  are switched on is controlled. During this operation, the current source  30  is arranged to keep its output current  31  substantially constant at a fixed level. 
         [0084]      FIG. 2  shows an alternative arrangement with a longer string of LEDs. The LEDs  101 ,  102 ,  103  are grouped in a LED segment  100 , all LEDs being arranged in series. The bypass switch  11  is arranged electrically parallel to the whole LED segment  100 , instead of to a single LED, and connects between node  100 T and  100 B of LED segment  100 . The LED segment  100  is electrically in series with a second LED segment  200 , of LEDs  201 ,  202 ,  203  in series, together forming the LED string. The operation is similar as that of  FIG. 1   a  and  FIG. 1   b.  In the example shown, the LED segment  100  consists of three LEDs  101 ,  102 ,  103  in series, but it can of course also have any other number of LEDs. It may, e.g., also consist of a single LED only. In describing  FIGS. 3 to 10 , we will refer to a LED segment of any number of LEDs as a LED segment  10  or  20 , with nodes  10 T and  10 B or  20 T and  20 B respectively. 
         [0085]      FIG. 3   a  shows an embodiment of the schematic arrangement of  FIG. 2 . The switches  11 ,  21  are implemented using MOSFET transistors  12 ,  22 . The bypass current through the top MOSFET transistor  12  from node  10 T to node  10 B is referred to as current  50 , the bypass current through the lower MOSFET transistor  22  from note  20 T to node  20 B is referred to as current  60 . The MOSFET transistors are depicted as NMOS transistors, but equally well be PMOS transistors or any other type of switch. The switches  12 ,  22  are controlled from a segment controller  36 , which drives the switches with control signals  70 ,  71 . We will refer to these control signals with the same reference numbers  70 ,  71  when we refer to their logical levels and when we refer to their electrical levels. The current source is implemented as a Buck converter  2001 , which is built from a power switch  31 , shown as a MOSFET transistor  31 , an inductive element  32 , a diode  34 , a resistor  33  and a Buck controller  35 . The Buck controller  35  drives the gate of the power transistor  31 , such that the inductor is charging and discharging at a high frequency. In an example, the arrangement has a total of 36 LEDs in series in the LED string, arranged in two segments of 18 LEDs each; the converter frequency is approximately 100 kHz with a DC-input voltage Vin of 150 V, and a value of the inductor of 5 mH. In the example, the gates of the bypass switches  12 ,  22  are operated at a frequency of approximately 200 Hz. It is to be noted that the segment controller  36  nor the switch mode controller  35  may not be shown in subsequent figures, but they are meant to be present for controlling the switches in the segment driver units and the power switches in the power supply respectively. 
         [0086]      FIG. 3   b  shows the electrical waveforms at various positions in the LED arrangement of  FIG. 2 . The upper curve shows a coil current  40 . The middle curve shows the current  50  through the upper LED segment  10 . The lower curve shows the current  60  through the lower LED segment  20 . The periodic modulation of the currents  40 ,  50 ,  60  is due to the operation principle of the switch mode driver, which charges and discharges the inductor  32  while periodically opening and closing the power transistor  31 . The LED current waveforms  50 ,  60  show a very deep modulation depth, varying periodically between, in this example, 0 mA and approximately 100 mA, at an average current of about 50 mA, i.e., with peak values that are twice the nominal value. This exemplary large modulation may be used to give power-efficiency and EMI advantages because of zero-current and zero-voltage switching during switch-on of the power transistor  31 . 
         [0087]      FIG. 3   c  shows a similar arrangement, but with a switch  34 ″ instead of the diode  34  of  FIG. 3   b.  By opening and closing the switch depending on the phase of the operation of the switch mode driver, the switch performs a similar function as the diode: it allows the coil current to discharge. 
         [0088]      FIG. 4   a  shows an embodiment of the circuit of  FIG. 2 , with an added filter capacitor  80  over the output of the Buck converter. The filter capacitor  80  reduces the current modulation to a smaller modulation depth, also called ripple. In this example, the capacitor  80  has a capacitor value of 15 nF. 
         [0089]      FIG. 4   b  shows the electrical waveforms for this example at various positions in the LED arrangement of  FIG. 2 . The upper curve shows a logical signal  71  controlling the gate of bypass transistor switch  22 . When the logical signal  71  is high, the switch  22  is closed, such that the current flows through the switch  22  and the lower LED segment  22  is switched off. When the logical signal  71  is low, the switch  22  is open such that the current flows through the lower LED segment  22  and the lower LED segment  22  is switched on. The middle curve shows a current  51  through the upper LED segment  10 . The lower curve shows a current  61  through the lower LED segment  20 , which is being switched by the bypass transistor  22 . It is observed that in the example the currents  51 ,  61  have a much smaller current modulation than the unfiltered currents  50 ,  60  of  FIG. 3   b,  with a current ripple  51 ,  61  of only about 10% at a nominal LED current of about 50 mA, due to the filter capacitor  80 . The maximum LED current is thus reduced with approximately 50%, resulting in a better lifetime of the LEDs compared to the unfiltered situation of  FIG. 3   a  and  FIG. 3   b.  However, around the switching moments, an unacceptable overshoot of about 300 mA and an undershoot of 0 mA is also observed in the LED current  51  through the upper LED  10 , i.e., the LED that is not switched but continues to stay on. These high transients can damage the LEDs. 
         [0090]      FIG. 5   a  shows an LED arrangement according to the present invention, with two LED segments  10 ,  20 . Each LED segment  10 ,  20  is driven from a LED segment driver  110 ,  210  which consists of not just a switch  12 ,  22 , but also a capacitor  13 ,  23  for each individual segment. The capacitors  13 ,  23  are connected electrically in parallel to the corresponding LED segment  10 ,  20 , as are the switches  12 ,  22 . I.e., the switch  12  and the capacitor  13  each connect between node  10 T and  10 B on either side of LED segment  10 , and the switch  22  and the capacitor  23  each connect between node  20 T and  20 B on either side of LED segment  20 . We also refer to the capacitors  13 ,  23  as segment capacitors. The segment capacitors  13 ,  23  are dimensioned such that the Buck output filter capacitor  80  is obsolete, and have a value of 30 nF each in this example, such that the same total capacitance is obtained from the series arrangement of capacitors  13  and  23  as the capacitance of capacitor  80 , resulting in the same current ripple. 
         [0091]      FIG. 5   b  shows the electrical waveforms for this circuit. The upper curve shows a logical signal  72  controlling the gate of bypass transistor switch  22 . The middle curve shows a current  52  through the upper LED segment  10 . The lower curve shows a current  62  through the lower LED segment  20 , which is being switched by the bypass transistor  22 . Comparing currents  52 ,  62  of  FIG. 5   b  to currents  51 ,  61  of  FIG. 4   a,  it is clearly observed that the current over- and undershoots are removed with the segmented capacitor. Also the ripple of the current is reduced. It is also observed in the lower curve showing current  62  that the switch-on of the dimmed segment takes longer compared to the current  61  in  FIG. 4   a.  This is because its segment capacitor  23  needs to charge from basically zero volt. This switch-on delay may be acceptable, as it is small compared to the drive period: in the example, the delay is about 40 μs vs. a drive period of 5 ms. When it is acceptable, the effect on the light output of the LED segment  20  can be ignored. In an alternative embodiment, the switch-on delay may be compensated for in the duty cycle of the signals  72  driving the bypass switches  12 ,  22 . The dead time may be calibrated for the LED arrangement, or monitored and automatically compensated for. Active monitoring and correction has the advantage that temperature and ageing effects are automatically taken into account, at the cost of some additional circuitry to measure the switching time and comparing the measured time with the required duty cycle. A further embodiment with a hardware solution will be described further below. 
         [0092]    We now turn to alternative embodiments with a Buck-boost converter employed in the driver arrangement. Compared to the previously described Buck converter, the ratio of peak LED current to average LED current can be even larger than 2 because of the discontinuous output current of a single-coil Buck-boost converter, that typically a filter capacitor is required to meet reliability and lifetime requirements of the LED. The Buck-boost topology is very well suited for the bypass driving of LEDs, as it will also continue to work well when the output voltage at any moment in time becomes smaller than the input voltage, which is the case when all bypass switches are closed and all LEDs are switched off. 
         [0093]    An example of such a topology is disclosed and its operation is described in detail in US patent application US 2004/0145320 A1. The description uses a single-coil Buck-boost converter, but is equally applicable for other topologies such as, e.g., a 4-switch auto-up-down, a Cuk, a SEPIC or a Zeta converter, as well as isolated implementations like flyback, forward or resonant converters. 
         [0094]      FIG. 6   a  shows a LED arrangement with a Buck-boost converter according to the prior art. The Buck-boost controller has a Buck-boost controller  35 ′, controlling the gate of a power transistor  31 ′, an inductive element  32 ′, a diode  34 ′ and a resistor  33 ′. 
         [0095]    In an example, the 
         [0096]      FIG. 6   b  shows a simulation of the electrical behaviour for an example with a converter frequency of again approximately 100 kHz, Vin=24 V and a total of 22 LEDs is placed in series in the LED string, arranged in two segments of 11 LEDs each. In the example, the inductive element  32 ′ with an inductor value of 500 μH. The coil current  43  shows a continuous triangular behavior. The LED currents  53 ,  54  however show a discontinuous saw-tooth behavior in which the LEDs carry a current during the secondary stroke of each supply conversion period when the inductive element  32 ′ is discharging over the diode  34 ′ and delivering a current to the LED string. In this example, for an average LED current of about 50 mA, the peak LED current is about 200 mA. 
         [0097]      FIG. 7   a  shows a LED arrangement with a Buck-boost converter with an output filetr capacitor according to the prior art. The Buck-boost controller has a Buck-boost controller  35 ′, controlling the gate of a power transistor  31 ′, an inductive element  32 ′, a diode  34 ′ and a resistor  33 ′, as in  FIG. 6   a.  A capacitor  80 ′ is placed over the converter in parallel to the LED string. This capacitor filters the discontinuous current with the large amplitude shown in  FIG. 6   b  to a current with a reduced ripple. In this example, the resulting ripple is about 10%. In this example, the inductive element  32 ′ has an inductor value of 500 μH, the converter output filter capacitor  80 ′ has a capacitor value of 150 nF, the converter frequency is again approximately 100 kHz, Vin=24 V and a total of 22 LEDs is placed in series in the LED string, arranged in two segments of 11 LEDs each. 
         [0098]      FIG. 7   b  shows a simulation of the electrical behavior. The upper curve shows a logical signal  74  controlling the gate of bypass transistor switch  22 . The middle curve shows a current  54  through the right LED segment  10 . The lower curve shows a current  64  through the left LED segment  20 , which is being switched by the bypass transistor  22 . Again, severe over- and undershooting LED currents are observed of approximately 300 mA and 0 mA at a nominal LED current of 50 mA in this example. The electrical components are dimensioned to get a current ripple of approximately 10%, as in the Buck-converter case. The discontinuous output of the Buck-boost converter required an increased amount of filtering, resulting in a somewhat longer rise time of current  64 , compared to the rise time of current  61  of the Buck converter of  FIG. 5   b.    
         [0099]      FIG. 8   a  shows a LED arrangement with a Buck-boost converter according to the invention. Comparing  FIG. 8   a  to  FIG. 7   a,  the Buck-boost converter output filter capacitor  80 ′ of  FIG. 7   a  is omitted and a first capacitor  13 ,  23  is applied for each of the LED segments. The first capacitors  13 ,  23  are connected electrically in parallel to the corresponding LED segment  10 ,  20 , as are the switches  12 ,  22 . I.e., the switch  12  and the capacitor  13  each connect between node  10 T and  10 B on either side of LED segment  10 , and the switch  13  and the capacitor  23  each connect between node  20 T and  20 B on either side of LED segment  20 . 
         [0100]    As an example,  FIG. 8   b  shows a simulation of the currents through the LEDs for a value of each of the first capacitors, of 300 nF, the filter capacitor is functionally replaced by serially connected first capacitors of the segments. The upper curve shows a logical signal  75  controlling the gate of bypass transistor switch  22 . The middle curve shows a current  55  through the right LED segment  10 . The lower curve shows a current  65  through the left LED segment  20 , which is being switched by the bypass transistor  22 . A larger switch-on delay for current  65  is observed, compared to the switch-on delay for the current  62  of the Buck converter of  FIG. 8 , due to the increased amount of filtering for the same current ripple of about 10%. This switch-on delay can be compensated for in the timing of the bypass switches, as described above in the discussion of  FIG. 5 . An alternative solution to prevent switch-on delay and to prevent the slow rise time is described next. 
         [0101]      FIG. 9   a  shows two LED segment drivers  110 ″,  210 ″ for two LED segments  10 ,  20  according to a further embodiment of the invention. The segment driver comprises a bypass switch  12 ,  22  and a segmented capacitor  13 ,  23 , and is also equipped with a second switch  14 ,  24  in series with the segmented capacitor  13 ,  23 . The series arrangement of the capacitor  13 ,  23  and corresponding second switch  14 ,  24  is connected electrically in parallel to the corresponding LED segment  10 ,  20 , as is the bypass switches  12 ,  22 . I.e., the series arrangement of the second switch  14  and the capacitor  13  connects between node  10 T and  10 B on either side of LED segment  10 , as does the bypass switch  12 . Likewise, the series arrangement of the second switch  24  and the capacitor  23  connects between node  20 T and  20 B on either side of LED segment  20 , as does the bypass switch  22 . The second switch and the segmented capacitor are operated to hold the voltage across the LED for the next switch-on phase after the LED is switched off. We thus also refer to the second switch and segmented capacitor as sample-and-hold switch and hold capacitor. 
         [0102]      FIG. 9   b  shows the electrical behavior of a logical signal  76  controlling the gate of bypass transistor switch  22 , a logical signal  86  controlling the gate of sample-and-hold transistor switch  23 , a current  56  through the upper LED segment  10  and a current  66  through the lower LED segment  20 , when the circuit of  FIG. 9   a  is implemented with the Buck-boost supply topology of  FIG. 8   a.  The simulation is done without any compensation in the control signals of the bypass switches  12 ,  22 . A fast and instantaneous switch-on of the current  66  is observed. 
         [0103]    To prevent short-circuiting of the segmented capacitor  13 ,  23  and sample-and-hold switch  14 ,  24  with the bypass switch  12 ,  22 , a non-overlapping clocking scheme is used, in which in a first phase A 1 , the voltage across LEDs is sampled by opening (i.e., put in a non-conducting state) the sample-and-hold switch  14 ,  24  and hold the voltage on the capacitor  13 ,  23 ; secondly, in a second phase P 1  bypass switch  12 ,  22  is closed (i.e., put in conducting state) to switch off the corresponding LED segment  10 ,  20 ; in a third phase P 2 , the bypass switch  12 ,  22  is kept closed for a certain PWM period; in a fourth phase P 3 , the bypass switch  12 ,  22  is opened (i.e., put in a non-conducting state) to switch on the corresponding LED segment  10 ,  20 ; and in a fifth phase A 2 , the filter and sample capacitor is connected again across corresponding LED segment  10 ,  20  by closing the sample-and-hold switch  14 ,  24 . 
         [0104]      FIG. 9   c  shows an alternative embodiment, with a pMOS transistor  14 ′,  24 ′ at the upper side of the segmented capacitor  13 ,  23 . This alternative embodiment is operated in a similar to that shown in  FIG. 9   a,  as a person skilled in the art will understand. 
         [0105]    During the small disconnect time of the segment capacitor the LED current gets filtered only by the parasitic capacitors of the LED itself. This disconnect time largely depends on the speed of the available devices in the IC process that is used to implement the drivers for the switches and consequently—it may be beneficial to add an additional (second) capacitor which is not sampled to the segment driver units of  FIG. 9   a  or  9   c.  This is depicted in  FIG. 10  with capacitors  15 ,  25 . As an example, the capacitors  15 ,  25  may each have a value of 1 nF, an order of magnitude smaller than the first capacitor. The capacitor  15 ,  25  is connected electrically in parallel to the corresponding LED segment  10 ,  20 . I.e., also capacitor  15  connects between node  10 T and  10 B on either side of LED segment  10 , and also capacitor  25  connects between node  20 T and  20 B on either side of LED segment  20 . 
         [0106]    In the description of the invention and its embodiments above, the physical arrangement of all components was not explicitly discussed. The arrangement may be built from discrete components on a single or on a plurality of carriers, e.g., printed circuit boards. The invention and its embodiments can be advantageously applied when the arrangement can be built from modular components with one or more of its specific components integrated in an assembly for each individual LED segment, or alternatively in an assembly for several LED segments together. In some embodiments, the assemblies are constructed on small printed circuit boards (PCBs) as small LED modules, each carrying all the LEDs for a single LED segment and one or more of the specific components needed in an arrangement according to the invention. Depending on the required size of the assembly for a specific application, the number of modules is then easily adapted. In some embodiments, the assembly is constructed on a submount, e.g., a silicon or ceramic carrier, and the assembly thus forms an active LED package. 
         [0107]    A LED assembly according to one embodiment of the invention comprises a LED  10  and a capacitor  13 . The capacitor  13  is arranged electrically in parallel to the LED  10 . 
         [0108]    A plurality of these assemblies can be easily put together with external switches and an external power supply to create the LED arrangement of e.g.,  FIG. 7 . Alternatively, a plurality of these assemblies can be put together to form a ladder network of LEDs and capacitors. This ladder network may then be connected to a plurality of external switches and an external power supply to create the LED arrangement of e.g.,  FIG. 7 .  FIG. 11   a  shows such a LED assembly, where the LED  10  and the capacitor  13  are mounted on a carrier  19 . 
         [0109]      FIG. 11   b  shows an alternative LED assembly where three LEDs  101 ,  102 ,  103  are mounted in a series arrangement as one LED segment  100 , together with a capacitor  13 , on a carrier. 
         [0110]      FIG. 11   c  shows another alternative LED assembly where a LED  10  (or a series arrangement  100  of LEDs  101 ,  102 ,  103  as in  FIG. 11   b ), a first capacitor  13  and a bypass switch  12  are mounted on a carrier  19 . The bypass switch  12  is connected electrically parallel to the LED  10  or LED segment  100  of several LEDs in series  101 ,  102 ,  103 . 
         [0111]      FIG. 11   d  shows again another alternative LED assembly where a LED  10  (or a series arrangement  100  of LEDs  101 ,  102 ,  103  as in  FIG. 11   b ), a first capacitor  13  and a sample-and-hold switch  14  are mounted on a carrier  19 . The sample-and-hold switch  14  is connected electrically in series with the first capacitor  13 , and together these are arranged electrically parallel to the LED  10  or LED segment  100  of several LEDs in series  101 ,  102 ,  103 . 
         [0112]      FIG. 11   e  shows again another alternative LED assembly where a LED  10 , a first capacitor  13 , a sample-and-hold switch  14  and a bypass switch  12  are mounted on a carrier  19 . The sample-and-hold switch  14  is connected electrically in series with the first capacitor  13 , and together these are arranged electrically parallel to the LED  10  and to the bypass switch  12 . 
         [0113]      FIG. 11   f  shows again another alternative LED assembly where a LED  10  (or a series arrangement  100  of LEDs  101 ,  102 ,  103  as in  FIG. 11   b ), a first capacitor  13 , a sample-and-hold switch  14  and a second capacitor  15  are mounted on a carrier  19 . The sample-and-hold switch  14  is connected electrically in series with the first capacitor  13 , and together these are arranged electrically parallel to the LED  10  and the second capacitor  15 . 
         [0114]      FIG. 11   g  shows again another alternative LED assembly where a LED  10  (or a series arrangement  100  of LEDs  101 ,  102 ,  103  as in  FIG. 11   b ), a first capacitor  13 , a sample-and-hold switch  14 , a bypass switch  12  and a second capacitor  15  are mounted on a carrier  19 . The sample-and-hold switch  14  is connected electrically in series with the first capacitor  13 , and together these are arranged electrically parallel to the LED  10 , to the bypass switch  12 , and to the second capacitor  15 . The switches  12  and  15  may be discrete switches, or integrated as part of an IC that also contains the driving electronics for the switch. 
         [0115]      FIG. 11   h  shows again another alternative LED assembly where a LED  10  (or a series arrangement  100  of LEDs  101 ,  102 ,  103  as in  FIG. 11   b ) and the second capacitor  15  are mounted on a carrier  19 . The second capacitor  15  is arranged electrically parallel to the LED  10 . 
         [0116]      FIG. 11   i  shows a LED assembly, where one LED  10  (or a series arrangement  100  of LEDs  101 ,  102 ,  103  as in  FIG. 11   b ) and one capacitor  13  are carried by a silicon submount carrier  19 . More specifically, the capacitor is implemented in the silicon submount itself instead of mounted as a separate electrical component on its surface. A plurality of these assemblies can be easily put together with external switches, external capacitors and an external power supply to create the LED assembly of, e.g.,  FIG. 7 . Also, additional electrical components, such as the sample-and-hold switches or capacitors may be integrated in the submount. 
         [0117]      FIG. 12  shows a light source  5000  with a LED assembly  1  in a housing  5001 . The housing  5001  is a metal box with reflective inner walls. The light generated by the LED assembly is reflected towards the front of the housing, which is covered with a diffusive transparent plate  5002 . The light source  5000  carries a power adapter  5010 , which supplies the LED assembly  1  with an input voltage Vin from an AC/DC converter, connected to the mains via a power cord  5011  with a power connecter  5012 , to fit a wall contact (not shown) with mains supply. 
         [0118]      FIG. 13  shows a method according to the invention to operate a LED arrangement according to the invention, e.g., the LED arrangement shown in  FIG. 5   a . The method comprises periodically executing a period comprising at least three subsequent phases P 1 , P 2 , P 3 . The first phase Pl, comprises closing the first switching element  12 ,  22  such that the current through the LED segment  10 ,  20  stops and the LED segment  10 ,  20  is switched off The subsequent second phase P 2  comprises keeping the first switching element  12 ,  22  closed for a specific duration of time for each individual drive period. The subsequent third phase P 3  comprises opening the first switching element  12 ,  22  such that the current flows through the LED segment  10 ,  20  and the LED segment  10 ,  20  is switched on. 
         [0119]    In an example, the period has a duration of 5 ms, corresponding to a frequency of 200 Hz. A current of 100 mA runs through the LED string and is routed by the first switching element  12  through the LED segment  10  such that the LED segment  10  emits light. At phase P 1  at the beginning of the period, the first switching element  12  closes and the current is routed through the first switching element  12 , bypassing the LED segment  10 , such that the LED segment  10  switches off The first switching element  12  remains closed during second phase P 2 , with a specific duration of time of, e.g., 2 ms. After this specific duration, during the third phase P 3  of the method the first switching element  12  opens again and the LED segment  10  is switched on for the remainder of the period and until the first phase P 1  of the next period starts. By varying the specific duration of time in each individual drive period, the time that the LED segment  10  emits light is varied and the amount of light emitted (averaged) over the drive period is varied. When the specific duration has the same duration as the drive period, the LED segment remains off. 
         [0120]    Second phase P 2  may comprise applying a compensation to the specific time for each individual drive period, the compensation compensating for the switching delay of the corresponding segment driver unit  110 ,  210 . As shown in, e.g.,  FIG. 5   b  and  FIG. 8   b,  a switching delay can occur when switching on a LED segment  10 ,  20 . In the examples shown in  FIG. 5   b  and  FIG. 8   b,  these delays are about  40  resp. 150 μs. This delay can be compensated for in the specific duration of time that the first switching element remains closed in P 3 . 
         [0121]      FIG. 14  shows a further method according to the invention, to operate a LED arrangement according to the invention, e.g., the LED arrangement with the segment driver units  110 ″,  210 ″ shown in  FIG. 9   a . In the LED arrangement to which this method applies, each segment driver unit  110 ″,  210 ″ comprises also a second switching element  14 ,  24 , arranged electrically in series with the first capacitor  13 ,  23 . 
         [0122]    The method comprises periodically executing a period comprising the at least three subsequent phases P 1 , P 2 , P 3 , and a first auxiliary phase A 1  prior to the first phase and a second auxiliary phase A 2  after the third phase. The first auxiliary phase A 1  comprises opening the second switching element  14 ,  24  such that the voltage over the corresponding LED segment  10 ,  20  is held by the first capacitor  13 ,  23 . The subsequent first phase P 1  comprises closing the first switching element  14 ,  24  such that the current through the LED segment  10 ,  20  stops and the LED segment  10 ,  20  is switched off. The subsequent second phase P 2  comprises keeping the first switching element  12 ,  22  closed for a specific duration of time. The subsequent third phase P 3  comprises opening the first switching element  12 ,  22  such that the current flows through the LED segment  10 ,  20  and the LED segment  10 ,  20  is switched on again. Last, the second auxiliary phase A 2  comprises closing the second switching element  14 ,  24 . 
         [0123]    It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. E.g., other topologies can be used for the switched-mode power supply, the diode  34 ,  34 ′ can be replaced by a switch  34 ″, p-type as well as n-type switches can be used, and other types of switches can be used, such as an IGBT instead of a MOSFET, without departing from the scope of the invention and the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.