Abstract:
The invention describes a driver arrangement ( 1 ) for driving a semiconductor lighting load ( 2 ), which driver arrangement ( 1 ) comprises a first power converter ( 10 ) arranged to drive a portion ( 2 _ 10, 2 _ 10 _ 11 ) of the lighting load ( 2 ); a second power converter ( 11 ) arranged to drive a portion ( 2 _ 11, 2 _ 10 _ 11 ) of the lighting load ( 2 ); a monitoring arrangement ( 12 ) realized to monitor an operating characteristic ( 12 A,  12 B,  12 C, I R30 )° f the second power converter ( 11 ); and a control signal generator (13) for generating a control signal ( 130 , V shunt ) for the first power converter ( 10 ) on the basis of the monitored operating characteristic ( 12 A,  12 B,  12 C, I R30 ). The invention also describes a lighting arrangement ( 100 ), which lighting arrangement ( 100 ) comprises a semiconductor lighting load ( 2 ); and such a driver arrangement ( 1 ) for driving the semiconductor lighting load ( 2 ). The invention also describes a method of driving a semiconductor lighting load ( 2 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The invention describes a driver arrangement, a lighting arrangement, and a method of driving a semiconductor lighting load. 
       BACKGROUND OF THE INVENTION 
       [0002]    Light-emitting diodes (LEDs) can be used in retro-fitting, i.e. replacing traditional light sources such as halogen, fluorescent, or incandescent light sources. The advantages of a retro-fit luminaire are that the lamp design remains essentially unchanged, and a significant reduction in power consumption is achieved. In order to use an LED light source (typically one or more strings of LEDs) in a “traditional” lamp, it must be adapted to existing design elements such as socket design, housing shape, etc. Furthermore, the traditional lamps were developed for different power supplies incandescent lamps can be used with a mains power supply, while a halogen lamp generally requires a transformer to lower the supply voltage. A lighting load based on semiconductor devices such as LEDs requires a highly controllable driver or power converter such as a switched-mode power supply to provide the necessary input power to the semiconductor devices. The power converter is configured to meet the requirements of the lighting load. However, the developing, testing and manufacturing of a dedicated driver for each possible application and for each possible power level in an application is prohibitively cost-intensive. Furthermore, in order to be able to use LEDs to replace a certain type of traditional light source, i.e. to retrofit a traditional lamp with LEDs, the driver for the LEDs must be realized to fit into the existing housing and must be able to deal with the power supply for which that lamp was originally designed. Since the physical size of a driver such as a switched-mode power supply generally increases with increased power rating, lamps with different power ratings might require different-sized housings for their drivers. For example, the magnetic component (coil or transformer) of a switched-mode powers supply is typically quite large, and an increase in power level means an increase in the size of the magnetic component. These may not fit into the available space in certain lighting applications. The requirement to provide larger lamp housings to accommodate larger components significantly adds to the overall design cost of such lighting applications. EP1691583 A1 discloses a LED driver for a lighting load. The lighting load comprises three single LEDs. The driver comprises a switch such that just one LED of the three LEDs is driven at a time. The driver comprises a switch mode driver and a linear mode driver and the two drivers provide in conjunction a single driver signal. 
         [0003]    Therefore, it is an object of the invention to provide an improved driver design that overcomes the problems outlined above. 
       SUMMARY OF THE INVENTION 
       [0004]    The object of the invention is achieved by the driver arrangement of claim  1 ; by the lighting arrangement of claim  11 ; and by the method of claim  15  of driving a semiconductor lighting load. 
         [0005]    According to the invention, the driver arrangement is configured to drive a semiconductor lighting load that comprises a number of semiconductor light sources, and comprises a first power converter configured to drive at least a portion of the lighting load; a second power converter configured to drive at least a portion of the lighting load; a monitoring arrangement configured to monitor an operating characteristic of the second power converter; and a control signal generator configured to generate a control signal for the first power converter on the basis of the monitored operating characteristic. The first power converter is configured to drive at least a first portion of the light load and the second power converter is configured to drive a second portion of the light load. 
         [0006]    In the context of the invention, the term “semiconductor light source” is to be understood to comprise a light source such as a light-emitting diode (LED), an organic light-emitting diode (OLED), a laser light source such as a vertical-cavity surface-emitting laser (VCSEL), etc., or any appropriate combination of such semiconductor light sources. Since a power converter in a lighting arrangement is used to drive the lighting load, the terms “power converter”, “driver” and “driver stage” may be used interchangeably in the following. 
         [0007]    An advantage of the driver arrangement according to the invention is that the first and second power converters act together to drive the lighting load in a more efficient manner from the point of view of design flexibility and cost than a prior art driving arrangement with only a single driver or a prior art driving arrangement with two cooperating drivers driving the entire lighting load, since the first power converter can be driven according to one or more operating characteristics of the second power converter. For example, the first power converter can be a favorably power-efficient and controllable driver such as a switched-mode power supply and may be regarded as the “main driver”, while the second power converter may be a more simple and economical, but less efficient and less controllable driver, and may be regarded as an “auxiliary driver” that assists the main driver. Since the total power to the lighting load can be provided by these two power converters, the first power converter does not have to supply the entire power to the lighting load, and may therefore be “smaller” and more economical than an equivalent single driver for that lighting load. Furthermore, the second power converter can be used to adjust the overall power provided to the lighting load so that one type of driver arrangement can be used to drive lighting loads with different power ratings. In this way, the driver arrangement according to the invention provides a favorably scalable power supply for a semiconductor lighting load. 
         [0008]    According to the invention, the lighting arrangement comprises a semiconductor lighting load and such a driver arrangement for driving the semiconductor lighting load. 
         [0009]    An advantage of the lighting arrangement according to the invention is that the combined cost of the first and second power converters is less than the portfolio of equivalent single main drivers that would be required in a prior art arrangement for driving a range of different lighting loads. This can favorably reduce the production costs of a luminaire comprising such a lighting arrangement. Furthermore, since a main driver is generally quite bulky, the combination of a smaller first power converter with the second power converter for augmenting the first power converter results in a combined driver arrangement that takes up less room than an equivalent driver of a prior art arrangement, so that the lighting arrangement according to the invention provides a simple and economical solution that overcomes the problem of costly design requirement, for example to accommodate large magnetic components as described above. The advantage of the more compact driver is that the overall luminaire can be smaller, and/or the luminaire may be realized to have a larger light-emitting area, as will be explained below. 
         [0010]    According to the invention, the method of driving a semiconductor lighting load comprises the steps of arranging a first power converter to drive a portion of the lighting load; arranging a second power converter to drive a portion of the lighting load; monitoring an operating characteristic of the second power converter; and generating a control signal for the first power converter on the basis of the monitored operating characteristic. The first power converter being configured to drive at least a first portion of the lighting load and the second power converter being configured to drive a second portion of the lighting load. 
         [0011]    An advantage of the method according to the invention is that the lighting load can be driven in a more efficient manner than in a prior art approach, since the lighting load can be driven mainly by the first power converter, and the performance of the first power converter can be augmented as required by the second power converter. As indicated above, the first power converter can be a high-performance and more complex driver, while the second power converter can be simpler in design, and taken together, these power converters can efficiently and reliably drive the lighting load while being more economical to realize. 
         [0012]    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. 
         [0013]    In the following, without restricting the invention in any way, it may be assumed that the semiconductor lighting load comprises a plurality of LEDs, and that a “portion” of the lighting load comprises any number of LEDs connected in any suitable manner, for example in series and/or in parallel. Of course, a lighting load “portion” can comprise the entire lighting load. A lighting load or a lighting load portion may be referred to in the following as a “light engine”. 
         [0014]    Preferably, the semiconductor lighting load of the lighting arrangement according to the invention comprises at least one LED string, wherein an LED string is arranged to be driven by a power converter of the driver arrangement, as will become clear in the following. 
         [0015]    As indicated above, the first power converter is a driver with a high degree of controllability. Various types of controllable drivers are available for driving a load comprising one or more semiconductor light sources. However, it is preferable to choose a driver that has a high degree of efficiency. Preferably, therefore, the first power converter comprises a switched-mode power supply (SMPS) realized in the form of a step-up or boost converter. Such a driver is characterized by a high power factor, and may even have a power factor that is close to unity. The first power converter therefore ensures a favorable power factor for the overall driving arrangement. A driver such as an SMPS can also deal with (i.e. compensate) mains voltage fluctuation, LED string voltage changes, etc. For example, for a lighting arrangement to be connected to the mains power supply, an SMPS driver can adjust its input power consumption over time so that the total power consumption satisfies mains harmonics regulation as laid down by appropriate regulations such as IEC 61000-3-2. When a lighting arrangement according to the invention is connected to a DC power supply such as a photovoltaic system, the SMPS driver can adjust its input power consumption to an appropriate target value. Such a driver is also capable of adjusting its output power delivery over time so that the total light output of the system complies with mains-related requirements such as modulation depth. An SMPS driver can also adjust its input-to-output signal conversion in order to balance the overall system performance. 
         [0016]    Also, as already indicated, the second power converter can be realized to simply provide additional power as needed, so that the total system (comprising driving arrangement and lighting load) can operate at a higher power level than could be achieved by the first power converter alone. Furthermore, since the first power converter can perform any necessary functions such as conforming to regulations, adjusting output power, etc., in a preferred embodiment of the invention the second power converter comprises a simple linear driver. Such a driver is generally characterized by a low power factor and would not be suitable on its own for driving a lighting load such as an LED arrangement, and may not even be approved for such use. The linear driver does not need to be able to perform the complex tasks mentioned above, since these are all dealt with by the first power converter. According to the invention, the driver arrangement effectively allows for a combination of power converters, for example one with a power factor close to unity and one with a low power factor, to achieve a significant range of power levels. From a small set of available driver stages, a large range can be ‘mixed’. For example, from the set of driver stages comprising 10.0 W and 20.0 W SMPS drivers; and 2.0 W, 5.0 W and 7.0 W linear drivers, it is possible to build drivers for 12, 15, 17, 22, 25 and 27 W lamps using an SMPS driver in conjunction with a linear driver (in addition to the 10 W and 20 W designs that use only an SMPS driver). Of course, any number of linear drivers can be deployed in a design, depending on the required power factor of the lamp. For example, a 24 W lamp can be powered from a 10.0 W SMPS driver in conjunction with two 7.0 W linear drivers. 
         [0017]    The monitoring unit can be realized to monitor any appropriate operating characteristic. For example, the monitoring unit could provide information about a relevant current waveform, an output characteristic such as a power level delivered to the lighting load, etc. In a preferred embodiment of the invention, the monitoring unit is realized to monitor an input current of the second power converter. The information already available to the first power converter together with the feedback supplied by the monitoring unit means that the first power converter is informed of a total input and/or output of the driver arrangement. The monitoring unit can comprise a means for measuring a signal directly, for example a resistor that can “measure” current by the voltage drop over the resistor. Equally, the monitoring unit can derive information from related signals, for example a setpoint command and a control loop error signal. Any number of monitoring units can be used in the driving arrangement according to the invention, depending on a desired degree of controllability and/or accuracy. 
         [0018]    The first power converter can respond to the feedback in an appropriate manner. In a preferred embodiment of the invention, the feedback delivered by the monitoring unit regulates the input power of the first power converter. In this way, the behavior of the first power converter is controlled on the basis of the operating characteristic monitored by the monitoring unit. 
         [0019]    As indicated above, the first power converter essentially acts as a “main driver” and takes care of all relevant functions such as compliance with regulations, etc., and the second power converter is essentially an “auxiliary driver”. In one preferred embodiment of the invention, therefore, the lighting load can be driven primarily by the first power converter, and the second power converter is preferably activated on the basis of a performance parameter of the lighting load. For example, since the light output of an LED decreases with increasing temperature, the second power converter may be activated in response to a detected temperature increase. Similarly, since the light output of an LED decreases over its lifetime, the second power converter may be activated after a predefined operating time of the LEDs is reached. 
         [0020]    In a preferred embodiment of the invention, the lighting load comprises a first LED string arranged to be driven by the first power converter only, and a second LED string arranged to be driven by the second power converter in conjunction with the first power converter. In this embodiment, the monitoring unit can comprise a means for measuring the input current of the second driver. This can be used to regulate the input power of the first driver. Such a realization is associated with a favorably straightforward design of the two driver stages. Furthermore, since the second LED string or light engine is driven by both drivers, relatively little effort must be expended to obtain a satisfactory mixing of the light output by both light engines. 
         [0021]    In an alternative embodiment of the invention, the lighting load comprises a single LED string or light engine arranged to be driven by the second power converter in conjunction with the first power converter, i.e. both drivers drive all LEDs of the single LED engine. The first driver in this case can also be an SMPS driver such as a boost converter. However, this embodiment should preferably comprise a means for limiting the supply voltage to the first driver during intervals in which the supply voltage is higher than the intended output voltage of the first driver. This can be achieved by using an appropriate semiconductor switch such as a MOSFET. With such an input regulating means, the boost converter cannot draw input current all the time, so that the power factor is lowered somewhat. However, the lower power factor may be weighed against the advantage of being able to use a single LED string or light engine, which will always provide a homogenous light output, and there is no need to invest any effort in mixing light from different light engines. 
         [0022]    In another alternative embodiment of the invention, the lighting load comprises a first LED string that is arranged to be driven by the first power converter only, and a second LED string that is arranged to be driven by the second power converter only, with a monitoring unit for driving the first power converter as described above. The advantage of such a design (with distinct first and second modules) is that a wide choice of driver designs is available for the first and second power converters. Furthermore, high-voltage and low-voltage LEDs can be combined in a single lighting arrangement. For example, a first module might comprise an SMPS driver and a first LED load in a well-defined or standardized module. A second module with linear driver and second LED load can then be chosen on the basis of lowest cost. While this realization is straightforward from an electrical point of view, some design effort may be required to mix the light output from both light engines. 
         [0023]    Alternatively, in another embodiment of the invention, the LEDs of the first LED string are interleaved with the LEDs of the second LED string. In this way, the light delivered by the overall LED arrangement is always favorably homogenous. This can be particularly favorable when the first and second power converters drive separate or distinct LED strings, since the LEDs of the separate strings might have noticeably different light output levels. By interleaving the LEDs of the LED strings, any such difference in light output levels is effectively hidden. 
         [0024]    In a particularly preferred embodiment of the invention, the lighting arrangement is realized as a tube LED, i.e. as a retro-fit application in which energy-efficient LEDs are used to replace a fluorescent light source. Here, the long tube is fitted with one or more LED strings. Since the driver arrangement according to the invention can be realized in a very compact manner, the components of the driver arrangement can fit into one or both tube ends, so that essentially the entire light-transmissive part of the tube can be used for illumination, i.e. there are essentially no unfavorable “dark ends” between the light-emitting elements and the tube ends. In a tube LED application, the diameter of the existing tube is a key limiting factor. However, as mentioned above, the physical size of the magnetic component of an SMPS driver is related to the power level. Beyond a certain power level, therefore, the magnetic component might not fit within the allowed diameter of the tube. In prior art solutions, this situation is dealt with by splitting the magnetic component so that two smaller parts can be used instead. However, this increases the overall cost and also increases the total length of the driver, leaving less room for the LED light sources in the tube, with the result that the undesirable “dark ends” tend to be more prominent in prior art tube LEDs. 
         [0025]    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 
         [0026]      FIG. 1  shows a block diagram of a lighting arrangement according to a first embodiment of the invention; 
           [0027]      FIG. 2  shows a simplified circuit diagram of a first embodiment of a lighting arrangement according to the invention; 
           [0028]      FIG. 3  shows exemplary current and voltage waveforms for the circuit of  FIG. 2 ; 
           [0029]      FIG. 4  shows a simplified circuit diagram of a second embodiment of a lighting arrangement according to the invention; 
           [0030]      FIG. 5  shows a simplified circuit diagram of a third embodiment of a lighting arrangement according to the invention; 
           [0031]      FIG. 6  is a schematic rendering of a retrofitted tube LED as a lighting arrangement according to the invention; 
           [0032]      FIG. 7  shows a block diagram of an embodiment of the driver arrangement according to the invention. 
       
    
    
       [0033]    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 
       [0034]      FIG. 1  shows a block diagram of a lighting arrangement  100  according to a first embodiment of the invention. The lighting arrangement  100  comprises a semiconductor lighting load  2 , in this case one or more strings of LEDs  20 . The lighting arrangement  100  is connected to a power supply  3 , for example a mains supply, a DC power supply, etc. The lighting arrangement  100  comprises a driving arrangement  1  according to the invention, with a first power converter  10 , a second power converter  11 , a monitoring unit  12 , and a control signal generator  13 . The first power converter  10  is a highly-controllable power supply such as an SMPS power supply, and has a high power factor close to unity. The second power converter  11  is relatively simple, and can be a linear power supply with a significantly lower power factor, and may even be a driver which would otherwise not be approved for use on its own in such a lighting arrangement  100 . During operation of the lighting arrangement  100 , the monitoring unit  12  monitors one or more operating parameters  12 A,  12 B,  12 C. An operating parameter  12 A can relate to an internal signal of the second power converter  11 . Further operating parameter(s)  12 B,  12 C may relate to an input power signal  12 B and/or an output signal  12 C of the second power converter  11 . The control signal generator  13  uses information provided by the monitoring unit  12  to generate an output control signal  130  which is used to control the performance of the first power converter  10 . Of course, although not shown in this diagram for the sake of clarity, the control signal generator  13  could also be supplied with monitored operating parameters of the first power converter  10  so that it can take internal, input and output characteristics of first power converter  10  into account. 
         [0035]    The lighting load  2  can comprise a single LED arrangement driven by both power converters  10 ,  11 . Equally, it may be divided into two or more LED arrangements that are driven separately or together by the power converters  10 ,  11 , as will be explained below. 
         [0036]      FIG. 2  shows a first embodiment of the driver arrangement  1  according to the invention in a simplified circuit diagram. For the sake of clarity, not all connections are shown in the diagram for reasons which will be understood by the skilled person. A power supply  3  and a rectification stage  30  supplies an input voltage V bus  to each driver  10 ,  11 . The first power converter  10  is an SMPS power supply  10 , represented here as a boost converter  10  with a power intake stage SMPS_in and a boosted output signal SMPS_out. The second power converter  11  is a linear driver  11  and drives a second LED string  2 _ 11 , while the SMPS power supply  10  drives a first LED string  2 _ 10  as well as the second LED string  2 _ 11 , so that a first current I 2    _   10  flows through the first LED string  2 _ 10  and a second current I 2   _   11  flows through the second LED string  2 _ 11 . Here, the monitoring unit  12  and control signal generating unit  13  are a combined unit realized to measure the input current I R30  drawn by the linear driver  11  and to generate a signal V shunt  that is used to limit the input power of the SMPS power converter  10 . Therefore, the input current of the SMPS driver  10  is shaped also according to the current drawn by the second driver  11 . The monitored input current I R30  therefore corresponds to the monitoring unit input signal  12 A of  FIG. 1 , while the control signal V shunt  corresponds to the output signal  130  of the control signal generating unit  13  as indicated in  FIG. 1 . The monitored input current I R30  can be measured using suitable circuitry such as a current mirror as will be known to the skilled person. Alternatively, the current measurement node in the second driver and the reference potential of the first driver can be chosen such that the input current information of the second driver is available to the first driver without requiring a current mirror. For example, the current in the return path can be measured. This current is the sum of the first driver&#39;s input current and the second driver&#39;s output current. Since the first driver may know its own output current, calculating the input current contribution (e.g. using simple analogue addition or subtraction circuitry) may be more straightforward and cost-effective than a current mirror. In such an embodiment, the analogue addition/subtraction circuit could act as control signal generation unit. 
         [0037]    In this embodiment, the linear driver  11  comprises a capacitor C 1  which is charged around peak mains voltage and which discharges over the LED string  2 _ 11 , thereby achieving a high light output of the LEDs of the second string  2 _ 11 . The effect of the capacitor C 1  on the light output of the second string  2 _ 11  is emulated by the combination of resistor R 25  and capacitor C 7 , so that the SMPS driver  10  obtains information about the performance of the linear driver  11 , and can adjust its input current accordingly. Therefore, the monitoring unit  12  and control signal generating unit  13  in this embodiment effectively take both input and output characteristics of the second driver  11  into account. 
         [0038]    Generally, the components of the monitoring unit  12  and control signal generating unit  13  can be “tuned” according to the choice of first power converter  10  and second power converter  11 , and according to required or desired performance of the lighting arrangement. This applies to all embodiments of the lighting arrangement  100  described herein. 
         [0039]      FIG. 3  shows exemplary current and voltage waveforms for the circuit of FIG. 
         [0040]      2 . Starting at the bottom of the diagram, the first part shows that the linear driver input current I R30  exhibits a series of pulses owing to the recharging of the capacitor C 1  around the peak of the mains voltage. The SMPS driver input current I SMPS   _   in  is reduced during input intervals of the linear driver input current IR30. 
         [0041]    The SMPS driver  10  outputs an LED current I_ 2 _ 10  for the LEDs of the first string  2 _ 10 ; and the linear driver  11  outputs an LED current I_ 2 _ 11  for the second string  2 _ 11 , as shown in the second part of the diagram. However, since the LED current I_ 2 _ 10  from the SMPS driver  10  is also fed to the second string  2 _ 11 , the light output of the second string  2 _ 11  can be greater than that of the first string  2 _ 10  in the case of identical LED strings  2 _ 10 ,  2 _ 11 . To obtain a homogenous light output, the strings  2 _ 10 ,  2 _ 11  can be balanced by using appropriate combinations of LED type and number. Alternatively, in a more straightforward and less costly approach, the LEDs of the two strings  2 _ 10 ,  2 _ 11  can simply be interleaved to mix the light from the two strings  2 _ 10 ,  2 _ 11 . 
         [0042]    The third part of the diagram shows the mains input U in  and the total input current I total , whose shape approaches a sinusoidal shape even though the output of the linear driver  11  is quite ‘peaky’. 
         [0043]    The output of the SMPS driver  10  is coupled to its input, and can have an efficiency exceeding 90%. The total input current I total  can have a favorably high power factor of about 0.97. In this embodiment, the total flux is characterized by a low level of flicker, as indicated in the top part of the diagram, where total light flux is plotted in arbitrary units. The excellent regulation performance of the SMPS driver  10  compensates for the poor input power quality and flicker characteristics of the simple linear driver  11 , so that the overall performance and flicker index of the lighting arrangement  100  is satisfactory. In the example given above, the total flicker index—i.e. the relative amount of light above the average light output—will not exceed 6%, which is a favorably low level for a lighting application. 
         [0044]      FIG. 4  shows a second embodiment of a lighting arrangement  100  according to the invention. Here, a single LED string  2 _ 10 _ 11  is driven by both drivers  10 ,  11  as a combined load. The diagram shows the first driver  10  connected to the beginning of the LED string  2 _ 10 _ 11 , but of course it could be connected to any point along the LED string  2 _ 10 _ 11  between two consecutive LEDs. In this embodiment also, the first power converter  10  can be a boost driver  10 , in which case the supply to the power converter  10  should be suppressed during the intervals in which the supply voltage V bus  is higher than the intended output voltage. To this end, a transistor switch M 1  such as a MOSFET is used in this embodiment, and can be controlled by the control signal generating unit  13 . The effect of this intermittent suppression, namely preventing the boost converter  10  from continuously drawing input current, lessens the power factor of the first driver  10 . In this embodiment, the power factor is lowered to about 0.86-0.9. However, in some applications, this side-effect can be regarded as minor compared to the advantages of being able to use a single LED string  2 _ 10 _ 11  and not having to take any steps to mix the light from two strings. In this embodiment, if the linear driver  11  has a well-defined input-to-output ratio that is known to the SMPS driver  10 , the SMPS driver  10  can detect a distortion (caused by the linear driver  11 ) on its own output. For example, the SMPS driver  10  can detect a voltage increase across the LED string  2 _ 10 _ 11  caused by the linear driver  11 , estimate the associated current delivered by the linear driver  11 , and can adjust its own input current wave shape to compensate. This ‘coupling’ between SMPS driver  10  and linear driver  11  is indirect, i.e. it takes place only over the load  2 _ 10 _ 11 , i.e. by sensing a load-related signal, so that the monitoring unit and control signal generation unit in this case are realized as part of the SMPS driver  10 . 
         [0045]      FIG. 5  shows a third embodiment of a lighting arrangement  100  according to the invention. Here, each driver  10 ,  11  drives its own separate LED string  2 _ 10 ,  2 _ 11 . In this embodiment also, the first power converter can be an SMPS driver  10  and the second power converter can be a simple linear driver  11 . This embodiment is particularly advantageous from the point of view of being able to use any of a wide variety of driver topologies. Furthermore, the LED strings  2 _ 10 ,  2 _ 11  can be different too, for example a high-voltage LED string  2 _ 11  can be driven by the second power converter  11 , and a low-voltage LED string  2 _ 10  can be driven by the first power converter  10 . As indicated above, the light from the two different strings can be mixed by interleaving the LEDs of the two strings  2 _ 10 ,  2 _ 11  in order to obtain a homogenous light output. Such an embodiment can make use of a well-defined module based on an off-the-shelf  10 W SMPS driver for which access to the control circuitry is possible, allowing control of the SMPS driver  10  using an output of the control signal generator  13 .The second driver  11  and its LED string  2 _ 11  can be chosen on the basis of lowest cost. This demonstrates the high degree of scalability of the driver arrangement  1  and lighting arrangement  100  according to the invention. 
         [0046]      FIG. 6  is a schematic rendering of a retrofitted tube LED  100  or “TLED”  100 . A tubular housing  60  and end pieces  61 ,  62  correspond to those of a standard fluorescent lamp. Here, two strings of LEDs  2 _ 10 ,  2 _ 11  are arranged along the length of the tubular housing  60 . The driving arrangement  1  comprising SMPS driver, linear driver, monitoring unit etc., can be any of the embodiments as described in the preceding drawings. The driving arrangement  1  is distributed within one or both end pieces  61 ,  62 , leaving essentially the entire transparent housing  60  to the LED strings  2 _ 10 ,  2 _ 11 . In this way, such a retro-fitted luminaire  100  is characterized by favorably short “dark ends”. To ensure a homogenous light output, the LEDs  20  of the LED strings  2 _ 10 ,  2 _ 11  are interleaved. For example, alternate LEDs  20  belong to one LED string. Of course, any interleaving pattern can be chosen. 
         [0047]    Alternatively, instead of splitting the driver arrangement and arranging separate parts in the tube ends  61 ,  62 , components of the linear driver can be arranged along the length of the tube. This is possible for a linear driver that does not need a bulky magnetic component. Such an arrangement frees up more space in the tube ends  61 ,  62  for the first driver  10 , resulting in even shorter dark ends. Besides the advantage of very short dark ends, the driving arrangement according to the invention also allows driver-related heat dissipation to be distributed along the length of the tube, rather than concentrating it in the tube ends. 
         [0048]      FIG. 7  shows an embodiment of the driver arrangement  1  according to the invention, with an activating means  7  for activating the second power converter  11 . The activating means  7  in this embodiment is based on the relationship between LED light output and temperature. As the pad temperature of an LED arrangement increases, the LED performance is adversely affected, and the light output drops accordingly. To detect this, the driver arrangement comprises a temperature sensor  70 . As long as the pad temperature of the lighting load  2  is below a threshold level, the first driver  10  performs in a stand-alone manner and the second driver  11  is inactive or disabled. When the temperature sensor  70  indicates that a temperature threshold has been exceeded, the activating means  7  enables the second driver  11 , for example by closing a switch  71  between power supply  3  and second driver  11 . The second driver  11  now augments the first driver  10  to increase the current through the LEDs  20 , so that the light output of the LED arrangement  2  remains constant even at higher temperatures. In an alternative embodiment, the activating means  7  could be realized to activate the second driver  11  after a certain portion of the LED lifetime has elapsed, since the light output of an LED is known to deteriorate slightly as the LED ages. Of course, instead of using a sensor to detect such an event, the activation means  7  could comprise a switch that is manually actuated as desired. 
         [0049]    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. 
         [0050]    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.