Abstract:
A driver system for driving at least one LED, the driver system includes an LED driver including a current source connected to the at least one LED for providing an LED drive current to the at least one LED, and a control module for controlling a flow of the LED drive current through the at least one LED. The driver system further includes a power supply having a power output terminal connected to a power input terminal of the current source for providing an electrical power to the current source. The control module includes a control signal output connected to a control signal input of the power supply for controlling an operation of the power supply.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage of International Application No. PCT/NL2012/050853 filed Dec. 5, 2012, which claims the benefit of U.S. Provisional Application No. 61/566,805, filed Dec. 5, 2011, the contents of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a driver system for driving at least one LED. 
     BACKGROUND OF THE INVENTION 
     A driver system may be applied for driving an LED assembly, the LED assembly may comprise at least one LED. It is well known for the skilled person to drive the LED assembly by means of a power supply providing for example a constant current to the at least one LED. 
     A prior art driver system is depicted in  FIG. 1 . In  FIG. 1 , a driver system  1  comprises a LED assembly  3 , a power supply  5  and a LED driver  7 . The LED assembly  3  comprises a plurality of LEDs  11 . At least one set of the LEDs  11  is formed in a serial LED chain  12  in the LED assembly  3 . The LED driver  7  comprises an (e.g. switched mode) current source  8  and a plurality of switches  9 . At least one set of the switches  9  is formed in a serial switches chain  10  whereby a parallel switch is provided for each respective LED or LED group. The power supply  5  in this example comprises a fly-back converter  13  and a power factor correction circuit  15 . 
     The power supply  5  is with an input thereof connected to an alternating current (AC) source  14  and is with an output thereof connected to the LED driver  7 . The LED driver  7  is with an input connected to the power supply  5  and is with an output connected to the LED assembly  3 . At least one of the switches  9  in the LED driver  7  is connected to one of the LEDs  11  or a group of LEDs in the LED assembly  3 . 
     The power supply  5  receives an alternating current (AC) from the AC source  14 . The fly-back converter  13  is arranged to convert a (rectified) mains voltage of the power supply  5  into a constant voltage  17  and transmit the constant voltage  17  to the LED driver  7 . The power factor correction circuit  15  is arranged to improve a phase relation between the input current and the input voltage in the power supply  5  so as to improve a power factor. 
     The LED driver  7  receives the constant voltage  17  from the power supply  5 . 
     The current source  8  is arranged to provide a LED drive current  19  to the LED assembly  3 . The switches  9  are arranged to switch ON/OFF the LEDs  11  in the LED assembly  3 . 
     The LEDs  11  in the LED assembly  3  are controlled by the respective parallel switches  9 . Each LED of the serial LED chain  12  is activated or de-activated by opening or closing of a respective corresponding switch of the serial chain  10 . 
     In the driver system  1 , when applying a constant LED drive current, a voltage across the entire LED assembly  3  will vary substantially with the switching ON/OFF of the separate LEDs or groups of LEDs  12  by the switches  10 . When no LEDs  12  are ON, the voltage across the entire LED system  3  will substantially drop to zero. When all LEDs  12  are ON, the voltage across the entire LED system  3  will equal the sum of the voltages of each independent LED group  12 . 
     The abovementioned driver system  1  comprising a cascading of two supplies (namely the, e.g. switched mode, power supply and the, e.g. switched mode, current supply that drives the LEDs) may exhibit shortcomings in its dynamic behavior when driving LED assembly  3 . 
     It is desirable to provide an improved driver system. 
     SUMMARY OF THE INVENTION 
     The present invention provides a driver system for driving at least one LED, the driver system comprising an LED driver comprising a current source connected to the at least one LED for providing an LED drive current to the at least one LED, and a control module for controlling a flow of the LED drive current through the at least one LED; a power supply having a power output terminal connected to a power input terminal of the current source for providing electrical power to the current source; wherein the control module comprises a control signal output connected to a control signal input of the power supply for controlling an operation of the power supply. 
     Here, the current source and the power supply may be any type of power supply, such as a switched mode power supply, e.g. of a buck-boost type or a boost-buck type or, more specifically, the fly-back type switched mode power supply. The current source and/or power supply may also be of a linear type, e.g. a regulated linear power supply. 
     The LED assembly may comprise any type of LEDs, such as high power illumination type LEDs, more specifically the LED assembly may comprise a group of LEDs having a same color, e.g. white light LEDs, or may contain LEDs having different colors. The LED driver may contain one or more switches (e.g. serial and/or parallel to the respective LED) that enable to switch on or off the respective LED associated with the particular switch. The switches may comprise a (e.g. semiconductor) switch, a switchable current source, a controllable impedance, etc. Alternatively, a value of the LED drive current may be varied, e.g. by means of setting different LED drive current levels or by pulsing the current using e.g. duty cycle modulation, pulse width modulation, etc. Still further, multiple current sources may be applied, such as a current source for each group of LEDs (e.g. each group of LEDs to emit its own color). The driver may provide a constant LED drive current (possibly in combination with switches that enable a selective switching of LEDs, thereby enabling for example a pulse width modulation) or may provide an LED drive current whose value may change over time, e.g. allowing a modulation of the effective LED light output by e.g. pulse width modulation of the LED drive current. 
     The control module may comprise any type of control, comprising e.g. digital control by means of a microprocessor, microcontroller, a programmable digital device, and/or other digital control electronics. Furthermore, or instead thereof, the control module may comprise analogue electronics, such as comparator(s), filter(s), pulse width control circuits, timer circuits etc. The control module may comprise a switches array comprising paralleled switches series or serial switches or any combination thereof. The control module may control a flow of the LED drive current through the at least one LED by means of a control of a magnitude of the LED drive current and or activation/de-activation of one or more of the LEDs. The control of the magnitude of the LED drive current may be performed by a pulse modulation (e.g. pulse width modulation, pulse frequency modulation, etc) of the LED drive current. The activation/de-activation of one or more of the LEDs may e.g. be performed by means of a pulse modulation, such as a pulse width modulation of switches associated with the LEDs (e.g. switches, switchable current sources or variable impedances parallel to the or each LED, switches, switchable voltage sources or variable impedances in series with the or each LED). The control signal may be provided by the control module to the power supply and may contain any type of control signal, such as an analogue control signal, a digital parallel or serial bus control signal, etc, and may for example contain a desired value of one or more parameters for the power supply. When the control signal is received by the power supply, the power supply may adapt its operation accordingly (examples of which will be provided below) so as to enable the cascading of the power supply and the current source to exhibit a more adequate dynamic behavior. The control module hence controls the driving of the at least one LED as well as—by means of the control signal—at least part of an operation of the power supply. The controlling the driving of the at least one LED by the control module may comprise controlling of the current source, controlling the switches, and/or controlling a load balancing between multiple current sources, learning by measuring long time statistics of the characteristics of the control loop and controlled quantities, so as to characterize components in a cascade that forms the control loop, including e.g. an estimate of the impedance of the mains supply, etc. A behavior of the cascaded power supplies may be improved. As an example, the power supply may establish a dynamic characteristic thereof in response to the control signal. The power supply may for example set its control loop bandwidth, its switching frequency, and/or its output voltage or output current in dependency of the control signal. Many effects may occur. For example, in case of a change in the load formed by the LEDs (for example a dynamic change in LED output power), the control module may by means of the control signal control the power supply so as to quickly adapt to the changed load condition. Thereby, when a transition in load is foreseen, the control bandwidth of the power supply may be increased. As another example, in load situations where a good power factor is of importance, for example at high power, a dynamic characteristic (e.g. a control loop bandwidth) of the power supply may be controlled via the control signal so as to optimize a power factor (for example by reducing a control loop bandwidth of the power supply so as to make the supply behave more like a resistive load, thereby improving a power factor), while at lower output power, other criteria such as a fast response, may take precedence, thereby for example sacrificing a (level of a) power factor to some extent in favor of other criteria. 
     In an embodiment of the present invention, the control module is arranged to estimate a load of the current source, and provide the control signal to the power supply based on the estimated load of the current source. 
     Based on the estimated load of the current source, the control module transmits a respective signal to the power supply to change a behavior thereof, for example a control loop bandwidth, switching frequency, output voltage, etc thereof so as to take account of the estimated load (e.g. enabling to taking account of a change in the load, a level of the load etc.). Thereby, dynamic changes in the load may for example establish a faster response of the power supply so as to more adequately (hence accurately) react to the change in load, hence allowing for example a faster settling time, resulting in an improved, e.g. more stable input to the current source, hence enabling the current source to more accurately comply to a desired dynamic change in light output. 
     Likewise, as explained above, the estimated load may be applied by the control module to control the power supply (by means of the control signal) for a high power factor in case a high load has been estimated and for example to control the power supply for a fast response otherwise, to prevent a hick-up mode, allow fast response to load changes, etc. 
     In an embodiment of the present invention, the control module is arranged to estimate the load of the current source from setpoint input data provided to the control module at a setpoint input thereof. The setpoint input data may represent a desired intensity, color, etc of the LEDs. The setpoint input may be a digital (e.g. bus) interface or an analogue input for correspondingly receiving digital or analogue setpoint data. The setpoint data (i.e. setpoint input data) may indicate e.g. the desired intensity or average light characteristics of the at least one of the LEDs of the LED assembly. Since the control module may arrange for the driving of at least one of LEDs  111  in the LED assembly based on setpoint data, the estimation of the load of the current source can be adequately determined from setpoint data. The estimation of the load may also be performed using (known) characteristics of the driver and LED assembly. These can be (partially) known by design or by means of a predetermined configuration data, such as a LED code. These characteristics may f.e. describe time characteristics such as a delay or aspects of the control behavior (overshoot, undershoot, etc.) of the current supply that may influence the load as “seen” by the voltage supply. Furthermore, setpoint data may be provided in the form of a sequence of setpoint values, allowing the control module to anticipate a desired light output level, hence a desired load. 
     In an embodiment of the present invention, the control module is arranged to estimate the load of the current source from a forward voltage over the at least one LED. 
     Here, the forward voltage may refer to the voltage across the entire LED assembly. The forward voltage over the LEDs (at a given, e.g. known or constant current) may provide an adequate measure for the load, as the total power may be adequately determined from the forward voltage and the LED drive current. This feature may be applied in case of a switching of the LEDs by means of respective switches associated with respective ones of the LEDs, as well as in case such switches are omitted and for example a pulsation of the LED drive current is applied. 
     In an embodiment of the present invention, the control module is arranged to anticipate a load change of the current source and to control the power supply in accordance with the anticipated load change. 
     Setpoint data may be provided in the form of a sequence of setpoint values, allowing the control module to anticipate a desired light output level, hence a desired load. Anticipation may for example be performed in the following causal way. A delay may occur for at least one of the following causes: From a moment when the control module receives a setpoint, successive actions take place that require some time before an effect of the new setpoint is substantially noticeable at the load (i.e. the LED&#39;s). Such actions include: Converting the setpoint to a drive signal for the FET or other switch of the driver current source. A response time of a level shifter in a gate of the FET, the FET itself, a coil that forms the inductance, an output capacitance, and the LEDs. It may be the case that the actual output of the driver is based on a previous setpoint value in order to enable an interpolation from the previous to the present setpoint. The interpolation provides more time to drive the power supply in order to anticipate a setpoint change. Also, such interpolation enables the power supply to more effectively control any (e.g. transient effects) as the effect is spread out in time as a result of the interpolation. A delay by one or more of the above causes may enable the control module to determine an effect on the power supply and provide a control signal (e.g. a sequence of control signal values) to the power supply in order to at least partly compensate for an effect of the (changed) setpoint on the power supply. Correspondingly, in an embodiment, the control module is arranged to anticipate the load change of the current source by determining the control signal to control the power supply during a delay between a providing of the setpoint input data to the control module and the load change resulting from the setpoint data. During the time between the providing of the setpoint (the delay due to one or more of the above causes) and the effect of the setpoint of an output of the driver, the control signal for controlling the power supply is generated, and the power supply is driven accordingly, so that dynamic effects may be anticipated and/or at least partly compensated by a corresponding control of the power supply. Based on the anticipated load change of the current source, the control module may transmit the control signal to the power supply to take account of the anticipated load, e.g. changing the e.g. output voltage thereof, changing a bandwidth thereof, changing a switching frequency thereof, etc. so as to at least partly compensate for a fluctuation due to the change of the load. 
     In an embodiment of the present invention, the control module is arranged to anticipate a power supply output ripple and to control the power supply in accordance with the anticipated power supply output ripple. 
     The power supply output ripple may be a repetitive effect. The control module may anticipate such repeating effect, and may transmit the control signal to the power supply to at least partly compensate for the anticipated load change, such as the ripple. Thereby, for example an output voltage setpoint of the power supply may be changed repetitively in order to at least partly compensate the ripple. 
     In an embodiment of the present invention, the power supply is arranged to set a control loop bandwidth of the power supply in response to a value of the control signal. 
     As explained above, the change of the control loop bandwidth of the power supply may be applied to set a dynamic behavior of the power supply as desired. 
     The control loop bandwidth may for example be set by activation of a filter, switching of a compensation network, or any other suitable means. 
     In an embodiment of the present invention, the control module is arranged to increase the control loop bandwidth of the power supply during a change of the load of the current source. 
     As explained above, this may render the power supply to quickly react to the load change and consequently may allow to have the LEDs respond more accurately to a desired change, such as a change in intensity, color, etc. 
     In an embodiment of the present invention, the control module is arranged to increase the control loop bandwidth of the power supply in dependency of a magnitude of the load of the current source. In a preferred embodiment, the control loop bandwidth is increased when the load is decreased and vice versa. As explained above, at a high load of the current source, the power factor may play a more dominant role, the power factor may be improved by a decrease of the control loop bandwidth of the power supply. At a low load, other requirements (fast response, preventing hick-up behavior at low load conditions, etc) may require a higher control loop bandwidth, possibly at a cost of a lower power factor, which may be more acceptable at a lower load level. Hence, a higher performance may be achieved possibly omitting additional elements that would be required otherwise, such as a filter or additional converter stage to improve power factor, etc. 
     In an embodiment of the present invention, the power supply is arranged to set a switching frequency of the power supply in response to a value of the control signal. 
     Here, the change of the switching frequency of the power supply may have an effect on a maximum output power, a dynamic behavior (e.g., bandwidth), a response time, a settling time, etc. of the power supply. 
     In particular, a combination of the above feature of the control loop bandwidth being increased when the load is decreased and vice versa, in combination with the feature of the control loop bandwidth being increased during a change of the load of the current source has proven to be effective: A high power factor (e.g. a higher power factor than could conventionally be obtained in a single stage power supply) may be obtained thereby at a high load (i.e. when a high power consumption is drawn from the mains supply), while a fast dynamic response and accuracy may be improved during transitions in power level as well as at lower load conditions where a high power factor is of less importance. 
     In general, the control signal provided by the control module to the power supply may have an effect on one or more of the following parameters of the power supply, such as but not being limited to: an output voltage setting, an output power setting, a bandwidth, a control loop frequency, proportional (P), integrating (I) and/or differentiating (D) factors, a damping, a control strategy/structure itself (f.e. from linear to non-linear or from non-cascaded to cascaded control, etc. Furthermore, the control signal provided by the control module to the power supply may have an effect on a feed forward gain factor of the power supply, whereby an anticipated load may become a feed-forward signal when fed-back but ahead of reality due to anticipation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail with reference to the appended drawings that are only intended to show embodiments of the invention and not to limit the scope of the invention. The scope of the invention is defined in the annexed claims and by its technical equivalents. 
       The drawings show: 
         FIG. 1  shows a schematic view of a prior art driver system. 
         FIG. 2  shows a schematic view of a driver system according to one embodiment of present invention. 
         FIGS. 3   a - 3   f  each depict a time diagram of a respective signal in the driver system in accordance with an embodiment of the invention. 
         FIGS. 4   a - 4   f  each depict a time diagram of a respective signal in the driver system in accordance with embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 2 , a driver system  100  comprises a LED assembly  103 , a power supply  105  and a LED driver  107 . The LED assembly  103  comprises a plurality of LEDs  111 . At least one group of the LEDs  111  is formed in a serial LED chain  112  in the LED assembly  103 . The LED driver  107  comprises a current source  108  and a control module  125 . 
     The control module  125  comprises a setpoint input  127  arranged to receive setpoint data. 
     In one embodiment, the control module  125  further comprises a plurality of switches  109 , e.g. ion parallel to each LED or group of LEDs, the switches may be used to selectively activate/de-activate the LED or group of LEDs to which the respective switch is connected in parallel. Alternatively, the switches  109  may be omitted. The LED drive current supplied by the current source may then be controlled so as to switch on the light, to switch off the light and to dim the light (eg. by reducing or pulsing the LED drive current). 
     In one embodiment, the power supply  105  is a fly-back type power supply. 
     The power supply  105  is with an input thereof connected to an alternating current (AC) source  114 , such as a mains supply voltage, and is with an output thereof (such as a regulated voltage output) connected to the LED driver  107  via a link  117 . The LED driver  107  is with an input thereof connected to the power supply  105  via the link  117  and is with an output thereof (such as an LED drive current output) connected to the LED assembly  103  via a link  119 . The control module  125  is connected to at least one of the LEDs  111  in the LED assembly  103 . The control module  125  is with a control signal output thereof connected to a control signal input of the switching voltage supply  105  via a link  121 . The control module  125  is in this embodiment with an output thereof connected to the current source  108  via a link  123  in order to enable the control module to control for example a level of the LED drive current. 
     In one embodiment, at least one of the switches  109  in the control module  125  is connected to one of the LEDs  111  in the LED assembly  103  via a link  129 . 
     [Function] 
     The power supply  105  receives an (e.g. rectified) alternating current (AC) from the AC source  114  and provides a voltage  118  to the LED driver  107  via the link  117 . 
     The power supply  105  receives a control signal  121  from the control module  125  via the link  121 . 
     In one embodiment, the voltage  118  provided by the power supply  105  is a constant voltage. In another embodiment, the power supply provides a constant current and in yet another embodiment, the power supply provides a constant power and in yet another a magnitude of one or more of those characteristics may be on demand. 
     The LED driver  107  receives the voltage  118  provided by the power supply  105 . The current source  108  in the LED driver  107  provides a LED drive current  120  to the LED assembly  108  via the link  119 . The current source  108  receives a second control signal  124  from the control module  125  via the link  123 . 
     In one embodiment, the LED drive current  120  provided by the current source  108  is a constant current. 
     The control module  125  provides the first control signal  122  to the power supply  105  via the link  121 . The setpoint input  127  of the control module  125  receives setpoint data. The setpoint input  127  may be a digital input, a bus interface or an analogue input for receiving setpoint data. The setpoint data may provide an indication of a operation of the at least one of the LEDs of the LED assembly  103 , e.g. the desired intensity or average light characteristics of the at least one of the LEDs of the LED assembly  103 . The control module  125  controls the current source and switches ON/OFF the at least one of LEDs  111  in the LED assembly  103  based on setpoint data. 
     In one embodiment, at least one of the switches  109  in the control module  125  switches ON/OFF the at least one of the LEDs  111  in the LED assembly  103 . 
     In one embodiment, the control module  125  provides the second control signal  124  to the current source  108 . 
     In one embodiment, the control module  125  estimates a load of the current source  108  in the LED driver  107 . The forward voltage over the LEDs (at a given, e.g. known, constant or switched current) may provide an adequate measure for the load, as the total power may be adequately determined from the forward voltage and the LED drive current It is noted that the forward voltage Vf may itself be the characteristic of the load that the control signal  122  is based on. For example when the forward voltage Vf is just 4V (only 1 LED switched ON), then it is wise for efficiency reasons to only require 4V+1 or 2 V=5 to 6 V from the front-end. It is thus noted that under the term load, not only power dissipation or current should be understood. All characteristics of load (voltage, current, power, delay, . . . ) may hence be used to determine the first control signal on. 
     A disturbance may be formed by a dip or peak at the output of the power supply at  117  due to a load change. Such a dip or peak could be anticipated, enabling to alter (e.g. heighten or lower) on beforehand the voltage supply in order to at least partially compensate for the dip or peak caused by the change in load. Due to delay effects and durations of disturbances and adjustment behaviors to load changes, the anticipation and subsequent control may lead to this type of (partial) compensation. 
     Furthermore, disturbances such as a ripple may at least partly be compensated. Such undesired disturbances may subsequently influence the LED assembly  103  and may deteriorate the expected result of the LED assembly  103 , e.g. the intensity or the average light characteristics of the LED assembly  103 . For example, an output voltage ripple of the power supply and/or an LED drive current ripple of the current source is measured by the control device (by any suitable measurement means or estimated from other measured parameters), the control module determines the control signal to at least partly compensate the disturbance: for example, a ripple may at least partly be compensated by the control module by means of the control signal affecting an output voltage input setpoint of the power supply so as to at least partly compensate such ripple. 
     In one embodiment, the estimation of the load of the current source  108  may be based on a forward voltage over the at least one LED  111  of the LED assembly  103 . Here, the forward voltage indicates the voltage across the entire LED assembly  103 . It is noted that some configurations may comprise multiple current sources loading a same power supply. In that case the power supply should be adequate for the multiple current sources typically causing it to be less than optimal for most individual current supplies. 
     In one alternative embodiment, the estimation of the load of the current source  108  may be based on set point input data at the set point input  127 . Note that the set point data from the set point input  127  may be available before providing the LED drive current to the LED assembly  103 . 
     Based on the estimated load of the current source  108 , the control module  125  can provide the first control signal  122  to the power supply  105  via the link  121  to control one or more parameters thereof. The first control signal  122  may represent or contain in any form the desired value of the parameters for the power supply  105 . When the first control signal  122  is received by the power supply  105 , the voltage  118  provided from the power supply  105  to the LED driver  107  is changed in response to the changes of the parameters of the power supply  105 . 
     The abovementioned parameters may for example be (further alternatives having been mentioned above):
         a parameter indicating the control loop frequency of the power supply  105 , and/or   one or more parameters indicating the transfer characters of the power supply  105 , and/or   a parameter indicating whether the power supply  105  is in a hick-up mode (or other mode) or not.       

     The estimation result of the load of the current source  108  can be classified into four categories: 
     1) the load of the current source  108  is a high value; 
     2) the load of the current source  108  is a low value; 
     3) the load of the current source  108  is decreasing; 
     4) the load of the current source  108  is increasing. 
     It is to be understood that the load may be understand so as to be characterized by one or more of its power, its resistance, its impedance, its voltage, its current, its sequential behavior, or any other characteristic or combinations of the mentioned characteristics. 
     For the first situation, the control module  125  transmits the control signal  122  to the power supply  105  in order to:
         decrease the control loop frequency of the power supply  105  so as to decrease the bandwidth of the power supply  105 , and/or   change the transfer characters of the power supply  105  so as to decrease the bandwidth thereof.       

     Consequently, the power factor of the power supply  105  may be improved due to the changing of the bandwidth of the power supply  105  from a relatively high value to a relatively low value. 
     For the second situation, the control module  125  transmits the control signal  122  to the power supply  105  in order to:
         increase the control loop frequency of the power supply  105  so as to increase the bandwidth of the power supply  105 , and/or   change the transfer characters of the power supply  105  so as to increase the bandwidth thereof, and/or   switch off the hick-up mode of the power supply  105 .       

     Consequently, the hick-up current of the power supply  105  is minimized due to the changing of the bandwidth of the power supply  105  from a relatively low value to a relatively high value and/or switching off the hick-up mode thereof. 
     For the third situation, the control module  125  transmits the control signal  122  to the power supply  105  in the following sequences:
         i) the control module  125  anticipates or detects a change of the load of the current source  108  from a high value to a low value;   ii) the control module  125  increases the bandwidth or maintains the bandwidth of the power supply  105  at a high level during the transitional period of the change of the load, e.g. the control module  125  does not transmit any signals to the power supply  105  during the transitional period;   iii) the control module  125  transmits the control signal  122  to power supply  105  for decreasing the bandwidth of the power supply  105  after the aforementioned transitional period.       

     Consequently, the fluctuation due to the change of the load of the current source  108  can be reduced. It also renders the power supply  105  to quickly react the load changes. 
     For the fourth situation, the control module  125  transmits the control signal  122  to the power supply  105  in the following sequences:
         i) the control module  125  anticipates or detects a change of the load of the current source  108  from a low value to a high value;   ii) the control module  125  transmits the control signal  122  to the power supply  105  for increasing the bandwidth of the power supply  105  before the transitional period of the change of the load. The control module could also (further) increase the output voltage and lower it again after the change.       

     Consequently, the fluctuation due to the change of the load of the current source  108  can be reduced. It also renders the power supply  105  to quickly react the load changes. 
     Note that the anticipation may be based on the predefined setpoint data that will be transferred to the setpoint input  127  of the control module  125 . 
     In the third and fourth situations, the control module  125  may also be adapted to detect or anticipate a power supply output ripple from the power supply  105 . Here, the power supply output ripple is a repeating effect. Based on this anticipated repeating effect, the control module transmits a respective signal to the power supply to change the power output (e.g. output voltage) thereof so as to compensate the load change 
     It is understood by the skilled person to change the bandwidth of the power supply  105  in a different way other than changing the control loop frequency or transfer characters of the power supply  105 . 
       FIG. 3   a )- 3   f ) depict the following signals versus time:
         a) received set-points   b) control signal to power supply   c) theoretical power supply output   d) LED current in case of no power supply control (not even at events  904  nor  905 )   e) Combined graph (normalized to resulting LED current), with signal c) [dotted] and d) [dashed] and their average [solid] which represents the resulting LED current.   F) resulting LED current (equal to average signal in e), now drawn separately)       
     Reverting to  FIG. 3   a , assume a steady state at event  901 , where a set-point is received that does not deviate from many previously received set-points. 
     At event  902  a larger set-point is received, causing the driver to send a higher LED current through the LEDs. After a delay  906  ( FIG. 3   d ) this causes a dip in the voltage supply. After the power supply has recovered, a steady state offset from the previous steady state occurs with magnitude  907  ( FIG. 3   d ). 
     At event  903  the previous set-point is received again causing a temporary rise in the power supply voltage and after that a return to the original steady state voltage. 
     Events  904  and  905  repeat the above scenario, but this time with control of the power supply through the control signal shown in  FIG. 3   b ). At event  904 , besides the control of the LED current, the control module also increases the control signal to the power supply to cause a compensating increase of the supply voltage as shown in  FIG. 3   c ). The waveform shown is a theoretical step-response of which only the delay  908  is explicitly shown. In practice the waveform will be less rectangular and the amplitude will not be exactly as designed. 
     At event  910 , the power supply is controlled to no longer compensate for the dip caused by the higher load, but to do compensate for the static effect of the higher load which would have caused a lower voltage at a magnitude of  907  would said compensation not be applied. This compensation causes the power supply to generate a voltage that is higher by magnitude  909  with respect to the original steady state voltage at  901 . 
     At event  905  the reverse takes place. 
     The effect on the LED current from the thus controlled power supply voltage is shown in  FIG. 9   e  as a dotted line. The effect of the set-points on the LED current without compensating via the power supply is shown in  FIG. 9   d . In  FIG. 9   e  it is shown that the waveform from  9   d  (dotted in  9   e ) is compensated for by the waveform from  FIG. 9   c  (dashed in  FIG. 9   e ) to average to a substantially unchanged resulting LED current (solid in  FIG. 9   e ). By dimensioning during design, calibrating/adjusting during production or installation and measuring/learning from the system during operation, the deviations caused by delays and amplitude deviations can be minimized. 
     In  FIGS. 4   a - f , as a basic example of influencing the control behaviour of a second order linear control in the power supply, 3 examples are shown of a simple PID control. The  FIGS. 4   a ,  4   b ,  4   c  show the actual controlled value versus the target value set at the start of the horizontal axis, the  FIGS. 4   d ,  4   e ,  4   f  show the resulting error. In  FIG. 4   a , critical damping is achieved by proper tuning of the P, I and D factors depending on, amongst others, the time constants of the controlled system. In  FIG. 4   b  a PID setting is used delivering undershoot while in  FIG. 4   c  a setting is used delivering overshoot. 
     The settings used in  FIG. 4   c  achieve a faster response to the change in set-point than in  FIG. 4   a  or  4   b . This can be used to advantage in case a large load change is expected. By switching to the settings used in  FIG. 4   c  just prior to the load change, will make the power supply react faster to the load change possibly at the expense of visible light effects at the output of the lighting system or a worse power factor at the input of the lighting system. 
     Using other control structures, such as cascaded control systems, employing techniques such as feed-forward, integrators at certain frequencies, prediction, adaption, etcetera, many of them made possible through digital control, enriches the ways in which to influence the control behaviour of the power supply from the control module. 
     Note that in an embodiment (where for example the cascaded power supply and LED driver are designed in an integrated fashion) the set-point for the desired light characteristics can also be fed directly to the power supply. In such embodiment, the control signal provided to the power supply may comprise the setpoint that represents a desired LED light intensity. The power-supply may then exhibit the desired behaviour for compensating load changes by control actions taken by its own internal controller. This controller may need to have knowledge of the LED driver and possibly of its load to perform optimally. Such knowledge can among others be fed to this controller by design, based on a model of LED driver and/or load, calibration, measuring/learning.