EP2958400B1

EP2958400B1 - A feedback arrangement for a LED driver

Info

Publication number: EP2958400B1
Application number: EP14172955.8A
Authority: EP
Inventors: Hannu Vihinen
Original assignee: Helvar Oy AB
Current assignee: Helvar Oy AB
Priority date: 2014-06-18
Filing date: 2014-06-18
Publication date: 2017-12-27
Anticipated expiration: 2034-06-18
Also published as: EP2958400A1

Description

FIELD OF THE INVENTION

The example embodiments of the present invention relate to a feedback arrangement in a light emitting diode (LED) driver.

BACKGROUND OF THE INVENTION

LED drivers supply the LEDs with an output current, the value (or effective value, in case the current is pulsed or otherwise varies over time) of which determines the observed brightness of the LEDs. In order to limit the output current below a selected maximum level, a LED driver is typically provided with a current feedback mechanism that monitors the output current and produces a limiting signal when the output current reaches the maximum value.
The voltage across the LEDs connected to the output of the LED driver depends on characteristics of the employed LEDs, in particular on forward voltage(s) of the employed LEDs. As an example, for LEDs connected in series the voltage across the LEDs is the sum of the forward voltages of the employed LEDs. Herein, the voltage across the LEDs connected to the output of the LED driver is referred to as an output voltage of the LED driver.
Various standards may regulate the maximum allowable output voltage. As an example, the Safety Extra Low Voltage (SELV) regulations set a maximum output voltage drawn from the LED driver to 60 volts. In order to ensure meeting this voltage limit (or any other voltage limit) the LED driver may be provided with a voltage feedback mechanism that monitors the output voltage and a voltage control mechanism that limits the output voltage in accordance with the voltage limit.
Another reason for limiting the output voltage may arise from a design of the power source of the LED driver: the power source is typically designed for a certain maximum output power and with a selected output current the output power varies with the output voltage. Consequently, the voltage control mechanism may be, additionally or alternatively, arranged to limit the output voltage in accordance with a voltage limit set in view of the maximum output power of the LED driver.
LED drivers that are capable of selectively supplying the output current at one of multiple different nominal output current levels are known in the art. The selection of the desired nominal output current may be provided e.g. by connecting an external component of suitable characteristics to the LED driver, by using a switch arranged in a housing of the LED driver for selection of the nominal output current or by issuing an external control signal of suitable characteristics to select the desired nominal output current.
For such a LED driver the voltage limitation mechanism outlined above is likely to provide suboptimal performance due to the voltage limit applied by the voltage control mechanism failing to account for the applied nominal level of the output current.
In related art, EP 2712272 A2 discloses a power supply device that includes a control circuit and a first circuit. The control circuit switches between a current control mode and a voltage control mode according to an inputted dimming signal and thus dims a light-emitting element. In the current control mode the control circuit controls an output current supplied to the light-emitting element to a target current, and in the voltage control mode the control circuit controls an output voltage supplied to the light-emitting element to a target voltage. The first circuit detects the output current and the output voltage, and sets the target voltage at which switching between the current control mode and the voltage control mode is carried out as a first voltage.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a technique that facilitates limiting the output voltage in dependence of the selected nominal output current level in a flexible and cost-effective manner.
The object(s) of the invention are reached by a driver and by a method as defined by the respective independent claims.
According to an example embodiment of the invention, a driver for operating one more LEDs is provided. The driver comprises an output for connecting the one or more LEDs, power source means for supplying power to said output, reference generation means for generating a common reference potential in dependence of a selectable component connectable to the driver, first feedback means for providing current feedback in dependence of said common reference potential, which current feedback is indicative of a level of an output current supplied to said output in relation to a nominal current level, second feedback means for providing voltage feedback in dependence of said common reference potential, which voltage feedback is indicative of an output voltage across nodes of said output in relation to an adaptive maximum output voltage, wherein the reference generation means is arranged to generate a common reference potential that serves to set the nominal current level and the adaptive maximum output voltage such that the higher value is selected for the nominal output current, the lower becomes the adaptive maximum output voltage and vice versa. The driver further comprises control means for controlling the power source means based at least on said current feedback and said voltage feedback such that the power supplied to said output does not exceed a predefined maximum power.
According to another example embodiment of the invention a method for operating one or more LEDs, using a driver comprising an output for connecting the one or more LEDs and a power source means for supplying power to said output is provided. The method comprises generating a common reference potential in dependence of a selectable component connectable to the driver, providing current feedback in dependence of said common reference potential, which current feedback is indicative of a level of an output current supplied to said output in relation to a nominal current level, and providing voltage feedback in dependence of said common reference potential, which voltage feedback is indicative of an output voltage across nodes of said output in relation to an adaptive maximum output voltage. In the method, generating the common reference potential comprises generating a common reference potential that serves to set the nominal current level and the adaptive maximum output voltage so that their product remains substantially constant, and the method further comprises controlling the power source means based at least on said current feedback and said voltage feedback such that the power supplied to said output does not exceed a predefined maximum power.
The exemplifying embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" and its derivatives are used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features described hereinafter are mutually freely combinable unless explicitly stated otherwise.
Some features of the invention are set forth in the appended claims. Aspects of the invention, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of some example embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically illustrates some components of a LED driver known in the art.
Figure 2 schematically illustrates some components of a LED driver in accordance with an example embodiment.
Figure 3 schematically illustrates some components of a current feedback means according to an example embodiment.
Figure 4 schematically illustrates some components of a current feedback means according to an example embodiment.
Figure 5 schematically illustrates some components of a voltage feedback means according to an example embodiment.
Figure 6 schematically illustrates some components of a voltage feedback means according to an example embodiment.
Figure 7 schematically illustrates some components of a LED driver in accordance with an example embodiment.
Figure 8 schematically illustrates some components of a reference generation circuit according to an example embodiment.
Figure 9 illustrates a relationship between output current, output voltage and output power.
Figure 10 schematically illustrates some components of a LED driver in accordance with an example embodiment.
Figure 11 illustrates a method in accordance with an example embodiment.

DETAILED DESCRIPTION

Figure 1 schematically illustrates some components of a driver 100 for driving one or more LEDs 120. The driver 100 comprises a power source portion 110 for generating an output current I _out to an output of the driver 100 using electric energy brought to an input marked as V _in in Figure 1. The output current I _out may comprise a direct current (DC) of desired characteristics that may be provided to the one or more LEDs 120 that are connectable to an output of the driver 100.
In order to enable keeping the output current I _out at or below a desired level, the driver 100 further comprises a current feedback means 130 for providing current feedback FB _I that serves as an indication of a measured level of the output current I _out in relation to a target current level. As an example, the current feedback FB_I may indicate a difference between the measured output current I _out and the target current level, or the current feedback FB_I may become active, or assume a particular value or character, when the measured output current I _out reaches a predetermined maximum level. The current feedback means 130 is provided with connectors for connecting a selectable external component, e.g. a resistor R _set illustrated in Figure 1. The selectable external component adjusts the operation of the current feedback means 130 such that the target current level applied therein is derived in dependence of characteristics of the selectable external component, thereby providing a flexible and low-cost approach for selectively setting the target current level.
The driver 100 further comprises a voltage feedback means 140 for providing voltage feedback FB _V that serves as an indication of a measured level of an output voltage V _out, i.e. the voltage across nodes of the output of the driver 100. As an example, the voltage feedback FB _V may indicate a difference between a predefined reference voltage and the measured output voltage V _out, which predefined reference voltage represents a predefined maximum allowable output voltage V _max. Alternatively or additionally the voltage feedback FB _V may become active, or assume a particular value or character, when the measured output voltage V _out reaches the predefined maximum allowable output voltage V _max. The maximum allowable output voltage V _max may comprise e.g. a maximum voltage defined by a relevant safety regulation or standard or a maximum voltage set to prevent exceeding a maximum output power of the driver 100.
The power source portion 110 comprises one or more power converters for generating the output current I _out and a control means for controlling the operation of the power converter(s). In particular, the control means may be arranged to operate the power converter(s) to generate the output current I _out as the direct current (DC) of desired characteristics in accordance with control information received via the control input CTRL and in accordance with the current feedback FB _V. Moreover, the control means may be further arranged to operate the power converter(s) in accordance with the voltage feedback FB _V such that the predefined maximum allowable output voltage V _max is not exceeded.
Figure 2 schematically illustrates some components of a driver 200 for driving the one or more LEDs 120. Along the lines described for the driver 100, also the driver 200 comprises a power source portion 210 for generating an output current I _out to an output of the driver 200 using electric energy brought to an input marked as V _in in Figure 2. The output current I _out may comprise a DC current of desired characteristics that may be provided to the one or more LEDs 120 that are connectable to an output of the driver 200.
The driver 200 comprises a reference generation means 250 for generating a reference potential V ₁. The reference generation means 250 is provided with connectors for connecting a selectable external component, e.g. a resistor R _set as illustrated in Figure 2. The selectable external component adjusts the operation of the reference generation means 250 such that the reference potential V ₁ is derived in dependence of characteristics of the selectable external component.
The driver 200 comprises a current feedback means 230 for providing the current feedback FB _I. The current feedback means 230 may be arranged to provide the current feedback FB _I in dependence of the reference potential V ₁ such that the current feedback FB _I is indicative of observed level of the output current I _out in relation to a selected nominal current level. The current feedback FB _I may be employed, for example, to operate the driver 200 as a constant current (CC) LED driver that is arranged to supply constant or substantially constant nominal current level as the output current I _out, or to limit the output current I _out below a predetermined maximum value.
The driver 200 further comprises a voltage feedback means 240 for providing the voltage feedback FB _V. The voltage feedback means 240 may be arranged to provide the voltage feedback FB _V in dependence of the reference potential V ₁ such that the voltage feedback FB _V is indicative of the output voltage V _out in relation to a maximum allowable output voltage V _max which, for reasons explained in more detailed below, can be called an adaptive maximum output voltage in the circuit of Figure 2. The voltage feedback FB _V may be employed, for example, to operate the driver 200 such that the output voltage V _out is not allowed to exceed the adaptive maximum output voltage V _max.
The reference potential V ₁ is hence a shared reference applied in both the current feedback means 230 and the voltage feedback means 240 and it is therefore (also) referred to as a common reference potential V ₁. Hence, a single selectable external component, e.g. the resistor R _set illustrated in Figure 2, that is connectable to the driver 200 may be applied to set or select the common reference potential V ₁ to a desired value. This, in turn, causes the current feedback means 230 being adjusted to operate such that the current feedback FB _I reflects a difference to a selectable target current level that depends on the common reference potential V ₁ and the voltage feedback means 240 being adjusted to operate such that the voltage feedback FB _V reflects a difference to an adaptive maximum output voltage V _max that also depends of the common reference potential V ₁. Instead or in addition to reflecting a difference, the current and/or voltage feedback may become active, or assume a particular value or character, when the measured output current or voltage reaches the predefined maximum allowable output current or voltage respectively.
Figure 3 schematically illustrates some components of an exemplifying current feedback means 330 that may be applied as the current feedback means 230 in the driver 200. The current feedback means 330 comprises current-to-voltage conversion means 332, e.g. a current-to-voltage conversion circuit, for deriving a potential V _I indicative of the level of the output current I _out. The current-to-voltage conversion means 332 may be arranged to derive the potential V _I as the voltage across a current sensing element (e.g. a current sensing resistor) arranged in the current path of the output current I _out. The current feedback means 330 further comprises a scaling means 334, e.g. a scaling circuit, for deriving a scaled reference potential V' ₁ on basis of the common reference potential V ₁. The scaling means 334 may be arranged to derive the scaled reference potential V' ₁ by scaling (e.g. multiplying) the common reference potential V ₁ by a predefined scaling factor k ₁. The current feedback means 330 further comprises a comparator 336 for deriving the current feedback FB _I that is indicative of a difference between the scaled reference potential V' ₁ and the potential indicative of the output current I _out level. This difference reflects the difference between the observed output current I _out and the nominal current level that would make the potential V _I equal to the scaled reference potential V' ₁ .
Figure 4 schematically illustrates some components of an exemplifying current feedback means 430 that may be applied as the current feedback means 230 in the driver 200. The current feedback means 430 comprises the current-to-voltage conversion means 332 for deriving a potential V _I indicative of the level of the output current I _out. The current feedback means 430 further comprises a scaling means 434, e.g. a scaling circuit, for deriving a scaled value V' _I of the potential V _I indicative of the level of the output current I _out. The scaling means 434 may be arranged derive the scaled potential V' _I as a combination of the potential V _I and the common reference potential V ₁ or by scaling (e.g. multiplying) the potential V _I by an adaptive scaling factor k ₂ that depends on the common reference potential V ₁. The current feedback means 430 further comprises a comparator 436 for deriving the current feedback FB _I that is indicative of a difference between the scaled potential V' ₁ and a predefined reference potential V _ref, which difference also reflects the difference between the observed output current I _out and the nominal current level that would make the scaled potential V' ₁ equal to the predefined reference potential V _ref. The predefined reference potential V _ref is, preferably, provided internally in the driver 200, e.g. by a DC voltage source arranged to supply the reference potential V _ref at a desired voltage level.
Figure 5 schematically illustrates some components of an exemplifying voltage feedback means 340 that may be applied as the voltage feedback means 240 in the driver 200. The voltage feedback means 340 comprises a scaling means 344, e.g. a scaling circuit, for deriving a scaled potential V' _out of the output voltage V _out. The scaling means 344 may be arranged derive the scaled potential V' _out as a combination of the output voltage V _out and the common reference potential V ₁ or by scaling (e.g. multiplying) the output voltage V _out by an adaptive scaling factor k ₃ that depends on the common reference potential V ₁. The voltage feedback means 340 further comprises a comparator 346 for deriving the voltage feedback FB _V that is indicative of a difference between the scaled output voltage V' _out and the predefined reference potential V _ref. This difference also reflects the difference between the observed output voltage V _out and the adaptive maximum output voltage V _max the occurrence of which at the output would make the scaled output voltage V' _out equal to the predefined reference potential V _ref.
The voltage feedback means 340 may be applied in the driver 200, for example, together with the current feedback means 330 or together with the current feedback means 430. In case a pair of the current feedback means 430 and the voltage feedback means 340 is applied, they may share the voltage source arranged to provide the predefined reference potential V _ref.
Figure 6 schematically illustrates some components of an exemplifying voltage feedback means 440 that may be applied as the voltage feedback means 240 in the driver 200. The voltage feedback means 440 comprises a scaling means 444, e.g. a scaling circuit, for deriving a scaled reference potential V' _ref on basis of the predetermined reference potential V _ref. The scaling means 444 may be arranged derive the scaled reference potential V' _ref as a combination of the reference potential V _ref and the common reference potential V ₁ or by scaling (e.g. multiplying) the reference potential V _ref by an adaptive scaling factor k ₄ that depends on the common reference potential V ₁. The voltage feedback means 440 further comprises a comparator 446 for deriving the voltage feedback FB _V that is indicative of a difference between the scaled reference potential V' _ref and the output voltage V _out, which difference also reflects the difference between the observed output voltage V _out and the adaptive maximum output voltage V _max that would be equal to the scaled reference potential V' _ref.
The voltage feedback means 440 may be applied in the driver 200, for example, together with the current feedback means 330 or together with the current feedback means 430. In case a pair of the current feedback means 430 and the voltage feedback means 440 is applied, they may share the voltage source arranged to provide the predefined reference potential V _ref.
While there a multiple ways for implementing current feedback means 330, 430 and/or the voltage feedback means 340, 440 outlined in the foregoing, Figure 7 schematically illustrates some components of an exemplifying driver 300 comprising a current feedback circuit 530 and a voltage feedback circuit 540. Herein, the current feedback provides an exemplifying implementation of the current feedback means 330, whereas the voltage feedback circuit 540 provides an exemplifying implementation of the voltage feedback means 340.
In the example of Figure 7, the current feedback circuit 530 comprises resistors R₂, R₃, R₄ and R_sense together with the comparator 336, whereas the voltage feedback circuit 540 comprises resistors R₅ and R₆ together with the comparator 346.
In the current feedback circuit 530 the resistors R₃ and R_sense constitute the current-to-voltage conversion means to provide the potential V _I indicative of the level of the output current I _out. Furthermore, the resistors R₂ and R₄ form a voltage divider that constitutes the scaling means 334 and define the scaling factor k ₁ (as k ₁ = R₂ / (R₂ + R₄)) for providing the scaled reference potential V' ₁ on basis of the common reference potential V ₁ as V' ₁ = k ₁ * V ₁. In the voltage feedback circuit 540 the resistors R₅ and R₆ form a voltage divider that constitutes the scaling means 344 and define the scaling for providing the scaled output voltage V' _out as a linear combination of the output voltage V _out and the common reference potential V ₁, e.g. as V' _out = (R₆ / (R₅ + R₆)) * V' _out + (R₅ / (R₅ + R₆)) * V ₁).
In Figure 7 the power source portion 210 is depicted with control means 212 that is arranged to receive the current feedback FB _I and the voltage feedback FB _V and further arranged to adjust operation of the driver 300 according to one or more predefined rules.
Figure 8 schematically illustrates some components of an exemplifying reference generation circuit 350 that may be that may be applied as the reference generation means 250, for example, in the drivers 200 and 300.
The reference generation circuit 350 comprises resistors R₇, R₈, R₉ and R₁₀ together with the voltage regulator IC₁. These components are arranged between an operating voltage source V _cc and a ground potential GND. In particular, resistor R₇ is connected between the operating voltage source V _cc and the cathode of the voltage regulator IC₁; resistor R₈ is connected between the cathode and the control input of the voltage regulator IC₁; and resistor R₉ is connected between the control input of the voltage regulator IC₁ and the ground potential GND, to which also the anode of the voltage regulator IC₁ is connected. Resistor R₁₀ is connected between the control input of the voltage regulator IC₁ and a first coupling node for an external selectable component, and the second coupling node for the external selectable component is connected to the ground potential GND.
The voltage regulator IC₁ is configured to provide a cathode potential on basis of an internal reference voltage V _int of the IC₁ in dependence of a selectable control voltage V _ctrl applied in its control input. The voltage in the cathode of the IC₁ is supplied as the common reference potential V ₁ for use by the current feedback means 230 and the voltage feedback means 240 described in the foregoing.
In particular, the IC₁ may be controlled to provide a desired value of the common reference potential V ₁ on basis of its internal reference voltage V _int such that the common reference potential V ₁ is selected between a minimum value V _{1_min} and a maximum value V _{1_max}. The selectable control voltage V _ctrl that causes selection of the common reference potential V ₁ in the range defined by the values V _{1_min} and V _{1_max} is defined by the output of the voltage divider formed by the resistors R₈, R₉ and R₁₀ of the reference generation circuit 350 and further by the external selectable component that may be coupled to the reference generation circuit 350, e.g. by the resistor R_set as illustrated in Figure 8.
In the exemplifying reference generation circuit 350 the relationship between the common reference potential V ₁ and the external selectable component R_set may be expressed as $V_{1} = V_{int} * (1 + R_{8} * \frac{R_{9} + R_{10} + R_{set}}{R_{9} * (R_{10} + R_{set})})$
Suitable selection of the resistors R₈, R₉ and R₁₀ hence enables providing a predefined mapping between the resistance of the external selectable component, e.g. the resistor R_set, and the resulting common reference potential V ₁.
The minimum value V _{1_min} of the common reference potential V ₁ is obtained by leaving an open current path between the connectors for connecting the selectable external component (thereby providing an infinite resistance), while the maximum value V _{1_max} of the common reference potential V ₁ is obtained by short-circuiting the connectors for connecting the selectable external component (thereby providing a zero resistance). This may be expressed e.g. as $V_{1_\max} = V_{int} * (1 + R_{8} * \frac{R_{9} + R_{10}}{R_{9} * R_{10}})$
$V_{1_\min} = V_{int} * (1 + \frac{R_{8}}{R_{9}})$
When the reference generation circuit 350 is applied in the driver 200 or 300, suitable selection of characteristics of the resistors R₇, R₈, R₉ and R₁₀ may be applied to provide desired value range V _{1_min} to V _{1_max} of the common reference potential V ₁ such that the minimum value V _{1_min} corresponds to the minimum setting for the nominal current level in the output current I _out and that the maximum value V _{1_max} corresponds to the maximum setting for the nominal current level in the output current I _out, whereas the external resistor R_set of suitable characteristics may be applied to select the desired value of the common reference potential V ₁ from this range.
In the following, an exemplifying approach is considered for pre-selecting the resistors R₂ and R₄ in context of the driver 300 such that the current feedback FB _I indicates the difference between the observed output current I _out and the nominal current level that would make the potential V _I equal to the scaled reference potential V'₁. Moreover, an exemplifying approach is considered for selecting the resistors R₅ and R₆ in context of the driver 300 such that the voltage feedback FB _V indicates the difference between the observed output voltage V _out and the adaptive maximum output voltage V _max that corresponds to the common reference potential V ₁.
Regarding the configuration of the current feedback circuit 530, as described in the foregoing, the scaled reference potential V' ₁ provided to the lower input of the comparator 336 (in the illustration of Figure 7) may be obtained as $V'_{1} = V_{1} \cdot \frac{R_{2}}{R_{2} + R_{4}},$
and the potential V _I provided to the upper input or inverting input of the comparator 336 (in the illustration of Figure 7) may be obtained as $V_{I} = I_{out} \cdot R_{sense} .$
In case the driver 300 is employed as a LED driver, the control means 212 may be arranged to cause the power source portion 210 to continuously adjust its operation when the output current I _out flows, such that the current feedback FB _I indicates zero difference or a difference that is as close as possible to zero, which indicates that the measured output current I _out exhibits a level that is at or close to the selected nominal current level. In other words, the control means 212 may be arranged to cause the power source portion 210 to adjust its operation to keep the V' ₁ and V _I equal. This corresponds to setting the right side of equation [4] equal to that of equation [5], which in turn enables deriving the following relationship between the output current I _out and the common reference potential V ₁: $I_{out} = V_{1} \cdot \frac{R_{2}}{R_{sense} \cdot (R_{2} + R_{4})} .$
In other words, nominal current level that is the desired level of the output current I _out is directly proportional to the common reference potential V ₁ in accordance with the resistances of the resistors R₂, R₄ and R_sense.
Regarding operation of the voltage feedback circuit 540, at a given value of the common reference potential V₁ (and hence at a corresponding value of the nominal current level defined according to the equation [6]) the adaptive maximum output voltage V _max is defined according to equation [7] below. The voltage feedback FB _V at the output of the comparator 346 indicates the difference between the observed output voltage V _out and the adaptive maximum output voltage V _max. $V_{out} = V_{ref} \cdot \frac{R_{5} + R_{6}}{R_{6}} - V_{1} \cdot \frac{R_{5}}{R_{6}} .$
In this regard, the control means 212 may be arranged to cause the power source portion 210 to continuously adjust its operation such that the measured output voltage V _out does not exceed the adaptive maximum output voltage V _max. In other words, the control means 212 may be arranged to cause the power source portion 210 to adjust its operation such that the output voltage V _out is limited or reduced in response to the voltage feedback FB _V indicating an observed output voltage V _out that exceeds the adaptive maximum output voltage V _max. This may be provided e.g. by reducing the amount of electric energy delivered to the output of the driver 300, which may mean e.g. decreasing a duty cycle of a cyclically operated switch that controls operation of the power source portion 210 or performing some other action that has the desired effect.
In order to operation of the driver 300 to actually limit the output voltage V _out in accordance with the predefined maximum output power P _max, the following condition must be encountered. $P_{\max} = V_{out} \cdot I_{out} .$
This is illustrated in Figure 9 that depicts as the bold solid line a curve that indicates the maximum allowable output voltage V _out (the y axis) as a function of the output current I _out (the x axis) in order to ensure adhering to the maximum output power P _max. In Figure 9 the output voltage V _max1 represents the adaptive maximum output voltage V _max that corresponds to the maximum setting for the output current I _out1, the output voltage V _max2 represents the adaptive maximum output voltage V _max that corresponds to the minimum setting for the output current I _out2, while the pair of V _x and I _x indicates an exemplifying point between the end points of the range(s) for the output current I _out and the maximum output voltage V _max.. The curve that indicates the P _max as a function of the output current I _out is a hyperbola. However, when a relatively narrow range of output currents I _out is considered, the hyperbola can be approximated at a reasonably good accuracy with a straight line (not shown) that passes via the end-points defined by the coordinates (I _out1, V _max1) and (I _out2, V _max2).
In view of the equation [8] and in consideration of the minimum and maximum values for the adaptive maximum output voltage V _out and the output current I _out, we get $V_{\max 1} \cdot I_{out 1} = V_{\max 2} \cdot I_{out 2} \to \frac{V_{\max 1}}{V_{\max 2}} = \frac{I_{out 2}}{I_{out 1}} .$
When we further denote the ratio between the minimum and maximum output currents I _out as rng we can write $rn g_{I} = \frac{I_{out 2}}{I_{out 1}} = \frac{V {}_{1_\min}}{V_{1_\max}} .$
Using the equation [7] we can rewrite the equation [10] into $rn g_{I} = \frac{V_{ref} \cdot \frac{R_{5} + R_{6}}{R_{6}} - V_{1_\max} \cdot \frac{R_{5}}{R_{6}}}{V_{ref} \cdot \frac{R_{5} + R_{6}}{R_{6}} - V_{1_\min} \cdot \frac{R_{5}}{R_{6}}} = \frac{V_{ref} \cdot \frac{R_{5} + R_{6}}{R_{6}} - \frac{V_{1_\min}}{rn g_{I}} \cdot \frac{R_{5}}{R_{6}}}{V_{ref} \cdot \frac{R_{5} + R_{6}}{R_{6}} - V_{1_\min} \cdot \frac{R_{5}}{R_{6}}}$
and further into $rn g_{I} \cdot (V_{ref} \cdot (1 + \frac{R_{5}}{R_{6}}) - V_{1_\min} \cdot \frac{R_{5}}{R_{6}}) = V_{ref} \cdot (1 + \frac{R_{5}}{R_{6}}) - \frac{V_{1_\min}}{rn g_{I}} \cdot \frac{R_{5}}{R_{6}},$
$V_{1_\min} \cdot \frac{R_{5}}{R_{6}} \cdot (\frac{1}{rn g_{I}^{2}} - 1) = V_{ref} \cdot (\frac{1}{rn g_{I}} - 1) \cdot (1 + \frac{R_{5}}{R_{6}}),$
$V_{1_\min} \cdot \frac{R_{5}}{R_{6}} \cdot (\frac{1}{rn g_{I}} + 1) = V_{ref} \cdot (1 + \frac{R_{5}}{R_{6}}), and$
$\frac{R_{5}}{R_{6}} = \frac{1}{\frac{V_{1_\min}}{V_{ref}} \cdot (\frac{1}{rn g_{I}} + 1) - 1} .$
Based on the equation [7] we can find another way of presenting the ratio of the resistances of the resistors R₅ and R₆: $V_{\max 2} = V_{ref} \cdot \frac{R_{5} + R_{6}}{R_{6}} - V_{1_min} \cdot \frac{R_{5}}{R_{6}} \to \frac{R_{5}}{R_{6}} = \frac{V_{\max 2} - V_{ref}}{V_{ref} - V_{1_min}} .$
Since both the equation [15] and the equation [16] may be applied to calculate the ratio of the resistances of the resistors R₅ and R₆, by setting these two equations equal we get $\frac{1}{\frac{V_{1_\min}}{V_{ref}} \cdot (\frac{1}{rn g_{I}} + 1) - 1} = \frac{V_{\max 2} - V_{ref}}{V_{ref} - V_{1_\min}} .$
The equation [17] enables computing the required predefined reference voltage V _ref as $V_{ref} = \frac{V_{\max 2} \cdot V_{1_\min} \cdot (\frac{1}{rn g_{I}} + 1)}{V_{\max 2} + V_{1_\min} \cdot \frac{1}{rn g_{I}}} = \frac{V_{\max 2} \cdot (V_{1_max} + V_{1_\min})}{(V_{\max 2} + V_{1_max})} .$
Now we can choose suitable values V _{1_min} and V _{1_max} e.g. such that the potential V _{1_min} = 3 V corresponds to the lowest selectable output current I _out2 at 1.05 A and that the potential V _{1_max} = 4 V corresponds to the highest selectable output current I _out1 at 1.4 A. Consequently, assuming that the maximum allowable output power P _max is set to 50 W we find that V _out2 = 50 W / 1.4 A = 35.7 V, which results in the predefined reference voltage $V_{ref} = \frac{V_{\max 2} \cdot (V_{1_max} + V_{1_\min})}{(V_{\max 2} + V_{1_max})} = \frac{35.7 \cdot (3 + 4)}{35.7 + 3} V = 6.46 V$
and in the ratio of the resistances of the resistors R₅ and R₆ $\frac{R_{5}}{R_{6}} = \frac{V_{\max 2} - V_{ref}}{V_{ref} - V_{1_\min}} = 11.88.$
Consequently, the adaptive maximum output voltage V _max set in accordance with the resistors R₅ and R₆ and the reference voltage V _ref in dependence of the common reference potential V ₁ can be arranged to follow the respective (constant or substantially constant) output current I _out at the nominal current level defined by the resistors R₂ and R₄ (see the equation [6]) (also) in dependence of the common reference potential V ₁.
In the following, an exemplifying driver 400 employed as a LED driver is described. The driver 400 is based on the driver 300 and only differences to the driver 300 are described in the following. Figure 10 schematically illustrates some components of the driver 400. In the driver 400 the comparator 336 is embodied as an operational amplifier (op-amp) 536 where the scaled reference potential V' ₁ is provided to the non-inverting input of the op-amp 536 and where the potential V _I indicative of the level of the output current I _out is provided to the inverting input of the op-amp 536. Moreover, the output of the op-amp 536 is fed back to the inverting input of the op-amp 536 via an impedance Z_fb1, producing the familiar inverting amplifier configuration. With such an arrangement the output of the op-amp 536 is configured to go the more negative, the more positive goes the potential in its inverting input.
Along similar lines, in the driver 400 the comparator 346 is embodied as an op-amp 546 where the scaled output voltage V' _out is provided to the inverting input of the op-amp 546 and where the predefined reference potential V _ref is provided to the non-inverting input of the op-amp 546. The output of the op-amp 546 is fed back to the inverting input of the op-amp 546 via an impedance Z_fb2, again producing the inverting amplifier configuration in which the output of the op-amp 546 goes the more negative, the more positive goes the potential in its inverting input.
In the driver 400 a galvanically isolated feedback path to the control means 212 is provided. A current path from a voltage source V _cc to the output of the op-amp 536 includes, in this order, a resistor R₁, a light source of an optoisolator O₁, and a diode D₂. A current path from the voltage source V _cc to the output of the op-amp 546 includes, in this order, the resistor R₁, the light source of the optoisolator O₁, and a diode D₃. The sensor side of the optoisolator O₁ is coupled to the control means 212, which is provided as part of the power source portion 210.
The galvanically isolated feedback path comprising the voltage source V _cc, the resistor R₁, the optoisolator O₁ and the diodes D₂ and D₃ is configured to operate such that an electric current flows from the voltage source V _cc towards the op-amp 536 in response to the current feedback FB _I indicating observed output current I _out that is higher than the selected nominal current level and towards the op-amp 546 in response to the voltage feedback FB _V indicating observed output voltage V _out that is higher than the adaptive maximum output voltage V _max. Consequently, the current flowing through the light source of the optoisolator O₁ invokes an electric current in the sensor side of the optoisolator O₁ and hence provides the control means 212 in the power source portion 210 with an indication that the amount of electric energy delivered to the load should be reduced. The indication thus takes the form of either current feedback FB _I or voltage feedback FB _V.
The arrangement of Figure 10 serves as a non-limiting example of making use of the current feedback FB _I and the voltage feedback FB _V for controlling operation of the driver 200, 300, 400. As an example, the feedbacks FB _I and FB _V may be provided to the control means 212 included in the power source portion 210 (along the lines depicted in Figure 7), and the control means 212 may be configured to adjust the operation of the power source portion 210 according to one or more predefined rules in view of the received feedback FB _I and FB _V. These rules may involve, for example, controlling the power source portion 210 to reduce the amount of electric energy delivered to the load in response to the current feedback FB_I indicating observed output current I _out level that is higher than the nominal current level, and/or in response to the voltage feedback FB _V indicating observed output voltage V _out that is higher than the adaptive maximum output voltage V _max.
At least some of the operations, procedures and/or functions assigned to the structural units described in the context of the driver 200, 300, 400 may be provided as steps of a method. As an example of this regard, Figure 11 illustrates a method 600 for operating the LEDs 120 using a driver apparatus that comprises at least the output for connecting the LEDs and the power source portion 210 for supplying output power to the output of the driver apparatus. The method 600 may be embodied e.g. in the driver apparatus or in a control module or control apparatus arranged control operation of the driver apparatus.
The method 600 comprises generating the common reference potential V ₁ in dependence of a selectable component, e.g. the resistor R _set, that is connectable to the driver apparatus (block 610). The method 600 further comprises providing the current feedback FB _I in dependence of the common reference potential V ₁ such that the current feedback FB _I is indicative of a level of the output current I _out supplied to the output of the driver apparatus in relation to a nominal current level (block 620) and providing the voltage feedback FB _V in dependence of said common reference potential V ₁ such that the voltage feedback FB _V is indicative of the output voltage V _out across nodes of the output of the driver apparatus in relation to the adaptive maximum output voltage V _max (block 630). The method 600 further comprises controlling the power source portion 210 based at least on the current feedback FB _I and the voltage feedback FB _V such that the power supplied to the output of the driver apparatus does not exceed the predefined maximum output power P _max. (block 640).
The method 600 outlined above may be varied in a number of ways, for example in accordance with the examples described in the foregoing in context of the driver 200, 300 and 400. Moreover, the steps of the method 600 may be provided in an order different from that illustrated in Figure 11.
A particular advantage of the embodiments of the invention described above is the ease at which a limitation of maximum output current can be combined with limiting also maximum output power. Simply connecting one selectable external component (like the resistor R_set in the embodiments described above) sets the nominal output current I _out, and simultaneously also sets the adaptive maximum output voltage V _max so that the product of I _out and V _max remains substantially constant. In other words, the higher value is selected for the nominal output current I _out, the lower becomes the adaptive maximum output voltage V _max, and vice versa, within certain range that is determined by internal components of the driver, 200, 300, 400. The interdependence between the output current I _out and output voltage V _out allows dimensioning the power source portion 210 so that it does not need to be able to deliver more electric power to the load than what is obtained as the product of I _out and V _max.
The manufacturer of a driver may provide a dedicated connector that is easily accessible from outside the driver, so that for example during the process of manufacturing a LED luminaire the luminaire manufacturer can install the selectable external component simply by pushing its leads to the dedicated connector. If the LEDs are to be coupled to the driver in the form of a LED module, the selectable external component may also be part of the LED module, in which case it becomes coupled to the driver at the same time when also other connections between the driver and the LED module are made.
The selectable external component does not need to be a resistor, although taken that the reference generation means typically generate a constant reference potential, the use of a resistor as a selectable external component is very straightforward. The selectable external component could be some other kind of passive component; for example, if the reference generation means handles oscillating signals, the selectable external component could be a capacitor or inductor that has a role in defining some resonance frequency, which in turn translates into a reference potential. A zener diode with a particular breakdown voltage can be used as a selectable external component. Also active components can be used as selectable external components, if their role in generating the reference in the reference generation means can be made unambiguous and accurate enough.
The embodiment of Figure 10 was based on the selection that the current feedback FB _I and the voltage feedback FB _V are only used in a limiting sense, so that excessive output current and/or excessive output voltage cause the light source in the optoisolator to light up and trigger the control means 212 to invoke a limiting action. However, a feedback signal from the output (i.e. the output current feedback or output voltage feedback) can also be used in enhancing mode, so that a feedback signal indicating that the measured quantity is smaller than a desired respective target value causes the amount of delivered electric energy to be increased. Such an approach is particularly applicable to the embodiment illustrated in Figure 7, where the control means 212 receives directly and separately the signals FB _I and FB _V.
Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

Claims

A driver (200, 300, 400) for operating one or more light emitting diodes (120), LEDs, the driver (200, 300, 400) comprising,
an output for connecting the one or more LEDs (120),
power source means (210) for supplying power to said output,
reference generation means (250, 350) for generating a common reference potential (V₁) in dependence of a selectable component (R_set) connectable to the driver (200, 300, 400),
first feedback means (230, 330, 430, 530) for providing current feedback (FB_I) in dependence of said common reference potential (V₁), which current feedback (FB_I) is indicative of a level of an output current (lout) supplied to said output in relation to a nominal current level, and
second feedback means (240, 340, 440, 540) for providing voltage feedback (FB_V) in dependence of said common reference potential (V₁), which voltage feedback (FB_V) is indicative of an output voltage (V_out) across nodes of said output in relation to an adaptive maximum output voltage (V_max),
characterized in that the reference generation means (250, 350) is arranged to generate a common reference potential (V₁) that serves to set the nominal current level and the adaptive maximum output voltage (V_max) such that the higher the value selected for the nominal current level the lower the value of the adaptive maximum output voltage and vice versa so that their product remains substantially constant, and that the driver further comprises
control means (212) for controlling the power source means (210) based at least on said current feedback (FB_I) and said voltage feedback (FB_V) such that the power supplied to said output does not exceed a predefined maximum power (P_max).
A driver according to claim 1, wherein the first feedback means (230, 330, 430, 530) is arranged to derive the current feedback (FB_I) as an indication of a difference between a first reference potential (V'₁) and a potential (V_I) indicative of the level of said output current (I_out), wherein said first reference potential (V'₁) is derived in dependence of said common reference potential (V₁).
A driver according to claim 1, wherein the first feedback means (230, 330, 430, 530) is arranged to derive the current feedback (FB_I) as an indication of a difference between a first reference potential (V_ref) and a potential (V'_I) indicative of the level of said output current (I_out), wherein said potential (V'_I) indicative of the level of said output current (lout) is derived in dependence of said common reference potential (V₁).
A driver according to claim 2, wherein said first feedback means (230, 330, 430, 530) is arranged to derive said first reference potential (V'₁) by scaling said common reference potential (V₁) by a predefined scaling factor.
A driver according to claim 3, wherein said first feedback means (230, 330, 430, 530) is arranged to derive said potential (V'_I) indicative of the level of said output current (lout) by combining said common reference potential (V₁) with a voltage (V_I) across an output current sensing element (R_sense).
A driver according to any of claims 1 to 5, wherein the second feedback means (240, 340, 440, 540) is arranged to derive the voltage feedback (FB_V) as an indication of a difference between a predefined reference potential (V_ref1) and a second reference potential (V'_out), wherein said second reference potential (V'_out) is derived in dependence of said common reference potential (V₁) and said output voltage (V_out).
A driver according to any of claims 1 to 5, wherein the second feedback means (240, 340, 440, 540) is arranged to derive the voltage feedback (FB_V) as an indication of a difference between a predefined reference potential (V_ref1) scaled in dependence of said common reference potential (V₁) and a second reference potential indicative of said output voltage (V_out).
A driver according to claim 6, wherein said second feedback means (240, 340, 440, 540) is configured to derive said second reference potential (V'_out) by combining a potential indicative of said output voltage (V_out) with said common reference potential (V₁).
A driver according to claim 7, wherein said second feedback means (240, 340, 440, 540) is configured to scale said predefined reference potential (V_ref1) by combining it with said common reference potential (V₁).
A driver according to any of claims 1 to 9, wherein said reference generation means (250, 350) comprises a voltage regulator (IC₁) arranged to generate the common reference potential (V₁) in dependence of a selectable control potential (V_ctrl) that is selectable in dependence of the selectable component (R_set).
A driver according to claim 10, wherein said control potential (V_ctrl) is defined by an output potential of a voltage divider arrangement, wherein the output potential of the voltage divider arrangement is set in dependence of the selectable component (R_set).
A driver according to any of claims 1 to 11, wherein said control means (212) is arranged to
control the power source means (210), on basis of said current feedback (FB_I), to supply constant or substantially constant output current (lout) at said nominal current level as a response to said voltage feedback (FB_V) indicating an output voltage (V_out) that does not exceed said adaptive maximum output voltage (V_max), and
control the power source means (210) to reduce the amount of electric energy delivered to its output as a response to said voltage feedback (FB_V) indicating an output voltage (V_out) that exceeds said adaptive maximum output voltage (V_out).
A method for operating one or more light emitting diodes (120), LEDs, using a driver (200, 300, 400) comprising an output for connecting the one or more LEDs (120) and a power source means (210) for supplying power to said output, the method comprising
generating a common reference potential (V₁) in dependence of a selectable component (R_set) connectable to the driver (200, 300, 400), providing current feedback (FB_I) in dependence of said common reference potential (V₁), which current feedback (FB_I) is indicative of a level of an output current (lout) supplied to said output in relation to a nominal current level, and
providing voltage feedback (FB_V) in dependence of said common reference potential (V₁), which voltage feedback (FB_V) is indicative of an output voltage (V_out) across nodes of said output in relation to an adaptive maximum output voltage (V_max),
characterized in that generating the common reference potential (V₁) comprises generating a common reference potential (V₁) that serves to set the nominal current level and the adaptive maximum output voltage (V_max) such that the higher the value selected for the nominal current level, the lower the value of the adaptive maximum output voltage and vice versa so that their product remains substantially constant, and that the method comprises controlling the power source means (210) based at least on said current feedback (FB_I) and said voltage feedback (FB_V) such that the power supplied to said output does not exceed a predefined maximum power (P_max).