WO2009044340A2

WO2009044340A2 - Method and circuit arrangement for determining the light output level of a led

Info

Publication number: WO2009044340A2
Application number: PCT/IB2008/053977
Authority: WO
Inventors: Gian Hoogzaad
Original assignee: Nxp B.V.
Priority date: 2007-10-02
Filing date: 2008-09-30
Publication date: 2009-04-09
Also published as: WO2009044340A3

Abstract

The invention provides a method for determining the light output level of a light emitting diode (LED), the LED being operated with a voltage over the LED and a current through the LED. The method uses obtaining (100) an LED input electrical power value (Pin) to the LED, obtaining (102) an a thermal power value (Pth), and determining (104) a LED output power from at least the input electrical power value (Pin) and the thermal power value (Pth). The thermal power value (Pth) is obtained using a temperature difference value between the temperature values measured at two different locations and a thermal resistance value (Rth). The invention also provides a circuit arrangement using the method, a LED driver IC using the circuit arrangement and a lighting system with at least one LED and such a circuit arrangement.

Description

Method and circuit arrangement for determining the light output level of a LED

FIELD OF THE INVENTION

The invention relates to a method for determining the light output level of a light emitting diode (LED). The invention further relates to a circuit arrangement for driving a LED. The invention further relates to a LED driver IC. The invention further relates to a LED lighting system.

BACKGROUND OF THE INVENTION

In lighting systems employing LEDs, it is generally required to know how much light is emitted by the LEDs. This requirement relates to a certain specification on light output like the brightness level of a LED backlight for a LCD monitor, requiring, e.g., a minimum or absolute brightness and/or a maximum brightness variation over the full area of the LED backlight. The requirement may also relate to the desire to have a stable color point from a system comprising a plurality of LEDs with different colours, e.g., the required white point of a LED lamp comprising red, green and blue LEDs. For such a LED lamp, the ratio of light output for the different colours needs to be accurate. However, the light output of a LED is generally not stable, but varies with, e.g., the operating temperature of the LED or the lifetime of the LED. The light output of LEDs may also show variation between individual LEDs of the same type, e.g. due to variations in manufacturing.

A variety of methods is known from the prior art methods to estimate or determine the optical flux output of LEDs. An overview is given in, e.g., P. Deurenberg et al, "Achieving color point stability in RGB multi-chip LED modules using various color control loops", Proc. SPIE 5914, 2005.

The prior art methods can be classified as direct or indirect type methods. Direct type methods use one or more optical sensors with sensor readout hardware and are consequently expensive. The optical sensors are used to measure the light output of one or several LEDs. The optical sensor may be a colour sensor, which is sensitive to only a part of the optical spectrum. The optical sensor may also be a flux sensor, which is sensitive to the whole optical spectrum. One sensor is required for each different colour when colour sensors are used. The colour sensors can be read out simultaneously. When a flux sensor is used, the different colours of the different LEDs are measured time-sequentially in order to discriminate between the contributions of each colour. The colour sensor and flux sensors are generally based on measuring dark current in a reverse-biased photo-diode. The optical sensor measurements may be used in a feed-back control system to keep the light output constant for each colour in an absolute or relative sense.

Systems employing these direct type methods may also use one or more thermal sensors to help compensating second-order effects of temperature-dependent parameters in a feed- forward way. As an example, the light output of a colour LED generally shows some spectral shift with temperature. This spectral shift is not detected by the flux or colour sensors. Using the known dependency of spectral shift as a function of temperature, the effect of these spectral shifts can be compensated for to still achieve a correct white point in a lamp employing red, green and blue LEDs. A direct type method based on feed-back control using one or more optical sensors, and optionally also a temperature sensor, may thus be too expensive. The indirect type methods do not use optical sensors and rely fully on temperature feed-forward colour control algorithms. These methods are significantly less expensive than the optical feed-back methods, but they generally perform less well. These indirect, full feed- forward methods use an assumed dependency of light output as a function of temperature. This dependency is however generally not very constant between different LEDs. Moreover, this dependency may change while the LEDs are ageing as the LED output may vary significantly while the LEDs are ageing, i.e., over the lifetime of the LEDs. As a consequence, an indirect type method based on only feed- forward control may not give satisfactory results, especially not over lifetime.

SUMMARY OF THE INVENTION

The present invention aims to provide a method to determine the optical output of one or more LEDs and which gives a reliable result and incorporates also ageing effects of the LEDs. The invention further aims to provide a circuit arrangement for driving a LED and arranged to apply such a method. The invention further aims to provide to a LED driver IC. The invention further aims to provide a LED lighting system.

For this purpose, the method according to the invention, with the LED being operated with a voltage over the LED and a current through the LED, comprises: obtaining a LED input electrical power value, obtaining a thermal power value, determining a LED output power value from at least the LED input electrical power value and the thermal power value.

Determining the LED output power value may further include an optical out- coupling efficiency.

The invention is based on the insight that LEDs do not or insignificantly, only less than 0.2%, convert input electrical energy into infrared or ultraviolet radiation. Substantially all input electrical energy is converted into heat or light. As a result, the total power that is consumed by the LED can be almost perfectly determined from the sum of two components, the thermal power and the light output power. I.e., when the total power consumed by the LED and the thermal power is known, the light output power can be determined from the difference between the total power and the thermal power. The method according to the invention exploits this relationship.

The optical out-coupling efficiency may be included in determining the LED output power value from at least the LED input electrical power value and the thermal power value, to, e.g., take losses into account of light generated in the LED but not leaving the system in which the LED is incorporated. Light may, e.g., be absorbed in a layer on top of an active layer of the LED, absorbed in an optical structure such as a lens on top of the LED, or absorbed somewhere in a package in which the LED is placed.

In order to obtain the thermal power value, the method may comprise: obtaining the thermal power value from a temperature difference value and a thermal impedance value.

The thermal impedance value is preferably a thermal resistance value.

In order to obtain the temperature difference value, the method may comprise: determining a first reference temperature value at a first location in a first thermal connection to the LED, determining a second reference temperature value at a second location in a second thermal connection to the LED, the second location being spaced apart from the first location, obtaining the temperature difference value from the first reference temperature value and the second reference temperature value. The first reference temperature value may be a temperature value measured at a location away from the LED junction. The first reference temperature value may, e.g., be determined at the position where the LED is soldered to a printed circuit board using, e.g., a thermocouple. This will be referred to as the soldering point temperature. The first reference temperature value could also be determined using another temperature-sensitive element mounted on the printed circuit board. Such a temperature-sensitive element may, e.g., be a resistor with a negative temperature coefficient (NTC) of which the resistance is dependent on the temperature, or a diode element with known temperature dependence e.g. silicon diode, or a diode-connected transistor e.g. a diode-connected bipolar transistor. The second reference temperature value is preferably measured at a location as close as possible to the position where the thermal power is generated, i.e., at the LED junction. It may however also be measured at a second location away from the LED.

The thermal resistance value used for obtaining the thermal power value from the temperature difference value and the thermal impedance value may relate to the thermal resistance between the first location and the second location.

In order to determine the first reference temperature value at the first location, the method may comprise: reading a readout value from a first temperature-sensitive element positioned at the first location, determining the first reference temperature value from the readout value from the first temperature-sensitive element.

The first temperature-sensitive element may, e.g., be a NTC, a diode or a thermocouple. Determining the first reference temperature value from the readout value from the first temperature-sensitive element may be done using, e.g., a lookup table (LUT) or a pre-determined function to translate the readout value to a temperature value.

In order to determine the second reference temperature value at the second location, the method may comprise: obtaining a LED forward voltage value, corresponding to a forward voltage over the LED, determining the second reference temperature value from the LED forward voltage value. Determining the second reference temperature value from the LED forward voltage value may further use a reference forward voltage value. The reference forward voltage value may be a predetermined value, e.g., obtained from a calibration measurement. A forward voltage difference value may be determined from the difference between the LED forward voltage value and the reference forward voltage value.

Determining the second reference temperature value from the LED forward voltage value and a reference forward voltage value may be done using, e.g., another lookup table (LUT) or a predetermined relation between the second reference temperature value and the LED forward voltage difference value. The dependency between the forward voltage difference on the temperature can be known from, e.g., the data sheet of the LED, or be obtained from a (off-line) measurement. The forward voltage variation as a function of temperature is typically - 2mV/K.

The second reference temperature thus derived thus relates to the temperature at the location of the LED junction.

In order to determine the second reference temperature value at the second location, the method may alternatively comprise: reading a readout value from a second temperature-sensitive element positioned at the second location, determining the second reference temperature from the readout value from the second temperature-sensitive element.

The second temperature-sensitive element may, e.g., be a second NTC, a diode or a thermocouple. Determining the second reference temperature value from the readout value from the second temperature-sensitive element may be done using, e.g., a further lookup table (LUT) or a further pre-determined function to translate the readout value to a temperature value.

The second location where the second temperature-sensitive element is positioned may be different from the location of the LED and the location of the LED junction.

In order to obtain the LED input electrical power value, the method may comprise: obtaining a LED voltage value, corresponding to the value of the voltage over the LED, obtaining a LED current value, corresponding to the value of the current through the LED, - obtaining the LED input electrical power value from a product of at least the

LED voltage value and the LED current value.

If the LED is operated with a LED duty cycle of the current through the LED in stead of continuously, the method may further comprise: obtaining a LED duty cycle value, corresponding to the value of the duty cycle of the current through the LED, and in obtaining the LED input electrical power value, the product of at least the LED voltage value and the LED current value may further include the LED duty cycle.

In order to obtain the LED voltage value, the method may comprise: measuring the voltage over the LED while the LED is switched on. In order to obtain the LED current value, the method may comprise: measuring the current through the LED while the LED is switched on, or using a current set-point value of a circuit supplying the current through the LED.

A value for the current set-point value may be obtained from a calibration of the circuit supplying the current through the LED while the LED is switched on, and stored in and retrieved from a memory.

In order to determining the LED output power, the method may comprise filtering of at least one of the LED voltage value, the LED current value, the LED input electrical power value, the first reference temperature value, the second reference temperature value, the temperature difference value and the thermal power value over a time period.

Filtering improves the accuracy of the corresponding value and thus improves the accuracy of the determined LED output power.

The method may further comprise adjusting the input electrical power to the

LED, in order to control the LED output power.

In order to adjust the input electrical power to the LED, the method may comprise adjusting at least one selected from the group of a duty cycle of the current through the LED and a magnitude of the current through the LED. The adjustment of the LED input electrical power may be carried out in comparison to a reference light output level value.

E.g., if the LED output power value is larger than a required reference light output level value, the duty cycle of the current through the LED is reduced, or the magnitude of the current through the LED is reduced.

The reference light output level value may be determined from a light output level value of at least one other LED, in order to balance the light output level between the LED and the at least one other LED. E.g., in a LED lighting system with a red LED, a green LED and a blue LED, the reference light output level value may be the light output level value corresponding to the LED output power value of one of the red, green and blue LEDs, and the ratio of the LED output power values of the red LED and the green LED may be kept constant.

E.g., in a LED lighting system with a first LED of one colour and a second LED of the same colour which are desired to have the same light output for a uniform light distribution, the reference light output level value may be the light output level value corresponding to the LED output power value of the second LED, and the ratio of the LED output power values of the first LED and the second LED may be kept constant.

Alternatively, the reference light output level value may, e.g., be determined from a pre-determined absolute light output level value.

E.g., the LED output power may be controlled so as to obtain a constant light output level at the pre-determined absolute light output level value, as, e.g., specified for an application. As an example, the LED output power of LED in a backlight for a LCD monitor may be kept constant so as to achieve a constant illumination of the LCD panel, such that a constant monitor brightness of, e.g., 300 cd/m² is obtained.

The circuit arrangement according to the invention comprises: output terminals arranged to electrically connect to the LED, - a supply unit arranged to supply the LED, when electrically connected, via the output terminals with a voltage over the LED and a current through the LED, a power detector circuit being connected to the supply unit and arranged for: -obtaining a LED input electrical power value, -obtaining a thermal power value, and a power processor unit being connected to the power detector circuit and arranged for:

-receiving the LED input electrical power value and the thermal power value from the power detector circuit, and determining a LED output power value from the received input electrical power value and thermal power value.

The circuit arrangement is thus equipped to operate the LED with a voltage, a current and to determine the LED output power. The LED may further be operated with a duty cycle of the current through the LED. For determining the LED output power value, an optical out-coupling efficiency may also be taken into account.

The circuit arrangement may further comprise: a temperature detector circuit arranged for: - determining a first reference temperature value at a first location in a first thermal connection to the LED,

- determining a second reference temperature value at a second location a second thermal connection to the LED, the second location being spaced apart from the first location, and obtaining a temperature difference value from the first reference temperature value and the second reference temperature value, wherein the power detector circuit is connected to the temperature detector circuit and is further arranged for: receiving the temperature difference value from the temperature detector circuit, and determining the thermal power value from the received temperature difference value and a thermal impedance value from the received temperature difference value.

The first location may, e.g., be a location on a PCB (printed circuit board) whereon the LED is mounted, and the second location may, e.g., be another location on the PCB whereon the LED is mounted.

In an embodiment of the circuit arrangement, the temperature detector circuit may be arranged for communicating with a first temperature-sensitive element positioned at the first location, wherein the temperature detector circuit is further arranged for acquiring a readout value from the first temperature-sensitive element positioned at the first location, and wherein the first reference temperature is determined from the acquired readout value from the first temperature-sensitive element.

The first temperature-sensitive element may be incorporated in the circuit arrangement itself, but may also be positioned outside it. The first temperature-sensitive element may be a dedicated component, such as an NTC or a thermocouple, or a functional unit in another electronic component or another circuit arrangement.

Determining the first reference temperature value from the readout value from the first temperature-sensitive element may be performed, e.g., by reading from a lookup table the temperature corresponding to a readout value of the NTC, or by calculating from a pre-determined function the temperature corresponding to a readout value of the NTC.

In an embodiment of the circuit arrangement, the temperature detector circuit may be further arranged for obtaining a LED forward voltage value, corresponding to a forward voltage over the LED, and wherein the second reference temperature is determined from the forward voltage value.

The forward voltage value relates to the voltage over a LED junction and varies with a junction temperature of the LED junction. The junction temperature can be used as the second reference temperature at the location of the LED junction. Determining the second reference temperature value from the LED forward voltage value may further use a reference forward voltage value. A forward voltage difference value may be determined from the difference between the LED forward voltage value and the reference forward voltage value.

Determining the second reference temperature value from the LED forward voltage value and a reference forward voltage value may be done using, e.g., another lookup table (LUT) or a predetermined relation between the second reference temperature value and the LED forward voltage difference value.

The dependency between the forward voltage difference on the temperature can be known from, e.g., the data sheet of the LED, or be obtained from a (off-line) measurement. The forward voltage variation as a function of temperature is typically -2mV/K.

In an alternative embodiment of the circuit arrangement, the temperature detector circuit may be arranged for communicating with a second temperature-sensitive element positioned at the second location, wherein the temperature detector circuit is further arranged for acquiring a readout value from the second temperature-sensitive element positioned at the second location, and wherein the second reference temperature value is determined from the acquired readout value from the second temperature-sensitive element. The second temperature-sensitive element may, e.g., be a second NTC.

As described above, the second location may, e.g., be the location where the LED is positioned, or another location on the printed circuit board on which the LED is mounted.

Determining the second reference temperature value from the readout value from the second temperature-sensitive element may be done using, e.g., a further lookup table (LUT) or a further pre-determined function to translate the readout value to a temperature value.

In order to obtain the LED input electrical power value, the power detector circuit may be further arranged for: obtaining a LED voltage value, corresponding to the value of the voltage over the LED, and obtaining a LED current value, corresponding to the value of the current through the LED, wherein the LED input electrical power value may be obtained from a product of at least the obtained LED voltage value and LED current value.

The LED voltage value may relate to the forward voltage value, but may include the value of a further voltage over a component in the LED which also causes a power dissipation in the LED, e.g., a resistive loss in a layer in the LED construction.

The power detector circuit may be further arranged for obtaining a LED duty cycle value, corresponding to the value of a duty cycle of the current through the LED, wherein the LED input electrical power value may be obtained from a product of at least the obtained LED voltage value, LED current value and LED duty cycle value.

The circuit arrangement may further comprise a controller, the controller being connected to the supply unit and the power processor unit, wherein the controller is further arranged for: receiving the LED output power value, and adjusting the LED input electrical power in dependence on the received LED output power value.

The controller can thus control the supply unit so as to obtain a required LED output power to obtain, e.g., a required absolute light level or a required balance in LED output power between different LEDs.

In order to adjust the input electrical power to the LED, the controller may be further arranged for: adjusting at least one LED supply parameter value selected from the group of a duty cycle value of the current through the LED and a magnitude value of the current through the LED, and providing the at least one adjusted LED supply parameter value to the supply unit.

Adjusting the duty cycle of the current through the LED or the magnitude of the current through the LED provide alternative ways to adjust the LED input electrical power and the LED output power.

The circuit arrangement may comprise an analogue circuit section, wherein the analogue circuit section is arranged for at least one of: generating the voltage that is supplied to the LED via the output terminals, - generating the current that is supplied to the LED via the output terminals, measuring the LED forward voltage value, measuring the LED current value, generating a bias signal for a temperature-sensitive element, acquiring the readout value from a temperature-sensitive element. The circuit arrangement may further comprise a digital circuit section, wherein the analogue section and the digital section are connected with at least one analogue- to-digital converter, and wherein the digital section comprises a programmable processor.

The analogue circuit section and the digital circuit section cooperate to operate the LED and/or to measure operating conditions of the LED, e.g., the LED forward voltage value or the LED current value and/or to determine the first reference temperature value and the second reference temperature value.

The digital circuit section may, e.g., provide a LED current setting for the analogue circuit section, the analogue circuit section may generate the LED current and supply the LED current to the LED via the output terminals. The analogue section may sense the LED forward voltage while the LED is being operated, a analogue-to-digital converter may convert the sensed value to a digital value, which the digital circuit section may use to calculate, e.g., the input electrical power to the LED from the product of the digital value of the sensed LED forward voltage value, the LED current value and, when the LED is operated with a duty cycle, the duty cycle. The digital circuit section may also receive temperature related parameters from the analogue part. Based on these and further measurements and/or settings, the digital section then determines the LED output power with one of the methods described above.

The functional hardware block, the programmable hardware block and the software processor may be equipped for carrying out, e.g., arithmetic functions, lookup table operations, control functions and interfacing functions.

The LED driver IC according to the invention comprises any one of the circuit arrangements described above.

The computer program according to the invention is arranged to be loaded in a processor and after being loaded allowing the processor to perform any one of the methods described above.

The computer program may be provided on a medium readable by a processor, the processor being associated with a circuit arrangement arranged for driving a light emitting diode, the computer program being arranged to be loaded in the processor and after being loaded allowing the processor to perform the method for determining the light output level of the LED according to any one of the method described above.

The circuit arrangement may be according to any one of the circuit arrangements described above. The processor may be incorporated in the circuit arrangement described above. The processor may alternatively be external to the circuit arrangement and cooperate with the circuit arrangement.

The invention also relates to a computer-readable medium. The computer- readable medium according to the invention is provided with the computer program described above.

A further embodiment of the invention relates to a LED lighting system comprising a LED and one of the circuit arrangements described above. The LED lighting system may be a brightness controlled LED-lamp, a color- variable LED lamp, a LED matrix light source, a LED matrix display, a large-sized LED information display for advertisement or moving images, a LED-backlight for a LCD-TV, a LED-backlight for a LCD-monitor, or any other lighting system in which the light output power of at least one LED is to be determined and optionally controlled, in accordance with aspects of the present invention as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the invention will be further elucidated and described in detail with reference to the drawings, in which corresponding reference symbols indicate corresponding parts:

Fig. 1 schematically shows a block diagram of an embodiment of a method according to the invention; Fig. 2 shows a block diagram of an embodiment of a first element of the method according to the invention;

Fig. 3 shows a block diagram of an embodiment of a second element of the method according to the invention;

Fig. 4a schematically shows a placement with a LED; Fig. 4b schematically shows am alternative placement with a LED;

Fig. 5 shows a block diagram of an embodiment of a third element of the method according to the invention;

Fig. 6 shows a block diagram of an embodiment of a fourth element of the method according to the invention; Fig. 7 shows a block diagram of an alternative embodiment of the fourth element of the method according to the invention;

Fig. 8 shows a block diagram of a further embodiment of a method according to the invention;

Fig. 9 schematically shows an embodiment of a LED driver IC comprising a circuit arrangement according to the invention;

Fig. 10 schematically shows an alternative embodiment of a LED driver IC comprising a circuit arrangement according to the invention;

Fig. 11 schematically shows a further circuit arrangement according to the invention; Fig. 12 shows an embodiment of a LED lighting system according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS Fig. 1 shows a block diagram of an embodiment of a method according to the invention. Upon driving 500 of a LED, an input electrical power value Pin is obtained 100 and a thermal power value Pth is obtained 102. A LED output power value Pflux is obtained 104 from the input electrical power value Pin and the thermal power value Pth, in the example shown by a subtraction of Pth from Pin:

Pflux = Pin - Pth (1)

The LED output power value Pflux is optionally analyzed and new drive parameters, e.g. a new current level or a new duty cycle value PWM, are generated 502. The generation 502 of the new drive parameters may be based on a reference level ref.level. The new drive parameters I, PWM may then be used for driving 500 of the LED.

Fig. 2 shows a block diagram of an exemplary embodiment of obtaining 100 of the input electrical power value Pin. Obtaining 100 of the input electrical power value Pin is implemented by first obtaining 400 a voltage value VLED corresponding to the voltage over the LED, obtaining 402 a current value ILED corresponding to the current through the LED and obtaining 404 a duty cycle value PWM corresponding to the duty cycle of the current through the LED. The LED may be driven with a variable duty cycle in order to change the effective brightness of the LED, while driving the LED with a fixed current level. Alternatively, the LED may be driven with a variable current level cycle in order to change the effective brightness of the LED and a fixed duty cycle, typically a duty cycle of 100% to have a continuous current through the LED.

The input electrical power value Pin is then obtained from the voltage value VLED, the current value ILED and the duty cycle value PWM by, e.g., a multiplication for obtaining 406 the input electrical power value Pin:

Pin = VLED * ILED * PWM (2) Fig. 3 shows a block diagram of an exemplary embodiment of obtaining 102 of the thermal power value Pth.

The obtaining 102 of the thermal power value Pth is implemented by first determining 200 a first temperature value Tl at a first location and determining 202 a second temperature value T2 at a second location. The first location 210 may be a location on, e.g., a printed circuit board 230 on which also the LED 1 is mounted, away from the LED 1 as shown in Figs. 4a and 4b. The second location 220, 221 may be a location on, e.g., the printed circuit board 230 on which also the LED 1 is mounted, away from the LED 1, at a close distance from the LED 1 , substantially at the same position as the LED 1 , or in the LED 1 itself, e.g., a position of a junction of the LED. Examples of alternative locations are shown in Figs. 4a and 4b. Then, a temperature difference value ΔT is obtained from the first temperature value Tl and the second temperature value T2 by a subtraction for determining 204 the temperature difference value ΔT:

- ΔT = T1 - T2 (3)

The thermal power value Pth is then obtained from the temperature difference value ΔT by a thermal modelling block 206. In the example shown, with a single LED, this thermal modelling block 206 is implemented by relating the thermal power value Pth to the temperature difference value ΔT and a thermal resistance value Rth. In the example shown, the thermal resistance Rth relates to the thermal resistance between the first location 210 and the second location 220, 221 and the thermal modelling block 206 relates the thermal power value Pth to the temperature difference value ΔT and the thermal resistance value Rth using of a division of the temperature difference value ΔT by the thermal resistance value Rth:

Pth = ( 1 / Rth ) * ΔT (4)

Fig. 5 shows an embodiment of a first element of the method according to the invention, i.e., of determining 200 the first temperature value Tl at the first location 210. The first location 210 is chosen to be thermally coupled to the LED 1, preferably with a small thermal influence of external influences, like e.g. air flow, compared to the thermal influence of the LED 1. The first location 210 is a location on, e.g., the printed circuit board 230 on which also the LED 1 is mounted, away from the LED 1 as shown in Fig. 4a and 4b. First, a read-out value of a temperature sensitive element is read 300. Such a temperature sensitive element is, e.g., a resistor with a negative temperature coefficient (NTC). Such an NTC acts as a temperature sensitive resistor. The NTC value can, e.g., be read either by measuring a voltage over the NTC when a known, typically fixed, current is applied through it, or alternatively by measuring the current through the NTC when a known, typically fixed, voltage is applied. The reading may comprise a single readout value, but preferably comprises reading a plurality of values over a short time period and filtering them. The short time period shall be shorter than the typical period with which the effective light output of the LED is changed. The first temperature value Tl is then determined 302 by obtaining a temperature value from a lookup table, relating NTC value to a temperature value. Alternatively, the first temperature value Tl could be obtained from e.g. a function which parametrizes the relation between the NTC value and the temperature value.

Fig. 6 shows an embodiment of a second element of the method according to the invention, i.e., of determining 202 the second temperature value T2 at the second location 221. The second location 221 may be a location of the junction of the LED 1, the LED 1 being mounted on the printed circuit board 230, as shown in Fig. 4b. The value of the temperature of the junction of the LED (in short, the junction temperature value) is used for the second temperature value T2. First, the forward voltage value VF over the LED is obtained 310 from a measurement of the voltage over the LED while the LED is turned on. Then, the junction temperature value of the LED is obtained 312 from a pre-determined relation between the LED forward voltage value VF and the LED junction temperature value.

The LED forward voltage value typically varies with -2 mV/K with the LED junction temperature, but is preferably obtained from a calibration measurement of the LED being used. The pre-determined relation between the LED forward voltage value and the

LED junction temperature value uses in this example the difference between LED forward voltage value and a reference forward voltage value corresponding to a reference junction temperature value. The reference forward voltage value and the reference junction temperature value may, e.g., have been obtained from a calibration measurement, stored in a memory and retrieved from the memory for use. The LED junction temperature is then obtained by dividing the difference between LED forward voltage value and the reference forward voltage value by the mentioned -2 mV/K, and adding the result to the reference junction temperature value. Fig. 7 shows an alternative embodiment of the second element of the method according to the invention, i.e., of determining 202 the second temperature value T2 at the second location 220, 221. In this embodiment, the second location 220, 221 is a location on, e.g., the printed circuit board 230 on which also the LED 1 is mounted, at a, preferably relatively, close distance from the LED 1. The second location 220, 221 is different from the first location 210, as shown in Fig. 4a. The second location 220, 221 is chosen to be thermally coupled to the LED 1 and thermally coupled to the first location 210, with a small thermal influence of external influences, like e.g. air flow, compared to the thermal influence of the LED 1. The second location 221 may also refer to the same location as the position of the LED 1 itself, e.g., a point where the LED is soldered to the board, the so-called soldering point. It functions similarly to the method described in reference to Fig. 5, by reading 320 a second NTC value of an NTC placed closely to the LED and determining 322 the second temperature value T2 from the NTC value using a lookup table or a function.

Fig. 8 shows a block diagram of an embodiment of a further method according to the invention. Upon driving 500 of an plurality of LEDs, i.e., driving 500R a red LED, driving 500G a green LED and driving 500B a blue LED, input electrical power values PinR, PinG and PinB of the red, green and blue LEDs respectively are obtained 100, 10OR, 10OG, IOOB and thermal power values PthR, PthG and PthB of the red, green and blue LEDs respectively are obtained 102. LED output power values PfluxR, PfluxG and PfluxB are obtained 104 from the LED input electrical power values PinR, PinG, PinB and the thermal power values PthR, PthG and PthB, in the example shown, by a subtraction 104R of PthR from PinR, a subtraction 104G of PthG from PinG and a subtraction 104B of PthB from PinB for the red, green and blue LEDs respectively. The LED output power values PfluxR, PfluxG and PfluxB are optionally analyzed and new drive parameters are generated 502, e.g., in order to keep the balance between the colors at a required level and/or the absolute light level at a required level, for example using algorithms like the algorithms described in the Deurenberg publication discussed above. The new drive parameters are used for driving 500R, 500G, 500B of the LEDs. The obtaining 100 of each of the input electrical power values, PinR, PinG and

PinB is implemented as a multiplication of the voltage value VLED over the LED, the current value ILED through the LED and the duty cycle value PWM of driving the LED, for each of the red, the green and the blue LEDs. In this example, the obtaining 102 of the thermal power values PthR, PhtG and PthB is implemented by first determining 200 a first temperature value Tl and determining 202R, 202G, 202B second temperature values T2R, T2G and T2B in relation to the red, green and blue LEDs respectively. The first temperature value Tl is measured at a common first location, e.g., by using a temperature-sensitive element at a central position on a printed circuit board. The second temperature values T2R, T2G and T2B are measured at different locations, e.g., at the junctions of the red, green and blue LEDs respectively, by using a method as described above in reference to Fig. 6. The second temperature values T2R, T2G and T2B may alternatively be measured at, e.g., different locations relatively close to the red, green and blue LEDs compared to the distance from the first location. Then, temperature difference values ΔTR, ΔTG, ΔTB relating to the red, green and blue LEDs respectively are obtained from the first temperature value Tl and each of the second temperature values T2R, T2G and T2B by subtractions 204R, 205 G, 204B. The thermal power values PthR, PthG and PthB are then obtained from the temperature difference values ΔTR, ΔTG, ΔTB by a thermal modelling block 206. The thermal modelling block 206 takes the thermal influence of each of the red, green and blue LEDs on the common first location into account. In the example shown, with a plurality of LEDs, this thermal modelling block 206 is implemented by relating the thermal power values, expressed as a vector vec(Pth) = (PthR, PthG, PthB)^T, to the temperature difference values, expressed as a vector vec(ΔT) = (ΔTR, ΔTG, ΔTB)^T, via a thermal resistance value matrix M(I /Rth):

vec(Pth) = M(l/Rth) *vec(ΔT) (5)

where the dimension of the coefficients in the thermal resistance value matrix M(I /Rth) is that of inverse thermal resistance.

The thermal resistance value matrix M(l/Rth) may, e.g., have been derived from a thermal model of the LEDs, their positions, the positions of the common first location and the second locations, and the thermal behaviour of the physical structure between those positions. Alternatively, the thermal resistance value matrix M(I /Rth) may, e.g., have been derived from a calibration measurement. The calibration measurement may, e.g., comprise the steps of applying a plurality of several thermal powers to the plurality of LEDs, measuring the resulting temperature difference values, and determining the coefficients of the thermal resistance value matrix M(I /Rth). The present invention is not limited in this respect. Other methods to obtain the thermal resistance value matrix M(I /Rth) may also be used.

Fig. 9 schematically shows an embodiment of a LED driver IC 1000 comprising a circuit arrangement according to the invention. The LED driver IC 1000 is connected to a LED 1 via output terminals 11, 12. The LED driver IC 1000 is also connected to a NTC 2 via NTC terminals 21, 22. The LED driver IC 1000 has an analogue section 1001 and a digital section 1002. Other sections may be present on the IC for other functions.

The analogue section has a LED drive unit 1006, generating a LED voltage, a LED current and a duty cycle. The LED voltage is also measured via a first analogue-to- digital (AJO) converter 13. The digital section 1002 obtains the digitized LED voltage measurement 4000 from the A/D converter 13 . The digital section also obtains 4020 a measurement value 4020 of the LED current ILED. The LED current value 4020 is multiplied with the duty cycle value PWM by a first multiplier 4061 to obtain an average LED current value. The average LED current value is multiplied with the LED voltage value 4000 by a second multiplier 4062 to obtain the LED input power value Pin of the LED 1.

The analogue section also generates a bias voltage Vbias for the NTC 2 positioned at a first location 210. The NTC current is measured via a second analogue-to- digital (A/D) converter 23. The digital section 1002 obtains the digitized NTC reading 3000 and obtains the NTC temperature value Tb from a first lookup-table 3020. In the example shown, the look-up table is implemented in the LED driver IC, but it may also be implemented in an external memory device. Alternatively, the temperature value Tb may be obtained from a predetermined function, defining the temperature value Tb as a function of the NTC reading. The digital section 1002 also obtains the LED voltage value, i.e., the LED forward voltage value 3100, of the LED 1 positioned at a location 221. The junction temperature value Tj of the LED is obtained from this LED forward voltage value from a second lookup-table 3120.

The NTC temperature value Tb and the junction temperature value Tj are passed to a arithmetic unit 2040 to obtain a temperature difference value ΔT = Tj - Tb. A second arithmetic unit 2060 determines the thermal power value Pth from the temperature difference value ΔT, by dividing the temperature difference value ΔT by a thermal resistance value Rth:

Pth = ΔT / Rth (6) A third arithmetic unit 1040 subtracts the thermal power value Pth from the LED input power value Pin. The result is multiplied with an optical output efficiency η in a third multiplier 1042, in order to obtain the LED output power value Pflux. The optical output efficiency η accounts for losses due to light being absorbed in, e.g., layers on top of the LED structure, or light not being captured by an optical out-coupling structure on top of the LED.

Fig. 10 shows schematically shows an alternative embodiment of a LED driver IC 1000. The LED driver IC 1000 is again connected to a LED 1 via output terminals 11, 12. The LED driver IC 1000 is also connected to a first NTC 2 via terminals 21, 22 and a second NTC 3 via terminals 31, 32. The LED driver IC 1000 has an analogue section 1001 and a digital section 1002.

The LED driver 1000 has a LED drive unit 1006 as described with reference to Fig. 9, and obtains the LED input power value Pin with the first A/D converter 13, a first multiplier 4061 and a second multiplier 4062 from the LED voltage value 4000, the LED current value 4020 and the duty cycle value PWM, in a similar manner as described above.

A first NTC temperature value TbI, corresponding to the value of a temperature of a first location 210 where the first NTC is positioned, is obtained using a second A/D converter 23 digitizing the NTC readout signal in a first NTC readout value 3000 and a first lookup table 3020. A second NTC temperature value Tb2, corresponding to the value of a temperature of a second location 220 where the second NTC is positioned, is obtained using a third A/D converter 33 digitizing the NTC readout signal in a second NTC readout value 3200 and a third lookup table 3220. The first NTC temperature values TbI and the second NTC temperature value Tb2 are passed to a arithmetic unit 2040 to obtain a temperature difference value ΔT = Tb2 - TbI . A second arithmetic unit 2060 determines the thermal power value Pth from the temperature difference value ΔT, by dividing the temperature difference value ΔT by a thermal resistance value Rth: Pth = ΔT / Rth.

A third arithmetic unit 1040 subtracts the thermal power value Pth from the LED input power value Pin. The result is multiplied with an optical output efficiency η in a third multiplier 1042, in order to obtain the LED output power value Pflux.

Fig. 11 shows a further circuit arrangement according to the invention, comprising a LED driver IC 1000, a red LED IR, a green LED IG, a blue LED IB, an NTC 2, and a controller IC 2000. The LED driver IC is connected to the red LED IR, the green LED IG and the blue LED IB, and drives the LEDs each with a current, a voltage and a duty cycle. The LED driver IC determines the LED output power values of each of the individual LEDs, PfluxR, PfluxG and PfluxB for the red, green and blue LED respectively, using the method described in reference with Fig. 8, and functional blocks similar to those described in reference with Figs. 9 and 7.

The LED driver IC 1000 is further connected to the controller IC 2000 via signal lines. The LED output power values PfluxR, PfluxG and PfluxB are received by the controller IC 2000 via the signal lines. The controller IC 2000 analyzes the LED output power values and generates new duty cycle values PWMR, PWMG, PWMB and/or new current level values ILEDR, ILEDG, ILEDG for the red, green and blue LEDs respectively, in order to achieve a required color point and/or light level when the LEDs are driven with these new parameters. The new duty cycle values and new current level values are received by the LED driver IC 1000 via the signal lines.

The LED driver IC 1000 as well as the controller IC 2000 may comprise one or more programmable processors 1002, 2002, such as a microcontroller, general-purpose CPU, DSP, FPGA or any other programmable processor, with a memory. The memory may, e.g., be a memory Ml, M2 integrated in the LED driver IC 1000 and the controller IC 2000 respectively, or in a separate memory device M3, M4 connected to the LED driver IC 1000 and the controller IC 2000 respectively. A computer program product arranged to perform any of the methods described above may be loaded in the programmable processor, e.g., via an interface connection connectable, directly or via intermediate units, to the programmable processor or to the memory of the programmable processor. The computer program product may be read from a computer-readable medium, e.g., a solid state memory such as a flash memory, EEPROM, RAM, an optical disk 3002 loaded in an optical disk drive 3000, a hard disk drive (HDD), or any other computer-readable medium. The computer-readable medium may be read by a dedicated unit, such as the optical disk drive to read the optical disk, directly by the programmable processor, such as a EEPROM M3 connected to the programmable processor 1002, or via other intermediate units.

Fig. 12 shows an example of a light source 5000 with a LED assembly 4000 in a housing 5001. The housing 5001 is a box with, preferably, reflective inner walls. The LED assembly 4000 comprises one or more LEDs and a circuit arrangement employing, during use, one of the methods described above. The light generated by the LED assembly 4000 is reflected towards the front of the housing 5001, which is covered with a diffusive transparent plate 5002. The light source 5000 carries a power adapter 5010, which supplies the LED assembly 4000 from a power converter, connected to the mains via a power cord 5011 with a power connecter 5012, to fit a wall contact (not shown) with mains supply.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments are conceivable without departing from the scope of the appended claims. E.g., LEDs with other colours than red, green and blue can be used, such as amber LEDs or white LEDs, or, e.g., the reference temperature can be measured at another location in the system than the locations explicitly described, or, e.g., a thermal conductance may be used where a thermal resistance was mentioned, or, e.g., the controller IC may also be incorporated in the LED driver IC itself, without departing from the scope of the invention and the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim.

Claims

CLAIMS:

1. A method for determining the light output level of a light emitting diode (LED), the LED being operated with a voltage over the LED and a current through the LED, the method comprising: obtaining (100) an LED input electrical power value (Pin) to the LED, - obtaining (102) a thermal power value (Pth), determining (104) a LED output power (Pflux) from at least the input electrical power value (Pin) and the thermal power value (Pth).

2. A method according to claim 1, wherein, in order to obtain the thermal power value (Pth), the method comprises: obtaining (206) the thermal power value (Pth) from a temperature difference value (ΔT) and a thermal impedance value (Rth).

3. A method according to claim 2, wherein, in order to obtain the temperature difference value (ΔT), the method comprises: determining (200) a first reference temperature value (Tl) at a first location (210) in a first thermal connection to the LED, determining (202) a second reference temperature value (T2) at a second location (220; 221) in a second thermal connection to the LED, the second location (220; 221) being spaced apart from the first location (210), obtaining (204) the temperature difference value (ΔT) from the first reference temperature value (Tl) and the second reference temperature value (T2).

4. A method according to claim 3, wherein, in order to determine the first reference temperature value (Tl) at the first location (210), the method comprises: reading (300) a readout value (3000) from a first temperature-sensitive element (2) positioned at the first location (210), determining (302) the first reference temperature value (Tl) from the readout value (3000) from the first temperature-sensitive element (2).

5. A method according to claim 3 or 4, wherein, in order to determine the second reference temperature value (T2) at the second location (221), the method comprises: obtaining (310) a LED forward voltage value (3100), corresponding to a forward voltage over the LED, determining (312) the second reference temperature value (T2) from the forward voltage value (3100).

6. A method according to claim 3 or 4, wherein, in order to determine the second reference temperature value (T2) at the second location (220; 221), the method comprises: reading (320) a readout value (3200) from a second temperature-sensitive element (3) positioned at the second location (220; 221), determining (322) the second reference temperature value (T2) from the readout value (3200) from the second temperature-sensitive element (3).

7. A method according to any one of the preceding claims, wherein, in order to obtain the LED input electrical power value (Pin), the method comprises: obtaining (400) a LED voltage value (4000), corresponding to the value of the voltage over the LED, - obtaining (402) a LED current value (4020), corresponding to the value of the current through the LED, obtaining (406) the LED input electrical power value (Pin) from a product of at least the LED voltage value (4000) and the LED current value (4020).

8. A method according to claim 7, wherein, to determine the LED input electrical power (Pin), the LED further being operated with a LED duty cycle of the current through the LED, the method further comprises: obtaining (404) a LED duty cycle value (PWM), corresponding to the value of the duty cycle of the current through the LED, and wherein in obtaining (406) the LED input electrical power value (Pin), the product of at least the LED voltage value (4000) and the LED current value (4020) further includes the

LED duty cycle (PWM).

9. A method according to claim 7 or 8, wherein, in order to obtain the LED voltage value (4000), the method comprises: measuring the voltage over the LED (1) while the LED is switched on.

10. A method according to claim 7, 8 or 9, wherein, in order to obtain the LED current value (4020), the method comprises: measuring the current through the LED (1) while the LED is switched on.

11. A method according to claim 7, 8 or 9, wherein, in order to obtain the LED current value (4020), the method comprises: using a set-point value of a circuit (1006) supplying the current through the LED (1).

12. A method according to claim 11, wherein the set-point value is obtained from a calibration of the circuit (1006) supplying the current through the LED (1) while the LED is switched on.

13. A method according to any one of the preceding claims, wherein, in order to determining the LED output power (Pflux), the method comprises filtering of at least one of the LED voltage value (4000), the LED current value (4020), the LED input electrical power value (Pin), the first reference temperature value (Tl), the second reference temperature value (T2), the temperature difference value (ΔT) and the thermal power value (Pth) over a time period.

14. A method according to any one of the preceding claims, wherein the method further comprises: adjusting the input electrical power to the LED (1).

15. A method according to claim 14, wherein, in order to adjust the input electrical power (Pin) to the LED, the method comprises: adjusting (502) at least one selected from the group of a duty cycle of the current through the LED and a magnitude of the current through the LED (1).

16. A method according to claim 14 or 15, wherein adjusting of the input electrical power to the LED (1) is carried out in comparison to a reference light output level value for the LED (I).

17. A method according to claim 16, wherein the reference light output level value for the LED (1) is determined from a light output level value (Pflux, PfluxR, PfluxG, PfluxB) of at least one other LED (1, IR, IG, IB).

18. A method according to claim 17, wherein the at least one other LED (1, IR, 1 G, 1 B) has a different colour than the LED ( 1 , 1 R, 1 G, 1 B).

19. A method according to claim 16, wherein the reference light output level value for the LED (1) is determined from a pre-determined absolute light output level value.

20. A circuit arrangement (1000) for driving a LED (1), comprising: output terminals (11, 12) arranged to electrically connect to the LED (1), a supply unit (1001) arranged to supply the LED (1), when electrically connected, via the output terminals (11, 12) with a voltage over the LED and a current through the LED, - a power detector circuit (2060, 4062) being connected to the supply unit

(1006) and arranged for:

- obtaining a LED input electrical power value (Pin),

- obtaining a thermal power value (Pth), and a power processor unit (1040, 1042) being connected to the power detector circuit (2060, 4062) and arranged for:

- receiving the LED input electrical power value (Pin) and the thermal power value (Pth) from the power detector circuit (2060, 4062), and determining a LED output power value (Pflux) from the received input electrical power value (Pin) and thermal power value (Pth).

21. A circuit arrangement according to claim 20, wherein the circuit arrangement further comprises: a temperature detector circuit (2040, 3020, 3120, 3220) arranged for:

- determining a first reference temperature value (Tl) at a first location (210) in a first thermal connection to the LED (1),

- determining a second reference temperature value (T2) at a second location (220; 221) a second thermal connection to the LED (1), the second location (220; 221) being spaced apart from the first location (210), obtaining a temperature difference value (ΔT) from the first reference temperature value (Tl) and the second reference temperature value (T2), and wherein the power detector circuit (2060, 4062) is connected to the temperature detector circuit (2040) and further arranged for: receiving the temperature difference value (ΔT) from the temperature detector circuit (2040), and determining the thermal power value (Pth) from the received temperature difference value (ΔT) and a thermal impedance value (Rth) from the received temperature difference value (ΔT).

22. A circuit arrangement according to claim 21, wherein the temperature detector circuit (2040, 3020, 3120, 3220) is arranged for communicating with a first temperature- sensitive element (2) positioned at the first location (210), and wherein the temperature detector circuit is further arranged for acquiring a readout value (3000) from the first temperature-sensitive element (2) positioned at the first location (210), and wherein the first reference temperature (Tl) is determined from the acquired readout value (3000) from the first temperature-sensitive element (2).

23. A circuit arrangement according to claim 21 or 22, wherein the temperature detector circuit (2040, 3020, 3120) is further arranged for obtaining a LED forward voltage value (3100), corresponding to a forward voltage over the LED (1), and wherein the second reference temperature (T2) is determined from the forward voltage value (3100).

24. A circuit arrangement according to claim 21 or 22, wherein the temperature detector circuit (2040, 3020, 3220) is arranged for communicating with a second temperature-sensitive element (3) positioned at the second location (220; 221), and wherein the temperature detector circuit is further arranged for acquiring a readout value (3200) from the second temperature-sensitive element (3) positioned at the second location (220; 221), and wherein the second reference temperature value (T2) is determined from the acquired readout value (3200) from the second temperature-sensitive element (3).

25. A circuit arrangement according to any one of the claims 20-24, wherein, in order to obtain the input electrical power (Pin) to the LED, the power detector circuit is further arranged for: obtaining a LED voltage value (4000), corresponding to the value of the voltage over the LED, and obtaining a LED current value (4020), corresponding to the value of the current through the LED, and wherein the LED input electrical power value (Pin) is obtained from a product of at least the obtained LED voltage value (4000) and LED current value (4020).

26. A circuit arrangement according to claim 25, wherein the power detector circuit is further arranged for: obtaining a LED duty cycle value (PWM), corresponding to the value of a duty cycle of the current through the LED, and wherein the LED input electrical power value is obtained from a product of at least the obtained LED voltage value (4000), LED current value (4020) and LED duty cycle value (PWM).

27. A circuit arrangement according to any one of the claims 20-26, further comprising a controller (2000), wherein the controller is connected to the supply unit and the power processor unit (1040, 1042), and wherein the controller is further arranged for: - receiving the LED output power value (Pflux; PfluxR, PfluxG, PfluxB), and adjusting the LED input electrical power in dependence on the received LED output power value ((Pflux; PfluxR, PfluxG, PfluxB).

28. A circuit arrangement according to claim 27, wherein, in order to adjust the input electrical power to the LED, the controller (2000) is further arranged for: adjusting at least one LED supply parameter value selected from the group of a duty cycle value (PWM; PWMR, PWMG, PWMB) of the current through the LED and a magnitude value of the current (I; IR, IG, IB) through the LED, and providing the at least one adjusted LED supply parameter value to the supply unit.

29. A LED driver IC (1000) comprising a circuit arrangement in accordance with any one of the claims 20-28.

30. A LED lighting system (5000) comprising at least one LED and a circuit arrangement in accordance with any one of the claims 20-28.

31. A computer program on a medium readable by a processor (1000, 2000, 1002,

2002), the processor being associated with a circuit arrangement arranged for driving a light emitting diode (LED), the computer program being arranged to be loaded in the processor and after being loaded allowing the processor to perform the method for determining the light output level of the LED in accordance with any one of the claims 1-19.

32. The computer program according to claim 31 , wherein the circuit arrangement is a circuit arrangement in accordance with any one of the claims 20-28.

33. A computer-readable medium being provided with a computer program in accordance with claim 31 or 32.