LED END OF LIFE DETECTION

Abstract

An LED driver measures a voltage across the LED and a temperature associated with the LED. The LED driver drives the LED to operate below a knee in the voltage/current curve of the LED and derives an expected voltage across the LED from the temperature of the LED, the LED driving current, and a predetermined relation between the expected LED voltage and at least one of LED temperature and LED driving current. The LED driver determines if the measured voltage across the LED exceeds the expected voltage across the LED, and establishes, based on the determination, if an LED approaching end of life warning is to be generated. Thus, an approaching end of life may be determined by the LED driver while the LED is still operational.

Claims

1. An LED driver for driving an LED, the LED driver comprising a power supply configured to power the LED, a control device configured to control the power supply to power the LED to operate the LED at a first electrical quantity and at a second electrical quantity, wherein the first electrical quantity is one of a voltage across the LED and a current through the LED and the second electrical quantity is the other one of the voltage across the LED and the current through the LED, wherein the control device comprises a measurement input connected to the LED and configured to measure the first electrical quantity, and a temperature sensor thermally coupled to the LED and configured to measure a sensor temperature indicative of a temperature of the LED, the temperature sensor comprising an output connected to a temperature measurement input of the control device wherein the control device is configured to: a) control the power supply to power the LED thereby driving the LED to operate at a predetermined value of the second electrical quantity, the predetermined value of the second electrical quantity being set to operate the LED below a knee voltage of the LED, b) derive an indicative temperature of the LED from the sensor temperature obtained from the temperature sensor, c) measure by the measurement input a value of the first electrical quantity, d) derive an expected value of the first electrical quantity from the indicative temperature of the LED, the predetermined value of the second electrical quantity, and a predetermined relation between the expected value of the first electrical quantity and at least one of LED temperature and the second electrical quantity, e) determine if the measured value of the first electrical quantity deviates from the expected value of the first electrical quantity, and f) establish, based on the determination, if an LED approaching end of life warning is to be generated, and g) output the LED approaching end of life warning in case the LED approaching end of life warning has been generated.

2. The LED driver according to claim 1, wherein the control device is further configured to in e), in case the first electrical quantity is the voltage across the LED and the second electrical quantity is the current through the LED, determining if the measured voltage increased in respect of the expected value, and in case the first electrical quantity is the current through the LED and the second electrical quantity is the voltage across the LED, determining if the measured current through the LED decreased in respect of the expected value, and wherein the control device is further configured to, in f), generate the end of life warning from an increase of the measured voltage in respect of the expected value, in the case the first electrical quantity is the voltage across the LED and the second electrical quantity is the current through the LED, respectively from a decrease of the measured current through the LED in respect of the expected value, in the case the first electrical quantity is the current through the LED and the second electrical quantity is the voltage across the LED.

3. The LED driver according to claim 1, wherein the temperature sensor is thermally connected to a heat sink of the LEDs, and the control device being configured to derive the indicative temperature of the LED by estimating a junction temperature of the LED from the temperature as measured by the temperature sensor and a thermal resistance from the LED junction to the heat sink, the thermal resistance from the LED junction to the heat sink being stored in a memory of the control device.

4. The LED driver according to claim 3, wherein the control device is further configured to estimate the junction temperature of the LED from an estimated dissipation of the LED, the control device being configured to estimate the dissipation from the measured value of the first electrical quantity and the predetermined value of the second electrical quantity.

5. The LED driver according to claim 1, wherein the comparing the measured value of the first electrical quantity to the expected value of the first electrical quantity comprises: determining if the measured value of the first electrical quantity exceeds the expected value of the first electrical quantity by a predetermined voltage threshold in case the first electrical quantity is the voltage across the LED, and determining if the measured value of the first electrical quantity underceeds the expected value of the first electrical quantity by a predetermined current threshold in case the first electrical quantity is the current through the LED.

6. The LED driver according to claim 5, wherein the predetermined voltage threshold respectively the predetermined current threshold is stored in a memory of the control device as a function of at least one of LED temperature and the second electrical quantity.

7. The LED driver according to claim 1, wherein the deriving the expected value of the first electrical quantity from the indicative temperature of the LED and the predetermined value of the second electrical quantity comprises: fetching the expected value of the first electrical quantity from the memory having stored therein the expected value of the first electrical quantity at the junction temperature and the predetermined value of the second electrical quantity.

8. The LED driver according to claim 7, wherein the control device is configured to store the measured value of the first electrical quantity in the memory upon receiving an calibration command.

9. The LED driver according to claim 1, wherein the control device is configured to perform a) - c) during a power-up procedure of the LED driver.

10. The LED driver according to claim 1, wherein the control device is configured to perform a) - c) during an LED off time in an LED modulationcycle.

11. The LED driver according to claim 1, wherein a relation between the first electrical quantity and temperature is stored in the memory of the control device, the control device further being configured to: derive a further indicative temperature of the LED from the measured value the first electrical quantity and the stored relation between the first electrical quantity and temperature, compare the indicative temperature of the LED to the further indicative temperature of the LED, and perform d) to g) in case a difference between the indicative temperature of the LED and the further indicative temperature of the LED is less than a predetermined temperature stability threshold.

12. The LED driver according to claim 1, wherein the control device is further configured to measure the value of the first electrical quantity at a first moment in time and to measure the value of the first electrical quantity at a second moment in time which is later than the first moment in time, to derive a time gradient of the first electrical quantity from a difference between the value of the first electrical quantity at the first moment in time and the value of the first electrical quantity at the second moment in time and an elapsed time between the first moment in time and the second moment in time, and to perform d) to g) in case the time gradient of the value of the first electrical quantity is lower than a predetermined gradient.

13. A method of driving an LED to operate the LED at a first electrical quantity and at a second electrical quantity, wherein the first electrical quantity is one of a voltage across the LED and a current through the LED and the second electrical quantity is the other one of the voltage across the LED and the current through the LED, the method comprising a) controlling a power supply to provide a predetermined value of the second electrical quantity to power the LED, the predetermined value of the second electrical quantity being set to operate the LED below a knee voltage of the LED, b) deriving an indicative temperature of the LED from a sensor temperature obtained from a temperature sensor, the temperature sensor being thermally coupled to the LED, c) measure a value of the first electrical quantity, d) deriving an expected value of the first electrical quantity from the indicative temperature of the LED, the predetermined value of the second electrical quantity, and a predetermined relation between the expected value of the first electrical quantity and at least one of LED temperature and the predetermined value of the second electrical quantity, e) determining if the measured value of the first electrical quantity deviates from the expected value of the first electrical quantity, and f) establishing, based on the determination, if an LED approaching end of life warning is to be generated, and g) outputting the LED approaching end of life warning in case the LED approaching end of life warning has been generated.

14. The method according to claim 13, wherein e) comprises: in case the first electrical quantity is the voltage across the LED and the second electrical quantity is the current through the LED, determining if the measured voltage increased in respect of the expected value, and in case the first electrical quantity is the current through the LED and the second electrical quantity is the voltage across the LED, determining if the measured current through the LED decreased in respect of the expected value, and wherein, in f), the LED approaching end of life warning is generated from an increase of the measured voltage in respect of the expected value, in case the first electrical quantity is the voltage across the LED and the second electrical quantity is the current through the LED, respectively from an decrease of the measured current through the LED in respect of the expected value, in case the first electrical quantity is the current through the LED and the second electrical quantity is the voltage across the LED.

Description

[0118] Further features, effects and advantages of the disclosed invention may become apparent from the appended drawing, showing a non-limiting embodiment of the disclosure, wherein:

[0119] FIG. 1 depicts a driver circuit in accordance with an embodiment of the invention;

[0120] FIG. 2a depicts a graph of LED forward voltage versus LED current

[0121] FIG. 2b depicts a graph of LED forward voltage versus LED current at different ages of the LED

[0122] FIG. 3 depicts a graph of LED forward voltage versus LED junction temperature at different forward currents.

[0123] FIG. 4 depicts a graph of LED forward voltage versus LED junction temperature at different ages of the LED.

[0124] FIG. 5 depicts a time diagram of LED driving and measurement.

[0125] FIGS. 6A and 6B depict thermal resistance networks modelling a thermal resistance.

[0126] It is noted that throughout the figures, the same or similar reference numerals refer to the same or similar features.

[0127] FIG. 1 depicts an LED driver DRV comprising a power input in the form of power input terminals IT, to be connected to a power supply such as an AC supply voltage, for example an AC mains voltage Vmains, a transformed AC mains voltage at a secondary side of a mains transformer, a DC supply voltage such as supplied by a battery or by a DC power supply network, etc. The LED driver comprises a driving output OT to be connected to an LED. In FIG. 1, the LED driver is connected to a series connection of LEDs: LED1, LED2, ... LEDn. The LEDs are mounted to a heat sink HS in order to sink heat generated by the LEDs during operation. A thermal resistance between the casing of the LEDS and the heat sink being denoted as Rch, a thermal resistance of the heat sink to ambient being denoted as Rha. The LEDs are electrically connected to the driver, whereby one terminal of the series connected LEDs (anode) is connected to a first output terminal OT1 of the driver while the other terminal of the series connected LEDs (cathode) is connected to a second output terminal OT2 of the driver. Alternatively, the other terminal OT2 of the LEDs is connected to ground, where the driver being single ended. The driver is configured to provide an LED drive current (also denoted as LED driving current or LED current or driver output current) to the LEDs. Thereto, in the present example, a current measurement device being comprised in the driver in order to measure the LED drive current as flowing through the LEDs. In the present example, the current measurement device is formed by the series Resistor Rs effectively connected in series with the LEDs. The driver may measure a voltage Vi across the series Resistor Rs. The current measurement may be performed at the low (return) side as depicted in FIG. 1, or alternatively, the current measurement may be performed at the high side, which may for example enable the diodes to be connected to ground at the cathode side.

[0128] The LED driver comprises a control device CD such as a microcontroller, for example a single chip microcontroller or other programmable device. The control device may be provided with suitable program instructions in order to perform the functions as will be described in more detail further below.

[0129] The control device is provided with a measurement input, such as a voltage measurement input to measure a voltage across the LED. A differential voltage Vf - Vi may be measured, for example in that the control device measures a voltage at the high (current supply) side of the LED, i.e. OT1, and at the low side (current return side) of the LED, i.e. OT2, which voltage Vi may be measured anyhow in case of presence of the depicted current measurement resistor. Alternatively, a differential measurement circuit may be provided, e.g. by means of a differential amplifier, to measure a differential voltage across the LED.

[0130] The control device is further provided with a temperature sensor input TS to which a temperature sensor TS is connected. The temperature sensor TS is in the present example mounted to the heatsink thereby being thermally coupled with the LED. The temperature sensor may for example comprise an NTC (i.e. a resistive element having a negative temperature coefficient) a PTC ((i.e. a resistive element having a positive temperature coefficient), a semiconductor junction (a forwardly biased semiconductor junction exhibiting a temperature dependent forward voltage), or any other suitable temperature sensor.

[0131] The control device may receive instructions for operating the LED in any suitable way. For example, the control device may be provided with a setpoint input SET (e.g. digital or analogue) to which LED setpoint data is provided. Alternatively, instructions may be transmitted to the LED driver, in particular to the control device, by a digital communication bus, such as an illumination system communication bus, such as Digital Addressable Lighting Interface, DALI, a wireless DALI, a Digital Signal Interface, DSI, or any other suitable communication interface.

[0132] FIG. 2A depicts a curve of LED forward voltage Vf versus forward LED current If. The curve is depicted at a junction temperature Tj of T1 and at a accumulated operational age of the LED of X hours. T1 and X may be suitable values within normal LED operating conditions. The curve of the forward voltage Vf versus the current If first shows a relatively fast rise of the forward voltage as the LED current rises from zero, until a voltage is reached where the LED diode junction starts to conduct, causing a less steep increase of the LED voltage as the LED forward current rises. Thus, a curve is shown that exhibits a knee at the transition from the non- conducting to the conducting state. Below the knee, the forward voltage typically relates to the forward current at a dVf/dlf (e.g. delta Vf / delta If ) of 0.25 to 0.35 Volt per Ampere for a typical LED.

[0133] FIG. 2B depicts a similar curve as FIG. 2A, adding by a dotted line the same curve for another age of the LED, i.e. the age Y. As shown in FIG. 2B, the dotted curve provides, at the same forward currents, for higher forward voltages. The age Y may exceed the age X, thus the forward voltage of the LED may increase, at a same forward current and temperature, at an increased age. The term age may be understood as an accumulated operational use of the LED. Use of the LED at elevated conditions (temperature, output power) may result in a more fast aging of the LED).

[0134] FIG. 3 depicts a curve of the LED forward voltage as a function of the LED junction temperature Tj. The curve (approximating a straight line over a certain range) shows that the Forward voltage Vf increases as the junction temperature decreases. The forward voltage typically relates to the junction temperature at a dVf/dTj (e.g. delta Vf / delta Tj ) of -0,03 Volt per degree Centigrade for a typical LED. As shown in FIG. 3, as the higher forward current, for example at the forward current If = b, higher forward voltages result at a same junction temperature Tj, thus providing for a higher curve of Vf versus Tj at a higher forward current.

[0135] FIG. 4 depicts a curve of LED forward voltage Vf versus junction temperature Tj at different ages of the LED. In the curves, the forward current If is kept constant. The curves show that the forward voltage decreases as the junction temperature increases, similar to FIG. 3, while further showing that the curve rises as the LED ages: the curve at age Y provides, at the same junction temperature, for a higher forward voltage Vf at the same junction temperature and at the same forward current.

[0136] Using the dependencies of LED forward voltage, LED forward current, LED junction temperature and LED age, as described above, an age of the LED may be estimated, as described in more detail below.

[0137] For example, a mathematical relation may be defined where Vf = R(If, Tj, age). This relation describes the Vf in dependency of all possible combinations of If, Tj and age across their ranges. For practical use, a threshold Vf-th may be determined from relation R which, when surpassed by a difference between the actual forward voltage and the expected forward voltage, i.e. Vfa-Vfe, may trigger a message to the user or service technician that LEDs or fixture need to be replaced to prevent End Of Life, EOL.

[0138] This Vf-th can be a function of Tj [assuming a constant Ifa = If-measurement (ifm)].

[0139] The Vf may be dependent on the junction temperature. The junction temperature may be strongly dependent on the forward current If and fluctuations therein. At currents below the knee of the LED Vf=f(lf) curve, steady state junction temperature may be rather constant. It may therefore be advantageous to measure at an If-measurement below the knee of this curve. A number of contemporary LED drivers may generate low currents which are under the knee current and where Vf is large enough to be measured accurately enough to detect changes in Vf caused by aging of the LEDs.

[0140] The present invention may use these dependencies in the driver as follows:

[0141] The driver, in particular the control device thereof, performs the following, [0142] a. generating an Ifm below the Ifknee. [0143] b. measuring the temperature of the temperature sensor and given the known thermal resistances, [0144] c. calculating the indicative junction temperature Tja of the LED chain. [0145] d. deriving the expected Vfe from the curve shown in FIG. 4 (or the curve shown in FIG. 2b) using the Tja [0146] e. measuring the actual Vfa [0147] f. comparing the difference Vfa-Vfe to a predetermined threshold Vf-th, which may be a function of the forward current and/or the junction temperature, i.e. f(If,Tj) [0148] g. signaling imminent end of life (EOL) to for example a user or a service technician

[0149] An embodiment is described with reference to FIG. 5. FIG. 5 depicts a diagram of the LED driving current, the LED voltage and the measured temperature versus time, as applied by the LED driver described with reference to FIG. 1. Applying a pulsed LED current, the junction temperature itself may change quickly, e.g. the junction temperature may rise during each current pulse and cool down after each pulse, while the measured temperature may respond more slowly to junction temperature changes, and may therefore show a more constant behavior.

[0150] The LED driver initially drives the LED to operate at in a modulation cycle, such as in a pulse width modulation, a pulse frequency modulation or any other suitable modulation. The LED thereby operates to provide an intensity in accordance with a received setpoint. For example, the LED may be a white LED, whereby the modulation serves to set an intensity level, As another example, the LED may be a cool or warm white LED in an assembly of a warm white and a cool white LED, whereby the modulation further serves to set a color temperature. At time T1, the LED current, i.e. the LED driving current as provided by the LED driver, is set to an LED driving current that provides that the LED operates at or just below the knee voltage Vknee of the LED (the knee voltage having been explained with reference to FIGS. 2a and 2b). Accordingly, the LED may not be emitting light. As the associated LED driving current is low, a junction temperature of the LED will decrease. At the time T2, a temperature is measured by the temperature sensor TS. Also, at or near T2, a voltage Vled across the LED is measured by the control device. The control device derives an estimated, indicative temperature of the LED, i.e. the LED junction, from the measured temperature. Thereto, the control device may use the thermal resistance between the LED junction and the LED casing, the thermal resistance between the LED casing and the LED heat sink, the thermal resistance between the Led heat sink and the temperature sensor. Furthermore, for a more accurate estimation, use may be made of the dissipation by the LED in order to model the dissipation by the LED junction as a result of the current that operates the LED just below the knee voltage. Hence, using the dissipation by the LED (in Watts), and the thermal resistances from the LED junctions to the temperature sensor (in degrees Centigrade per Watt), the indicative LED junction temperature may be calculated from the measured temperature of the temperature sensor, the thermal resistance between temperature sensor and LED junction the dissipation at the LED junction. Due to thermal capacitance, it may take some time after the LED temperature has stabilized, as represented by the time between T1 and T2.

[0151] Having determined the indicative temperature of the LED junction, the control device determines an expected voltage across the LED. Thereto, the determined junction temperature and the LED driving current are applied as input parameters, as following the behavior explained with reference to FIGS. 2-4, the LED forward voltage is temperature and current dependent. The control device may store in a memory thereof a mathematical relation between voltage across the LED, junction temperature and LED driving current. Alternatively, a lookup table may be stored in memory in order for the control device to look up an expected forward voltage. In an embodiment, the expected forward voltage may be expressed as a function of the LED temperature, the LED driving current or both LED temperature and the LED driving current. The LED temperature alone may suffice in case the determination is always performed at a same LED driving current (which may amongst others be the case when all LEDs as may be applied exhibit the knee in the V, I curve at substantially the same point). The LED driving current alone may be applied in case the LED temperature would either be held constant, e.g. by active cooling, or in case the thermal relations between cooling body and LED would be designed in such a way that the temperature would directly follow from the LED current (for example at a relatively high LED driving current, e.g. above the knee, a dissipation as a result of the LED driving current and the thermal behavior of the heat sink, may set the LED temperature).

[0152] The control device compares the expected forward voltage Vfe of the LED to the actual, measured forward voltage Vfa of the LED, as depicted in FIG. 4. As described with reference to FIG. 4, the LED voltage may rise as the LED ages. Thus, in case the LED voltage exceeds the expected voltage by a threshold, a warning that an end of life of the LED may be approaching, may be generated and output. The threshold may be a single, predetermined value. In another embodiment, the threshold may be dependent on temperature and LED current, thus to take account of the behavior of the LED at different temperatures and LED currents, as follows from FIGS. 2 - 4. The dependency may be stored in a memory of the control device in a form of a mathematical function or a lookup table.

[0153] The warning may be output by the control device in the form of a message sent via a communications interface, e.g. a DALI interface, to a remote server, such as a server running a remote maintenance application. The message may trigger a service technician to exchange the LED before the end of life is reached, or may result in the LED driver to operate the LED at lower intensity in order to, for example, use a remaining lifetime as effective as possible thus to extend a lifetime until replacement of the LED takes place. The warning may also be output optically by the driver, e.g. making the LED to signal that it needs service, e.g. by blinking or any other suitable signaling.

[0154] After having measured the voltage across the LED and the temperature, the control device may proceed to resume normal operation of the LED driver thereby driving the LED to emit light, as schematically indicated in FIG. 5 where the pulsed operation of the LED is resumed at T3. In order to keep the “off time” from T1 to T3 as short as possible, the remaining steps as described above may be performed after T3 if desired. As a temperature change of the LED junction may result in a relatively large change of the voltage across the LED, the above described age determination shows to be highly sensitive to temperature errors. Hence, verification of the (measured) temperature of the LED junction is desired. The determination of the temperature if the LED junction may be verified in various ways.

[0155] As a first example of verification of the temperature measurement, after having set the LED driving current to the knee current or below the knee current, the voltage across the LED may be repetitively measured. The control device may then determine if the voltage across the LED is stable. As the change of the temperature of the LED reflects into a change of voltage across the LED (keeping the LED driving current constant) a short term fluctuation in LED voltage reflects a short term temperature fluctuation. Thus, the control device may wait until the voltage across the LED appears to be constant, thus indicating that the temperature of the LED appears to be constant. As a result, effects of thermal delay, e.g. a long thermal time constant of the heat sink, resulting in a relatively slow decay, or effects of the dissipation of nearby other LEDs on the temperature of the heat sink, affecting a junction temperature of the LED, may be taken into account.

[0156] As a second example of verification of the temperature measurement, the fact that the LED forward voltage depends on temperature may be applied to measure the temperature of the LED. The control device may make use of a stored curve that expresses the LED forward voltage as a function of temperature (and optionally as a function of LED current). Thereto, the same curve or lookup table as described above to estimate the expected voltage across the LED may be applied, however in “reverse” direction, i.e. from voltage to temperature instead of from temperature to voltage. Accordingly, when the LED voltage has been measured by the control device, the control device may apply the stored relation between voltage and temperature to obtain an indication of the temperature from the voltage measurement (thereby for the time being disregarding age effects). In case the thus approximated junction temperature equals or nears the junction temperature as derived from the temperature sensor measurement, the temperature measurement by the temperature sensor may be considered validated. Otherwise, in case the temperature as derived from the voltage measurement appears to deviate from the junction temperature as derived from the temperature sensor output, the control device may wait for the temperature to stabilize and then repeat the determination, or proceed with normal tasks to perform the above described processes at another moment in time.

[0157] As described above, the determination is performed during an LED off time in the LED modulation scheme. Alternatively, the determination may be performed during a power up procedure, i.e. an initialization procedure whereby the LED driver is starting operation when the power is switched on.

[0158] In order to take account of individual tolerances of the LED forward voltage, the forward voltage may be measured, e.g. when the LED is taken into operation for the first time, or after a certain amount of operating hours counted from the moment that the LED is taken into operation for the first time. The measured forward voltage at a known LED forward current, i.e. LED driving current and at a known junction temperature, may be stored in the memory of the control device. The stored value may be used as a parameter to calibrate a stored relation between LED forward voltage, temperature and current, for example by multiplying the stored relation (function or lookup table) by a calibration factor derived from e.g. the stored value in respect of the expected value of the voltage according to the stored relation between LED forward voltage, temperature and current.

[0159] As a reference for thermal resistance networks:

[0160] For determining the junction temperature Tj, the thermal resistance network as depicted in FIG. 6A may be applied, associated with a dimensioning of heatsinks.

[0161] Where: [0162] Tj = the junction temperature of the considered component (here LED) in [K] [0163] Tc = the case temperature of the considered component in [K] [0164] Th = the temperature of the heatsink in [K] [0165] Ta = the ambient temperature in [K] [0166] (more exact would be to also model the thermal resistance between the heatsink and the driver enclosure and between the driver enclosure and ambient). [0167] Rjc = thermal resistance between junction and case in [K/W] [0168] Rch = thermal resistance between case and heatsink in [K/W] [0169] Rha = thermal resistance between heatsink and ambient in [K/W] [0170] Rja = thermal resistance between junction and ambient in [K/W]

[0171] Similarly the following thermal resistance network as depicted in FIG. 6B may be applied, associated with deriving Tj when TNTC is known.

[0172] Where: [0173] Tj = the junction temperature of the considered component (here LED) in [K] [0174] Tc = the case temperature of the considered component in [K] [0175] Th = the temperature of the heatsink in [K] [0176] TNTC = the temperature as measured by the NTC in [K] [0177] Rjc = thermal resistance between junction and case in [K/W] [0178] Rch = thermal resistance between case and heatsink in [K/W] [0179] RhNTC = thermal resistance between heatsink and NTC in [K/W] [0180] RjNTC = thermal resistance between junction and NTC in [K/W]