# Patent

## Title
Method of characterising an LED device

## Abstract

A method of characterizing an LED, as well as an integrated circuit using this method, based on a so-called characteristic resistance, in which the LED is operated at a first, relatively low, operating current and then at a second, relatively high, operating current. From the ratio between the difference between the forward voltages at these two operating currents, and the difference between the operating current, the characteristic resistance is determined. The characteristic resistance is measured at two or more moments during the operational lifetime of the device, and a prediction or estimate is made in relation to the total operational lifetime of the devices, from the evolution or change of the characteristic resistance.

## Background
This application claims the priority under 35 U.S.C. §119 of European patent application No. 11165352.3, filed on May 9, 2011, the contents of which are incorporated by reference herein.
FIELD OF THE INVENTION
This invention relates to method of characterising a light emitting diode (LED) device. It further relates to LED drivers configured to operate such a method.
BACKGROUND OF THE INVENTION
Due to their known advantages, such as high efficiency in terms of lumens per watt, small form factor and durability, LEDs are used as light sources in high performance lighting fixtures. LEDs are increasing preferred light sources in difficult-to-replace lighting fixtures, such as street lights, traffic signal lights and in fixture that require high reliability, such as automotive lights, for instance for safety reasons.
Similar to many other light sources, the light output from an LED decays over time, ultimately leading to LED failure. In order to avoid complete failure, LEDs are typically replaced according to a fixed schedule. However, since the replacement schedules generally try to completely avoid pre-replacement failure, and there is a significant spread in the time at which an LED may be expected to fail, many LEDs are replaced considerable before a likely failure, which is clearly wasteful; alternatively, if the replacement schedule is extended in order to reduce such waste, some LEDs are likely to fail before being replaced, which is generally inconvenient and could be dangerous.
In order to predict, and thereby where appropriate prevent, the failure of LEDs, it is known to monitor or measure the light output by means of external optical sensors such as photodiodes. Whilst this method is generally robust, it requires additional components, circuitry and wiring, and is thus undesirable. Further, in non-ideal lighting environments, such as where there may be interference from other LEDs or extraneous light sources, the method may be inaccurate.
There is thus an ongoing requirement to provide other methods of predicting the failure of LEDs, and characterising their performance in general.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided a method of characterising an LED device, the method comprising: determining a first value and a second value of a characteristic-resistance Ron, by determining a first and second voltage (Vh, Vl) across the LED device whilst a respective first and second current (ih, il) is passing through the device; determining the characteristic-resistance from the ratio of the difference between the first and second voltage, and the first and second current, according to Ron=(Vh−Vl)/(ih−il); and predicting an end of an operational lifetime of the device from the first value (Ron1) and second value (Ron2) of the characteristic-resistance.
Thus, according to this aspect, the change in the value of the characteristic-resistance may be considered as a proxy for, or may be indicative of, the deterioration to the output light intensity from the LED; knowledge of the deterioration to the output light intensity may be used to make predictions about the remaining life of the LED. In some cases, the prediction may be a straightforward extrapolation of the change in characteristic-resistance; however in more complex environments in which the operating conditions of the LED have altered or been modified over time, the prediction may be more involved, or may take into account changes in operating conditions.
In embodiments, the first value of the characteristic-resistance may be determined at the start of an operational lifetime of the LED device. In embodiments, the second value of the characteristic-resistance may be determined after a part of an operational lifetime of the LED device. In such embodiments, the first value of the characteristic-resistance may be determined after a further part of the operational lifetime of the LED device, and the method may further comprise extrapolating to estimate a value of the characteristic-resistance at a start of the operational lifetime of the LED device.
The method may further comprise determining at least one further value of the characteristic resistance after respectively at least one further part of the operation life, and predicting an end of an operational lifetime may comprise extrapolating an evolution or slope of the characteristic resistance against operational lifetime. This may involve a linear extrapolation, or a non-linear extrapolation particularly where the operating conditions have altered or been modified. In general, the more measurements of the characteristic resistance are made, the more accurate is likely to be the prediction of the end of life, since a better fit may be made to the data, and changes in operating conditions may more readily be taken into account.
In embodiments, the method further comprises storing at least a value of the characteristic resistance in a memory. Thus, for example, an initial value of the characteristic may be stored, or a series of values may be stored, in order to better monitor the evolution or change of the characteristic resistance. Alternatively or in addition and without limitation, one or more parameters which represent or are indicative of the evolution may be stored.
In embodiments, the end of an operation lifetime is predicted to be when the characteristic-resistance Ron differs from its value at the start of the operational lifetime, by a predetermined amount. Without limitation, the predetermined amount may be in the range 0.6 to 1.6 Ohms, more particularly may be in the range 0.8 to 1.2 Ohm, or may be approximately 1 ohm.
In embodiments, the first current is larger than the second current by between 4 and 6 orders of magnitude, or in particular by 5 orders of magnitude.
In embodiments, the method may further comprise providing a warning signal indicative of the predicted end of an operational lifetime.
In embodiments, the method further comprises selecting one of a plurality of performance bins based on the predicted end of lifetime. Thus the characterisation may involve a pre-screening of LEDs, and categorising them according to their expected lifetime, so that LEDs with similar lifetimes can be “binned” together, thereby simplifying for instance, replacement during preventative maintenance operations or similar since LEDs in the same performance bin may be expected to deteriorate in a broadly similar manner. This is analogous to other binning of LEDs to provide, for example, wavelength matching.
According to another aspect there is provided an integrated circuit configured to drive an LED device and to operate the method of any preceding claim.
These and other aspects will be apparent from, and elucidated with reference to, the embodiments described hereinafter.



BRIEF DESCRIPTION OF DRAWINGS
Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which:
FIG. 1 is a graph of typical forward bias current-voltage (IV) plots for an LED (light emitting diode);
FIG. 2 plots the change in an effective resistance, at a particular operating condition, against operational lifetime of an LED, at a relatively high drive current at FIG. 2(a), and at a relatively low drive current at FIG. 2(b);
FIG. 3 shows the change in ΔRon against normalised output light intensity—which is representative of an operational lifetime of a high-power blue LED;
FIG. 4 is a table of experimental measurements showing the relationship between the relative optical output and lifetime;
FIG. 5 shows the relationship between the relative light output and ΔRon, for the devices listed in FIG. 4; and
FIG. 6 shows a flow diagram of a method according to embodiments.



It should be noted that the Figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these Figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar feature in modified and different embodiments.
DETAILED DESCRIPTION OF EMBODIMENTS
FIG. 1 is a graph of typical forward bias current-voltage (IV) plots for an LED. Curve 10 is a typical curve for a “new” or pristine LED, that is to say, an LED which is at the start of its operational lifetime. The curve is characterised by having low leakage current. This is shown towards the left of the curve a region 11, below the ‘knee’ of the curve. Further, above the ‘knee’, the curve is generally steep as shown at region 12.
The figure further shows a corresponding IV-characteristic 20 of a typical aging LED. Relative to the new, or pristine, LED curve 10, this curve 20 has a somewhat higher leakage at low forward bias, shown at region 21, and typically a somewhat less steep slope at higher forward bias, as shown at 22.
The curve may be understood as follows: as the LED gets older, the metal—semiconductor contact of the LED may deteriorate, leading to extra resistance at those contacts. This increased contact resistance leads to an increased effective resistance at larger driving currents. At the same time, there are typically new leakage paths forming within the LED. This corresponds to increased non-radiative recombination of the carriers at the p-n junction; such non-radiative recombination generally increases over the operational lifetime of the LED due to changes in the crystallography of the semiconductor, together with electro-migration or thermal diffusion of impurities and dislocations. Consequently, the light output of the device decreases, and it consumes more power—which typically results in hotter operation, and even hotter temperature if the operating conditions are adjusted to maintain the same output luminosity.
FIG. 1 also shows two current levels (horizontal lines) corresponding to 2 driving levels of the LED. A first driving level (ih) is well above the “knee” of the curve, so the diode is switched on and providing an optical output. The second driving level (il) is significantly lower, but importantly is non-zero and positive. Since this driving level is below the knee of the curve, the LED is effectively, that is, optically, switched off (that is to say, the radiative recombination is negligibly low or zero).
As shown in the figure, the operating voltage of the LED at the higher driving level ih is higher for the aged device, whilst the operating voltage of the LED at the low driving level il is lower for the aged device. This has been measured experimentally, as shown in FIG. 2, in which is plotted the change in an effective resistance, at a particular operating condition, against operational lifetime of an LED. By “effective resistance” is meant, at particular operating conditions (in this case, fixed current), a value of the ratio of voltage to current. Since the LED has a non-linear current-voltage response, Ohms law does not apply, so the measurement is not a true resistance, but none-the-less yields a useful value, in other words figure of merit, which is termed herein “effective resistance”.
FIG. 2(a) shows the change over operational lifetime of the effective resistance, determined according to vl/il, of an LED with a high drive current (in this case 1A). The normalised light output over time is plotted on the x-axis (or abscissa); this is taken as being representative of the aging of the device. The effective resistance is plotted on the y-axis (or ordinate). The effective resistance is seen to increase with operation of the device. As discussed above, this may be explained in terms of increased series resistance. With reference to FIG. 1, this is equivalent to the operating point at a high drive current ih moving to the right as the device ages.
FIG. 2(b) shows the change over operational lifetime of an effective resistance, determined according to vl/il, of an LED with a low drive current (in this case 10 μA). The axes are the same as in FIG. 2(a)—although in this case, the vertical scale is from 18-22 kOhms, in contrast to the 0-7 Ohms shown in FIG. 2(a). In both cases the normalised output is plotted from 100% (ie a new or pristine LED) on the left, to 50%, for a heavily aged device, on the right. The effective resistance is seen to decrease with operation of the device. As discussed above, this may be explained in terms of increased leakage paths. With reference to FIG. 1, this is equivalent to the operating point at a low drive current il moving to the left, as the device ages.
A characteristic value of a parameter, which has dimensions of Ohms, and thus may be termed a characteristic-resistance, may be derived from the above measurements. Herein this characteristic-resistance will also be described as an “on-resistance” Ron for the device, where Ron is calculated according to

Ron=(Vh−Vl)/(ih−il).

It will immediately be appreciated, that Ron is a function of the currents chosen, that is to say Ron−Ron(il, ih). Furthermore, it will be apparent that Ron is equal to the inverse of the slope of the line joining the operating points at high and low drive currents, shown at 30 for the pristine device and at 40 for the aged device.
As shown in FIG. 1 and explained above in terms of the loss mechanisms, the slope of the line joining the operating points at high and low drive current becomes shallower as the device ages. Thus its inverse increases. This increase may be denoted ΔRon and is generally positive. ΔRon is a function of the devices life, and the value of ΔRon at any moment in a device's life may be defined as the difference between Ron measured at that moment in the device's lifetime, and Ron of the device when pristine.
That is, considered as a function of operational time t:

ΔRon(t)=ΔRon(t)−ΔRon(0),

ΔRon(t)=[(Vh(t)−Vl(t))−(Vh(0)−Vl(0))]/(ih−il).

As already discussed, the magnitude of Ron is a function of the chosen drive currents ih and il, and thus so is ΔRon. However, as the present inventors have made the surprising realisation and experimentally verified, ΔRon follows a linear relationship with the reduction in the normalised light output over the aging of a device, and this is to some extent independent or nearly independent of the operating conditions.
FIG. 3 shows the change in ΔRon over an operational lifetime of a high-power blue LED. The figure plots, on the y-axis or ordinate, the normalised light output of the device, against on the x-axis or abscissa the increase in characteristic-resistance, that is to say ΔRon, which has the dimensions of ohms. As shown, there is a linear relationship between these two properties. Provided, then, that this relationship holds, or at least that the relationship is predictable or deterministic, by finding the ΔRon of an LED at a particular moment during its operational lifetime, it may thus be possible to deduce the relative light output of the LED at that moment, as well as to predict the remaining LED lifetime, until total failure may be expected, based on the elapsed time and past operating conditions.
As an example, consider a high-power blue LED which has been operational for an operational time t1 of 10,000 hours, and has a ΔRon of 0.2 ohm. As shown at the dashed line on the FIG. 3, we can calculate that the light output of this LED at t1 is 92.5% of its pristine value. From a simple linear extrapolation, since it took 10,000 hours to reduce the LED light output from a 100% to 92.5%, it will take a total of 40,000 hours to reduce it to 70%. If a light output reduction to 70% of its nominal or pristine value is taken to indicate the nominal end of the operational lifetime of the device, it can then be calculated that there is 30,000 hours remaining of the operational lifetime based on the past operation conditions.
It will be appreciated that other extrapolation algorithms for lifetime prediction, such as an exponential equation, Arrhenius equation, Black equation for meantime through the prediction, are also possible.
It will also be appreciated that, by measuring ΔRon at several moments during the operational life of the LED, a more accurate prediction of the remaining life can be achieved, and that as the device ages, the accuracy of the prediction will generally increase.
The relationship between the relative optical output and lifetime, has been experimentally verified, as shown in FIG. 4. Several high-power blue LEDs were tested under various conditions, and the figure shows a table, presenting, for each device (#), the test conditions being the temperature T, in ° C., of the heatsink, and the current I, in mA, under which the LEDs were stressed, the total operational lifetime L, in s, of the device before total failure, and a value of φ(end)/φ0, corresponding to the last optical output (normalised to pristine light output) just before the LED went dead, that is, at the end of its total operational lifetime. From the figure it will be observed that the total lifetime of the devices varied, even for devices operated under the same nominal conditions, such as devices #8, #9 and #11.
FIG. 5 shows the relationship between the relative light output of the devices listed in FIG. 4, plotted on the y-axis, and the increase in characteristic-resistance, that is to say ΔRon, plotted on the x-axis and measured in ohms. Despite the fact that the LEDs have very different total lifetimes, as demonstrated in FIG. 5, the relationship between each LED's ΔRon and its relative light output follows a remarkably similar linear curve. It is thus possible to use this relationship to determine, from the measured values of ΔRon, an estimate of the relative light output. For instance, it may be predicted that if ΔRon is approximately 1 ohm, the LED is approaching the end of its economic the lifetime if the latter is defined to be 70%±3% of its pristine light output.
FIG. 6 shows a flow diagram of a method according to embodiments. The method includes the following steps:

    
    
        At a first known moment during the LED's operational lifetime, shown at 610, a first value of a characteristic-resistance Ron1 is determined, by determining a first and second voltage (Vh, Vl) across the LED device whilst a respective first and second current (ih, il) is passing through the device, at 612 and 614 respectively; determining, at 616, the characteristic-resistance from the ratio of the difference between the first and second voltage, and the first and second current, according to Ron=(Vh−Vl)/(ih−il);
        At a second known moment during the LED's operational lifetime shown at 620, a second value of a characteristic-resistance Ron2 is determined, by determining a first and second voltage (Vh, Vl) across the LED device whilst a respective first and second current (ih, il) is passing through the device, at 622 and 624 respectively; determining, at 626, the characteristic-resistance from the ratio of the difference between the first and second voltage, and the first and second current, according to Ron=(Vh−Vl)/(ih−il);
        Predicting an end of an operational lifetime of the device from the first value (Ron1) and second value (Ron2) of the characteristic-resistance, as shown at 630.
    
    


The skilled person will appreciate that the values of Vh and Vl at the second known moment may be different to those at the respective first known moment.
The prediction of the end of the operation lifetime of the device may be based on the difference between the values of Ron at the first and second known moments (if the first moment is at the start of the operational life, that is to say may be based upon a value of ΔRon). The first known moment may be the start of the operational life of the device: in these embodiments the difference between the values of Ron at the first and second known moments, is equal to the value of ΔRon. The prediction of the end of the operational life may be a linear extrapolation from ΔRon.
Further values of Ron may be determined at a third or subsequent known moment or moments during the LED's operational lifetime in order to improve the prediction of the operational life.
Based on the prediction of the operational life of the LED, further actions may be taken: these include providing compensation of the operating conditions of the LED, in order to either maintain the original or another predetermined optical output, or to increase be life of the LED, for instance by reducing the stress on the LED by operating it at lower power. Another action which may be taken is for instance to provide a warning related to the expected end of life.
It will be appreciated that, as used herein the term “operational life” and the like are to be construed broadly, so as to include burn-in periods or pre-screening periods. The prediction of a total operational lifetime may thus be made as a result of a pre-screening operation or a pre-stressing operation, before the LED is used in it's normal operating environment or expected application.
As has already been discussed above, the predicted total operational lifetime, and thus the predicted end of life of an LED depends on the operating conditions under which it has been operated: it will be appreciated by the skilled person that ΔRon depends on the cumulative operational flux, and the evolution of ΔRon depends on the operating conditions during that evolutionary period, rather than on the specific momentary operating conditions. Since it is not possible to know future operating conditions with certainty, the total operational lifetime prediction is subject to errors based on changes in the operating conditions. Needless to say, the accuracy will improve towards the end of lifetime, since even a significant change to the rate of deterioration then has a relatively less significant effect on the overall cumulative operating conditions.
It will be appreciated that the exact value of current chosen for either the first current or the second current is not critical. The higher current should in general be large enough to distinguish changes in contact resistance of the device. It may conveniently be chosen to be the nominal operating current of the LED itself, This may typically be between 100 mA and 1 A, or in general, of the order of 106 μA/mm2. The lower current should be chosen to adequately distinguish changes in the leakage path or paths through the device, and may be about 10 μA or of the order of 101 μA/mm2, Thus the higher current may conveniently be larger than the lower current by a factor of between 104 and 106, and in particular by a factor of 105. Since the contact resistance may be of the order of Ohms, and the leakage resistance of the order of 100 kOhm, it will be appreciate that a factor of 105 ratio between the currents is convenient.
Seen from one viewpoint, then, a method of characterising an LED is disclosed herein, based on a so-called characteristic resistance, in which the LED is operated at a first, relatively low, operating current and then at a second, relatively high, operating current. From the ratio between the difference between the forward voltages at these two operating currents, and the difference between the operating current, the characteristic resistance is determined. The characteristic resistance is measured at two or more moments during the operational lifetime of the device, and a prediction or estimate is made in relation to the total operational lifetime of the devices, from the evolution or change of the characteristic resistance. An integrated circuit configured to operate such a process is also disclosed.
From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of LED devices, and which may be used instead of, or in addition to, features already described herein.
Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.
Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.
The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.
For the sake of completeness it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, a single processor or other unit may fulfill the functions of several means recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims.

## Claims

1. A method of characterising an LED device, the method comprising:
determining a first value and a second value of a characteristic-resistance Ron, by measuring a first voltage and a second voltage across the LED device whilst a respective first current and a second current is passing through the LED device;
determining the characteristic-resistance from a ratio of a difference between the first and second voltages, and a difference between the first and the second currents, according to


Ron=(Vh−Vl)/(ih−il); and


predicting an end of an operational lifetime of the device from the first value and the second value of the characteristic-resistance.

2. The method of claim 1, wherein the first value of the characteristic-resistance is determined at a start of an operational lifetime of the LED device.

3. The method of claim 1, wherein the second value of the characteristic-resistance is determined after a part of an operational lifetime of the LED device.

4. The method of claim 2, wherein the first value of the characteristic-resistance is determined after a further part of the operational lifetime of the LED device, and further comprising extrapolating to estimate a value of the characteristic at a start of the operational lifetime of the LED device.

5. The method of claim 1, further comprising:
determining at least one further value of the characteristic resistance after respectively at least one further part of the operation life, and wherein predicting an end of an operational lifetime comprising extrapolating an evolution of the characteristic resistance against operational lifetime.

6. The method of claim 1, further comprising storing at least a value of the characteristic resistance in a memory.

7. The method of claim 1, wherein the end of an operation lifetime is predicted to be when the characteristic-resistance Ron differs from its value at the start of the operational lifetime, by a predetermined amount.

8. The method of claim 1, wherein the first current is larger than the second current by between 4 and 6 orders of magnitude.

9. The method of claim 7, wherein the first current is larger than the second current by 5 orders of magnitude.

10. The method of claim 1, further comprising providing a warning signal indicative of the predicted end of an operational lifetime.

11. The method of claim 1, wherein the method further comprises selecting one of a plurality of performance bins based on the predicted end of lifetime.

12. An integrated circuit configured to drive an LED device and to operate the method of claim 1.