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03-01-2012

Understanding LED Performance

Rudi Hechfellner, Steve Landau

Philips Lumileds

Understanding and comparing LED performance appears straightforward: get the datasheets, compare numbers for light output, efficacy and lumen maintenance, and make a decision. Unfortunately, any purchase and design decision based simply on the top-line numbers – the specs on the early pages – without analysis of how the LEDs will perform under real operating conditions can lead to unsatisfactory results and significant business risks. This article describes the tools contained in datasheets that can be used to show how an LED will perform under real operating conditions.

LED Lamp

Use of these tools is best illustrated by way of an example. Let us imagine that you are to design a single-LED desk lamp with the highest possible light output. The average lamp must be capable of producing light output after 50,000 hours of operation at a level that is at least 70% of output when the lamp was new. A key part of this design project will be to choose an appropriate brand of power LED as the light source.

This example compares high-performance power LEDs from four leading suppliers, identified here as MFR 1-4. It uses only publicly available datasheet information as provided by each of the manufacturers for their own LED. The example headline light output figures for each device are shown in Table 1.

Table 1. Example headline product specifications for minature power LEDs
Manufacturer
Datasheet
Min. Flux
Datasheet
drive current
Datasheet
test temp
Datasheet
test time
MFR1
91 lm
350 mA
TA 25C
25 ms
MFR2
107 lm
350 mA
TJ 25C
25 ms
MFR3
130 lm
700 mA
TA 25C
25 ms
MFR4
100 lm
350 mA
TPad 25C
25 ms
TA – Ambient Temperature
TJ – Junction Temperature
TPad – Solder Pad Temperature

This data does not allow a like-for-like comparison, as the MFR 3 part is specified at 700 mA. The brief, however, was to maximise light output consistent with the lifetime goal of 50,000 hours. By driving the LEDs at 350 mA (as per the datasheet figures), we would not be maximising light output, so let us instead compare all four LEDs at the higher figure of 700 mA (see Table 2).

Table 2. Raw comparison of LEDs at 700 mA drive current
Manufacturer
Datasheet
Min. Flux
Normalize
to drive current
Normalized
Min. Flux @ 700 mA
Datasheet
test temp
Datasheet
test time
MFR1
91 lm
700 mA
164 lm
TA 25C
25 ms
MFR2
107 lm
700 mA
182 lm
Tj 25C
25 ms
MFR3
130 lm
700 mA
130 lm
TA 25C
25 ms
MFR4
100 lm
700 mA
165 lm
TPad 25C
25 ms

For three of the parts, this means applying the ‘flux normalisation’ graph found in each datasheet (see Figure 1 for a typical example). The graph will provide a factor to use for each specific LED to calculate the light output produced at the higher current.

Example of Flux normalization graph
Figure 1. Example of Flux normalization graph.

As we now see in Table 2, the MFR 3 emitter is no longer the leader in light output, but the comparisons here are still some way from being like-for-like, since we are not yet comparing the LEDs’ performance in actual operating temperatures.

For this we need the ‘temperature derating’ graph provided in every manufacturer’s datasheet. First, we must specify the conditions in which our LEDs will operate: the ambient temperature, and the thermal resistance of the luminaire. Using conservative assumptions (an ambient temperature of 25°C and a small heat sink), the light output comparison in Table 3 has changed strikingly when compared with Table 2.

Table 3. Comparison of light output under real operating temperature conditions
Manufacturer
Datasheet
Min. Flux
Actual
drive
current
Normalized
Min. Flux
@ 25C
Datasheet
TJ max.
Datasheet
TJ (calculated)
@25CA,
Rth50K/W
Determine Flux
De-rating Factor
Actual
Flux
MFR1
91 lm
700 mA
164 lm
145C
135C
72%
118 lm
MFR2
107 lm
700 mA
182 lm
150C
128C
 
142 lm
MFR3
130 lm
700 mA
130 lm
125C
141C
78%
 
MFR4
100 lm
700 mA
165 lm
150C
130C
81%
133 lm

The first interesting point to note is that the MFR 3 part cannot be used at all under these conditions: the high system thermal resistance drives the temperature at the LED junction up to 141°C, 16°C above its maximum rated value. Also interesting is the rate at which the output from the MFR 1 part declines under these conditions.

We now have a much more realistic basis for comparing different brands of LED. But we still have not taken into account the requirement for 70% lumen maintenance after 50,000 hours.

Again, all datasheets provide lumen maintenance information and it is important to look carefully at the operating conditions that apply to valid data sets (see Table 4). For the MFR 4 emitter, these operating conditions are consistent with the lumen maintenance conditions: the device is able to provide 50,000 hours of use at a junction temperature of 135°C; in the desk lamp example, the LED will actually run at 130°C. So we now know that the MFR 4 LED will produce at least 133 lumens when new, and will still provide at least 70% of peak output after 50,000 hours.

Table 4. LED output consistent with 50,000-hour lifetime requirement
Manufacturer
Calculated
Lumens
Lumen
Maintenance
L70 Claim
Datasheet
TJ max.
L70/50KH
condition
Actual
Operating
TJ
(calculated)
Calculated
current
to achive
lumen
maintenance
Final
calculated
Lumens
MFR2
142 lm
50,000 hours
150C
TJ ≤ 85C
TA ≠ 25C
128C
407 mA
107 lm
MFR4
133 lm
50,000 hours
150C
TJ ≤ 135C &
if ≤ 700 mA
TA – N.A.
130C
700 mA
133 lm

Table 4 also shows the conditions under which the MFR 2 part can provide 70% lumen maintenance at 50,000 hours: the junction temperature – the temperature at the LED itself – must be 85°C or less. But in our example, when driving LEDs at 700 mA for high light output, the MFR 2 device runs at a much higher 128°C.

Simple Comparisons

The simplest way to compare the MFR 2 emitter with the others in our example while achieving a 50,000-hour lifetime is to lower the drive current to a value at which junction temperature is 85°C. To achieve this, current must be reduced to 407 mA, and at this low current the LED only produces 107 lumens, versus the 133 lumens from the MFR 4 LED at the full 700 mA. Additionally, off-the-shelf LED drivers are generally available only for 350 mA or 700 mA. Since 407 mA is not a standard value, a custom solution would likely have to be created, which could add cost to the solution.

The conditions that apply in the raw statements of performance shown in Table 1 and typical of LED datasheets are very different from those that apply in real luminaires. Only through analysis of the LED performance metrics based on the actual application and intended environment can an appropriate selection decision be made.

philipslumileds.com

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