ULT 2.0 Tutorial
ULT
2.0
Tutorial
2010.01
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ULT 2.0 Tutorial
2010.01
ULT 2.0 Tutorial
What is ULT?
The Usable Light Tool (ULT) is a lighting analysis tool used to provide a true evaluation and comparison of high power LEDs. High
power LED datasheets publish typical flux and other performance values at a lab controlled junction temperature of 25°C. The ULT
takes into account the effects of 7 critical relationships (see Figure 1) between current, forward voltage, dissipated power, and heat to
determine the expected “real world” level of light from an LED.
Figure 1 Different Factors Affecting the Light Output
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Working Principles
The Usable Light Tool automatically determines the optimal drive current to maximize the usable light output of an LED, while
respecting the maximum junction temperature. However, it is important to note that the “optimal” current may not necessarily be
maximum current. The models created for each LED are based on values obtained directly from the LED manufacturer’s datasheets.
The ULT uses 3 major sources of data to optimize the analysis:
Data source 1: Flux vs. Forward Current
The light output of an LED is directly influenced by the drive
current (see Figure 2). However, as the drive current increases, the
power consumption of the LED increases as well, which will in turn
generate additional heat.
Figure 2 R
elative Flux vs. Forward Current at Tj=25°C From Philips Lumileds Datasheet – DS64
Data source 2: Forward Current vs. Forward Voltage
Increasing the drive current will increase the forward voltage (see Figure 3).
The power consumption of an LED is a direct product of forward current and
the forward voltage. As a result, a rise in the forward voltage will also increase
the power consumption, which will in turn increase the amount of heat
generated.
Figure 3 Forward Current vs. Forward Voltage - From
Philips Lumileds Datasheet – DS64
Data source 3: Luminous Flux vs. Junction Temperature
An increase in junction temperature of an LED will decrease its light output
(see Figure 4). This is known as the Hot/Cold factor.
Figure 4 N
ormalized Flux vs. Temperature Tj – From
Philips Lumileds Datasheet – DS64
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New to the Tool
While maintaining its original graphical user interface, ULT 2.0 provides the user with additional options to enhance the analysis.
In order to facilitate the description of the system’s thermal management, it has been divided into 2 parts: circuit board and heat sink.
This also makes the comparative simulation of the systems with a different LED count more accurate.
Standard Board Type
The user has the option to select one of following 4 available
boards in the analysis: FR4 with Filled & Capped Vias, FR4
with Opened Vias, Standard MCPCB, and Custom Circuit
Board. Figure 5 shows the drop-down menu under the
Standard Board Type field.
Upon selection, the thermal resistance (Rth) of the board will be
displayed under Standard Board Rth for 1 LED. If the Custom
Circuit Board is selected, the user must manually specify the
thermal resistance of the board for 1 LED.
The characteristics of the selectable standard board types
are obtained by simulation or from the LED manufacturer’s
documentation. The characteristics of standard LUXEON
Rebel board types are described in Philips Lumileds’
“LUXEON® Rebel Assembly and Handling Information”
application brief (AB32).
Figure 5 Selecting the Board Type
Heat Sink Shape
This feature enables the user to utilize one of the following 3
heat sink shapes in the analysis: Rectangular, Circular and
Custom Solution. Depending on the shape selected, the
user must choose the Heat Sink Width/Diameter and the
Heat Sink Height, (and the Heat Sink Length, if Rectangular
shape chosen), from the respective drop-down menus. The
incorporated heat sink dimensions are obtained from Aavid
Thermalloy’s product datasheets. The ULT has incorporated
a Heat Sink Diagram field, which displays the mechanical
specifications of the heat sink selected. Figure 6 displays a
case where an FR4 board with opened vias has been selected.
Notice that upon selection of the board and its properties,
ULT displays the calculated thermal resistance of the heat sink
under Heat Sink Rth and Properties.
Hovering the curser over the “details icon” on the right hand
side permits the user to view the selected heat sink’s part
number and its properties.
As most applications are based on non-standard heat sink
designs, the user must carefully select a standard heat sink
shape and size that best approximates his/her own design. If a
Custom Solution is selected, the user must specify the thermal
resistance of the heat sink.
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Figure 6 Selecting the Heat Sink Shape and Properties
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Datasheets
The Usable Light Tool enables the user to access the
datasheets of the selected LEDs directly from the tool. A
datasheet is accessed by clicking the link (underlined LED
part number) in the LED Part Number and Datasheet field. As
illustrated in Figure 7, this will open the datasheet in a new
window.
Figure 7 Accessing the LED Datasheets
Analysis:
5 LXML-PWN1-0090 LEDs vs. 5 LXM3-PW51 LEDs
The ULT is used to perform a thorough assessment and comparison of high power LEDs. To illustrate different functionalities of
the tool, a detailed comparison between the LUXEON Rebel General Purpose White (LXML-PWN1-0090) and the LUXEON Rebel
Illumination Portfolio (LXM3-PW51) will be performed.
Required Inputs
Required Inputs represent the set of required application
parameters to carry out the simulation. Figure 8 displays
the Required Inputs section of the ULT populated with the
design specifications. Once the Required Inputs parameters
have been entered, the tool automatically calculates the heat
sink thermal resistance. In this case, the heat sink thermal
resistance was calculated to be Rth = 14°C/W. It is important
to note that the ULT defines the Ambient Temperature as the
temperature of the air in contact with the system heat sink.
Now, if the LED system is directly exposed to the ambient
environment, then the Ambient Temperature is essentially the
same as the true ambient temperature (i.e. typically 25°C).
However, if the LED system is placed within an enclosure, the
user must estimate the actual temperature of the air within the
enclosure, when specifying the Ambient Temperature. For the
purpose of this analysis, it is assumed that the LED systems
are exposed to room temperature, hence 25°C.
For Power LED 2 (LXM3-PW51), the user is required to specify
the LED manufacturer, LED product family, the part number,
optimization algorithm and the number of LEDs; the tool will
use the same parameters as in Power LED 1 to populate the
rest of the fields.
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Figure 8 Specifying the Parameters of LED 2
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Optional Inputs and Datasheet Reference Values
At this stage the user has the option to begin the simulation,
or to continue on with the Optional Inputs and Datasheet
Reference Values section to fine-tune the analysis. As shown
in Figure 9, the user has the option to override the Maximum
Current, Typical Flux @ Nominal Current, Typical Vf @ Nominal
Current, and the Lumen Maintenance Tj. For instance, if the
user wishes to set a ceiling for the drive current, let’s say
500mA, he/she will override the Maximum Current to 500mA.
It is important to remember that the ULT increases the drive
current (up to the maximum or overridden value), as long
as the lumen maintenance junction temperature has not
been reached. However, if the lumen maintenance junction
temperature is reached before attaining the overridden
maximum current, the tool will clip the current and will display
the results up to that value. Similarly, if the user is utilizing
a specific forward voltage bin or a flux bin, and wishes to
study the behavior of the LED(s) at that specific bin value, he/
she can override the Typical Flux and/or Typical Vf @ Nominal
Current to fine tune the analysis. For the purpose of this
analysis, the maximum current will be overridden to 350mA
(see Figure 9). Clicking the Analyze Power LEDs button will
start the simulation.
Figure 9 Overriding the Nominal Parameters
Power LED Analysis Report
As part of the analysis, the ULT calculates the selected LEDs’ performances, and outputs the results in the Analysis Report page. The
results page consists of 3 sections: Input Variables, Calculated Results and the Primary/Secondary Charts section.
Input Variables
The Input Variables section provides a summary of the entered
input parameters. As shown in Figure 10, LXML-PWN1-0090
has a 10 lm head start (95 lm vs. 85 lm) over its counterpart
at Tj = 25°C. To make any adjustments to the input fields, the
user has the option to Return to Input Page, and reenter the
variables.
Figure 10 Input Variables at a Glance
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Calculated Results
The Calculated Results section displays the calculated drive current, forward voltage, LED and array power consumption, radiometric
flux and the LED efficiency. In addition, the tool calculates the junction-to-ambient thermal resistance Rth (which is essentially the total
system thermal resistance) and the junction temperature Tj. As far as the light output is concerned, the ULT calculates the single and
array LED flux, as well as the usable efficacy.
As shown in Figure 11, the drive current of both LEDs was
calculated to be 350mA. As long as the junction temperature
of an LED is kept below lumen maintenance junction
temperature (i.e. Tj = 135°C)., the ULT increases the drive
current (up to the maximum or overridden value) to maximize
the light output. In this analysis, the LEDs were driven at the
maximum overridden current (350 mA), since the junction
temperature limitations were never reached. Furthermore, the
results show that due to negative “temperature coefficient”
of the LUXEON Rebel LEDs, the forward voltages of LXMLPWN1-0090 and LXM3-PW51 have dropped (from 3.15V for
LXML-PWN1-0090 and 3.0V for LXM3-PW51) to 2.97 V and
2.84 V, respectively. The calculated results demonstrate that
both LEDs have the same calculated Junction-to-Ambient
Rth (17.4°C/W). The junction-to-ambient thermal resistance
represents the total thermal resistance of the system and is
calculated using Equation 1:
Figure 11 Calculated Results
Equation 1 Sytem Thermal Resistance
R
R
Rth-system = th-LED
+ th-Board
+ Rth-heat sink
N
N
Where, N is the LED count
Rth-system= 17.4°C/W
Therefore, since both systems use the same number of LEDs, with the same type/size of a heat sink and board, the Junction-toAmbient Rth was calculated to be 17.4°C/W for both systems. Nevertheless, the results state that systems 1 and 2 have different
junction temperatures. The junction temperature of a system is determined using Equation 2:
Equation 2 Junction Temperature Relation
Tj = TA + (Pd)(Rth-system)
Where,
TA= Ambient Temperature (in our case 25°C)
Pd = Power Dissipated
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The dissipated power is the amount of the consumed power that is not converted into light. In the past, the amount of the power
converted into light was not a significant factor to consider, since most of the LED’s consumed power was converted into heat.
However, recent improvements in LED efficiency have necessitated the need to distinguish the amount of power that is converted into
light versus heat. In this analysis, the Calculated LED Efficiency was calculated via complex and recursive algorithms to be 23.1% and
24.3% for systems 1 and 2, respectively. In other words, 76.9% of system 1’s input power and 75.7% of system 2’s input power has
been converted into heat.
Therefore, following Equation 2:
Tj-system 1= 25°C + [(5.19 W * 0.769) * 17.4°C/W ]
Tj-system 1= 95°C
Similarly,
Tj-system 2= 25°C + [(4.96 W * 0.757) * 17.4°C/W ]
Tj-system 2= 90°C
It is important to note that both LEDs have a calculated junction temperature below lumen maintenance junction temperature
(i.e. Tj = 135°C). This means that the LXML-PWN1-0090 and the LXM3-PW51 LEDs can both be driven at a higher current while
maintaining a junction temperature of 135°C or below.
As previously stated, the light output decreases as the junction temperature increases. The light output degradation is a direct function
of the junction temperature. Nonetheless, depending on the test environment and the epitaxial technology used, LEDs are affected
differently by the change of temperature. LXML-PWN1-0090 and LXM3-PW51 have typical flux values of 95 lm and 85 lm at a junction
temperature of 25°C, respectively. Nevertheless, simulated in a “real-life” environment, the ULT calculates the Usable LED Flux to be
80 lm for both LEDs.
The results also illustrate that Systems 1 and 2 have different usable efficacies as they consume different amounts of power. System
1 has produced a total usable array flux of 399 lm consuming 5.19 W (efficacy = 399/5.19 = 76.89 lm/W), while system 2 produced a
total usable array flux of 402 lm consuming 4.96W (efficacy = 81.06 lm/W).
Note: T
he discrepancy between the calculated total array luminous flux (399lm and 402 for systems 1 and 2, respectively) and the
theoretical value (5 * 80lm/LED = 400lm) is due to the fact that the ULT rounds off the calculation results.
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Primary and Secondary Charts
The ULT generates up to 40 charts to provide results under different operating conditions. These charts reduce the analysis time
considerably. The ULT charts enable the user to extract significant information from the curves without having to run multiple iterations
of the same analysis. The ULT has categorized the charts into 5 groups; the name of group represents the x-axis variable against
which, the results have been plotted. Figure 12 and 13 displays the 8 Primary Charts, plotted as a function of Forward Current.
Figure 12 Primary Charts as a function of Forward Current - 1
Figure 13 Primary Charts as a function of Forward Current – 2
Depending on the parameters of interest and the target application, some of the ULT charts may be more relevant than others to the
users. For the purpose of this study, a subset of the charts will be examined to demonstrate the functionality of the tool.
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Primary Charts – By LED Forward Current
The ULT charts display the results from 100 mA, and increases the drive current until
the LED reaches the absolute maximum junction temperature. The absolute maximum
junction temperature of the LUXEON Rebel InGaN white LEDs is 150°C. Therefore,
the ULT will clip the current plot right before the LED reaches a junction temperature
of 150°C. It is absolutely NOT recommended to operate the LEDs anywhere near
the maximum junction temperature. Operating at such high temperatures will affect
the lifetime and the light output of the LEDs significantly. In addition, it can lead to
transient thermal overstress, which may result in a catastrophic failure.
Figure 14 illustrates that the maximum allowable forward current for LXM3-PW51 and
LXML-PWN1-0090 were calculated to be 624 mA and 590 mA, respectively. Moving
the cursor along the Usable Flux vs. Current curve, the user can view the luminous flux
of LXML-PWN1-0090 and LXM3-PW51 at the desired current. As illustrated in Figure
14, the array of LXM3-PW51 LEDs can produce a maximum light output of 539 lm, if
the LEDs are driven at 624 mA. If the user wishes to verify these results, he/she can
return to the input page, override the maximum current (see Figure 6) to 624mA and
run the analysis again.
Figure 14 Usable Flux vs. Forward Current
As depicted in Figure 14, the light output increases as the forward current increases.
However, as the forward current increases, the power consumption increases as
well. Moreover, as the forward current increases, while maintaining the size of heat
sink, the junction temperature of the LED rises too, which will in turn reduce the
light output. As a result, the usable efficacy drops as the forward current of the LED
increases. The LED efficacy is further deteriorated by the “droop effect,” which causes
the LED to generate a smaller amount of incremental light as the drive current is
increased. Therefore, even with a theoretical infinite heat sink that would maintain the
junction temperature constant, the efficacy would still decrease as the LED current is
increased. The effect of current increase on the usable efficacy is shown in Figure 15.
“Hot/Cold” factor is defined as the amount of light output degradation as a function
of junction temperature. From Figure 16, it is noticeable that LXM3-PW51 has a
better “Hot/Cold” factor than its counterpart. As a result, at the calculated junction
temperature, the luminous flux of LXM3-PW51 has only dropped to 94.77%, while the
luminous flux of LXML-PWN1-0090 has degraded to 84.41% of its initial value. This
is the primary reason why LXM3-PW51 produces the same light output (i.e. 80 lm) as
LXML-PWN1-0090, even though it has a lower nominal flux value.
Figure 15 Usable Efficacy vs. Current
Figure 16 Relative Flux vs. Junction Temperature
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Secondary Charts – By Ambient Temperature
Secondary Charts can be accessed by clicking on the Expand link on the right hand
side.
Figure 17 depicts the derated current characteristic of LXML-PWN1-0090 and
LXM3-PW51 LEDs, where the boundary condition is the lumen maintenance junction
temperature. To protect the LEDs from overheating and to maintain the power
dissipation within its limits, the forward current must be reduced right before the
LED reaches its lumen maintenance junction temperature (in this case, 135°C). For
instance, the Derated Current vs. Ambient T curve (Figure 17) illustrates that in the
case of LXML-PWN1-0090, the current must be reduced if the ambient temperature
reaches 66.6°C or above. As long as the ambient temperature increases, the current
derating curve will continue to drop until it reaches the minimum value (100mA) at the
ambient temperature of 119°C.
Figure 17 D
erated Current vs. Ambient
Temperature
Secondary Charts – By Heat Sink Thermal Resistance
The junction temperature of an LED is directly
affected by the thermal resistance of the
system. As an example, increasing the thermal
resistance of the heat sink (i.e. decreasing
the size of the heat sink) will increase the LED
junction temperature, resulting in a lowered
usable light. Furthermore, since the power
consumption would remain relatively constant
(there will be slight decrease due the effect of
junction temperature on forward voltage), the
efficacy will also decrease. This can be seen
from ULT’s Usable Efficacy vs. Heat Sink Rth
curve (Figure 18). This curve also demonstrates
the superior Hot/Cold Factor characteristics
of LXM3-PW51. Comparing the two LEDs, it
is clear that LXM3-PW51 has a better usable
efficacy, since it has an enhanced light output
performance as the junction temperature of the
LED increases.
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Figure 18 Actual Tj & Efficacy vs. Heat Sink Rth
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Secondary Charts – By Nominal Vf Bin Range
The usable flux of an LED is directly related
to the junction temperature. The junction
temperature is itself linked to the power
consumption of the system. Furthermore, an
increase in the forward voltage will increase the
power consumption. As a result, an increase
in the forward voltage will decrease the total
usable flux and the usable efficacy of the LED.
Figure 19 illustrates that using LEDs (in this
case LXML-PWN1-0090) from a lower voltage
bin will produce a higher light output, while
increasing the usable efficacy.
Figure 19 Usable Flux & Efficacy vs. Nominal Vf
Secondary Charts – By Nominal Flux Bin Range
Using an LED with a higher luminous flux permits the user to drive the LED at a lower
current while meeting the specification. Additionally, the higher the nominal flux, the
greater is the portion of the consumed power that is converted into light vs. heat.
A drop in the amount of heat generated decreases the junction temperature while
maintaining the same current level. Consequently, as shown in Figure 20, using an
LED from a higher flux bin will decrease the junction temperature, which will improve
the overall performance of the system.
Figure 20 J
unction Temperature vs. Nominal
Flux
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Closing Remarks
The Usable Light Tool 2.0 (ULT) developed by Future Lighting Solutions is a lighting analysis tool used to provide a true evaluation and
comparison of high power LEDs in “real-world” application conditions. The tool takes into account the significant effects of current,
forward voltage, dissipated power, and heat to determine the expected “real world” level of light from an LED. As part of the analysis,
the Usable Light Tool generates various charts to provide results under different operating conditions. The ULT charts enable the user
to extract significant information from the curves without having to run multiple iterations of the same analysis. Depending on the
parameters of interest and the application in hand, the user can make use of different charts to suit his/her design requirements. In this
analysis, a detailed comparison between the LUXEON Rebel General Purpose White Portfolio (LXML-PWN1-0090) and the LUXEON
Rebel Illumination Portfolio (LXM3-PW51) was performed. Using the calculation results/charts, it was illustrated that the light output
performance of an LED is directly affected by different factors such as, the drive current, forward voltage, junction temperature and the
Hot/Cold factor.
Contact Information
www.FutureLightingSolutions.com
In North America:
1-888-LUXEON2
Americas@futurelightingsolutions.com
In Europe:
00-800-44FUTURE
Europe@futurelightingsolutions.com
In Asia:
+800-LUMILEDS
Asia@futurelightingsolutions.com
In Japan:
+81-0120-667-013
Japan@futurelightingsolutions.com
Future Electronics and Future Lighting Solutions make no representation or warranty, express or implied, as to the accuracy, completeness, reliability
or timeliness of the information provided herein. Any and all information is furnished on an “as is” basis and only as an accommodation to customers
and not based upon any legal or contractual obligation to do so. Conclusions drawn from or actions undertaken on the basis of such information are
the sole responsibility of the user. Future Electronics and Future Lighting Solutions specifically disclaim any liability or responsibility for any information
provided herein or for any loss or damage resulting from the use of or reliance upon any such information.
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