Application Note

Light Emitting DiodeHow to Calculate the Junction Temperature
for the Nichia 123G Series LEDs

Table of Contents
1. Overview .......................................................................... 2
2. Applicable Part Numbers ................................................. 2
3. Terms and Descriptions .................................................... 3
4. TMP Measurement ............................................................. 5
5. How to Calculate the Junction Temperature (TJ) ............. 5
6. Calculation Example of the Junction Temperature (TJ) ... 6
7. Considerations and Suggestions when Using aThermocouple................................................................... 68. TMP Measurement Considerations .................................... 7
9. Summary .......................................................................... 8The Nichia part numbers NFSW123G, NFSA123G, NJSW123G, and NJSA123G within thisdocument are merely Nichia’s part numbers for those Nichia products and are not related nor bearresemblance to any other company’s product that might bear a trademark.

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-211111
1/9Feb. 22, 2022

Application Note
1. Overview

Light Emitting Diode

When designing applications using LEDs, it is necessary to consider the heat generated from theLEDs during operation. The junction temperature (TJ) of the LED is an important aspect to beconsidered in selecting the operating conditions of the application. If the absolute maximum rating
TJ is exceeded even for a short period of time, it may cause an adverse effect on the performance ofthe LED; in the worst case, it could cause the LED not to illuminate. Note that the LED must beoperated to ensure that the absolute maximum rating TJ is not exceeded. To prevent the TJ fromexceeding the absolute maximum rating TJ, it is necessary to know how high the TJ will be whenthe LED is operated in the conditions/environments in which the LED will actually be used.
This application note provides how to estimate the TJ by measuring the temperature at themeasurement point Nichia specifies with a thermocouple.

2. Applicable Part Numbers
This application note applies to the LEDs listed in Table 1.
Table 1. Applicable Part Numbers
Series Name

Nichia 123G Series

Wattage

0.5W

1W

Part Number

NFSW123G

NFSA123G

NJSW123GNJSA123GEmitted ColorWhiteAmber

White

Amber

2.0×3.0×0.7

2.0×3.0×0.7

2.0×3.0×0.7

2.0×3.0×0.7

Appearance

OutlineDimensions (mm)This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-211111
2/9Feb. 22, 2022

Application Note
3. Terms and Descriptions
Figure 1 shows a cross-sectional view of the LED mounted to a PCB and a simple model of thethermal resistance.
Table 2 shows the terms and descriptions that are used herein regarding temperatures and thermalresistances of different points/areas.

TJ: Junction Temperature
Thermal Resistance RθJMP

Light Emitting Diode

TMP: Measurement Point
Temperature

LEDThermal Resistance RθJS_elSolder

PCB

Copper Layer
Thermal Resistance RθSMP

TS: Solder Joint Temperature

Figure 1. Cross-sectional View of the LED Mounted on a PCB
and a Simple Thermal Resistance Model

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-211111
3/9Feb. 22, 2022

Application Note
Table 2. Terms and Definitions
TermSymbol

Junction
Temperature
Solder Joint
Temperature

Description
Temperature of the chip.

TJThe absolute maximum rating TJ of the Nichia 123G Series LEDs is 150°C; thistemperature must not be exceeded.

TSTemperature of the solder bonded to the cathode electrode (i.e. solder joint).
The TS cannot be measured. The TS may be used in a thermal simulation.
Temperature measured at the measurement point Nichia specifies.
The measurement point is on the solder that is applied on the soldering pad

Measurement
TMP

Point

Light Emitting Diode

Temperaturepattern on the cathode electrode side. The TMP is measured with the LED
mounted on a PCB. See Figure 2 in section 4.
The TMP is used to estimate the TJ. See the following sections for detailedinformation.
Thermal resistance from the chip to the solder joint. The RθJS_el is obtained
according to the measurement/calculation methods detailed in JESD 51.

RθJS_elThe RθJS_el may be different depending on the part number; refer to the applicablespecification for the typical and maximum RθJS_el values of the LED.
The RθJS_el is not used in the TJ estimation method discussed herein.
Thermal resistance from the chip to the solder joint. The RθJS_real is determinedwith taking the power conversion efficiency (ηe)1 of the LED into consideration.
The RθJS_real is not provided in Figure 1.
The RθJS_real is obtained with the following equation:

RθJS_realRθJS_real=RθJS_el/(1-ηe/100)
The RθJS_real value may be different depending on the part number; refer to theThermal

applicable specification for the typical and maximum RθJS_real values.

Resistance

The RθJS_real may be used in a thermal simulation.
The RθJS_real is not used in the TJ estimation method discussed herein.
Thermal resistance from the solder joint to the TMP measurement point.
RθSMP

The RθCMP can vary depending on the heat dissipation performance (i.e. thermalconductivity) of the PCB used, the heatsink used, the operating temperature, etc.
Thermal resistance from the chip to the TMP measurement point.
As shown in Figure 1, the RθJMP is the sum of the RθJS_el and the RθSMP.

RθJMP

For the RθJMP value of each LED part number, refer to the annex: Thermal
Resistance Values of the Nichia 123G Series LEDs.
The RθJMP is used to calculate the TJ using the TMP value. See the following
sections for detailed information.

1

The power conversion efficiency is the ratio of the electrical energy output as light to the input electrical energy.
It can be determined with the following equation:
Power Conversion Efficiency (%) = Radiant Flux (W)/Input Power (W) × 100
For LEDs, all the electrical energy that has not been converted into light becomes heat.
This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-211111
4/9Feb. 22, 2022

Application Note
4. TMP Measurement
The TJ estimation can be calculated by measuring the TMP of the LED once it has been soldered toa PCB. The measuring current should be the maximum operating current (IF) that will be applied tothe LED under the conditions in which the LED will actually be used. See Figure 2 for the TMP
measurement point.
To measure the TMP, solder the temperature sensing part on the tip of the thermocouple to thesoldering pad pattern (i.e. copper layer) of the PCB on the cathode electrode side; ensure that thetemperature sensing part is attached at a point closest to the soldering pad pattern.
TMP Measurement Point

Light Emitting Diode

Cathode MarkPCB

TMP Measurement Point
Solder

LEDSolder

Thermocouple

SolderLED

Soldering Pad Pattern

PCB

Thermocouple

Figure 2. TMP Measurement Point

5. How to Calculate the Junction Temperature (TJ)
When the product using the LED is operated, the TJ can be calculated with Equation 1 below.
Equation 1: TJ = TMP + RθJMP × W
TJ: Junction Temperature (˚C)
TMP: Measurement Point Temperature (˚C)
RθJMP: Thermal Resistance from the Chip to the TMP Measurement Point (˚C/W)
W: Input Power (IF × VF) (W)IF=Forward Current (A), VF=Forward Voltage (V)The RθJMP value may be different depending on the part number. For the RθJMP value of each partnumber, refer to the annex: Thermal Resistance Values of the Nichia 123G Series LEDs. For theLEDs whose RθJMP values are not listed in this annex, contact a local Nichia sales representative.
This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-211111
5/9Feb. 22, 2022

Application Note
6. Calculation Example of the Junction Temperature (TJ)This section provides an example of the TJ calculation using the measured TMP.
Example: The NFSW123G LED is operated at an input power of 0.5W.
The measured TMP is 45˚C.

Light Emitting DiodeThe RθJMP value of the NFSW123G LED is 31.6˚C/W (refer to the annex: Thermal Resistance Valuesof the Nichia 123G Series LEDs).
Using Equation 1 (TJ = TMP + RθJMP × W), the following calculation is obtained:
TJ = 45(˚C) + 31.6(˚C/W) × 0.5(W) = 60.8(˚C)

7. Considerations and Suggestions when Using a Thermocouple
It is recommended to use a thermocouple with wires that are as thin as possible. If the wires are too
thick, they may create thermal paths causing measurement errors.
If the temperature sensing part of the thermocouple is too large and/or is not located at the tip of thethermocouple, the temperature sensing part may not contact the TMP measurement point; if thetemperature sensing part is attached too far away from the TMP measurement point and/or in contact
with a place that is not the TMP measurement point, it may affect the measurement accuracy.

Correct

Incorrect

Temperature
Sensing PartTemperature
Sensing Part

The wires are thin.

The wires are thick.

The temperature sensing part contacts theThe temperature sensing part is too large.

TMP measurement point.

Figure 3. Examples of Correct/Incorrect Thermocouples(Before soldering to the soldering pad pattern)

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-211111
6/9Feb. 22, 2022

Application Note

Light Emitting DiodeThe temperature sensing part of a thermocouple is at the base of the bonding/contacting area of thewires. Figure 4 (b) shows a thermocouple twisted near the base. With this type of thermocouple,
perform the temperature measurement at the base of the twisted part, not at the tip of thethermocouple. With a thermocouple whose temperature sensing part is not at the tip, the TMP
measurement may be lower than the actual TMP even when the tip is in contact with the TMP
measurement point; ensure that the temperature sensing part is attached to contact the TMP
measurement point.

Correct

Incorrect

（a）

（b）

Incorrect（c）

TemperatureSensing PartTemperature
Sensing Part

Temperature
Sensing PartFigure 4. Temperature Sensing Part of Thermocouples

8. TMP Measurement Considerations
Even under the same operating conditions, the TJ of the LED may vary depending on the heatdissipation conditions around the LED. The TMP should be measured once the saturation temperature
at the junction has been reached while ensuring that the LED is incorporated in the chosen
application at the finished product level and in a manner that takes into consideration theconditions/environments in which the LED will actually be used, oriented the way it will actually
be used, and the LED is operated at the maximum possible ambient temperature after aging. Notethat the measurement may be affected by the material of the PCB, the thickness of the copper layer,
the design of the soldering pad pattern, the location of the LED, the environment/components
adjacent to the LED, the heatsink used, etc.
Taking into consideration the possibility of measurement variations it is recommended that theevaluation is performed with more than one LED. The more measurements that are used in theevaluation, the easier it will be to judge the accuracy of the TMP measurement.

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-211111
7/9Feb. 22, 2022

Application Note
9. Summary

Light Emitting DiodeIn this application note, Nichia has provided how to calculate the estimated value of the T J for theLED mounted on a PCB using the thermal resistance RθJMP and the measured TMP. To estimate theTJ more accurately, perform the evaluation as per the precautions/suggestions provided herein
regarding how to attach the thermocouple to the soldering pad pattern and the TMP measurement
conditions/environments.
Additionally, ensure that the chosen design has a sufficient margin to not exceed the absolute
maximum rating TJ by taking into consideration the heat dissipating conditions (i.e. the material,design, etc. of the components used with the LED, mounting conditions of the LED, etc.) and thevariation of the heat dissipation performance of each individual component.

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-211111
8/9Feb. 22, 2022

Application NoteDisclaimer
This application note is a controlled document of Nichia Corporation (Nichia) published to provide
technical information/data for reference purposes only. By using this application note, the user
agrees to the following:・ This application note has been prepared solely for reference on the subject matters incorporated
within it and Nichia makes no guarantee that customers will see the same results for their chosenapplication.

Light Emitting Diode・ The information/data contained herein are only typical examples of performances and/orapplications for the product. Nichia does not provide any guarantees or grant any license underor immunity from any intellectual property rights or other rights held by Nichia or third parties.
・ Nichia makes no representation or warranty, express or implied, as to the accuracy, completenessor usefulness of any information contained herein. In addition, Nichia shall not be liable for any
damages or losses arising out of exploiting, using, or downloading or otherwise this document,or any other acts associated with this document.
・ The content of this application note may be changed without any prior or subsequent notice.
・ Copyrights and all other rights regarding the content of this document are reserved by Nichia or
the right holders who have permitted Nichia to use the content. Without prior written consent of
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hereof, is strictly prohibited.

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TOKUSHIMA 774-8601, JAPAN
Phone: +81-884-22-2311 Fax: +81-884-21-0148

NICHIA CORPORATION
http://www.nichia.co.jp

This document contains tentative information, Nichia may change the contents without notice.
SP-QR-C2-211111
9/9
Feb. 22, 2022