Application Note

Light Emitting Diode

Thermal Design - Selecting an Appropriate
Heatsink for the COB-Z Series LEDs

Table of contents
1. Overview............................................................................................. 2
2. Evaluation Objectives .................................................................... 2
3. Evaluation Method ......................................................................... 2
4. Evaluation Results ........................................................................... 4
5. Summary ............................................................................................ 8

The Nichia part numbers NJCxS024Z, NVCxL024Z, and NVExJ048Z within this document
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.
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Application Note
1. Overview

Light Emitting DiodeIn recent years, many luminaire designers have started to use COB LEDs in theirapplications and COB LED-based luminaires have become available on the market. Whendesigning a COB LED-based luminaire, one of the key requirements is to understand thethermal management of COB LEDs.
COB LEDs can produce high-power outputs and are designed to have multiple LED die
assembled close to each other in a single COB LED, causing heat to concentrate in a narrow
area. If COB LEDs become excessively hot during operation, it will cause the luminous flux
to decrease (i.e. temperature characteristics of COB LEDs) and/or shorten the lifetime. To
avoid the performance/reliability degradation, proper thermal management of COB LEDs
will be required, especially for the heatsink as it will play the primary role in dissipating theheat of the COB LED.
Nichia performed evaluations of heatsinks with an emphasis on the relationship between
the volume of a heatsink and the heat dissipation performance of the heatsink (i.e. junction
temperature of the COB LED). This application note provides information and data fromthe evaluations to select an appropriate heatsink for the COB-Z series LEDs.

2. Evaluation Objectives
The evaluations were performed to determine:
1. the change in the junction temperature (TJ) depending on the volume of the heatsink
when the COB LEDs were operated in a constant current mode,2. the heatsink volume required to ensure that the absolute maximum junction
temperature (TJMax) is not exceeded.

3. Evaluation Method
A COB LED was secured to an evaluation heatsink via COB LED holders and screws. Then,the assembly was installed in an enclosure1 with no ventilation and operated at thesorting/absolute maximum current of the COB LED (i.e. the constant current). Once thesaturation temperature at the junction had been reached, the temperature at the TC
measurement point (i.e. TC) was measured to calculate the TJ. For the part numbers andspecifications of the evaluated COB LEDs, refer to Table 1 below. For the evaluation
environment and the specifications of the assembly components except for the COB LEDs,
refer to Figure 1 below. For the position of the TC measurement point, refer to Figure 2below.

Note:1
Enclosure size (mm): L=350 x W=350 x H=350
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Application Note
Table 1. Specifications of the evaluated COB LEDs

Light Emitting DiodePart Number

IF [mA]

VF [V]Sorting Current

Max. Rating Current

Typ.

NJCxS024Z

500

1000

35.8

NVCxL024Z

1200

2200

35.4

NVExJ048Z

1800

3300

47.2

TJmax [°C]150

Enclosure1 (with no ventilation)
COB LED secured to a heat sink
via holders25052 aluminum-alloy Heatsink3

Figure 1.

Evaluation environment

TC measurement point

Figure 2.

Position of the TC measurement point

Note:
2
Thermal grease was used between the COB LED and the heatsink.
3
Different volumes of heatsinks were used
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Application Note
4. Evaluation Results
4.1 The NJCxS024Z COB LEDs
4.1.1 Change in the TJ depending on the volume of the heatsink
Refer to Figure 3 for the change in the TJ vs. the volume of the heatsink when operated at:- IF = 500mA (i.e. the sorting current), which is equivalent to 17W- IF = 1000mA (i.e. the absolute maximum rating current), which is equivalent to 36W

If the COB LEDs are operated at IF = 1000mA, a heatsink with a volume more than 50cm 3is required to ensure that TJ does not exceed 150°C (i.e. TJMax).

NJCxS024Z Volume of Heatsink vs. TJ

175

150°C (TJMax)

150

IF=1000mA(equivalent to 36W)
IF=500mA(equivalent to 17W)

125

TJ[°C]

Light Emitting Diode4.1.2 Minimum volume to ensure that the TJMax is not exceeded

100
7550

TA=25°C

25050

100

150

200

250

300

350

Volume of Heatsink [cm3]

Figure 3. Evaluation Result for the NJCxS024Z COB LEDs

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Application Note
4.2 The NVCxL024Z COB LEDs
4.2.1 Change in the TJ depending on the volume of the heatsink
Refer to Figure 4 for the change in the TJ vs. the volume of the heatsink when operated at:- IF = 1200mA (i.e. the sorting current), which is equivalent to 41W- IF = 2200mA (i.e. the absolute maximum rating current), which is equivalent to 75W
4.2.2 Minimum volume to ensure that the TJMax is not exceeded

NVCxL024Z Volume of Heatsink vs. TJ

175

150°C (TJMax)

150

IF=2200mA(equivalent to 75W)
IF=1200mA(equivalent to 41W)

125

TJ[°C]Light Emitting DiodeIf the COB LEDs are operated at IF = 2200mA, a heatsink with a volume more than 1100cm3
is required to ensure that TJ does not exceed 150°C (i.e. TJMax).

1007550

TA=25°C

25300

500

700

900

1100

1300

1500

Volume of Heatsink [cm3]

Figure 4. Evaluation Result for the NVCxL024Z COB LEDs

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Application Note
4.3 The NVExJ048Z COB LEDs
4.3.1 Change in the TJ depending on the volume of the heatsink
Refer to Figure 5 for the change in the TJ vs. the volume of the heatsink when operated at:- IF = 1800mA (i.e. the sorting current), which is equivalent to 81W- IF = 3300mA (i.e. the absolute maximum rating current), which is equivalent to 150W

As shown in Figure 5 below, when the COB LEDs were operated at IF = 3300mA, increasing
the volume of the heatsink did not have a significant effect on reducing the TJ. Since there
are cases where this approach is not effective, alternative solutions (e.g. use with coolingfans, heatsink with heat pipes, etc.) may be required to ensure that the TJ does not exceed
150°C (i.e. TJMax). The following section discusses the effectiveness of heatsinks with heatpipes as a solution to the issue.

NVExJ048Z Volume of Heatsink vs. TJ

200

IF=3300mA(equivalent to 150W)
IF=1800mA(equivalent to 81W)

175
150

150°C (TJMax)
125

TJ[°C]

Light Emitting Diode4.3.2 Minimum volume to ensure that the TJMax is not exceeded

100

75
50

TA=25°C

25500

750

1000

1250

1500

1750

2000

Volume of Heatsink [cm3]

Figure 5. Evaluation Result for the NVExJ048Z COB LEDsThis document contains tentative information, Nichia may change the contents without notice.
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Application Note
4.4 Evaluation using a Heatsink with Heat Pipes
Nichia performed an evaluation to determine the effectiveness of heatsinks with heat pipes
in dissipating heat from the COB LED, especially in cases where increasing the volume ofthe heat sink is not sufficient (e.g. driving the NVCxL024Z/ NVExJ048Z COB LEDs at highwattages).
Refer to 4.4.1 for the evaluation method and conditions used and 4.4.2 for the results.

Light Emitting Diode4.4.1 Evaluation Method/Conditions
A COB LED was secured to a heatsink with heat pipes via a COB LED holder and screws.
Thermal grease was used between the COB LED and the heatsink. Then, the assembly wasinstalled in an enclosure with no ventilation and operated at the absolute maximum currentof the COB LED (i.e. the constant current). Once the saturation temperature at the junction
had been reached, the temperature at the TC measurement point (i.e. TC) was measured tocalculate the TJ.
LED Part No.
LED/Heatsink
Orientation
Cooling MethodAmbient Temp.
Heatsink withHeat Pipes

HolderScrews
Thermal Grease

NVCxL024Z, NVExJ048Z
See the image to the left.
Natural cooling (No ventilation
openings/gaps within the enclosure)25°C
HYC200 manufactured by Furukawa
Electric Co., Ltd.
143 x 130 x 220 [mm]Aluminum, Copper
Manufactured by TE Connectivity Ltd.
M3 pan-head screws tightened with atorque of 0.5N∙m (without washers)
SPG-30B manufactured by Fuji
Polymer Industries Co., Ltd.
Conductivity: 3.1W/m∙K

Figure 6. Conditions of the evaluation using a heatsink with heat pipes

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Application Note
4.4.2 Evaluation Results
As shown in Table 2 below, Nichia verified that when the NVCxL024Z COB LEDs and theNVExJ048Z COB LEDs were operated at the absolute maximum rating currents while being
cooled by the heatsink with heat pipes, the TJ did not exceed the TJMax.
If these COB LEDs or similar/successor COB LEDs are designed into a high-wattage
application, determine the appropriate thermal solution (e.g. use with cooling fans,heatsink with heat pipes, etc.) for the application and perform a sufficient verification toensure that there are no issues with the thermal design since the COB LED can produce asignificant amount of heat.
Table 2. Specifications for the evaluated COB LEDs and evaluation results

Light Emitting Diode

Evaluation LED
NVCxL024Z

NVExJ048Z

IF [mA]
2200mA(i.e. absolute maximum
rating current)
3300mA(i.e. absolute maximum
rating current)

TJ4 [°C]

TJMax [°C]

117
150
128

5. Summary
Since the volume of the heat sink has an effect on the TJ for COB LEDs, it is very importantto choose a heatsink of sufficient volume to improve the performance and the reliabilityof the chosen application.
To optimize the thermal design of the chosen application, it is critical to determine the T J
when the assembly/system and the COB LED are being operated. Since this T J can varydepending on the heatsink being used and Nichia COB LEDs have different thermal
resistances (i.e. RθJC), an actual measured TC should be used to calculate the TJ.
To maximize the heat dissipation for a chosen heatsink, the luminaire designer should also
consider how the COB LED is attached to the heatsink to ensure sufficient adhesion (e.g.
determining the correct tightening torque, thermal interface material [TIM], and theamount of the TIM, etc.). For more details, refer to Nichia’s application note entitled Heatdissipation performance according to the adhesion strength of COB and housing.

Note:
4The TJ when the saturation temperature at the junction had been reached. These T J are calculated values.
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Application Note

Light Emitting DiodeDisclaimer
This application note is a controlled document of Nichia Corporation (Nichia) published toprovide technical information/data for reference purposes only. By using this applicationnote, 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 chosen application.
 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 licenseunder or immunity from any intellectual property rights or other rights held by Nichia orthird parties.
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 The content of this application note may be changed without any prior or subsequent
notice.
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http://www.nichia.co.jp

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