Application Note

Light Emitting Diode

Important Factors to Consider When
Using LEDs for Retrofit Lamps

Table of contents
1. Overview ........................................................................2
2. Structure of a Typical LED Retrofit Lamp ............2
3. Lifetime of a LED Retrofit Lamp .............................4
4. Thermal Evaluation of the LEDs for a LED
Retrofit Lamp ...............................................................6
5. Evaluation of the Lifetimes of Selected LED
Retrofit Lamps .............................................................9
6. Thermal Design Considerations for a LED
Retrofit Lamp ............................................................ 10
7. Summary ..................................................................... 12

The part number NFSx757D-V1 and NF2x757DR-V1 in this document are
the part number of Nichia’s products, and do not have any relevance orsimilarity to other companies' products that may have trademark rights.

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Application Note
1. Overview
As the performance and reliability of LEDs has improved in recent years, the number of light bulbs using
LEDs (i.e. LED retrofit lamp) has increased; much of this volume is due to replacing LEDs in light bulbs that
had previously used conventional light sources (i.e. incandescent bulbs and compact fluorescent lamp
[CFL]).
LED retrofit lamps offer both economic and ecological benefits; two major advantages of LED retrofit
lamps over conventional lamps are:•Low energy consumption•

Long lifetime (e.g. commercially available LED retrofit lamps with a long lifetime of 40,000hours).

However, depending on the thermal design of the LED retrofit lamp (i.e. how to dissipate the heat fromthe LEDs and keep the LED temperature sufficiently low), the lifetime may become shorter than theLight Emitting Diodedesigned lifetime. Considering that LED retrofit lamps are designed to be compact, lightweight, and canbe used for special applications (e.g. hermetically-sealed luminaire, luminaire in contact with or
enclosed/covered by thermal insulation material), careful consideration should be given to ensure
appropriate thermal management.
This application note provides information for understanding the lifetime of LEDs and details on how toestimate the useful life of an LED retrofit lamp.

2. Structure of a Typical LED Retrofit Lamp
Refer to Figure 1 for the structure of a typical LED retrofit lamp and Table 1 for the descriptions of its key
components.
The performance/reliability of the LED may vary depending on the material of the components; ensurethat sufficient verification is performed for the chosen application.

Lamp Cover

LED Light Engine(i.e. LEDs soldered to a PCB)
Housing (Heat sink)
Power Supply Circuit(i.e. LED Driver)
Base

Figure 1. Structure of a Typical LED retrofit lamp
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Application Note
Table 1. Structure of a Typical LED retrofit lamp
Key Component

Function/Feature

Notes

Lamp CoverA lamp cover is a means ofproviding protection for theLEDs and diffusing the light. For
further control of the lamp light(e.g. directing the light upwards,
collimating the light, etc.),
additional lenses may beinstalled inside the lamp cover.

The lamp cover may affect the luminous flux of the lamps
depending on its visible transmittance. To minimize thereduction, a lamp cover with a high visible transmittanceshould be used. However, this lamp cover may not beeffective to diffuse the LED light, increasing the glare of thelamp (i.e. allowing more light to enter the eyes and cause
difficulty seeing objects).

LED Light Engine

A light engine serves as the lightsource for an LED retrofit lamp.
It is a PCB populated with LEDs
and protection devices.

It is important for LED retrofit lamps to dissipate the heatfrom the LEDs effectively; metal core printed circuit boards(i.e. MCPCB) with a high thermal conductivity may berequired depending on the design of the chosen
application.

Light Emitting DiodeThe copper layer should be sufficiently large to ensure thatthe heat from the LEDs spreads evenly and effectively.
To extract more light from the light engine and improve
the luminous flux of the lamp, a material with highreflectivity should be used for the solder mask.
Housing(Heat Sink)

A heat sink is a means ofdissipating the heat from theLEDs and provides mechanicalsupport for the lamp.

The light engine temperature can be high and this is more
likely to occur in the LED retrofit lamp than other lighting
applications. To keep the temperature sufficiently low, athermal interface material (e.g. thermal grease, thermalfilm) with a high thermal conductivity should be used todissipate the heat from the LEDs.
It is possible to improve the heat dissipation capability of
the LED retrofit lamp by optimizing the shape of the heatsink (e.g. heat sink with pins/fins to dissipate heat more
effectively); however, this will increase the lamp’s weight.

Power Supply
Circuit(i.e. LED Driver)

A LED driver holds the power
supply circuit to transform thegeneral-purpose alternating
current (i.e. AC) to the direct
current (i.e. DC) for the LEDs.

The LED driver’s AC-DC transformation efficiency has asignificant effect on the luminous efficacy of the LED
retrofit lamp.

A base (also sometimes referred
to as a lamp cap) is a means toelectrically and mechanicallyconnect the lamp to the lamp
holder of a luminaire.

There are standards/specifications (e.g. shape) for bases
and sockets by the type of luminaire1. For more details,
refer to standards/specifications corresponding to thechosen application.

Base

LED retrofit lamps are designed to be compact and the LED
driver is positioned close to the LED light engine; electronic
components in the LED driver are more likely to beaffected by the heat from the LEDs and become warm. If
this occurs in the electrolytic capacitor, it will extremely
shorten the component’s life and accordingly the lamp
life/performance will be significantly affected.
To provide insulation between the LED driver and the heatsink and allow for better heat dissipation of the LED retrofit
lamp, the LED driver should be completely surrounded by aresin material (e.g. silicone resin).

Note:
1

JIS C 7709-1 Lamp caps and holders together with gauges for the control of interchangeability and safety, Part 1 Lamp caps

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Application Note
3. Lifetime of a LED Retrofit Lamp
As discussed in the Overview section, as the junction temperature of the LEDs increases the lifetime of theLEDs will shorten, causing the life of the LED retrofit lamp to decrease. Understanding the effect oftemperature on lifetimes is critical to ensure good thermal management for the chosen application. For
more details, refer to the following section.

3.1. Factors that affect the life/performance of the LED retrofit lamp
Unlike incandescent light bulbs which stop illuminating suddenly, LED retrofit lamps instead lose theirbrightness over time as the lamp is used. The lifetime of an LED retrofit lamp is defined by the JapaneseIndustrial Standards2 as the point at which the light output (i.e. lumen) has depreciated to 70% of theLight Emitting Diodeinitial light output (i.e. rated luminous flux) when the lamp is operated under specified conditions.
When designing a LED retrofit lamp, the design life may be determined based on the lumen maintenancedata for the LED being used. However, it is important to consider the lifetimes of the other components
in the lamp when estimating the lifetime of the system. For example, electrolytic capacitors are one of thecomponents that are commonly used in the LED driver and are particularly susceptible to heat. Ifelectrolytic capacitors are used where the ambient temperature (i.e. TA) is high, it can cause thecapacitance to significantly decrease causing the component’s life to decrease. The life of an electrolytic
capacitor may be halved for every increase of 10°C in the component. Once the electrolytic capacitor has
reached its end-of-life, it may cause the LED retrofit lamp to flicker or not to illuminate.

3.2. Factors that affect the LED lifetime
LEDs consist of a semiconductor device (i.e. LED die) and other component parts/materials. While LED
dice have very long lifetimes, LED component parts/materials can discolor over time as they are exposed
to high ambient temperature and this causes the LED’s light output to decrease. For the NICHIA 757 series
LEDs, the following changes have a significant effect on the amount of light extracted from the package,leading to a reduction over time in the luminous flux:•

Encapsulating resin discolors and it causes the transmittance to decrease•

Package materials discolor and it causes the reflectance to decrease•

Die-bonding resin discolors and it causes the reflectance to decrease•

Lead frame discolors and it causes the reflectance to decrease

For the internal structure of a NICHIA 757 series LED and the positional relationship between thesepart/materials and the LED die, refer to Figure 2 below.
The LED die temperature (i.e. LED junction temperature) affects the LED lifetime. The higher the LED die
temperature, the higher the temperatures of the surrounding parts/materials and the more rapidly
discoloration progresses in these parts/materials. The following section provides more details on how the
LED junction temperature affects the LED lifetime.
Note:
2

JIS C 8157: 2011 Self-ballasted LED-lamps for general lighting services > 50V – Performance requirements.

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Application Note
Encapsulating Resin

Semiconductor device(i.e. LED die)

Package Materials
Die Bonding Resin

Lead Frame
Figure 2. Internal Structure of a NICHIA 757 series LED

3.3. How to Estimate the LED Lifetime
Nichia provides technical data/information that is required to estimate the LED lifetimes upon request(See Figure 3 below). This includes the Junction Temperature (i.e. TJ) vs. Lifetime data that shows how the
TJ affects the lifetime (i.e. L70 lifetime: time at which the LED has degraded to 70% of its initial luminousflux). For example, if the NFSx757D-V1 or NF2x757DR-V1 LED is used to design a LED retrofit lamp with aLight Emitting Diodelifetime of ≥ 40,000 hours, ensure the TJ ≤ 95°C per the TJ vs. Lifetime data for the LED model (see Figure
3 below). There are important issues to consider when using this technical data/information for the chosen
application:• The lifetimes in the technical information/data are estimates using an average of measured data; themeasurement conditions/environments may differ from those for the chosen application. Ensure athermal design with sufficient margin/tolerances is used for the chosen application.
• If the LEDs are used at junction temperatures (TJ) that exceed the absolute maximum TJ (See Figure3), it may cause issues (e.g. the LED performance to degrade, damage to the LED).
• The LED lifetime can be extended until it reaches a certain point of time; however, it is limited by thecomponent parts/materials. For example, the maximum estimated lifetime of the NFSx757D-V1 or
NF2x757DR-V1 LED is 50,000 hours (See Figure 3).
• If the period of use exceeds the maximum estimated lifetime, it may cause issues (e.g. the LED
performance to degrade, damage to the LED).

Part No.
型番

Maximum Lifetime

Estimated Lifetime

●

⇙

TJ

Absolute
Maximum TJ

Figure 3. Example of the Technical Data (NFSx757D-V1/NF2x757DR-V1 Presumption of Lifetime)
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Application Note
4. Thermal Evaluation of the LEDs for a LED Retrofit Lamp
The lifetime of an LED is determined by the TJ; to estimate the lifetime of the LEDs, the correct TJ value isessential. This section provides details on how to determine the TJ and cautions/suggestions when
performing thermal evaluations.

4.1. How to Determine the LED Junction Temperature (TJ)
For information on how to determine the TJ, refer to Nichia’s Application Note SP-QR-C2-210274 Thermal
Design of the LEDs that details the following two recommendations:• Measuring the temperature at the TS Measurement Point (i.e. TS) or the temperature at the TC
Measurement Point (i.e. TC) to calculate the TJ (i.e. Equation 1)

Light Emitting Diode• Operating the LED under the specified conditions to measure the VF according to Nichia’s Application
Note SP-QR-C2-210274 Thermal Design of the LEDs and calculate the TJ using the VF data.
For other information specific to each LED model that is necessary to perform these calculations (i.e.
location to attach the thermocouple, RθJS/RθJC), refer to both the specification for the LED model and thetechnical data/information as mentioned in Section 3.3. Note that the RθJS/RθJC may vary depending on
the LED; to calculate the TJ use the maximum RθJS/RθJC, instead of the typical RθJS/RθJC.
Equation 1:

TJ = TS + RθJS W（or TJ = TC + RθJC W）

TJ = LED junction temperature: °C
TS = Soldering temperature: °C
TC = LED case temperature: °C
RθJS(RθJC) = Thermal resistance from junction toTS(TC) measurement point: °C/W
W = Input power (IF×VF) : W

4.2. Cautions/Suggestions when performing a thermal evaluation
Ensure that the evaluation LED light engine/conditions meet the following requirements/
recommendations prior to measuring the TS/TC:
1. Verify that the temperature is distributed evenly over the surface of the evaluation LED light engine
2. Estimate the maximum TA for the chosen application and use this value for the evaluation
3. Install the evaluation LED light engine in the actual LED retrofit lamp and measure the TS/TC with all
the lamp component parts (e.g. lamp cover) in place
4. Operate the evaluation LED retrofit lamp to ensure the saturation temperature at the LED junction
has been reached
5. Set the evaluation LED retrofit lamp according to the intended position/orientation
For more details, refer to Sections 4.2.1 to 4.2.5 below.

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Application Note
4.2.1. Verifying the Temperature Distribution over the Evaluation LED Light Engine
The temperature of the LED light engine may not be distributed evenly (See Figure 4-b below) when:• there is a defect in the parts/materials (e.g. housing/heat sink, thermal interface material) on thethermal path between the LED and ambient air• there is an issue with the LED configuration design and it causes an excessive amount of electricalcurrent to a specific LED, causing this LED to generate excessive heat.
These may cause the TJ to exceed the expected value causing the LED life to decrease; the LED retrofit
lamp/LED light engine should be designed to ensure that the temperature is distributed as evenly as
possible on the LED light engine (see Figure 4 below).

Light Emitting Diode

Note that the thermal imaging results represent the LED’s surface temperature (i.e. temperature of theencapsulating resin) instead of the actual TJ of the LED; thermal imaging cameras should be used only toverify the temperature distribution and determine the LED with the highest surface temperature. To
calculate the TJ to estimate the lifetime of the LED retrofit lamp, use this LED and measure the TS/TC.

Figure 4. Example of an Even
Temperature Distribution

Figure 5. Example of an Uneven
Temperature Distribution

Figure 5 shows that there is a significant difference in the LED surface temperature between the left sideand right side of the evaluated LED light engine, indicating that heat is dissipated less efficiently on theleft side than the right.
4.2.2. Using the maximum ambient temperature
The TS/TC of the LEDs changes according to the TA. For special applications (e.g. a hermetically-sealed
luminaire, luminaire in contact with or enclosed/covered by thermal insulation material), the TA is likely tobe higher than the TA in other applications. Before measuring the TS/TC, determine the maximum TA forthe chosen application and use this value for thermal evaluation. Otherwise the estimated lifetime may
be incorrect and longer than the lifetime when used in actual environments.

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Application Note
Refer to Table 2 below for an example of TA/TS measurement data for an LED operated under different
environmental conditions. When there was a difference of 15.2°C in the TA between these twoenvironmental conditions, the difference in the TS between the LEDs in these environments was 15.7°C;
this result indicates that the increase in the TA has a direct effect on the TS.
Table 2. TA/TS Measurement Results
TA (°C)TS (°C)

Environment Condition 1

32.0

71.7

Environment Condition 2

47.2

87.4

Difference in temperature

15.2

15.7

4.2.3. Measuring the TS/TC of the LEDs installed in the chosen LED retrofit lamp

Light Emitting Diode

Ensure that the evaluation LED light engine and all the other component parts of the LED retrofit lamp
are installed in place. Do not dismount the light engine from the lamp and measure the TS/TC of the LEDsor remove the lamp cover from the lamp and measure the TS/TC of the LEDs for the convenience ofmeasurement (i.e. to attach a thermocouple to the LEDs). Otherwise this may cause the thermal path tochange, affecting the TJ calculation. If an opening for the thermocouple is made in the lamp cover/housing
and/or tape is used to secure the thermocouple, ensure that there are no adverse effects on the heatdissipation.

4.2.4. Ensuring that the saturation temperature at the junction has been reached
Operate the evaluation LED retrofit lamp for a sufficient period of time and measure the TS/TC of the LEDs
once the saturation temperature at the junction has been reached.

4.2.5. Positioning the LED retrofit lamp for its intended use
For accurate measurement of the TS/TC, set the evaluation LED retrofit lamp according to the intended
position/direction. The TJ may vary depending on the installation position/orientation of the LED retrofit
lamp. Generally, hot air rises and cold air sinks and this may occur inside the LED retrofit lamp; if theevaluation LED retrofit lamp is not positioned/oriented in the intended manner/direction, it may affectthe internal temperature distribution of the lamp causing the calculated TJ to be inaccurate.

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Application Note
5. Evaluation of the Lifetimes of Selected LED Retrofit Lamps
This section provides an analysis of the lifetimes of four 60W LED retrofit lamps with different shapes. All
these selected lamps use either a Nichia 757 series LED or a competitor’s equivalent LED for its lightengine. For the measured electrical/optical data and temperatures of the selected lamps, refer to Section
5.1. For the analysis of the lifetimes of these LED retrofit lamps, refer to Section 5.2.

5.1. Characteristics/temperature measurement
The electrical/optical characteristics and temperature measurements were performed on the selected LED
retrofit lamps prior to estimating the lifetimes.

5.1.1. Measurement method

Light Emitting Diode

1. The selected LED retrofit lamps were operated at TA=25°C.
2. Once the saturation temperature was reached, the electrical/optical characteristics of the selected
lamps were measured.
3. The LED with the highest surface temperature among the LEDs on the light engine was determined
and the TS was measured.
4. The temperature of the LED driver was measured.
5. The TJ of this LED was calculated using a thermal resistance (RθJS) of 17°C/W (max.)3.

5.1.2. Measurement results
Table 3. Measurement results of selected LED retrofit lamps

Lamp

LED
LED Driver

Lamp A

Lamp B

Lamp C

Lamp D

Luminous Flux

(lm)

881

855

923

820

Electric Power

(W)

8.7

10.4

12.1

7.5

TS

(°C)

74

88

101

74

TJ4

(°C)

81

95

113

81

Temperature

(°C)

68

69

80

66

5.2. Evaluation of the lifetimes
The lifetimes of the selected LED retrofit lamps were estimated using the calculated TJ in Table 3 above toverify that the LED retrofit lamps have a lifetime of 40,000 hours as claimed for many 60W LED retrofit
lamps.
Note that the LED TJ vs. estimate lifetime characteristic of the NF2x757DR-V1 LED was used for thefollowing evaluation of all the selected LED retrofit lamps’ lifetimes.
Note:
3

Since the selected lamps use either a Nichia 757 series LED or a competitor’s equivalent LED for its light engine, the thermal resistance(RθJS) of the NF2x757DR-V1 LED was used for all these lamps.

4

Calculated based on the measured TS for each lamp

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Application Note
5.2.1. Evaluation results
Lamp A and Lamp D
The estimated lifetime was 50,000 hours, exceeding the targeted lifetime (i.e. 40,000 hours) with asufficient TJ tolerance (i.e. 14°C). The measured TJ was no more than 81°C (See Table 3) while the TJ at
which these LED retrofit lamps (i.e. Lamp A and Lamp D) can last to 40,000 hours is 95°C (See Figure 6).
Lamp BThe estimated lifetime reached 40,000 hours with little TJ tolerance. Depending on the variation of the
LED/LED driver characteristics, the actual lifetime may be shorter than 40,000 hours. Note that the TA
may be higher and accordingly, the lifetime may be shorter than 40,000 hours when the LED retrofit
lamp (i.e. Lamp B) is used in a special application (e.g. a hermetically-sealed luminaire or a luminaire inLight Emitting Diodecontact with or enclosed/covered by thermal insulation material).
Lamp CThe measured TJ significantly exceeded 95°C (i.e. TJ=113°C per Table 3) and accordingly the estimatedlifetime was no more than 20,000 hours. Additionally, the measured temperature of the LED driver was
the highest among the selected LED retrofit lamps; this may cause the electrolytic capacitor life to
decrease, adversely affecting the lifetime of the lamp.

Lamp A・D●●● Lamp B● Lamp C
81°C

95°C

▲

▲

▲
113°C

Figure 6. Lifetime Estimation of Selected LED Retrofit Lamps

6. Thermal Design Considerations for a LED Retrofit Lamp
The following are items that should be considered when designing a LED retrofit lamp to ensure goodthermal management of the LEDs.

6.1. Performing the thermal evaluation with an accurate TJ
Sufficient verification is required at the design stage to ensure that there are no issues with the thermal
design. If the LED retrofit lamp allows the TJ to exceed the design value while in actual use, it may cause
issues (i.e. color shift, decrease in the luminous flux) before the end of its designed lifetime. For the detailsof how to determine the TJ and lifetime of the LEDs, refer to Section 4.

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Application Note
6.2. Ensuring the thermal design with a sufficient tolerance
For LEDs/LED drivers, there are variations in their characteristics; this requires that the design of the chosen
LED retrofit lamp have a sufficient tolerance for the target T J (i.e. the maximum possible TJ for the target
lifetime of the LED retrofit lamp per the TJ vs Lifetime data for the LED being used). Otherwise, while the
LED retrofit lamp is operated in actual use the TJ of the LEDs may exceed the target TJ and this may cause
the LED retrofit lamp to reach its end-of-life earlier than the target lifetime. To improve the lifetime of thislamp, appropriate thermal management measures should be taken to ensure that the thermal design has
a sufficient tolerance. Examples are:
• Review the component parts that dissipate the heat (e.g. housing/heat sink, PCB for the LED lightengine)Light Emitting Diode• Reduce the input power

6.3. Ensuring that the absolute maximum TJ is not exceeded
The thermal design must ensure that the absolute maximum TJ is not exceeded under any circumstances.
Otherwise the reliability/performance of the LEDs will be jeopardized (e.g. performance degradation,
damage to the LED). If the LED retrofit lamp is designed for specific applications (e.g. a hermetically-sealed
luminaire, luminaire in contact with or enclosed/covered by thermal insulation material), the resulting
effect on the LEDs (i.e. increase in the TJ) must be considered and necessary thermal management
measures are incorporated into the design.

6.4. Ensuring that the absolute maximum current is not exceeded
The circuits in the LED driver and the LED light engine must be designed to ensure that the absolute
maximum current is not exceeded under any circumstances. Otherwise the reliability/performance of theLEDs will be jeopardized (e.g. performance degradation, damage to the LED). If the LED driver produces acurrent waveform with noticeable periodic variation (i.e. ripple), the average value of the current flowing
through the circuit may be lower than the absolute maximum current for the LEDs and yet the peak current
may not. To ensure that this does not occur in the circuit of the chosen application, use an oscilloscope toverify that any part of the current waveform does not exceed the absolute maximum current.

6.5. Taking measures to protect the LEDs from surges
Measures should be taken to protect the LEDs from surges (i.e. voltage/current spikes). If excess
voltage/current is applied to the LEDs, it may cause the LED to be damaged. This may occur when the LED
retrofit lamp is first turned on (i.e. inrush current) or if electrostatic discharge occurs (e.g. two electricallycharged objects come in contact with each other). Nichia recommends using a protection device for theLEDs in both the LED driver and LED light engine circuits.

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Application Note
7. Summary
Since the lifetime of an LED can vary depending on the TJ, the thermal management of the LEDs is criticaland careful consideration should be given to the thermal evaluations (e.g. measuring the T S/TC andcalculating the TJ). When determining the overall system design, ensure that other factors that affect theTJ (e.g. variations in the LED/LED driver characteristics) are considered and the thermal design has asufficient margin for the target lifetime.
For more details on the information/data and technical documents outlined in this application note,Light Emitting Diodecontact Nichia’s sales team.

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Application Note

Light Emitting Diode

Disclaimer
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incorporated within it and Nichia makes no guarantee that customers will see the same
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