docs/XLampThermalManagement.pdf — Oasis datasheet

APPLICATION NOTE

Thermal Management of XLamp® LEDs

Figure 1: Thermal path of an XLamp® LED

TABLE OF CONTENTS

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Fundamentals of Heat Transfer

Conduction

Convection...

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XLamp LED Componentt..........cceceeeseseeeeseeeteeeeeeseeeeeeeeeeeeeeseneeeees 9

Printed Circuit Board ou... cece cece eeeseeseeteeeseeseseeeeeeeees 9

Thermal Interface Material ..0........0. cece eeneseeneseeeseeneenees 9

Heat Sink.

Fans/Active Coolin

Thermal Measurement

Thermocouples .....

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INTRODUCTION

XLamp® power LEDs lead the industry in brightness and reliability, enabling the LED lighting revolution with energy-efficient, environmentally friendly light. To take full advantage of the benefits of XLamp LEDs, proper thermal management must be understood and employed. This application note explains the importance of thermal management, discusses basic thermal fundamentals and details some general guidelines and design recommendations. A basic overview of methods to measure and simulate thermal performance is also provided.

Cree LED / 4001 E. Hwy. 54, Suite 2000 / Durham, NC 27709 USA / +1.919.313.5330 / www.cree-led.com

© 2004-2023 Cree LED. The information in this document is subject to change without notice. Cree®, the Cree logo, the Cree LED logo, and XLamp® are registered trademarks of Cree LED. Other trademarks, product and company names are the property of their respective owners and do not imply

specific product and/or vendor endorsement, sponsorship or association. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree-led.com.

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THERMAL MANAGEMENT OF XLAMP® LEDS

IMPORTANCE OF PROPER THERMAL MANAGEMENT

A main cause of LED failures is improper thermal management. Many performance characteristics of LED components are influenced by the operating temperature, so LED system designers need a basic understanding of thermal design and performance. All XLamp LED product data sheets detail the performance characteristics that are impacted by temperature. Some performance characteristics experience a recoverable change, such as light output, color and voltage, while others, such as lifetime, can experience a non-recoverable degradation due to high operating temperatures. However, exceeding the maximum operating temperature specification, which is typically a 150 °C junction temperature, can cause permanent and/or catastrophic damage to XLamp LEDs, so care must be taken to operate LEDs

below this limit.

Light Output

Elevated junction temperatures cause recoverable light output reduction, which is plotted on each of XLamp LED data sheets. As the junction temperature increases, the light output of the LED decreases, but recovers when the LED cools. Below in Figure 2 is a chart showing the relative flux versus junction temperature from the XLamp XB-D LED data sheet. The XB-D LED, among many other new XLamp LEDs, is binned at 85 °C, so the relative flux data is based on 100% light output at an 85 °C junction temperature.

120%

100%

80%

60%

40%

Relative Luminous Flux

20%

0% 25 50 75 100 125 150

Junction Temperature (°C)

Figure 2: XLamp XB-D LED relative flux vs. steady-state junction temperature

Color

With increasing junction temperatures, the color of all LEDs shifts. While Cree LED manufactures the industry's best components with the smallest amount of color shift, this effect needs to be understood for proper system design. Shown below in Figure 3, again from the XB-D data sheet, are the CCx and CCy shifts relative to solder point temperature (T,,). It is worth noting that the data in this plot shows that there is less than a 0.004 shift in either direction for the full range of operating temperatures. This will not be the case with most competitor’s LEDs on the market which typically shift drastically further than this.

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THERMAL MANAGEMENT OF XLAMP® LEDS

u.uzu

T_ = 350mA F

0.015

0.010

0.005

0.000

— ACCx

-0.005

= = Accy

-0.010

0.015

-0.020

0 20 40 60 80 100 Tsp (°C)

120

140 160

Figure 3: XLamp XB-D LED color shift vs. solder point temperature

Voltage

Forward voltage decreases as the junction temperature of an LED increases. This is shown on each of the XLamp LED data sheets as the

temperature coefficient of voltage, and varies slightly depending on the color and package type. This value varies from approximately -1 to -4 mV/°C per LED. It is important to understand the full operating conditions for an LED system so the driver can accommodate the potential range of drive voltages over the operating temperature of the system. An example of this is shown later in this document.

Reliability

The reliability of any LED is a direct function of junction temperature. The higher the junction temperature, the shorter the lifetime of the LED. Data from an IES LM-80- 08 report can be used to predict the lumen maintenance of an LED under various temperature and drive current operating environments. Cree LED has published an LM-80 summary for its XLamp LEDs and full LM-80 data can be obtained by contacting a sales representative.

Cree LED Components . IES LM-80-2008 Testing Results RIVA Revision: 81 (March 30, 2023) Yes

Figure 4: XLamp LED LM-80 summary

ATM-21 calculator from the Environmental Protection Agency (EPA) can be used with the LM-80 data to predict lumen degradation and

expected lifetime under specific operating conditions.

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THERMAL MANAGEMENT OF XLAMP® LEDS

Performance Example

An XLamp XB-D LED from the Q4 flux bin (100 Im at 350 mA, 85 °C) is sourced at 700 mA in an initial ambient temperature of 25 °C and heated to a 60 °C junction temperature. At what lumen output and voltage did the lamp start and end?

This is shown below in Table 1.

Table 1: Performance characterization vs. temperature

85 °C (binned) 172.3 3.087 79.7 25 °C (initial room temperature) 196.1 | 3.237 | 86.5 60 °C (steady state) | 182.2 | 3.149 | 82.7

A few lessons can be learned from this example, including the drop in lumens and lumens per watt as the temperature increases. Additionally, the decrease in voltage can become significant when a large number of LEDs are included in a system, especially in very low initial ambient temperatures. When designing luminaire systems, all of this needs to be considered for proper final designs.

HEAT GENERATION

LEDs generate visible light when current passes across the junction of the semiconductor chip. However, LEDs are not 100% efficient; much of the power running through an LED is output as heat, which thus needs to be dissipated. Royal blue XLamp LEDs are over 50% efficient and white XLamp LEDs are over 40% efficient. That is, under normal operating conditions, approximately 50% to 60% of the input power is output as heat, while the rest of the input power is converted to light. To be conservative, assume LEDs convert 25% of the input power to light and output 75% of the input power as heat. This estimate varies depending on current density, brightness and component, but is a good estimate for thermal design. Equation 1 below shows how to calculate the thermal power.

P,=0.75 V, |, Equation 1: Thermal power calculation where: P, is the thermal power (W) V, is the forward voltage of the LED (V) |, is the source current to the LED (A)

The V, and I, can be measured directly or calculated, so the thermal power can easily be calculated. This is the amount of power the system/heat sink must dissipate.

FUNDAMENTALS OF HEAT TRANSFER

There are three basic modes of heat transfer: conduction, convection and radiation. Each plays a role in LED performance and final system design and must be understood for proper thermal management.

Conduction Conduction is the transfer of heat through a solid material by direct contact. This is the first mode of heat transfer to get thermal power from the LED junction to the heat sink. Metals are typically the best conductors of heat. The heat conduction potential of all materials can

© 2004-2023 Cree LED. The information in this document is subject to change without notice. Cree®, the Cree logo, the Cree LED logo, and XLamp® CLD-APS 4 are registered trademarks of Cree LED. Other trademarks, product and company names are the property of their respective owners and do not imply REV 5

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be expressed as thermal conductivity, typically abbreviated as k. Equation 2 below shows how to calculate the quantity of heat transferred via conduction.

AT Qoong =k A zz Equation 2: Fourier's law of heat conduction where: Quong iS the amount of heat transferred through conduction (W) k is the thermal conductivity of the material (W/m K) Ais the cross sectional area of the material through which the heat flows (m7?) AT is the temperature gradient across the material (°C) Ax is the distance for the heat must travel (m)

Convection

Convection is the transfer of heat through the movement of fluids and gases. In LED systems, this is typically the transfer of heat from the heat sink to the ambient air. There are two sub-categories of convection: natural and forced. Natural convection occurs with no artificial source of field movement and is due to the buoyancy forces induced by thermal gradients between the fluid and solid. Forced convection occurs when an external instrument such as a fan, pump, or other device is used to artificially move the fluid or gas. In LED cooling systems, convection is the main mode of heat transfer to remove the generated heat from the LED system and heat sink. Equation 3 below shows how to calculate the quantity of heat transferred via convection.

Qo = A AT Equation 3: Newton's law of cooling (convection) where: Q.ony is the amount of heat transferred through convection (W) his the heat transfer coefficient (W/m?K) Ais the surface area (m?) AT is the temperature gradient across the material (°C), typically the difference between the surface temperature and

ambient air temperature

The fundamental challenge in calculating the heat transfer via conduction is determining and computing the heat transfer coefficient (h). Typical values for h can vary significantly depending on boundary conditions, geometry and many other factors. However, for natural convection h will usually be in the range of 5-20 W/mK, while for forced convection h can be as high as 100 W/m°K for air and up to 10,000 W/m°K for water. Typically, for natural convection in air, a value of 10 W/mK is a good assumption for an initial rough calculation.

Radiation

The transfer of thermal energy through an electromagnetic field is the third component of heat transfer, radiation. The magnitude of radiation heat transfer is based on the emissivity of the material, which is the ratio of how closely the surface approximates a blackbody. In an LED system, radiation typically has a very small effect on the net system heat transfer since the surface areas are typically fairly small and surface temperatures are relatively low, to keep the LED junction temperatures below the maximum rated temperature of 150 °C. Equation 4 below shows how to calculate the quantity of heat transferred via radiation.

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THERMAL MANAGEMENT OF XLAMP® LEDS

Q.g=e& 6 A (TT) Equation 4: Radiative heat transfer equation

rad

where: Q,,4 is the amount of heat transferred through radiation (W) € is the emissivity of the surface (dimensionless) ois the Stefan-Boltzmann constant (5.67 x 10° W/m?K‘) Ais the surface area (m?) T, is the surface temperature of the material (°C) T, is the fluid temperature of the medium (°C), typically referenced to the ambient air temperature

THERMAL PATH/MODEL

The thermal path of an LED system can be illustrated by a simple resistor network similar to an electrical circuit. Thermal resistances are represented by the resistors, the heat flow is approximated by the electrical current, and the corresponding temperatures within the system correspond to the electrical voltages. Below in Figure 6 is a resistor network representation of a multiple-LED system on a printed circuit board (PCB) mounted to a heat sink in ambient air.

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THERMAL MANAGEMENT OF XLAMP® LEDS

junction

Theat sink Ohs-a

ambient

Figure 5: Thermal circuit of an LED array (not to scale)

where: T is the temperature at each corresponding location (°C) @,,, is the thermal resistance from point a to point b (°C/W) nis the number of LED components on a single PCB

To summarize the heat path illustrated in the Figure 6 above, heat is conducted from the LED junctions through the LED components to the PCB, through the thermal interface material (TIM) to the heat sink and then convected and radiated to the ambient air.

The nodes in the circuit represent the individual sections within the system and the locations where temperatures may be measured. For example, the solder point temperature (T,,,., point) represents the location on the board, as specified in the corresponding data sheet for each XLamp LED, where the temperature on the top of the PCB can be measured. This can be used to calculate the junction temperature,

which is detailed in a later section.

The resistors represent the thermal resistances of the individual contributors. For example, 0, ., represents the thermal resistance of the LED component from junction to solder point.

The system is divided into a network of parallel connections for the multiple LEDs and series connections for the singular components. If the system includes only one LED, or n =1 in Figure 5, the entire thermal path is simply in series.

The individual thermal resistances described above can be calculated from Equation 5 below.

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THERMAL MANAGEMENT OF XLAMP® LEDS

T,-T,

O.,= P, Equation 5: Individual thermal resistance calculation where: @,,, is the thermal resistance from point “a” to point “b” (°C/W) T, is the temperature at point “a” (°C) T,, is the temperature at point “b” (°C) P, is the thermal power as calculated in Equation 1

The thermal resistance of the entire system can also be compared to an electrical circuit in series, where the system thermal resistance can be calculated as shown below in Equation 6. O..027 O.+ O,,+ + 9, Equation 6: System thermal resistance calculation where:

©.,.a2 18 the system thermal resistance from point “a” to point “z” (°C/W)

@,,, is the thermal resistance from point “a” to point “b” (°C/W)

@, , is the thermal resistance from point “b” to point “c” (°C/W)

©,,, is the thermal resistance from point “y” to point “z” (°C/W)

In an LED system, the total system-level thermal resistance is typically defined as “junction to ambient”, or © ,,. This quantifies how well each component transfers thermal power. Each XLamp LED has a current de-rating curve in its data sheet that gives the maximum drive current versus ambient temperature for a few system thermal resistances, or ©, ,. Shown below in Figure 7 is the current de-rating curve for an XLamp XB-D LED from the XB-D data sheet.

1200 1000 =< £ 300 = i= o 5 g 600 = é = 400 g mm F-a = 10°C/W ——Rj-a = 15°C/W 200

——Rj-a = 20°C/W ——Fj-a = 25°C/W

0 20 40 60 80 100 120 140

Ambient Temperature (°C)

Figure 6: XB-D current de-rating curve

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THERMAL MANAGEMENT OF XLAMP® LEDS

To use the current de-rating curve in Figure 7, the system-level thermal resistance must be calculated, as detailed above in Equation 6. The ambient temperature must also be known, and from this, the maximum drive current for this thermal design can be determined from the graph in Figure 7.

THERMAL STACK

For purposes of thermal analysis, an LED system typically consists of a multi-component assembly, called a thermal stack, in which all components contribute in varying degrees to the total system thermal performance. In a typical system, the LED is soldered to a PCB, either metal core or FR4, which is then usually attached to a heat sink. It is critical to maximize heat transfer between the heat sink and PCB, so a good TIM is needed to fill any air voids. The best method to enhance the thermal path is to minimize the number of materials in the thermal stack and use the most thermally conductive materials available. Below is a description of this typical LED system with some suggestions and comments to consider for each element.

XLamp LED Component Heat is generated at the junction of the LED chip within the XLamp LED component. The amount of heat can be calculated from Equation 7 below based on the measured T,,, and the thermal resistance of the LED, as stated on its data sheet.

T,= Typ + O,, P, Equation 7: Junction temperature calculation

total

where: T, is the junction temperature (°C) T,, is the measured solder point temperature (°C) @,, is the thermal resistance of the component (°C/W) P.,ta1 iS the total power (W) input to the LED (I, x V,)

All XLamp LEDs must not exceed their maximum junction temperature of 150 °C, as specified on each data sheet.

Printed Circuit Board

Most XLamp LEDs are required to be mounted on a PCB to provide electrical and mechanical connections to additional components such as the driver and heat sink. The thermal effect of the PCB can be significant, so care must be used when choosing or designing a PCB. Cree LED has published a technical article, “Optimizing PCB Thermal! Performance” that details PCB design recommendations and offers tips. Please consult this article for further information and advice.

Thermal Interface Material

The thermal interface material can play a large role in the system thermal performance, depending on the design choices made. TIMs are critical to minimize the air gaps between the heat sink and the PCB. TIMs not only provide a thermal interface between the PCB and the heat sink, but depending on the application these can have other functions as well, such as electrical insulation or making a mechanical connection. Many types of TIMs are used in LED systems including greases, tapes, pads, and epoxies. Each has its advantages and disadvantages depending on the application. Table 2 below shows some of the benefits and drawbacks of various materials.

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THERMAL MANAGEMENT OF XLAMP® LEDS

Table 2: Relative properties of TIMs'

Adhesive films Good Poor Fair Fair Poor Fair

Adhesives Good Good Excellent Excellent Poor Fair

Compounds Good Excellent Excellent Excellent Good Excellent

Encapsulants Excellent Good Good Excellent Good Good

Gap fill pads Excellent Fair Poor Fair Excellent Good

Gels Good Good Good Excellent Good Excellent

Phase changes Excellent Excellent Fair Excellent Good Excellent Many characteristics must be considered when selecting a TIM, not just the thermal conductivity. Often overlooked is the bond line thickness of the material, and as shown below in Equation 8, the thermal resistance of the material is highly dependent on this thickness. The TIM manufacturer will provide these basic characteristics on their own data sheet, and it is important to understand how all the

characteristics work together and to decide which is the most important for each specific application.

Sometimes a thinner TIM with poor thermal conductivity has a lower thermal resistance than a thicker TIM with better thermal conductivity. Both these attributes must be considered when selecting a TIM and their relative effects can be quantified with Equation 8 below. However, though a TIM may have better thermal conductivity than air, its conductivity will not be nearly as good as metal’s, so the approach is not to add material between metal components but rather to fill the voids that are typically occupied by air. Just remember, thin to win!

eo. = —_ TIM kA

Equation 8: TIM thermal resistance calculation where: ©, is the thermal resistance of the TIM Lis the thickness of the TIM (m) k is the thermal conductivity of the TIM (W/m K) Ais the contact area (m?)

Heat Sink

The heat sink is the last and most influential part of the thermal stack, and is needed to first conduct heat away from the LEDs and then to convect and radiate heat to the ambient air. Thus, the first task of the heat sink necessitates that the heat sink be fabricated from a high thermal conductivity material to conduct heat away. The second task requires that the heat sink have a large surface area to convect heat to ambient and also have good emissivity so it can radiate heat away. In some cases, heat sinks are coupled to other heat dissipating devices such as housings, enclosures, etc. For this document, we group these devices under the general term “heat sink”, but this should not be overlooked in system design as it can contribute significantly to the performance of the entire LED system.

Table 3 below shows the thermal conductivity of some common materials as well as a rough range of their emissivity, which can vary significantly depending on the material finish. Choosing the highest thermal conductivity and/or emissivity is obviously not always

1 www.dowcorning.com/content/etronics/etronicswet/relativepropertiespop.asp?popup=true

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possible because of other factors that must be considered such as weight, cost, and manufacturability. Each of these must be evaluated for each application to determine the best material and manufacturing process.

Cast or forged aluminum is typically used for heat sinks. Anodizing aluminum gives a heat sink a much higher (to about 0.8) emissivity than standard aluminum and helps with radiative heat transfer to the environment.

Table 3: Thermal conductivity and emissivity for various materials at 25 °C

Thermal Conductivity (W/m K) Approximate Emissivity

Acrylic 0.2 0.94

Air 0.024 Not applicable Aluminum 120 - 240 0.02 - 0.9 (depending on finish) eSeanites Aluminum ntide, 100-200 area Conductive polymers 3~20 Not applicable

Copper 401 0.05 - 0.8 (depending on finish) Diamond 2000 1.0

FR4 0.2 0.7-0.8

Glass 1.05 0.6 - 0.97

Silicon 150 0.6

Silicon carbide 350 0.85

Silver 429 0.02 - 0.074 Stainless steel 16 0.1 - 0.9 (depending on finish) Thermal grease/epoxies/pads 0.1~10 Not applicable

Water 0.58 0.85 - 0.99

Wood 0.17 0.8-0.9

Heat sink design can be very complicated and limited by many restrictions such as space constraints, cost, weight, manufacturability and countless other requirements. There is no one right answer to heat sink design and each application must be approached on a case-by- case basis, but the following general guidelines can help in the design process.

+ Maximize the surface area of the heat sink to maximize its ability to convect heat away from the LED source.

+ Arough estimate of approximately 5-10 in? of heat sink surface area per watt of heat can be used for a first-order estimate of heat sink size.

+ Do not restrict the airflow between the fins. Understand the orientation of the heat sink in the application in which it will be used and maximize the airflow through the heat sink and around as much surface area as possible.

+ Choose a material that has good thermal conductivity to spread heat away from the LEDs.

+ Use high surface emissivity heat sinks to maximize thermal radiation heat transfer. Anodizing dramatically increases the emissivity of an aluminum heat sink.

+ Passive (natural convection) heat sinks are always preferred for many reasons, but if appropriate, actively cooled heat sinks can significantly improve performance.

+ Use of thermal modeling can alleviate repetitive prototyping and indicate design deficiencies and potential areas of improvement early in the design process.

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THERMAL MANAGEMENT OF XLAMP® LEDS

A key aspect of heat sink design to account for is the manufacturing method that will be used. These methods vary significantly and can produce extremely different heat sinks, which can serve different applications and their specific needs. Some aspects of the more common processes are compared below in Table 4.

Table 4: Heat sink manufacturing process comparison

Maximum fin height 6.0 in 3.0in 3.0in 2.0in 5.0in 1.5in 2.0in Minimum fin thickness 0.032 in 0.070 in 0.040 in 0.010 in 0.020 in 0.016 in 0.007 in Maximum aspect ratio 60:1 10:1 10:1 40:1 35:1 16:1 41 - Straight . Not perfectly - Type Straight Any Goes Straight Any straight Straight Cooling factor 0.8x 1.4x x 0.7x 1.5x 1.2x 1.4x Base cost factor 1.3x 0.8x x 1.2x 1.3x 1.2x 0.7x Tooling cost factor 1.5x 4.0x x 1.8x 3.0x 1.2x 2.8x 4 Al6063 Al6063 Al6063 Al6063 Al6063 Metals cu1100 (bes AEE cu1100 cu1100 cu1100 cu1100

Fans/Active Cooling

When the thermal load of an LED system is too high to be properly dissipated by passive means, active cooling may be the only solution. There are many types of actively cooled systems, from fans to liquid cooling to heat pipes to other exotic methods. The effectiveness, reliability, noise, cost, added power (and thus lower system efficiency) and maintenance of these devices need to be weighed against the benefits of an actively cooled system. Very few active cooling devices can equal the long LED lifetimes of many thousands to hundreds of thousands of hours, so care must be taken to not compromise system lifetime with inept active cooling solutions. An LED system is only as good as its weakest link, and active cooling can be this link without careful selection. Table 5 below briefly summarizes aspects of several active cooling systems.

© 2004-2023 Cree LED. The information in this document is subject to change without notice. Cree®, the Cree logo, the Cree LED logo, and XLamp® CLD-APS 12 are registered trademarks of Cree LED. Other trademarks, product and company names are the property of their respective owners and do not imply REV 5

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Table 5: Active cooling examples?

Type Thermal Example Image yp D ition Power P 9

Fan sink <40W Fan mounted directly on heat sink. Additional power needed, therefore less Im/W.

One big negative: if the fan fails, the entire unit must be replaced. . Heat pipes do not dissipate heat; they move it to another location, so a heat sink is

REEEphe Oy still needed elsewhere.

Liquid cooling <200W High heat flux applications. Expensive, typically 10 times the cost of a heat pipe solution.

Peltier devices <80W Inefficient, limited cooling, expensive. Additional power needed, therefore less Im/W.

ae ‘ Cooling possibilities similar to a fan, but less noise and better reliability. Special heat Synthetic jet cooling ae sink design needed to best utilize this technology. THERMAL MEASUREMENT

Accurate temperature measurements are required to appropriately design a thermal system and to evaluate and assess an existing design. Whether for a final design or a prototype, the measurement process is the same and requires due diligence to make sure realistic and accurate measurements are made. LED reliability is a major advantage compared to traditional light sources, so proper and realistic measurement procedures should be used so this benefit is not jeopardized.

When performing thermal measurements, it is critical to set up the test subject as close as possible to the real-life, worst-case scenario to which the system may be subjected. Ensure that the measurement setup accounts for similar airflow, material properties, orientation,

2 www.vettecorp.com/support/downloads/presentations/HeatSink_Basics_Cisco_Core_v6.pdf qats.com/cms/2011/06/22/how-to-use-synthetic-jets-for-local-thermal-management/

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THERMAL MANAGEMENT OF XLAMP® LEDS

ambient conditions and any additional heat sources such as power supplies or contributory heat loads. This ensures that the temperatures measured correspond to real-world, worst-case scenarios and could identify potential problems that best-case scenarios may miss.

Another factor to note is the time required for the system to thermally stabilize. Depending on the size of the heat load and the mass and effectiveness of the heat sink, some systems take only a few minutes to stabilize while others take hours. It is best to monitor the thermal stability and wait one hour at the very least for each thermal measurement. It is also recommended to monitor the ambient temperature and look exclusively at the difference between the measurement point and ambient temperature, as any change in ambient will be reflected in the measured data.

Various methods to measure temperature exist, and for LED systems the most common methods are simple thermocouples, infrared (IR) microscopes, and pulsed voltage/transient response monitoring. The latter two methods require expensive, accurate and specific tools that are beyond the scope of this document. Simple thermocouples are the most common and simplest method to obtain accurate data and are recommended for precise absolute LED system measurements.

Thermocouples

The soldering and handling application note Cree LED publishes for each XLamp LED details the location and process for attaching a thermocouple. Below in Figure 8 is an image from the XLamp XB Family LED Soldering and Handling document showing the proper location for a thermocouple. The general guideline for thermocouple attachment is to place the thermocouple as close to the LED as possible, mounted directly on a metal pad connected to the neutral thermal trace, if possible. Thermally conductive epoxy or solder is recommended to ensure good heat transfer from the board to the thermocouple. All thermocouples must be out of the optical light path or photons will interfere with the readings, giving extremely inaccurate results.

Thermocouple site if no separate pad

Pad for thermocouple

Figure 7: XB-D T,,, measurement location

Infrared Camera

IR camera measurements can be useful to get a quick visual representation of the heat spreading through an LED system to see any potential hot spots and for other relative comparisons. However, using an IR camera for absolute temperature measurements can be very complex and lead to inaccurate results. Knowing the exact emissivity of the material is crucial for accurate results, and this often is not precisely known. One way to get this is to take a measurement with a thermocouple and then adjust the emissivity setting on the IR camera to match these results.

Figure 9 shows the same IR image captured with four different emissivity values, all within a normal small range for typical materials. The upper left image has an emissivity setting of 0.95, the upper right has 0.8, the lower left has 0.7 and the lower right has 0.5. The same

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point on the LED board has a temperature of 86.0 °C, 95.7 °C, 104 °C and 129 °C, respectively. Thus, a huge difference in temperature is

seen with just a small emissivity difference.

Spot 86.0~

Atm

Figure 8: Same IR image with different emissivity values Upper left: 0.95, upper right: 0.8, lower left: 0.7, lower right: 0.5

THERMAL SIMULATIONS

For early design validation, before investing in expensive prototyping and multiple design revisions, it is highly recommended to perform thermal simulations. Thermal simulations can show problematic areas and hot spots within a design and can allow for iterative design adjustments to address problems and optimize the system. Computational fluid dynamics (CFD) analysis further enhances thermal simulations by solving for conduction, convection, whether natural or forced, and radiation to fully evaluate the total system design and the effect of the fluid flow around the system. Simulations are good for quick and inexpensive design adjustments and give a good visual

representation of spreading, potential bottlenecks, and thus possible areas of improvement.

Care must be taken when performing thermal simulations as many assumptions must be made and operating conditions must be known and understood. A simulation will always give an answer; however, depending to the approach taken, it may not always be the right answer. After simulating a system and optimizing a design, building a final prototype to accurately measure its performance is always

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THERMAL MANAGEMENT OF XLAMP® LEDS

recommended. However, thermal simulations can be used to minimize the iterative steps of seemingly infinite prototyping by proactively reducing the number of options and focusing the design work instead of investing in prototypes, testing and repeating.

Below in Table 6 are results from CFD simulations of comparative performances of two different types of heat sinks versus their orientation. An extruded heat sink shows significantly inferior performance when the fins are oriented perpendicular to gravity (Horizontal - 90° case), but comparable performance in the Horizontal - 0° and Vertical - Down cases in which the airflow is not restricted and is allowed to flow upward along the fins away from the source. Due to their symmetry, the radial heat sinks show identical performance when mounted horizontally, and much better performance when mounted vertically so that the air around the fins can flow upward. Remember, heat rises, so always try to orient a heat sink’s fins such that the airflow is not impeded in the upward direction, opposite gravity.

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Table 6: Heat sink orientation effects computed by thermal simulation

Extruded Heat Sink Radial Heat Sink

Thermal Resistance (Scaled to Source Orientation Horizontal

Source Orientation

. 0°) Horizontal - 0° 1.00 x 1.00 x Horizontal - 90° 2.15 x 1.00 x Vertical - Up 1.23 x 0.74x Vertical - Down 1.03 x 0.74x

ee®, the Cree logo, the Cree LED logo, and XLamp® CLD-APS 1 7 erty of their respective owners and do not imply REV 5

ided for informational purposes only and is not a ;cree-led.com.

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THERMAL MANAGEMENT OF XLAMP® LEDS

CONCLUSION

Thermal management of LEDs is extremely critical and understanding it is essential when designing and developing LED systems. To prolong their lifetime and improve their performance, LEDs must be kept cool under all drive and operating conditions. The fundamentals of heat transfer and a full understanding of the heat path and thermal stack is needed to properly design an LED system. Thermal simulations and testing should be used to optimize and measure the performance of each LED system.

© 2004-2023 Cree LED. The information in this document is subject to change without notice. Cree®, the Cree logo, the Cree LED logo, and XLamp® CLD-APS 18 are registered trademarks of Cree LED. Other trademarks, product and company names are the property of their respective owners and do not imply REV 5

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THERMAL MANAGEMENT OF XLAMP® LEDS

RESOURCES

Below are some web links for various heat management resources. These are not explicitly recommended by Cree LED nor is this intended to be an all-encompassing list, but rather offers some resources for thermal design assistance.

Heat Sink Companies

Aavid Thermalloy

Advanced Thermal Solutions, Inc.

Asia Vital Components Co. (AVC) Cooler Master Ltd., Inc.

Cooling Source Inc.

Huizhou Taisun Precision Parts Co., Ltd. Nuventix, Inc.

Wakefield Solutions, Inc.

Wisefull Technology Ltd.

www.aavid.com/ www.gats.com/ www.avc-europa.de/ odm.coolermaster.com/ www.coolingsource.com/ www.hztaisun.com/en/index.asp www.nuventix.com/ www.wakefield.com/

www.wisefull.com/

3M Arctic Silver, Inc. Bergquist Company

Chomerics Dow Corning Corporation

GrafTech International Ltd. Indium Corporation

Laird

Shin-Etsu Chemical Co., Ltd.

TiMtronics

TIM Companies solutions.3m.com/wps/portal/3M/en_US/AdhesivesForElectronics/Home/Products/ThermalSolutions/ www.arcticsilver.com/ www.bergquistcompany.com/thermal_materials/index.htm www.chomerics.com/products/thermal/index.html

www.dowcorning.com/content/etronics/etronicspadsfilm/ www.dowcorning.com/content/etronics/etronicswet/default.asp

www.graftech.com/MARKETS/Lighting-Thermal-Management.aspx

www.indium.com/TIM/ www.lairdtech.com/Products/Thermal-Management-Solutions/Thermal-Interface-Materials/ www.silicone.jp/e/products/notice/heat/index.shtml

www.timtronics.com/

Bergquist Company Copper Wave, Inc.

Milplex Circuit Inc. Multilayer Prototypes, Inc.

Saturn Electronics Corporation

PCB Companies www.bergquistcompany.com/thermal_substrates/index.htm www.copperwavepcb.com/ www.milplexcircuit.com/ www.mpi-peb.com/

saturnelectronics.com/thermalmanagement.htm

Advanced Thermal Solutions, Inc. ANSYS, Inc.

Autodesk, Inc.

Informative Design Partners (IDP)

ThermoAnalytics, Inc.

Thermal Simulation Companies www.gats.com/ www.ansys.com/ www.autodesk.com/simulation-cfd www.informativedp.com/

www.thermoanalytics.com/

are registered trademarks of Cree LED. Other trademarks, product and company names are the property of their respective owners and do not imply specific product and/or vendor endorsement, sponsorship or association. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at www.cree-led.com.

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