Effective Thermal Management of LED Arrays

The performance, reliability, and design life of LED arrays depend heavily on proper thermal management. This article examines a variety of conduction, convection, and radiation techniques to help ensure optimal performance and long life for your next lighting design.

Introduction

The Bridgelux family of LED Array products delivers high performance, compact, and cost-effective solid-state lighting solutions to serve the general lighting market. These products combine the higher efficiency, lifetime, and reliability benefits of LEDs with the light output levels of many conventional lighting sources.

As noted in Bridgelux LED Array product datasheets, several performance characteristics of the LED Array products, including flux, forward voltage, color, and reliability are dependent upon temperature. This is a common characteristic for all LEDs, a characteristic of the semiconductor-based technology. As temperature increases, several performance parameters experience a temporary and recoverable shift. With increasing temperature, light output (or flux) decreases, forward voltage (or V_f) decreases, and the color temperature shifts towards blue.

Furthermore, absolute maximum ratings, such as maximum case temperature and maximum junction temperature, must not be exceeded. Exceeding the absolute maximum ratings, as listed in the product datasheets, may irreversibly damage the product and cause permanent shifts in performance.

Optimization of performance and reliability in a lighting system using Bridgelux LED Arrays requires proper thermal management. Although a critical design parameter, thermal management is not as difficult as many would believe. Understanding the basics allows every lighting designer to optimize his product and meet specification requirements.

This article describes basic thermal management concepts and guidelines for proper use of Bridgelux LED Arrays in a lighting system. Included is an overview of basic heat transfer concepts, a description of a thermal model, a sample calculation using this thermal model, a description of various thermal components, and recommendations for measuring the case temperature of the Bridgelux LED Array to validate the performance of the thermal management solution.



Heat generation

When voltage is applied across the junction of an LED, current flows through the junction generating light. It is a common misconception that LEDs do not generate heat. While essentially no heat is generated in the light beam (unlike conventional light sources), heat is generated at the case of the LED Array and must be effectively managed. As LEDs are not 100 percent efficient at converting input power to light, some of the energy is converted into heat and must be transferred to the ambient. The amount of heat generated from the LED Array that must be transferred to the ambient may be conservatively estimated to be the power that is applied to the LED Array, or P_d, and may be calculated by multiplying the forward voltage (V_f) times the current (I_f). This is described in Equation 1.

Where:
	Pd is the thermal power dissipated
	Vf is the forward voltage of the device
	If is the current flowing through the device

The maximum dissipated power for each of the Bridgelux LED Array products is listed in Table 3 of this article. It should be noted that any input power (Vf * If) that is converted to light is usually ignored when calculating the total thermal load. Ignoring the devices thermal efficiency provides a safety margin in the design of the thermal solution.

Heat generated by additional sources, such as a power supply located near the LED Array, must also be managed. In order to reduce the size and cost of the thermal management solution and to minimize the amount of additional heat added to the system, power supplies and other heat generating components should not be located in close proximity to the LED Array.

Thermal path

Heat generated at the LED junction must be transferred to the ambient via all elements that make up the thermal management solution. These elements include the LED Array, the thermal interface material used between the LED Array and heat sink, the heat sink, the luminaire enclosure (if applicable), and other components that come in contact with the lighting assembly. These elements transfer heat to the ambient through conduction, convection, or radiation. These heat transfer modes will be discussed in greater detail in the next section of this application note. For a simple thermal management solution that consists of an LED Array mounted to a heat sink, we consider that all the heat from the LED junction is typically transferred to the ambient through the following thermal path:

1. Heat is conducted from the semiconductor chip within the LED Array to the elements that make up the LED Array, including the metal board and the silicone resin.
2. Heat is then conducted from the LED Array through a thermal interface material to the heat sink of the lighting system. This is a critical component in transferring the heat. We will discuss this point in detail later.
3. Heat is then conducted through the heat sink.
4. Heat is then convected to the air around the heat sink and is radiated to the ambient.

Modes of heat transfer

There are three basic modes of heat transfer; conduction, convection, and radiation. All heat flow is driven by temperature gradients; heat moves from hot to cold. Each heat transfer mode plays an important role in transferring heat away from the LED junction to the ambient. Table 1 provides a summary describing the various heat transfer modes and the equations that govern them.

The first of these, conduction, is the transfer of heat between adjacent molecules of a material, usually a solid. Equation 2 provides the basis for our first set of guidelines for designing meaningful thermal solutions.

Considerations for heat sink design or selection:

• Minimize the distance heat must travel (dx). In practical terms, this means that if a heat sink is too large, it may lose effectiveness. However, this distance should not be so small that a bottleneck is created, constricting the flow of heat.
• Select heat sinks made of materials that have a high thermal conductivity (k). As a reference, Table 2 compares thermal conductivity of various metals typically used for heat sinks and the thermal conductivity of air. Although aluminum is not as an effective heat conductor as copper, it is frequently the material of choice as it minimizes the cost and the weight of the thermal solution.
• Use heat sinks with large surface areas (A).
• Eliminate air gaps and voids between the LED Array and the heat sink. Air is a very poor conductor of heat. During assembly, the flat bottom surface of the LED Array should be in full contact with the flat surface of a heat sink. If air gaps or voids exist between the two, a thermal interface material should be used to fill the gaps.

Mode	Governing Equation	Definition of Variables
Conduction	QCOND= -k A dT/dx Equation 2: Heat Conduction	QCOND is the heat that is conducted (°C/Wm) k is the thermal conductivity of the material (W/mK) A is the cross sectional area of the medium the heat is conducted through (m2) dT is the temperature gradient across the material (°C) dx is the distance the heat is traveling through the material (m)
Convection	QCOND= h A (Ts — T∞) Equation 3: Heat Convection	QCOND is the heat convected h is the convection heat transfer coefficiency (W/m2K) A is surface area Ts is the surface temperature of the hot surface T∞ is the temperature at infinity, typically the ambient air temperature
Radiation	QRAD= εσ A (T4s — T4∞) Equation 4: Heat Radiation of Gray Bodies*	QRAD is radiated heat ε is emissivity and provides a measure for how a surface emits energy relative to a blackbody σ is the Stephan-Boltzmann Constant, 5.6703 10-8 (W/m2K4 A is the area of the emitting body Ts is the surface temperature T∞ is the temperature at infinity, typically the ambient air temperature

* In the real world, the ideal models of Physics do not absolutely apply. Typically, engineers assume that heat radiation does not depend on the sample’s surface conditions. In these cases, the heat source is known as a “Gray Body."

Nature has two great thermal insulators. The first great insulator is a vacuum, and the second is air. Therefore, it is critical to have a sufficient volume of the thermal interface material to displace the air. However, if there is any excess thermal interface material, for example more than 0.3mm, then the system thermal resistance will begin to increase. On the surface, this may not appear to make sense. What one needs to consider is that while the thermal interface material is better at removing heat than air, it is much worse than the metal in the heat sink that conducts the heat away from the LED Array.

Material	Thermal Conductivity (W/mK)
Iron	79.5
Aluminum	205
Copper	385
Air (at 0°C)	0.024

Convection describes heat transfer due to random molecular motion and bulk motion of a fluid. In other words, convection is the heat transfer from the heat sink to air (or water) and is directly dependent upon the amount of flow of the air or water. In the case of the natural convection in air, where, for example, the movement of air molecules in not aided by a fan, the convection heat transfer coefficient ranges from 2 to 15 W/m²K.

Forced convection is a result of movement in the fluid (water or air), which results from other forces, such as the use of a fan or pump. With forced convection, the convection heat transfer coefficients range from 25 to 250 for gases and from 100 to 20,000 for liquids. Both natural convection and forced convection may be used to effectively convect heat away from the LED Array. Equation 3 can be used to develop guidelines for enabling heat transfer through convection.

Considerations for heat sink design or selection:

• Use heat sinks with the largest surface area (A) that is physically or economically feasible. As a general rule of thumb, for a well-ventilated heat sink, there should be 10 square inches of heat sink for every 1 watt of power dissipated. The use of pinned heat sinks, however, is not recommended.
• Orient heat sink fins in a manner that allows hot air to flow upward and away from the heat sink and cold air to flow onto the surfaces of the heat sink.
• Avoid constricting the airflow.
• If possible, use natural convection to transfer heat from the heat sink to the ambient. Doing so avoids potential reliability issues of fans.
• If natural convection is insufficient, then consider using fans, heat pumps, or liquid cooling elements that can dramatically increase the convection heat transfer coefficient and hence dramatically increase heat transfer.
• Use heat sinks with surfaces that have high emissivity values.
• Radiation is energy transfer by electromagnetic waves. By analyzing Equation 4, guidelines for enabling heat transfer through radiation can be developed.
• Radiation heat transfer has a very strong dependency on temperature. The hotter the heats sink, the more significant the heat transfer through radiation. However, as the maximum case temperature of the LED Array is 105°C, heat transfer due to radiation is very small when compared to other heat transfer modes.

Keeping in mind how heat is transferred and consciously optimizing specific heat transfer modes should make the task of designing a cost effective and optimally sized thermal management solution easier. In a typical single LED Array lighting assembly using simple heat sinks, the amount of heat that is conducted, convected, and radiated is a function of the heat sink geometry, surface area, material properties, surface properties, fin geometry (including thickness and spacing), and heat sink temperature. While this may sound complicated, the application of effective heat sink design is straightforward.

Thermal model

A simple thermal model or thermal circuit can illustrate the heat flowing through an LED Array. This model is analogous to an electrical circuit where heat flow is represented by current, voltages represent temperatures, heat sources are represented by constant current sources, and resistors represent thermal resistances. Figure 1 shows a thermal circuit for a single LED Array mounted to a heat sink.

Where:
	Q is heat flowing from hot to cold through the LED
	Tj is the temperature at the junction of the device
	Tc is the temperature at the case of the LED Array
	Th is the temperature at the point where the heat sink is attached to the LED Array
	Tamb is the ambient air temperature
	Rθjc is the thermal resistance from junction to case of the LED Array
	Rθch is the thermal resistance between the case of the LED Array and the heat sink
	Rθha is the thermal resistance of the heat sink

With the exception of T_j, all temperatures listed above can be easily measured. For Bridgelux LED Arrays, case temperature measurements are made in the area noted in the mechanical drawings section of the product datasheets. Note that this area is on the same surface of the LED Array as the resin area, providing an easy to measure location after assembly to the heat sink. Although traditionally case temperature measurements are conducted on the back side of device, the difference in temperature between the defined case temperature measurement point location on the top of the Bridgelux LED Array and bottom are very small. Bridgelux has characterized the difference between these two points to be typically 1°C or less and considers the difference negligible.

The thermal resistances that make up the thermal model may be calculated and solved for using the following equation:

Where:
	Rθxy is the thermal resistance from x to y, where x and y are points along the thermal circuit
	Tx is the temperature at x
	Ty is the temperature at y
	Q is heat flow and may be approximated to be Pd (see Equation 1)

Substituting “x” with “j” and “y” with “c,” we get Rθ_jc, or the thermal resistance from junction to case of the LED Array. Rθ_jc values are included in the Product Data Sheets for all Bridgelux LED Arrays and therefore do not need to be calculated. Instead, by knowing Rθ_jc values and by using Equation 5, we may solve for T_j, the temperature at the junction.

By substituting “x” and “y” with appropriate values, both Rθ_ch and Rθ_ha may be solved for using Equation 5. When using thermal interface materials, such as thermal pastes and adhesives, Rθ_ch describes the thermal resistance of the thermal interface material. Thermal interface materials and their impact on thermal resistance will be discussed in greater detail later in this article. Rθ_ha describes the thermal resistance of a heat sink or heat sink assembly. Once maximum operating conditions are known, including drive current, forward voltage, and maximum ambient temperature, this value is solved for, and a heat sink may be sized and selected to achieve this value.

Rules governing a thermal circuit are also analogous to those of an electric circuit. Equation 6 depicts the method for adding series thermal resistance values.


Where Rθ_ja is typically referred to as the system thermal resistance.

Furthermore, when assembling multiple LED Arrays on a single heat sink, the rule of parallels applies. This is depicted in Equation 7.


In Equation 7, “n” refers to the number of LED Arrays mounted onto a single heat sink and Rθ_{n jh} is the thermal resistance from the LED junction to the heat sink of each of the individual LED Arrays. It should be noted that when using this model the total power (sum of all LED Arrays mounted to the single heat sink) must be multiplied by the system thermal resistance to calculate the case or junction temperature.

Thermal Model Example

Design Challenge:

A luminaire is to be designed using the BXRA-C1202-00000 LED Array driven at 1050 mA. The maximum ambient temperature will be 40°C and the case temperature cannot exceed 70°C in the application. A thermal paste, with a thermal conductivity of 4 W/mK and a resulting thermal resistance of 0.07 vC per watt has been selected. Shifts in forward voltage due to temperature are assumed negligible. Given this information, the thermal resistance of the heat sink required for this design must be calculated and the minimum surface area for an extruded aluminum heat sink must be determined.



Solution:

To solve for the thermal resistance of the heat sink, we use equation 5, where “x” is “h” and “y” is “a”:


Here, T_a is 40°C and T_h is unknown at this time, but may be solved for. Q is the dissipated power, which is the product of the voltage and current. In order to design a thermal management system to account for the entire range of product performance, the maximum forward voltage of the LED Array must be used (see Table 3 or the relevant product datasheet).


As the thermal resistance of the thermal interface material is known, to solve for Th we use equation 5, substituting “x” with “c” and “y” with “h”:



Now solving for Rθ_ha :

The surface area of a black anodized aluminum extruded heat sink with at thermal resistance of 2°C/W is estimated to be at least 101.5 square inches for a heat sink with fins that are oriented vertically or 145 square inches if the fins are oriented non-vertically. While these values may appear large, this is the total surface area required and may consist of surfaces that make up the luminaire itself such as the housing and reflector, in addition to the surfaces that make-up the heat sink. The final required surface area of the heat sink depends on many variables, including, but not limited to, fin orientation, the ability of hot air to move away from the heat sink, the ability of cold air to enter and flow through the heat sink, and the existence of other heat sources near the LED Array.

Thermal solution components — heat sinks

There are many commercially available components, including heat sinks and heat pipes, which may be used with Bridgelux LED Arrays. The most commonly used components are heat sinks, typically made of aluminum or copper. Heat sinks conduct heat from a heat source and then convect the heat to the ambient. The size of a required heat sink depends on many variables, including the temperature requirements for the application (such as maximum ambient and case temperature), the material of the heat sink, the surface characteristics of the heat sink, and the physical constraints for the application.

Table 3 lists minimum surface areas and sample dimensions for reference heat sinks in simple cases where a single LED Array is used in a light fixture, and natural convection and radiation are used to transfer heat to the ambient. Calculated thermal resistance values of the reference heat sinks (R_ha) in Table 3 are based on the following:

• The maximum ambient temperature of the application is 40°C.
• The maximum case temperature of the array is 70°C.
• The thermal interface material used has a thermal conductivity of 4W m/K (like that of Dow Corning TC-5022). The thermal resistance contributions for various thermal interface materials are shown in Table 5. These values vary by LED Array product and range from 0.19°C/W for the LS Array Series products to 0.035°C/W for the RS Array Series products.
• Maximum dissipated power values have been calculated using forward voltage values reached at elevated junction temperatures, those reached when the LED Arrays reach a case temperature of 70°C.

The surface area and dimension values listed in Table 3 are based on the reference heat sink (R_ha) values.

Table 3 illustrates that when relying on natural convection to transfer heat to the ambient, the minimum required surface area for an extruded aluminum heat sink with a black anodized finish is 10 in² per watt of dissipated power. This area may be provided as a flat plate or via a finned extruded heat sink to minimize the volume of the thermal management solution. The required heat sink surface area must be determined and validated by the luminaire designer and will depend on many luminaire design variables, including fin orientation and cold and hot air flow paths.

In some applications, space requirements may not allow for such a large heat sink. In these cases, designers should consider using forced convection elements, such as fans. Consult heat sink suppliers to explore forced convection heat sink options. Heat sink suppliers can provide detailed technical data on their heat sinks to customers. Data supplied includes heat sink temperature rise above ambient as a function of air flow speed and heat sink thermal resistance as a function of air flow speed. This information aids in the design and heat sink selection process.

Part Number		Electrical Characteristics				Reference Heat Sink Characteristics With Free Convection
		Maximum Voltage at Tj=25°C (V)	Maximum Voltage at Tcase=70°C (V)	Current (mA)	Power(W)	Rha (°C/W)	Surface Area (in2)	Surface Area (mm2)	Dimensions (mm3)
Warm White	BXRA-W0240	14.3	13.9	350	4.9	6.15	48.8	31477	38.1x16x84
	BXRA-W0241	7.3	7.1	700	5.0	6.02	49.8	32155	39.6x19x105
	BXRA-W0260	13.6	13.2	350	4.6	6.47	46.3	29897	38.1x16x84
	BXRA-W0261	6.8	6.6	700	4.6	6.47	46.3	29897	38.1x16x84
	BXRA-W0400	10.6	10.3	900	9.3	3.23	93.0	59981	88.9x21.6x84
	BXRA-W0401	10.3	10.0	700	7.0	4.27	70.2	45297	57.1x20.6x91
	BXRA-W0402	9.7	9.4	700	6.6	4.54	66.0	42587	57.1x20.6x91
	BXRA-W0403	31.7	30.9	250	7.7	3.88	77.2	49822	50.4x25x105
	BXRA-W0800	14.3	13.9	1300	18.1	1.66	181.2	116916	73.3x25.4x91
	BXRA-W0802	13.2	12.8	1050	13.5	2.23	134.8	86980	76.1x57.1x91
	BXRA-W1200	17.8	17.4	1600	27.8	1.08	277.6	179096	100x40x210
	BXRA-W1202	16.3	15.9	1200	19.0	1.58	190.2	122709	73.3x25.4x91
	BXRA-W1203	19.8	19.3	1050	20.2	1.48	202.2	130471	66x40x210
	BXRA-W3000	28.3	27.6	2100	57.9	0.52	579.2	373664	160x40x308

Impact of fin orientation on heat sink performance

Thermal performance presented by heat sink manufacturers usually pertains to the most favorable heat sink orientation, which is with the fins of the heat sink oriented vertically (see Table 4). In this orientation, hot air rises readily, allowing cool air to circulate through the fins. Other orientations give varying results. Rules of thumb on the impact of varying fin orientation are shown in Table 4. However, please note that the actual performance of a heat sink is dependent upon many variables such as heat sink location within an assembly, the location of other heat generating elements such as a power supply, effective airflow, fin spacing, fin height, fin thickness, base thickness, base surface area, shape, fin geometry, and overall length. Consequently, the effectiveness of a heat sink mounted in varying orientations is not a fixed number, and depends on the inter-relationship between many variables. A much larger poorly positioned heat sink will not be as effective as a properly oriented thermal system.

Part Number		Electrical Characteristics				Reference Heat Sink Characteristics With Free Convection
		Maximum Voltage at Tj=25°C (V)	Maximum Voltage at Tcase=70°C (V)	Current (mA)	Power(W)	Rha (°C/W)	Surface Area (in2)	Surface Area (mm2)	Dimensions (mm3)
Cool White	BXRA-C0360	14.3	13.9	350	4.9	6.15	48.8	31477	38.1x16x84
	BXRA-C0361	7.3	7.1	700	5.0	6.02	49.8	32155	39.6x19x105
	BXRA-C0400	10.6	10.3	600	6.2	4.84	62.0	39987	53.3x11.8x140
	BXRA-C0402	10.3	10.0	500	5.0	5.98	50.2	32355	39.6x19x105
	BXRA-C0603	31.7	30.9	250	7.7	3.88	77.2	49822	57.1x20.6x140
	BXRA-C0800	14.1	13.7	900	12.4	2.43	123.7	79780	76.2x38.1x140
	BXRA-C0802	13.7	13.3	700	9.3	3.21	93.4	60245	88.9x21.6x84
	BXRA-C1200	14.3	13.9	1300	18.1	1.66	181.2	116916	73.3x25.4x91
	BXRA-C1202	13.8	13.4	1050	14.1	2.13	141.1	91045	76.1x57.1x91
	BXRA-C2000	18.0	17.7	1750	31.0	0.97	310.3	200177	100x40x210
	BXRA-C2002	17.5	17.1	1500	25.6	1.17	255.8	165000	80x40x280
	BXRA-C4500	28.3	27.6	2100	57.9	0.52	579.2	373664	160x40x308

Where:
	L is the thermal interface material thickness (mm)
	K is the thermal conductivity (W/m-K)
	Ac is the contact area (mm2)

For design purposes, the thermal interface material thickness typically ranges from 0.15 to 0.30 mm, depending on the LED Array product and the thermal interface material selected. This range of thicknesses assumes that the planarity of the bottom surface of the LED Array is maintained at or below 0.1 mm for all products except RS Array Series products. The planarity for the RS Array Series products is maintained at or below 0.25 mm. Hence, when selecting a thermal interface material ensure that the thickness of the material is sufficient to fill gaps between the base of the LED Array and the heat sink while at the same time minimizing the thickness. Thermal conductivities of interface materials vary from product to product. Table 5 lists a few examples and the impact on the thermal resistance of a lighting system for various Bridgelux LED Array products.

Part Numbers	Thermal Interface Material Product	Thermal Conductivity (W/mK)	Thickness (mm)[1]	Estimated Interface Resistance[2] (°C/W)
BXRA-W0240 BXRA-W0241 BXRA-W0260 BXRA-W0261 BXRA-C0360 BXRA-C0361	Aavid Thermalloy Sil Free 1020	0.79	0.15	0.96
	Berquist A1500	2	0.254	0.64
	Chomerics T777	2.2	0.15	0.34
	Omega OT-201	2.24	0.15	0.34
	Laird Tgrease 2500	3.8	0.15	0.20
	Dow Coming TC-5022	4	0.15	0.19
BXRA-W0400 BXRA-W0401 BXRA-W0402 BXRA-W0403 BXRA-N0400 BXRA-N0402 BXRA-C0400 BXRA-C0402 BXRA-C0603	Aavid Thermalloy Sil Free 1020	0.79	0.15	0.63
	Berquist A1500	2	0.254	0.42
	Chomerics T777	2.2	0.15	0.23
	Omega OT-201	2.24	0.15	0.22
	Laird Tgrease 2500	3.8	0.15	0.13
	Dow Coming TC-5022	4	0.15	0.12
BXRA-W0800 BXRA-W0802 BXRA-N0800 BXRA-N0802 BXRA-C0800 BXRA-C0802 BXRA-W1200 BXRA-W1202 BXRA-W1203 BXRA-N1200 BXRA-N1203 BXRA-C1200 BXRA-C1202 BXRA-C2000 BXRA-C2002	Aavid Thermalloy Sil Free 1020	0.79	0.15	0.34
	Berquist A1500	2	0.254	0.22
	Chomerics T777	2.2	0.15	0.12
	Omega OT-201	2.24	0.15	0.12
	Laird Tgrease 2500	3.8	0.15	0.07
	Dow Coming TC-5022	4	0.15	0.07
BXRA-W3000 BXRA-N3500 BXRA-C4500	Aavid Thermalloy Sil Free 1020	0.79	0.3	0.176
	Berquist A1500	2	0.254	0.059
	Omega OT-201	2.24	0.3	0.062
	Laird Tgrease 2500	3.8	0.3	0.036
	Dow Coming TC-5022	4	0.3	0.035

1. For all products except RS Array series products, flatness of the back surface of the LED Array is maintained at < 0.1 mm across the LED Array. For RS Array Series products, flatness is maintained at < 0.25 mm across the LED Array. Examples shown assume this worst-case scenario.
2. Thermal interface material thickness is assumed to be 0.05 mm for all materials except the Berquist A1500 Sil-Pad. The Berquist A1500 Sil-Pad is 0.254 mm thick.

All thermal interface materials must be applied in a way that ensures the entire bottom of the LED Array is covered and that air gaps or voids between the LED Array and the heat sink are filled. One method of achieving this, which may be used in volume production, is to silkscreen precise quantities of a dispensable thermal interface material on a heat sink surface prior to attaching the LED Array.

Silkscreen templates are made in sizes that mirror the base of the LED Array or are scaled to a slightly larger size. The final thickness of the interface material is controlled by the force applied on the LED Array from the top when pressing it down on the thermal interface material. These forces can vary from a few hundred grams, which pick and place equipment is capable of handling, to several kilograms, which would require special tooling. If pick and place equipment is used for this processes, consider using “Scrub” mode, while pressing the LED Array onto the heat sink. In all cases, care must be taken to avoid contact with the resin area of the LED Array during assembly. Please consult application note AN11 — Handling and Assembly of Bridgelux LED Arrays, for further information.

The customer must evaluate the performance of the thermal interface material to ensure adequacy in terms of thermal performance, manufacturability, and durability.

Use of current derating curves

Current derating curves are included in the Bridgelux LED Array Product Data Sheets. These curves provide guidance to customers in developing effective thermal management solutions that meet system design requirements.

A derating curve is included for each Bridgelux LED Array product. When using these graphs, the required system thermal resistance can be estimated when the LED Array is used at the rated test current under various ambient conditions. An example of one of these derating curves is included in Figure 3. Please consult the relevant product datasheets for the most recent versions of these derating curves.


The thermal resistance values indicated on the derating curves are total system thermal resistance values (junction to ambient). Although limited options are included, it is possible to interpolate between these curves for approximation purposes. The safest approach, however, is to calculate the required system thermal resistance using the equations contained in this application note.

The thermal resistance for the BXRA-W0400 product from the Bridgelux LED Array product datasheet is listed as 1.0°C/W. If in a given lighting system the thermal resistance from case to ambient is designed to deliver 3°C/W, the curve in Figure 3 labeled 4°C/W would be applicable for this lighting system (sum of junction to case and case to ambient thermal resistance values).

In this example, as long as the ambient temperature is maintained below 80°C, the maximum temperature ratings of the product will not be violated at the rated forward current of 900 mA. If, however, the ambient temperature was to rise to 85°C, the forward current would need to be reduced to 700 mA based on the 4°C/W system thermal resistance. Alternatively, a heat sink could be designed to deliver a case-to-ambient thermal resistance of 2°C/W. This would result in a system thermal resistance of 3°C/W, allowing for 900 mA operation at the 85°C ambient condition.

Measuring effectiveness of a thermal solution

After a thermal management solution has been designed, it is critical to experimentally validate the effectiveness of the solution. This is typically done by building a prototype, simulating the worst case use conditions, and measuring T_case. When simulating worst case use conditions ensure the following:

• Convection conditions are realistic.
• Material properties and dimensions, including wall thicknesses, surface areas, and component sizes are representative of the design.
• Surface properties, including color and roughness properties, are representative of the design.
• Additional heat sources that may impact the thermal performance of the device are included (such as a power supply that is placed inside a luminaire enclosure or imbedded into a heat sink).

Once the representative prototype is built, and realistic use conditions are simulated, T_case may be measured to validate the design.

Special care is required when measuring the case temperature to ensure an accurate measurement. The following approach is recommended to minimize measurement errors for attaching the thermocouple to the case temperature measurement point of the LED Array:

• Use 36 gauge or smaller diameter K-type thermocouples.
• Ensure that the thermocouple is properly calibrated.
• Attach the thermocouple bead, or junction, to the area on top of the LED Array in the prescribed area (refer to the mechanical drawings section in the relevant product datasheet).
• Attach the thermocouple to the LED Array using an adhesive that has high thermal conductivity. To do this, first place the thermocouple bead on to the prescribed area. Temporarily secure the thermocouple using Kapton tape. Next, using the back of a thin diameter wood stick, such as a tooth pick or the wooden end of a cotton swab, press on the thermocouple bead, ensuring contact with the LED Array board. Lastly, apply a small amount of a fast curing, low viscosity, and thermally conductive adhesive around the base of the thermocouple bead. Allow the adhesive to cure. Remove as much as the wooden stick as possible.
• Note that it is critical that the entire thermocouple bead be secured tightly against the case. There should be no air gaps between the thermocouple tip and the case of the LED Array.

After turning on the LED Array, T_case will increase with time as the assembly heats up. Eventually, the lighting assembly should reach a steady state temperature. The time required to reach a steady state temperature depends on the time-constant of the assembly, but is likely to be in the range of 45 minutes to an hour. Maintain the Bridgelux LED Array T_case at or below the maximum case temperatures listed in the product datasheet to ensure functionality and reliability.