978-1-61284-736-8/11/$26.00©201 1 IEEE 292 27th IEEE SEMI-THERM Sympo sium

Method for Heat Flux Measurement on LED Light Engines

Theo Treurniet, Karel Joop BosschaartPhilips Lighting, Eindhoven, The Netherlands

theo.treurniet@philips.com

AbstractIn order to ensure the exchangeability of LED light

engines in LED based luminaires, the Zhaga consortium develops standard specifications for the interfaces of LED light engines. The complete interface definition consists of the description of a mechanical, optical, electrical and thermal interface.

The thermal interface has to ensure a good thermal contact between the engine and the fixture. Next to that, the heat spreading capabilities of both the engine and the fixture have to be taken into account in order to ensure sufficient heat spreading capabilities of complete luminaire.

In order to come to a practical interface definition, a number of tests and test devices are proposed. Engines and fixture have to pass these tests in order to become Zhaga compliant.

One test is the heat flux measurement on the LED light engine in order to determine the amount of heat that has to be transferred from the engine via the fixture.

The second test is a test with a reference thermal engine in order to determine the heat spreading capabilities and the thermal resistance of a fixture.

The final test is a test with a reference luminaire in order to determine the heat spreading capabilities of the LED light engine.

With these three tests, we can realize a practical thermal interface definition.

KeywordsLED Light Engine, Heat Flux Measurement, Thermal

Interface, Standardization, Zhaga

NomenclatureLED light engine – A LED based module that acts as the

light source in a LED based lighting fixtureFixture – The LED lighting system, excluding the LED

Light EngineLuminaire – The combination of a LED light engine with

the fixturePthermal – The amount of thermal power that has to be

transferred from the LED light engine via the thermal interface and the fixture

Theatsink – The reference temperature of the heat sink, measured at reference position at the interface between the LED light engine and the fixture

1. IntroductionThe LED Lighting market is addressed in several ways.

One approach is to replace existing lamps with LED based lamps, while maintaining the existing lighting fixtures and infrastructure. With limited cost and effort, one can realize considerable energy savings and other advantages of LED

lighting this way. However, existing lighting fixtures are not fully optimized for LED lamps. There is usually no provision to remove the heat from the LED lamps in an optimal way. As a result, this solution is not able to fully utilize the advantages of LEDs.

From a thermal point of view, it is much more efficient to integrate the thermal solution at lighting fixture system level. A LED lighting fixture can be optimized to remove the heat to the ambient air in a much more efficient way than a LED lamp that is placed in a lighting fixture that is designed for conventional light sources. As a result, there is a growing request from lighting fixture manufactures for standardized LED light engines that can be applied in optimized LED lighting fixtures. One of the requirements of these LED light engines is that they have a clearly defined thermal interface, next to the optical, electrical, control and mechanical interface.

In order to standardize these interfaces, the Zhaga consortium has been formed [1]. Zhaga is an industry-wide open and global industry consortium to create standardized interfaces for LED light engines. One part of this interface definition is the thermal interface. In order to standardize this interface, test methods are required to test both the LED light engine and the lighting fixtures.

This paper describes the proposed interface definition and the thermal test methods that are developed for both the LED light engine and fixtures. One of the challenges is to measure the actual heat that is generated by the LED light engine that should be transferred by the lighting fixture. One of the approaches is to estimate the heat by subtracting the optical power that is generated from the electrical input power. However, this approach has the disadvantage that an optical measurement is required. Further, it does not take into account heat dissipation to the ambient air, directly from the LED light engine and thus overestimates the heat passing through the interface from engine to fixture. As a result, the accuracy of this method is limited. This paper describes a method to measure the generated heat with a heat flux sensor and shows the results and accuracy of this method.

Further it describes a method to test the thermal capabilities of a lighting fixture by means of the thermal test device and how these tests are used in the standardization of an actual LED light engine.

2. Standardization of LED Light EnginesA prerequisite of a broadly accepted LED light engine

standard is a clear description of the different interfaces. Within the lighting industry, there is a long history of standardization of light sources. Most of the conventional light sources are standardized in IEC documents. These standards describe the electrical, mechanical and optical interface of the light sources. Next to these interfaces, LED

Treurniet, Method for Heat Flux Meas urments on LED Light ... 27th IEEE SEMI-THERM Symposium

light engines need a well described thermal interface. One of the challenges we face is to come to a description of the thermal interface that has a sound basis and takes into account the relevant physical phenomena like heat spreading and the additional complexity of systems that generate light, but on the other hand can be applied in practice by engineers in the field, without over-specifying both light engines and fixture and adding unnecessary complexity and cost.

2.1. Zhaga ConsortiumSince the existing standardization bodies cannot follow the

pace of the industry at the moment, the Zhaga consortium is formed to generate industry standards. This consortium consists of a large number of industrial players and will develop standard specifications for the interfaces of LED light engines. Zhaga will enable interchangeability between products made by diverse manufacturers.

Interchangeability is achieved by defining interfaces for a variety of application-specific light engines. Zhaga standards will cover the physical dimensions, as well as the photometric, electrical and thermal behavior of LED light engines. For this paper, we use the Fortimo Twistable Light Engine. The Fortimo Twistable Light Engine is a socketable integrated light engine, which is one of the first engine types that will be covered by a Zhaga standard specification.

2.2. Example: Fortimo Twistable Light EngineThe Fortimo Twistable light engine is a family of

modules, developed for a down lighting application thatcovers a range of color temperatures and currently generates ca. 1100 lm of light. It currently uses about 18W of electrical input power. The driver electronics is integrated in the engine. The engine consists of two parts. One is the actual engine; the other part is a lamp holder that is mounted in the fixture. This results in an engine that can be easily exchanged. In order to ensure a reliable thermal contact, a thermal interface material is applied between the LED light engine and the fixture. This material is supplied by the engine supplier and considered to be part of the engine.

Figure 1. Fortimo Twistable Light Engine. Example of a light engine that will be covered by a Zhaga standard specification.

3. Description of the Thermal Interface3.1. System Level Thermal Management

András Poppe and Clemens Lasance indicated in a number of publications [2], [3] and [4] the importance to develop a standard to characterize LED components thermally. One can argue that the need for standardization at LED Light Engine level is even more urgent. This is the interface that a large amount of LED luminiare manufactures have to deal with.The actual architecture of the LED light engine can be very different between LED light engine makers. However, there is a clear market need for standardization of the thermal interface at engine level, due to a large amount of LED luminaire manufacturers that demand standardized solutions from the light engine suppliers.

In a large majority of cases, an LED system has to transfer its heat to the ambient air via convection. Often passive cooling via natural convection is used, but in a number of cases, active cooling technologies like fan cooling or synthetic jet cooling is used as well. At LED die level, the thermal power density is in the order of 1 W/mm². At the surface of the heat sink, this is reduced more than 3 orders of magnitude. This shows that the main thermal function in the interior is heat spreading, i.e. reducing the thermal power density to make cooling with air possible. The heat spreading function is either done by the light engine, the luminaire or a combinationof the two. The generated heat has to be spread is the most effective way, with minimal thermal spreading resistance, resulting in a limited temperature gradient from the LED junction to the heat sink surface. Next to that, the design of the heat sink is of course crucial for optimal performance of the whole system.

In the majority of systems, the heat spreading is performed by conduction. Materials that are used a lot are aluminum and copper. Next to that, ceramic materials are applied, either in the LED package or as pcb material. The advantage of ceramic materials is that it combines good thermal properties with electrical insulation. If conduction does not provide sufficient heat spreading capabilities, heat pipes or vapor chambers can be applied to improve the heat spreading capabilities of the system.

It is important to note that the thermal interface from LED light engine to fixture crosses the path from junction to heat sink surface. Therefore, the heat spreading capabilities of both the LED light engine and the fixture have to be taken into account in order to describe the spreading capabilities of the whole system. Lasance [5] shows that the heat spreading in the LED light engine and the heat transfer over a thermal interface cannot separated from a mathematical point of view. They are coupled and should be treated that way.

The LED light engine will be available in several luminous flux and power levels. Typical luminous flux level required for the down light application, the main application for the Fortimo Twistable Light Engine, is 1000 – 3000 lumen. This results in a dissipated thermal power of 15 –45W. It should be noted that this figure will decrease as the LED efficiency increases. Next to that, there will be a range of fixtures on the market that differ in application, design andthus in thermal capabilities. The challenge is to come to an interface definition that is general enough to cover different engine and fixture designs and power levels, while on the other hand, it ensures that modules can be exchanged. The

Treurniet, Method for Heat Flux Meas urments on LED Light ... 27th IEEE SEMI-THERM Symposium

interface description should not be too restrictive. That would lead to over-specification of engines and fixtures, resulting in unnecessary cost and added weight to the systems.

Figure 2 Schematic Description of a LED luminaire

Figure 2 shows a schematic picture of a LED light engine, mounted in a fixture. The light engine applies a thermal load to the lighting fixture with a certain power distribution. The thermal interface material is part of the light engine. The exact load depends on the light engine parameters like the thermal power that is generated by the engine, but also on the heat spreading capabilities of the engine. To make things more complicated, the actual power distribution on the lighting fixture also depends on the thermal performance of the thermal interface material and on the spreading capabilities of the lighting fixture.

The most natural way to define the thermal interface is to describe the heat load that the light engine generates Pthermal.Next to that, we have to describe the thermal capabilities of the fixture by defining the thermal resistance of the lighting fixture as Rth = (Theatsink – Tambient)/Pthermal as indicated in Figure 2.

Conceptually, this is the most intuitive way of defining the thermal interface. In practice however, this raises a lot of questions on the exact definition of the different parameters in this equation. In the sections 3.2, 3.3 and 3.4, we propose a definition of each of these parameters.

Even if we have a clear definition of all these parametersand know how to measure them, we face a number of challenges if we want to make a clear interface definition. The issue is that most of the parameters depend on properties of both the lighting fixture and the light engine. The thermal load that the light engine gives on the lighting fixture depends on properties of the light engine like the generated thermal powerand the spreading capabilities of the engine. But it also depends on the thermal performance of the thermal interface material and the spreading capabilities of the lighting fixture.

On the other side, the heat sink temperature depends on heat sink performance of the lighting fixture and the spreading capabilities of the lighting fixture, but also on the performance of the thermal interface material and the spreading capabilities of the light engine. So the challenge we face is to come to a

workable description of the interface, while the critical thermal parameters at the interface depend on the properties of both the lighting fixture and the light engine.

3.2. Ambient Temperature definitionLighting luminaires can be applied in a number of

different ambient conditions. Even for indoor applications, there is a large variation in ambient conditions. E.g. a suspended luminaire underneath a ceiling will typically have an ambient environment in the order of 25 °C. However, if a luminaire is recessed in a ceiling, the ambient conditions can be higher. In the end, it is up to the luminaire manufacturer to specify the exact ambient conditions of the luminaire.However, what can be standardized is a test that can be used to determine the thermal capabilities of the fixture. Within this test, the ambient temperature can be clearly specified in a way that it can be related to temperature measurements in the actual application.

There are a number of items that need to be taken into account in the test definition. Since most systems are cooled by natural convection, forced convection due to air conditioning systems or draught should be avoided. Therefore the test should be performed and a still air chamber. If we assume a maximum thermal power of 50W and do not want an increase of the temperature inside the still air chamber of more than 2 °C, relative to the temperature outside the chamber, a 1x1x1 m3 box is a good size. The ambient temperature outside the still air chamber should be the same as the maximum ambient temperature conditions in the application, to be specified by the luminaire manufacturer.

With increasing LED efficiency, we should take care that the optical radiation does not impact the measurements. Preferably, the optical radiation should exit the still air chamber via a transparent wall and is absorbed outside the chamber in a way that is does not impact the temperature of the air chamber. This means that it either has to be dissipated at a relatively large distance or on a surface that is cooled andstays at a temperature that is comparable to the wall temperature of the chamber. Next to that, the sensor to measure the temperature inside the chamber should not be influenced by that optical radiation. Therefore, the sensor should not be placed in the optical beam coming from the luminiare, but next to the luminiare.

3.3. Heat Sink Temperature definitionThe thermal capabilities of a fixture will be determined by

a thermal reference engine. This device has a mechanical interface that fits into the fixture. It generates a thermal load to the fixture by means of a resistance heater. The thermal reference engine is isolated in order to minimize the heat losses directly to the ambient air. The advantage of this approach is that the thermal load can be easily estimated fromthe electrical input and no optical measurements are required. Next to that, electrical measurements can be performed with high accuracy. As already indicated in section 3.1, the exact temperature at the thermal interface is determined by both the fixture and the engine. Therefore, the thermal test device for testing the fixture should apply a relatively concentrated heat load. This way, the fixture is also tested for its heat spreading

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capabilities. Good spreading capabilities are translated into better thermal resistance values.

On the other hand, it is crucial to also perform heat spreading close to the LEDs, where it is most effective. Therefore, we can expect an LED engine to have certain minimal heat spreading capabilities. The thermal test device should not have worse spreading capabilities than this one in order to avoid too high thermal loads at the interface and over-specification at system level.

3.4. Heat Flux DefinitionFor a good description of the thermal interface, we need to

know the thermal load from the engine on the fixture. A method that is often used to estimate the thermal load of anengine is to measure the electrical input power and the optical output power of the device. The difference between the two is than taken as the thermal losses of the engine. While it is correct to state that this difference results in the total thermal losses, this approach has some issues. First it is difficult to measure the optical output very accurately. If an optical measurement is performed according to CIE standard 121-1996 we typically get accuracies in the order of 5%. With increasing light engine efficiency, this will result in an increasing error in the estimate of the thermal load as is indicated in Figure 3. This will lead to errors in the estimate of the thermal load that easily exceed the 5% that we consider an acceptable limit.

Figure 3. Accuracy of the Thermal Power estimate as a function of the optical efficiency for a 2% and 5% error in the optical measurement. Al other error sources are neglected.

Next to this effect, there is another issue. Part of the heat that is generated by the light engine will be transferred to the ambient air directly from the engine and not via the light engine – fixture interface. The light engines itself can be relatively large and might have a relatively high temperature. This is specially the case of an engine contains a remote phosphor. With a remote phosphor solution, the phosphor is not applied directly in the LED package, but remotely. Within the Fortimo Twist module, for example, the phosphor is located at the optical window, opposite of the LEDs as indicated in Figure 1. Next to the increased efficiency, remote phosphor has another advantage. The Stokes losses in the phosphor contribute a considerable part of the total losses in a LED system. These losses are about 30% of the blue light that

is emitted by the LEDs. With the current state of LED, the blue LEDs can have an efficiency of 50% and this will increase over time. As a result 15% of the energy that goes into the LED is converted into heat in the phosphor. And this contribution will increase as the LED efficiency will increase.

The remote phosphor is located at the exit aperture of the light engine and can withstand a relatively high temperature. Currently, the maximum remote phosphor temperature is around 100 °C and can increase to about 180 °C if inorganic carriers like glass or ceramic are applied. As a result, almost all thermal power is transferred directly from the remote phosphor to the ambient air via radiation and convection and thus does not put any load on the fixture via the thermal interface of the light engine.

The most effective way to determine the thermal load of the light engine on the fixture is to measure the heat flux from the engine to the fixture directly. In order to do that, we developed a heat flux measurement method, which is described in section 4.1.

4. Test Devices4.1. Heat Flux Measurement

In order to determine the thermal load on the fixture, aLED engine thermal power test device is developed. The thermal power test device measures the part of the thermal power that is transported by conduction to the mounting platform of the LED engine.

The setup is developed by the company Hukseflux Thermal Sensors [6]. A schematic view of the setup is drawn in Figure 4. The LED engine is mounted on a heat spreader that distributes the heat and eliminates sensitivity to the position of the engine. Beneath that an internal resistor serves as a calibration source as well as a heat source to control the engine temperature during the measurement. Beneath another heat spreader, a heat flux sensor measures the heat flux through the setup. A water cooled metal block serves as the heat sink.

Figure 4. Schematic view of the LED Engine Thermal Power Test Device

A control system keeps the heat flux through the setup constant by adjusting the power dissipation in the internal resistors. This way, the temperature distribution in sensor stays constant with different thermal loads, which increases the accuracy. The difference between the dissipation in the

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internal resistors without and with additional thermal load from the LED engine should be equal to the thermal load from the LED engine

The setup is used upside down (LED shining downwards)to simulate worst case conditions. The LED engine operates at a predetermined, stabilized temperature. In most cases, a temperature of 65 °C is defined as interface temperature.

The heat flux sensor is tested with different thermal test dummy engines. The dummy engines are equipped with a resistive thermal load. The two dummies differ in heat spreading capability in order to test the effect of heat spreading on the heat flux sensor. The results of these tests are plotted in Figure 5.

Figure 5. Measured heat flux vs. Electrical power.

The graph shows a very good correlation between the measured flux and the applied electrical power. The accuracy of the heat flux sensor is better than 5%

4.2. Reference Thermal EngineIn order to test a fixture, a Thermal Test Engine is

developed. This Test Engine consists of the resistance heater,placed on a heat spreader with limited spreading capabilities. A Thermal Interface Material is part of the Test Engine. The purpose of the Test Engine is to determine the Rth of the fixture. This Rth includes the spreading capabilities of the luminaire.

4.3. Reference LuminaireIn order to test LED light engines, a Thermal Test Fixture

is developed. The heat flux sensor is used to determine the thermal load on the fixture. However, in order to get an accurate measurement of the heat flux, the sensor has very good heat spreading capabilities. The main goal for the test fixture is to determine if the engine has sufficient spreading capabilities

The reference luminaire consists of a heat sink with limited, but sufficient heat spreading capability. The combination of the reference thermal engine and the reference luminaire is designed in such a way that they are on the edge of what is allowed in term of spreading capabilities. On the reference luminaire, the temperature at the interface is

measured at a number of points according to the distribution indicated in Figure 6.

Figure 6. Measuring points to estimate the temperature distribution over the thermal interface

If we define a uniformity parameter Rij= (Ti-Tj)/Pthermalbetween each of the measurement points, the uniformitybetween any pair of the measurement points should be at most 0.2 K/W. If a LED light engine with insufficient spreading capabilities is applied to the reference luminaire, the max temperature becomes too high, resulting in a too high temperature gradient over the interface and an engine not passing the test.

5. ConclusionsAn approach to standardize the thermal interface between

a LED light engine and a LED based fixture is proposed. This approach consists of three items. First, a heat flux measurement in order to determine the thermal load of the light engine on the fixture. Second, a reference thermal engine to determine the thermal capabilities of the LED based fixture. Third a reference luminaire in order to test the LED light engine on heat spreading capabilities. With these three tests, a standards complying thermal interface can be defined.

References[1] http://www.zhagastandard.org/[2] András Poppe and Clemens J.M. Lasance,” On the

Standardization of Thermal Characterization of LEDs”Proceedings of the 25th SEMI-THERM Symposium, San Jose, Calif., 2009, pp. 151-158

[3] Lasance C., "On the standardisation of thermal characterisation of LEDs Part I: Comparison with IC packages and proposal for action", Proc. 14th THERMINIC, Rome, Italy, 24-26 September 2008, pp. 208-212

[4] Lasance, C. and Poppe, A., On the standardisation of thermal characterisation of LEDs Part II: Problem definition and potential solutions, Proc. 14th THERMINIC, Rome, Italy, 24-26 September 2008, pp. 213-219

[5] Clemens J.M. Lasance, ”Heat spreading: Not a trivial problem”, Electronics Cooling, May 2008

[6] http://www.hukseflux.com/

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