Understanding Heat Dissipation of a Remote Phosphor Layer in an LED System

Indika U. Perera and Nadarajah Narendran Lighting Research Center, Rensselaer Polytechnic Institute

21 Union Street, Troy, NY 12180 narenn2@rpi.edu, (518) 687-7100

Abstract

This study investigated how passive and active cooling methods dissipated the heat from a remote phosphor layer in an LED system. The focus was on quantifying the amount of cooling contributed by each heat dissipation mechanism, namely, conduction, convection, and radiation.

The conductive heatsink surrounding the phosphor layer acted as an extended surface for dissipating the heat. In this study, the effect of increasing the heat extraction area between the phosphor layer volume, changes in convection coefficients, and changes in surface emissivity of the heatsink on phosphor layer temperature were investigated. The surface temperature of the remote phosphor layer was measured using an infrared imaging camera.

At the first stage the metal heatsink removed the heat from the phosphor layer by conduction, then the heat from the metal heatsink was dissipated to the ambient via convection and radiation. The results indicated a significant reduction in phosphor layer temperature. Additionally, active cooling further reduced the phosphor layer temperature. The results also showed that the temperature distribution on the phosphor layer improved with the metal heatsink configuration.

Keywords: solid-state lighting, light-emitting diode, remote phosphor, thermal management, IR thermography, heat transfer, conduction, convection, radiation, extended-surface

Introduction A phosphor-converted white light-emitting diode (LED)

has a phosphor layer placed around the semiconductor chip to convert the narrowband short-wavelength radiation emitted from the LED chip into a broadband white light [1], [2]. White LED package efficacies are targeted to go beyond 250 lumens per watt by 2020 [3]. During the past several years many methods have contributed to improving the efficacy of phosphor-converted white LEDs. Remote phosphor is one such method where significant improvement in overall LED package efficacy was achieved by reducing the absorption of photons in the package and by reducing the phosphor quenching due to high heat at the LED chip [4]-[6]. These improvements are attributed to the displacement of the phosphor layer from the LED chip. Demand for smaller light engines and increased light output are constraining higher radiant energy densities on the remote phosphor layers, thus increasing heat densities in these phosphor conversion layers. This increase in heat density in the phosphor conversion layer negatively affects the performance. Ultimately, the increase in temperature in the phosphor layer affects luminous efficacy, color shift, and life of the remote phosphor LED system [7], [8].

Past research has observed increased heat generation in the phosphor layer in remote phosphor applications [2], [9], [10]. This heat is generated due to conversion inefficiencies and trapped photons in the phosphor layer. One past study mentions that the generated heat in the phosphor layer can be about 13% of the electrical input to the system [9]. Other studies have stated that the generated heat due to these conversion inefficiencies and trapping of photons can increase the phosphor layer operational temperature up to 150°C [11].

The LED industry defines the life of an LED based on lumen depreciation. For the most part, it is the degradation of the encapsulant that causes lumen depreciation in white LEDs. Excessive heat and short-wavelength visible radiation are known to cause encapsulant material degradation in phosphor-converted white LEDs [12]. Heat at the phosphor layer is one main cause for rapid lumen depreciation. Therefore, proper thermal management of the phosphor layer can improve remote phosphor white LED performance.

Methodology In a phosphor-converted LED light source there are two

stages where energy is transformed from one form to another. First, electrical energy is converted to short-wavelength visible radiant energy by the semiconductor chip. Next, the short-wavelength visible radiation is converted to broadband long-wavelength visible radiation by the phosphor material [1], [2]. At both of these conversion stages, heat is generated due to inefficiencies of the conversion processes, as discussed earlier.

Conductive material embedded in the phosphor layer can reduce the operational temperature by effectively transferring the heat in the phosphor layer to the ambient [13]. Effective heat dissipation methods have been used to transfer the generated heat in the remote phosphor layer to the ambient environment. Past research showed that by embedding the phosphor layer in a perforated, thermally conductive heatsink, the phosphor layer temperature can be reduced [14].

The schematic shown in Figure 1 illustrates the possible energy transfer paths of a phosphor layer embedded in a thermally conductive heatsink. The heat transfer mechanisms include:

Conduction – heat transfer along the phosphor layer to the heatsink and along the heatsink following temperature gradients;

Convection – convective heat transfer from the phosphor layer and the heatsink to the ambient and the LED reflector cavity;

Radiation – radiative heat transfer from the phosphor layer and the heatsink to the surrounding and the LED reflector cavity.

978-1-4799-5267-0/14/$31.00 ©2014 IEEE 186 14th IEEE ITHERM Conference

The LEDs are placed on a heatsink (LED-hs). The reflector cavity is not illustrated in the diagram but the collective is represented at the LED heatsink as LED cavity/LED heatsink. The radiant energy emitted by the LEDs is denoted by Evis,in while the visible energy emitted by the phosphor layer is denoted by Evis,out. The difference between the visible radiations in and out across the phosphor layer cause inter-heat generation and is denoted by Eg.

The generated heat in the phosphor layer transfers to the surround using three basic heat transfer mechanisms, including conduction, convection, and radiation. The heat conducted from the phosphor layer to the phosphor heatsink is denoted as Eph,cond→hs. The convective heat transfer from the phosphor layer to the ambient and to the LED cavity/LED heatsink is denoted by Eph,conv→amb and Eph,conv→cav, respectively. Similarly, the thermal radiation emitted from the phosphor layer towards the surrounding is denoted as Eph,rad→sur, and the thermal radiation emitted towards the LED cavity/LED heatsink is denoted as Eph,rad→LED,cav,LED-hs. The phosphor layer heatsink material is assumed not to absorb any visible radiation energy emitted from the LEDs because it is painted white to reflect all energy. The heating of the phosphor layer heatsink is due to conduction from the phosphor layer (Eph,cond→hs). In addition to this, thermal radiation incident on the phosphor layer and the heatsink from the surrounding (Esur→ph,hs) and the LED, LED cavity/LED heatsink (ELED,cav,LED-hs→ph,hs) are also present. The heatsink material dissipates heat via convection to the ambient (Ehs,conv→amb) and LED cavity (Ehs,conv→amb) and via thermal radiation to the surrounding (Ehs,rad→sur) and LED cavity (Ehs,rad→LED,cav,LED-hs).

Figure 1. Schematic of the energy transfer in the LED

remote phosphor layer with heatsink. Hereafter in this paper, the visible radiation will be called

light and the heat will be called thermal radiation. Due to experimental constraints, the radiation and

convection effects towards the LED side from the phosphor layer and heatsink material were not considered in the present study. To reduce the light absorption in the heatsink material, the side facing the LEDs were painted white to increase light reflection.

Experiment An array of 12 LEDs with peak wavelength near 448 nm

was used as the source for irradiating the phosphor layer and the heatsink. The LED array was fabricated in-house to have a uniform irradiance incident on the phosphor layer heatsink when placed on the reflector cup, as illustrated in Figure 2. The beam uniformity on the bottom surface of the phosphor layer and heatsink was analyzed using LightTools®, a commercial ray-tracing simulation software.

Figure 2. Schematic diagram of the LED source used for

irradiating the phosphor layer heatsinks. The LED operational temperature was maintained at a

constant value for all subsequent testing with the use of a temperature-controlled heatsink. The operational temperature was determined by monitoring the temperature at a reference location on the LED package prescribed by the LED manufacturer.

A number of aluminum plates were machined with geometrical configurations identical to the phosphor layer heatsink plates used in a previous study [14]. The table below describes the physical details of the phosphor layer heatsinks.

Table 1. Details of phosphor layer heatsinks.

Thickness of heatsink = 1.5 mm

Heatsink #1

Heatsink #2

Diameter of the hole (mm) 19.1 7.2

Number of holes 1 7

Light emitting surface area (mm2) 285 286

Light emitting surface area ratio 1.0 1.0

Heat conduction area (mm2) 90 238

Heat conduction area ratio 1.0 2.6

Figure 3. Geometric configurations of the phosphor layer

heatsinks. Figure 3 illustrates the geometric configurations of the

phosphor layer heatsinks. As explained in an earlier paper [14], the increase in heat conduction area of heatsink #2 increases the transfer of heat generated in the phosphor down-conversion process. The heat conduction area is defined, for the purpose of the study, as the interface area between the phosphor and epoxy mixture layer and the aluminum material and is used to describe the said area throughout this paper.

The phosphor-to-epoxy mixing ratio in the phosphor and epoxy layer was kept at 1:10 by weight ratio in the present study, similar to the ratio used in the earlier study [14].

To increase the light output from the reflector cup, the aluminum heatsink and sides of the cup irradiated by the LEDs were all coated with a white paint (ρ≈0.8). This coating assisted in extracting light out of the reflector cup emitted by the LEDs due to reduced surface absorption of photons at the aluminum heatsink.

The temperature on the light-emitting surface of the phosphor layer heatsink was monitored and recorded by a JENOPTIK IR-TCM 384 infrared camera module. The infrared camera is sensitive in the 7.5–14 μm wavelength range and therefore is not affected by the light (0.38–0.78 μm) emitted from the phosphor layer surface. The target emissivity of the phosphor and epoxy surface was determined based on surface emissivity characterization procedures recommended/followed by the infrared camera manufacturer/infrared thermography community [15], [16].

Two cooling approaches were taken to understand the relative magnitudes of the heat transfer due to different mechanisms. One was to introduce active cooling on to the phosphor layer heatsink while the other was to enhance radiative cooling by increasing the surface emissivity of the phosphor layer heatsink.

A fan operated with a DC power supply was used to create active cooling of the phosphor layer heatsink and was operated at constant conditions for all relevant experiments. In order to enhance the surface emissivity of the aluminum heatsink, after the low radiative experiments (uncoated heatsinks on the side exposed to the ambient) were concluded, the heatsinks were coated with the same white paint (ε≈0.92) that was used earlier to coat the surface exposed to the ambient.

Figure 4 is a schematic of the experimental setup used for the present study. The fan was oriented to generate a constant

air-flow rate over the phosphor layer heatsink. The infrared enclosure was designed not to obstruct the air-flow.

Figure 4. Schematic diagram of the infrared thermal

imaging camera setup for temperature measurements All measurements were acquired at steady-state. This was

ensured by monitoring temperature and current values to the LED. Six successive measurements taken within five minutes being less than 2% of the maximum variation compared to the maximum measured value within the five minutes ensure steady state. Ten infrared camera images were acquired at a frequency of 1 Hz for post processing on JENOPTIK VarioAnalyze proprietary software. These post processed image data were used in the subsequent temperature analysis.

Results and Discussion The maximum temperature at the phosphor layer heatsink

surface, indicated by the surface area enclosed by the discontinuous circumferences of the circles in Figure 5, and the temperature distribution along the surface from the geometric center of the plate to the plate edge, indicated by the arrows in Figure 5, were both used in the temperature analysis.

Figure 5. Phosphor layer heatsink schematics used for

temperature analysis. The maximum temperature on the phosphor layer heatsink

reduced as the heat conduction area was increased (Figure 6). The heat conduction area of the respective heatsinks was normalized to the heat conduction area of heatsink #1 (Table 1).

For comparison purposes, experimental results obtained in a similar earlier study [10] were used. In that study, the

phosphor was placed on acrylic heatsinks embedded with a very similar phosphor and epoxy mixture irradiated with a similar setup to that used in the present study, and those results were used as a reference in the present study. The reason for this comparison was to investigate the complete thermal management system. This includes the conduction of the generated heat in the phosphor to the aluminum phosphor layer heatsink which is dissipated via convection and radiation mechanisms to the ambient, as explained in the methodology section.

Figure 6. Experimental results on phosphor layer temperature change with heat conduction area.

An increase in the heatsink conduction area decreased the

maximum phosphor layer temperature provided the heatsink material had higher thermal conductivity compared to the phosphor layer. At steady-state, the heat transferred along the heatsink material was dissipated to the environment via both natural convection and radiation.

While it is beneficial to reduce the peak temperature in the phosphor layer, it is also important to have uniform temperature distribution in the phosphor layer. In order to check for temperature uniformity, the temperature distribution on the phosphor surface was analyzed. Figure 7 and Figure 8 show the temperature distribution on the phosphor and epoxy surface of heatsink #1 and heatsink #2, respectively. In both figures, the acrylic data from [14] is overlaid as a reference. The discontinuous vertical lines in the figures represent the phosphor layer and heatsink material interfaces. The surface temperature data on the aluminum/acrylic material are not plotted in Figures 7 and 8.

Figure 7. Temperature distribution on the phosphor and

epoxy surface of heatsink #1.

Figure 8. Temperature distribution on the phosphor and

epoxy surface of heatsink #2. The above figures indicate aluminum heatsink #2 achieved

a lower maximum phosphor layer temperature compared to its acrylic counterpart (~45°C) and ~20°C compared to aluminum heatsink #1 while maintaining a better temperature uniformity in the phosphor layer as a whole. The max-min temperature difference was <10°C for aluminum heatsink #2 compared to ~20°C in acrylic heatsink #2 and ~30°C in aluminum heatsink #1.

In order to assess the effect of active cooling on the heatsinks, the maximum phosphor layer temperature (Figure 9) and the temperature distribution on the phosphor layer surface (Figure 10 and Figure 11) were used.

Figure 9. Passive cooling vs. active cooling.

The phosphor layer maximum temperature further

decreased from the passive cooling temperatures by 25°C and 20°C, respectively, for heatsink #1 and heatsink #2, respectively. The air-flow rate generated by the active cooling system was ~0.4 m/s velocity not accounting for boundary layer effects at the heatsink surface. Considering the benefit it delivers the implementation of an active cooling system in an application can be justified provided the heat generated in the phosphor layer is excessively high.

Figure 10. Heatsink #1 temperature distribution under

passive and active cooling. The temperature uniformity on the phosphor layer surface

is not affected by the active cooling system, which still indicated relatively large temperature gradients across the phosphor layer in heatsink #1 (>20°C). The max-min temperature difference in heatsink #2 was ~6°C.

Figure 11. Heatsink #2 temperature distribution under

passive and active cooling. The increase in surface emissivity of the heatsink indicated

moderate benefits, as illustrated by Figure 12. The maximum temperature on the phosphor layer reduced by ~5°C in both heatsink configurations. The temperature distribution on the phosphor layer indicated almost a constant offset of approximately 5°C with the increased surface emissivity of the heatsink (Figure 13 and Figure 14).

Figure 12. Radiative cooling effect under natural

convection. Figure 13 and Figure 14 illustrate the temperature

distribution on the phosphor layer surface for four test conditions with the acrylic reference temperature profile. The active cooling with white paint coating (increased emissivity) temperature profile data points are marginally below (~2°C) the active cooling with no white paint temperature data for both heatsink configurations. The maximum temperature of the phosphor layer was 78°C and 61°C, respectively, for heatsink #1 and heatsink #2.

The aluminum heatsink #1 outperforms the acrylic heatsinks. This is due to the higher conductivity of the aluminum, which manages to maintain a lower phosphor layer

temperature by better dissipating heat to the ambient. The contribution of convection is larger compared to radiation in the dissipation mechanism and as the temperature is lowered, this disparity between convection and radiation mechanisms becomes larger. This is the reason for the marginal reduction in the temperature profile associated with active cooling between those conditions with and without white paint.

Figure 13. Heatsink #1 temperature distributions.

The thermal management capability of heatsink #2

becomes evident when Figure 14 is compared with Figure 13. The passive cooling with white paint condition of heatsink #2 has a lower maximum temperature (~75°C) than the active cooling with white paint condition of heatsink #1(~78°C). The minimum temperature is lower for heatsink #1, closer to the aluminum heatsink material, in the same comparison conditions with a large temperature dispersion.

Figure 14. Heatsink #2 temperature distributions.

Conclusions The study systematically investigated the absolute

temperature reduction in a phosphor layer using both passive and active cooling strategies. The use of conductive heatsinks for reducing phosphor layer temperature was found to be a feasible method. The use of a distributed heatsink configuration (heatsink #2) compared to a non-conductive

heatsink reduced the maximum temperature to ~50°C while reducing the temperature disparity to below 10°C using passive cooling strategies. It was possible to further reduce the maximum phosphor layer temperature by adopting active cooling strategies. The reduction in temperature using active cooling compared to passive cooling was approximately 15°C.

Acknowledgments The authors would like to acknowledge the financial

support of the Lighting Research Center at Rensselaer Polytechnic Institute, Henkel, and the Besal Lighting Education Fund provided by Acuity Brands Lighting; and the contribution and support of Yi-wei Liu, Martin Overington, Howard Ohlhous, and Jennifer Taylor at the Lighting Research Center.

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