Elements of cooling systems of power LEDs

ELEMENTS OF COOLING SYSTEMS OF POWER LEDS

Gdynia Maritime University

Faculty of Marine Electrical Engineering Poland

ABSTRACT

In the paper the problem of cooling power LEDs is considered. Modern solutions of packages of power LEDs and elements which improve the removal of heat generated in these devices are presented. Especially, much attention is paid to geometrical dimensions of the packages and construction elements of the heat flow path in free and forced cooling systems.

1. INTRODUCTION

Power LEDs are more and more frequently used in lighting [1, 2, 3, 4] and automotive [5, 6, 7] applications. In the last several years it has been possible to observe a significant improvement of operating parameters of semiconductor lighting sources and a significant decrease in the cost of purchasing a single element [8, 9]. Properties of semiconductor lighting sources produced in present-day make it possible to obtain high values of luminous flux, high luminous efficiency and long life time counted in few thousand hours of continuous operation [9]. Thanks to the selection of semiconductor material in a production process one can obtain different colours of light emitted by LEDs [10]. In turn, the application of lenses of different shape makes it possible to obtain different characteristics of light distribution.

The papers [11, 12] present long life time of LEDs as a significant advantage, however the important problem is the removal of heat generated in the considered devices [13, 14, 15]. In the papers [16, 17, 18] the influence of the thermal phenomenon and mutual thermal interactions on reliability, electrical and optical parameters of semiconductors lighting sources is described. Significant dependence of the emitted luminous flux by LED lamps on a thermal phenomenon is observed [16, 17, 18]. The value of this parameter at junction temperature equal to 100°C can decrease by even about several percent [18]. The elevated junction temperature of the device negatively influences also life time of LEDs and in short time it can

lead to displacement of spectral characteristics of the device [19]. Therefore effective cooling of LEDs is so important. Cooling is optimized by the producers of LEDs and on the application level – by the designer of the device cooling systems [1, 3, 4, 8, 11, 12, 13, 19].

The aim of this paper is a presentation of today’s constructions of packages of power LEDs, arbitrary selected examples of these packages used by primary producers of semiconductor lighting sources and a description of free and forced systems of cooling both the single diodes and LED modules.

The second section describes mechanical construction of the LEDs. The section three presents packages of the LEDs in historical background and packages use in modern solutions to apply on lighting and automotive market. The fourth section describes elements of free cooling systems of the diodes and LED modules. Finally, in the fifth section forced cooling systems mainly used in high power LED lamps are shown.

2. CONSTRUCTION ELEMENTS OF LEDS

The package of the power LED diode should realize two tasks: provide effective transfer of heat generated in the semiconductor chip and to shape the emitting light beam in a desirable manner at the least optical waste [15]. Effectiveness of heat removal is described by thermal resistance Rth. The smaller value of this parameter, the more effective removal of generated heat to the surroundings [15, 17, 18]. There are also a few parameters characterizing optical properties of the package, one is the lighting absorption coefficient.

Figure 1 presents the package of the power LED dedicated to devices with admissible power not exceeding 5 W [20]. The substrate of the LED structure is most often made a ceramics or epoxides materials. Thermal interface between the substrate and the LED structure is most often made of solder paste or thermo conductive glue. Lenses are made of plastic or silicone. The lenses enable obtaining different angles of lighting distribution.

Figure 2 describes the package of the power LEDs Dragon series by OSRAM Company. This package was designed in such a way as to take

into account improvement of thermal conditions, which means heat removal from the semiconductor chip. The package of this diode includes modified plastic with an integrated heat sink and electrical contacts [21].

Plastic Lens

Reflector

LED Chip

Substrate

Fig. 1. Typical outline of the LED diode with nominal power

Improvement of thermal conditions is constructing diodes from Dragon series that positively influences the value of thermal resistance (junction-case) RthJ-C, which is equal only to 2.5 K/W. The next significant element of the power LED is built-in in the structure special ESD (Electrostatic Discharge) protection circuit, the operation of which is based on bidirectional limit of the maximum voltage occurring on the p-n junction. This circuit minimizes electrostatic overvoltage effects, which are very sensitive to power LEDs. In the package presented in Figure 2 it is possible to install even several single LED structures [21].

Fig. 2. Package of the Dragon diode optimized thermally [21]

We can describe the package of the LED diode with the help of a layer outline shown in Figure 3 [15]. Chip Ceramic Metal Core PCB Thermal interface 1 Thermal interface 2

Fig. 3. Layer construction of the latest power LED packages [15]

The first layer in the outline presented in Figure 3 is the semiconductor chip, which emits light as a result of the flowing current. The LED structure was installed in a soldering process on the ceramic substrate. This substrate was installed with the use of thermally conductive glue to the MCPCB (Metal Core Printed Circuit Board) [22, 23, 24]. The use of MCPCB substrate allows decreasing the thermal resistance Rth value of power LEDs. In Figure 3 a special thermal interface, which plays a very important role in decreasing the thermal resistance Rth value is marked. The choice of equivalent thermally conductive pastes and solder pastes cause effective improvement of heat removal from the LED chip [15]. The next section describes the selected packages of power LEDs.

3. REVIEW OF PACKAGES OF POWER LEDS

The first LEDs, assigned to operate as indicators appeared on the market in the year1962. The p-n junction of this diode is situated in a glass pipe. These diodes were characterized by the thermal resistance Rth value reaching several hundreds of K/W [4]. The following step in evolution of LEDs used also as indicators, were those installed in plastic cases. These diodes were characterized by thermal resistance of the value Rth = 200 K/W [4].

Figure 4 presents the package of the power LED of the type Emitter produced since the beginning of 2000. Typical value of thermal resistance of this diode is equal to 10 K/W and makes possible effective heat removal at power not exceeding 5 W [4].

Contacts LED

Plastic Lens LED structure

Plastic Package

Fig. 4. Package of power LEDs of the type Emitter at power equal to 5W [12]

Fig. 5. Package TO220 of the diode LP9KLB [25]

Figure 5 shows the package of the LP9KLB diode by LEDTECH [25] company. The type of the package is standard TO-220. The light sources by LEDTECH were situated in TO-220 packages of dimensions 16.55 × 9.8 mm and TO-220 packages soldered to STAR substrates of the diameter 20.2 mm and thickness 1.75 mm. This device can operate with the maximum power PTOTMAX = 1 W, the forward voltage UF = 3.2V and the maximum luminous flux ΦV = 52 lm at the forward current IF = 0.35A. The diode can be characterized by thermal resistance (junction-case) RthJ-C = 15 K/W [25].

3.1. LEDs by Philips Lumileds

In this section parameters of semiconductors LED light sources from Philips Lumileds Company are described [26]. This company as the first on the market of semiconductors lighting sources used LEDs in automotive industry [27]. Table 1

describes basic construction parameters of the selected LEDs by Philips Lumileds Company. In Figure 6 the arbitrary selected packages of LEDs by Philips Lumileds Company are shown.

In Figure 6a the case of Luxeon Rebel K2 LED produced by Philips Company is shown. The device in the Emitter package was mounted on the STAR type MCPCB surface [22, 24]. This diode was mainly used to illuminate architectonic elements. A typical forward voltage UF is equal to 3.72 V at the forward current IF = 1 A. This diode is installed in the Emitter package of dimensions 7.35 × 7.35 × 6 mm on the STAR surface of the diameter equal to 20.2 mm and thickness equal to 1.75 mm. This diode is characterized by thermal resistance (junction-case) RthJ-C equal to 13 K/W for the STAR type package and RthJ-C equal to 9 K/W for the Emitter type package. The maximum luminous flux ΦV of this diode is equal to 140 lm at the forward current IF = 1.5 A. The maximum value of the power supply PTOTMAX is equal to 5 W [29].

a) b) c) d) e)

Fig. 6. Package of LEDs by Philips Lumileds company [28, 29, 30, 31, 32] Table 1. Basic parameters of LEDs by Philips Lumileds company [28, 29, 30, 31, 32]

Diode type RthJ-C

[K/W] [K/W] RthC-A Package type

Dimensions (mm) (length x width x

height)

PTOTMAX

[W]

Luxeon REBEl Plus 9 --- Ceramic SMD 3 x 4.5 x 2.1 2

Luxeon Q 7 --- Ceramic SMD 3.5 x 3.5 x 2.12 3

Luxeon Rebel K2 STAR 13 --- STAR 20.2 x 20.2 x 7.75 5

Luxeon Rebel K2 EMITER 9 --- EMITTER 7.35 x 7.35 x 6 5

Luxeon Altilon 1 x 2 --- 17 Automotive 17.1 x 16.1 x 5 7

Luxeon Altilon 1 x 4 --- 8.5 Automotive 17.1 x 16.1 x 5 13.7

Luxeon LHC1-5770-1208 0.22 --- COB 24 x 20 x 1.5 70

Figure 6b shows the Luxeon Altilon diode dedicated to the use in automotive industry and at present waiting for commercial applications. This device is in the phase of laboratory tests to the admitted use as the automotive head-light [28]. The Luxeon Altilon diodes are produced in two different configurations of connection of the LED structures. For the bi-structure configuration, the typical value of the forward voltage UF is equal to 7 V and thermal resistance (case-ambient) RthC-A is equal to 17 K/W. For the four-structure configuration the forward voltage UF is equal to 13.7 V and thermal resistance (case-ambient) RthC-A isequal

to 7.5 K/W at forward current IF equal to 1 A and at the case temperature TC of the diode equal to 25°C. The structures of LEDs are connected in series. Both versions of this device possess ESD circuit protection [30].

Figure 6c illustrates the package of the Luxeon Rebel Plus diode characterized by high luminous efficiency equal to 110 lm/W at the forward current not exceeding 0.35A. The maximum forward current IF of this diode is equal to 0.7A. This device is characterized by thermal resistance (junction-case) RthJ-C equal to 9 K/W. This device is a continuation of the series of the Luxeon Rebel diodes used mainly to illuminate architectonic elements and it is compatible with optical solutions used in the Luxeon Rebel diodes series. The package of the Luxeon Rebel Plus diode is mounted in SMD 4530 standard case of dimensions 4.5 × 3 mm on the ceramic substrate [31].

Figure 6d shows the Luxeon Q diode characterized by the forward voltage UF equal to 2.99 V at the forward current IF = 1 A and the maximum value of the junction temperature TJ equal to 135°C. The diode can be supplied by the maximum forward current IF = 1050 mA. This device is characterized by thermal resistance (junction-case) RthJ-C = 7 K/W and it is used in lighting and automotive technique. The Luxeon Q diode was the first diode mounted in the TFFC (Thin Film-Flip Chip) standard [33]. The package of this device is made in SMD 3535 standard of dimensions 3.5 × 3.5 mm on the ceramic substrate. In designing the package of the Luxeon Q diode, the CSP (Chip Scale Design) technology was used [32].

Figure 6e presents the package of the Luxeon LHC1-5770-1208 diode of COB type, which is characterized by the forward voltage UF = 35.5 V at forward current IF = 0.9 A and the maximum junction temperature TJmax = 125°C. This LED can operate with the maximum forward current IFMAX = 1.8 A and dissipates the maximum power PTOTMAX = 70 W. This device is characterized by the very small value of thermal resistance (junction-case) RthJ-C, which is equal to 0.22 K/W and the high value of luminous efficiency ηopt = 130 lm/W. This diode is mounted in the small package of dimensions 24 × 20 × 1.5 mm and it is characterized by the high luminous flux ΦV = 4100 lm and the value of the CRI (Colour Rendering Index) equal to 73 [28].

3.2. LEDs by Osram Semiconductors

The next producer of semiconductor lighting sources is Osram Semiconductor [34] company. Figure 7 presents the arbitrary selected packages of the diodes by Osram Semiconductor Company, however in Table 2 the basic construction parameters of the selected elements are collected.

a) b) c)

Fig. 7. Packages outlines of the selected diodes by Osram Company [35, 36, 37]

Particular attention should be paid to the diode from Ceramos series. The semiconductor structure of this diode is mounted in the ceramic SMD package, which is shown in Figure 7a. This diode is characterized by the typical value of the luminous flux ΦV equal to 120 lm and the forward voltage UF = 4 V at the maximal forward current IFMAX = 0.5A and at thermal resistance (junction-solder point) RthJ-S equal to 28 K/W for the package dimensions 2.2 × 1.75 × 0.85 mm. The semiconductor structure is made in the ThinGaN technology [33]. The LUWC9SP diode is used mainly in illumination architectonic elements and the matrix TFT (Thin Film Transistor) backlight circuit [35].

Table 2. Construction parameters of the selected diodes by Osram company [35, 36, 37]

Diode type RthJ-S [K/W] Package type Dimensions (mm) (length x width x height) PTOTMAX [W]

OSRAM Ceramos 28 Ceramic SMD 2.2 x 1.75 x 0.85 1.5

OSLON SSL 80 6.2 Ceramic SMD 3 x 3 x 2.3 3

DURIS S8 GWP9LRS1 --- SMD 5.8 x 5.2 x 1 4

DURIS S8 GWP9LMS1 --- SMD 5.8 x 5.2 x 1 6

OSTAR LE UW S2LN 4 Ceramic SMD 5 x 5 x 2.3 10

Figure 7b presents the diode Oslon SSL80 series LCW CR7P.EC allows pointing lighting systems. The producer assures that this diode possesses its biggest luminous flux ΦV equal to 140 lm at the forward current IF = 0.35 A and the highest value of luminous efficiency ηopt which is equal to 108 lm/W at small package dimensions relation. This diode can operate at the maximum forward current IF = 0.8 A. The device is characterized by the small value of thermal resistance (junction-case) RthJ-C which is equal to 6.2 K/W. The package of this device designed in the SMD 3030 standard has dimensions 3 × 3 mm and possesses the ceramic substrate and silicon lens [36].

Figure 7c shows the package of the LED Ostar Lighting Plus from series LEUWS2LN. The diode is mounted in the SMD 5050 standard of dimensions 5 × 5 mm and contains four LEDs structures in one package. This emitting luminous flux is equal to 425 lm at the forward current IF = 0.35 A. This diode is characterized by value of the colour temperature CCT = 6500 K, the luminous efficiency of this diode equal to 100 lm/W and the forward voltage UF = 3.2 V at the forward current IF = 0.35 A. The device can operate with the maximum forward

current equal to 0.7A per single structure and it possesses the small value of thermal resistance (junction-case) RthJ-C, which is equal to 4 K/W. The structure of this diode is located on the ceramic substrate [37].

3.3. LEDs by CREE

The next producer of LED lighting sources, which is one of most famous on the semiconductor market, is Cree Company [38]. These diodes are characterized by the high value of luminous efficiency at small package dimensions and these use the latest technologies to produce the semiconductor structure. Figure 8 presents the arbitrary selected packages of LEDs by Cree Company. In Table 3 basic construction parameters of the diodes are collected.

The diode shown in Figure 8a belongs to Xlamp XR-E product family. This diode is characterized by the maximum luminous flux equal to 107 lm at the forward current IF = 0.35 A. This diode emits light of the colour temperature CCT in the range from 2600 K to 10000 K. This device has the maximum forward current IFMAX equal to 1 A and thermal resistance (junction-case) RthJ-C equal to 8 K/W. The package of this device is made in the SMD 9070 standard and has dimensions 9 × 7 mm. This package is mounted on the substrate made of aluminium and FR-4 laminate layers [39].

a) _{b) c)}

Fig. 8. Outline of the packages of LEDs by CREE [39, 40, 41] Table 3.3. Construction parameters of LEDs by CREE [39, 40, 41]

Diode type RthJ-C

[K/W] Package type

Dimensions (mm)

(length x width) PTOTMAX[W]

Xlamp XR-E 8 SMD, Al., FR4 7 x 9 4

Xlamp MC-E 3 PLCC 7075 7 x 7,5 10

Xlamp XM-L2 2.5 Ceramic SMD 5 x 5 10

Figure 8b presents the LED from Xlamp XM-L2 product family. This diode has the value of the luminous flux ΦV equal 728 lm at the forward current IF = 2 A and junction temperature equal to 85°C. This device is characterized by the very small value of thermal resistance (junction-case) RthJ-C which is equal to 2.5 K/W. This diode can operate at the maximum forward current IF = 3 A. This diode is used in LED lamps and lighting system supplied from photovoltaic’s panels. The package of the device is made in the SMD 5050 standard and it has dimensions 5 × 5 mm and the ceramic substrate [40].

Figure 8c describes the LED from Xlamp MC-E product family, which is characterized by the value of the luminous flux on the level at 430 lm at the value of the forward current equal to 0.35 A. This diode emits the luminous flux at the high value compared to small package dimensions. This device generates the luminous flux at colour temperature CCT in the range from 2600 K to 10000 K. The construction of the device allows independent supply of each LED structure with the maximum forward current IFMAX = 0.7 A. The diode MC-E contains two LED structures emitting light of colour temperature CCT equal to 2700 K and two structures of the CCT equal to 6500 K. This device allows a dynamic shift of colour of emitting light with the use of the control circuit based on the PWM (Pulse Width Modulation) supply signal. This diode is characterized by the small value of thermal resistance (junction-case) RthJ-C equal to 3 K/W. The package of this device is made of plastic in the PLCC7570 standard at dimensions 7.5 × 7 mm [41].

4. FREE COOLING SYSTEMS

In order to limit a temperature rise, the materials of high thermal conductivity are used. Such materials are printed circuit boards with the metal core MCPCB and natural graphite. They are very good thermal conductors, which correctly used, limit an increase of temperature and keep up its uniform distribution in the structures of power LEDs [22].

Figure 9 shows the structure of the board dissipating heat, which contains the core made of aluminium MCPCB. On the other hand, in Figure 10 the board outline with the core of natural graphite is presented. The case study of the value of the thermal conductive coefficient λ for natural graphite is equal to 4840-5300 W/mK and for the aluminium alloy 200 W/mK where appears supremacy board heat transfer made of natural graphite [22, 23, 24]. Modern methods of heat removal allow increase by the range of operating temperature of the specific device, yet thermal monitoring systems allow keeping up the value of the luminous flux and colour temperature on the same level. Using special digital integrated circuits (dedicated microcontrollers) to measure currents and voltages of LEDs is better than traditional methods of measuring thermal parameters and regulation of necessary parameters [22].

Metal Core (Aluminium) LED Chip

Substrate Solder Mask

Dielectric

Fig. 9. Outline of the LED on the heat-sink board made of aluminium alloy [22]

LED Chip Substrate Thermal Interface Dielectric Thermal Via Natural Graphite

Fig. 10. Outline of the LED on the board dissipating heat made of natural graphite [22]

Figure 11 describes the outline of the substrate with thermal vias in laminate FR-4 [42]. In Table 4 basic parameters of the modified substrate FR-4 are shown. The biggest part of the substrate FR-4 is a dielectric of the very small value of thermal conductivity. On the vertical direction of this figure the thermal vias, which enable heat transfer form the LED structure and additional electrical contacts [42] are contained. Thermal Vias Top Layer Cu FR4 Dielectric Bottom Layer Cu Solder Mask

Fig. 11. Outline of the modified FR4 substrate with thermal vias [42]

In the paper [43] the basic parameters of MCPCB boards dedicated to operate with the LED Golden Dragon series are described. Figure 12 presents the layers outline of this substrate structure. The board consists of three layers: 35–200 µm copper, 75–100 µm polymer dielectric and 1–3 mm aluminium alloy. Using the superficial copper layer allows making electrical contacts with the LED and preliminary transfer of heat generated by the high value of thermal conductivity λ of copper equal to 400 W/mK. The layer of polymer dielectric allows insulating electrical contacts of the LED diode of the aluminium layer, whose thermal conductivity is equal to λ = 200 W/mK [43].

Table 4. Thickness and parameters of the modified FR4 substrate [42]

Layer/ Material Thickness _[µm] Thermal Conductivity _[W/mK]

SnAgCu solder 75 58

Top layer Copper 70 398

FR4 1588 0.2

Filled Vias (SnAgCu) 1588 58

Bottom Layer Copper 70 398

Fig. 12. Substrate board dedicated to operate with the diodes from Golden Dragon series [43]

5. FORCED COOLING SYSTEMS

Applications with high power LEDs demand the use of additional cooling systems. In the case of the strongest light emitters at the power value from 30 to 60 W, the free cooling systems, described in section four do not do their task and the forced cooling systems. One of the main producers of such systems is SUNON [44] company. Figure 13 presents the special compact cooling modules to remove heat from power LEDs. The cooling fan in forced cooling systems is supplied by the dc voltage equal to 5 or 12 V [45].

The forced cooling system, realized by the system consisted of the fan and the heat-sink makes heat removal possible four times more effectively than the free cooling system. It allows a considerable decrease in size and weight by the lighting sources construction. Therefore, the production costs of all lamps are considerably decreased. Sunon Company assures support in cooling system modules for LEDs at power supply from 10 to 180 W [44]. The cooling modules are compatible with power LEDs that are produced by: Philips Lumileds [26], Osram Semiconductors [34] or Cree [38].

Fig. 13. Forced cooling systems for power LEDs [45]

Assuring constant and low operating temperature of the diode is a key-condition for keeping up long lifetime of this element of about 50000 hours. In the case of the forced cooling system the biggest problem can be lifetime of the cooling fan,

but the unique construction of Sunon allows obtaining lifetime equal to 70000 hours at the ambient temperature equal to 60°C [44]. For this purpose a few modern solutions are used: porous bearing (Vapo) and rotor hanging on the magnetic pad, which allow decreasing friction and extension of lifetime of cooling modules. Additionally, it allows a decrease in vibration and noise to only 16 dB [45].

The other groups of cooling system are the cooling modules TA003-10001 and TA003-10003 designated for point light sources Fortimo SLM 2000 produce by Philips Lumileds [26]. They also have the forced cooling system for automotive head lights. Figure 14 shows the outline drawing of the cooling module to the Fortimo SLM 2000 lamp.

Luminous Flux Inlet

Outlet

Fig. 14. Construction of the cooling module to Fortimo SLM lamps by Philips [45]

6. CONCLUSIONS

In this paper the problem of cooling power LEDs is considered. Particularly the constructions of the used packages of the considered devices offered by primary producers are described. Free cooling systems using substrates made of natural graphite and containing the aluminium alloy core (MCPCB) are presented. The modified substrate FR4 built up with dielectric layer and thermal vias made of copper, which makes easier heat transfer from the LED package to the second site of the printed circuit board is described. The forced cooling system, described in this paper, allows effective cooling of semiconductor lighting sources, whose value of dissipated power supply achieves even to 180 W.

The information collected in this paper shows the evolution of construction of power LEDs aiming at limiting geometrical dimensions of their elements and increasing luminous efficiency. The measurements results of the characteristics

of power LEDs obtained by authors [18] prove that cooling system of the device significantly influences luminous efficiency of the considered light sources.

The considerations described in the paper can be useful for designers of cooling systems with LEDs and for designers of cooling systems of semiconductor devices.

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