Thermal Management of OSRAM OSTAR Projection Light Source Application Note

Transcription

1 Thermal Management o OSRAM OSTAR Projection Light Source Application Note Abstract This application note provides an introduction to the thermal undamentals o the compact LED high power light source OSRAM OSTAR Projection rom OSRAM Opto Semiconductors. In addition, the general handling o the new light source will be briely covered, and the requirements or the necessary heat dissipation and cooling will be described. In order to achieve reliability and optimal perormance o the light source, appropriate thermal management is imperative. Introduction The recently introduced high-power LED OSRAM OSTAR Projection rings in a new era or compact semiconductor light sources. With a luminous lux o more than 200 lumens at RGB-white, it pushes the standard to a new record level regarding brightness, and surpasses customary LED light sources by great lengths. Based on semiconductor chips o the newest thin ilm technology with pure surace emission, it exhibits exemplary Lambertian radiation characteristics and oers uniorm, angle-independent luminance within the smallest space. Available as a single color, RRBB or RGGB module with a small size o 30 x 10 mm, the OSRAM OSTAR Projection oers an extreme measure o design reedom and lexibility and permits advancement into new application areas such as general lighting, medical technology, projection or automotive technology. The OSRAM OSTAR Projection is designed or a power consumption o 10 W or more. As with all semiconductor components, the unction o the LED is inluenced by temperature. Handling o the OSRAM OSTAR Projection Light Source In order to protect the semiconductor chip rom environmental inluences such as moisture, the OSRAM OSTAR Projection is equipped with a clear silicone encapsulant which additionally provides a positive inluence on the reliability and lietime. Moreover, the silicone encapsulant permits operation at a higher junction temperature compared to that o standard LEDs. Due to the elastic properties o the encapsulant, however, mechanical stress to the silicone should be minimized or avoided as much as possible during assembling o the LED (see Application Note "Handling o Silicone Resin LEDs"). Accordingly, care must be taken with the black Globe-Top encapsulant o the connecting pins. Exertion o pressure on the Globe-Top connector can lead to a spontaneous ailure o the light source. To prevent damage to or puncturing o the encapsulant, the use o all types o sharp January, 2014 Page 1 o 16

2 objects should generally be avoided, since this can damage the component. In addition, it should be assured that suicient cooling is available when the compact light source is placed into operation. Continual operation without cooling, even at low current, can lead to overheating, damage or ailure o the module. Construction o the OSRAM OSTAR Projection Light Source During design o the OSRAM OSTAR Projection light source, special attention was given to the thermal optimization o the module. The module core consists o our highly eicient semiconductor chips soldered to a ceramic base. For optimal heat dissipation, the ceramic is directly mounted to the aluminum surace o the isolated metal core PCB (IMS-PCB). This MC-PCB acts as a heat spreader, providing a large surace area or eicient thermal contact to the system heat sink. In addition, an NTC resistor is mounted on the OSRAM OSTAR Board which permits implementation o a eedback loop or temperature monitoring in the driver circuitry. The NTC temperature can provide a good approximation o the average temperature on the underside o the board. Basic Relationships Operation o LEDs is basically limited by several actors which on the one hand are dependent on the material and on the other are contingent on the technology. An important parameter o practical interest is the temperature o the active semiconductor layer or junction layer (T junction ), since this inluences the unction and lietime o the component. In order to prevent damage to the device during operation, the manuacturer recommended maximum junction temperature should not be exceeded. The goal o thermal management is to keep the junction temperature or a given application as low as possible. The irst line o support or the development o thermal management is the thermal resistance value (Rth). Analogous to Ohm's Law, this is deined by the ollowing ormula: U I R T P R D th (1) Figure 1: Construction o the OSRAM OSTAR Projection The power dissipation P D at the active layer o the semiconductor chip is conducted by the ceramic and rom there, distributed to the aluminum board. The heat is urther transerred rom the aluminum board by means o direct contact with the heat sink. Heat transer occurs rom the ree suraces o the heat sink to the environment by means o radiation and convection. For an OSRAM OSTAR Projection light source with a heat sink, the thermal resistance R th,ja can be deined as ollows: TJunction Ambient Rth, Junction Ambient (2) P where: D January, 2014 Page 2 o 16

3 T [ K] TJ ( unction) TA( mbient) P U I D [ W ] U [ V ] [ A] I orward power dissipation voltage orward current The relevant junction temperature is then obtained by: The issue is simpliied by the ollowing resistance model (Figure 2) which makes use o analogies to electronics: The heat dissipation P D close to the chip surace is simulated by a current source. T J R, P T (3) th JA D A The "resistor network" consists o a parallel circuit and a series circuit to the ambient temperature. The thermal resistance R th,ja in turn is represented by the sum o the individual resistances along the thermal path rom the active semiconductor layer to the outside ambient temperature. For the OSRAM OSTAR Projection, the total resistance can be divided into an internal and an external (application speciic) thermal resistance. The ambient temperature is represented by a voltage source. R th, JA Rth, JB Rth, BA (4) R R th, J ( unction) B( oard) th, B( oard) A( mbient) The internal thermal resistance R th,jb is deined by the physical construction o the OSRAM OSTAR Projection LED. It describes the thermal characteristics o the LED starting rom the junction layers o the our highly eicient semiconductor chips to the ceramic and inally to the underside o the aluminum base plate. The values or R th,jb can be ound in the corresponding data sheets or the OSRAM OSTAR Projection LED. The outside thermal resistance R th,ba deines the connection o the OSRAM OSTAR Projection to a heat sink and the thermal properties o the application speciic cooling. It is strongly inluenced by various actors such as component layout, material/properties o the LED connection, cooling design, ans etc. January, 2014 Page 3 o 16 Figure 2: Static equivalent circuit In general, the thermal management o the OSRAM OSTAR Projection light source can be divided into two categories: internal and external thermal management.

4 The internal management addresses the thermal resistance rom the junction layer to the carrier board while the external thermal management addresses thermal resistance rom the connection o the board to the environment. Because o the predeined values or the internal resistance, the external management plays a signiicant role which includes the selection o an application speciic cooling method, the design o the cooling element, the material selection as well as the mounting process. Transient Thermal Resistance Z* th or Pulsed Operation Conditions While the thermal characteristics or stationary states can be described by the thermal resistance R th, dynamic, pulsed processes additionally require that the thermal capacitance o all components be taken into account. Due to the complex interrelations and transient behavior o pulsed systems, one usually considers the thermal state at the end o a pulse in order to obtain an impression o the maximal change in temperature. To calculate the maximum junction layer temperature, a irst approximation can be obtained by: T Junction T Board, Bottom Z * th P D where P U I D [ W ] U [ V ] [ A] I orward orward power dissipation voltage current Z* th describes the transient thermal resistance dependent on the repetition rate and the duty cycle. I Z* th is multiplied by the maximum power dissipation during the pulse, an upper estimate or the junction temperature can be obtained. Figures 3 to 5 show the characteristics o Z * th or the OSRAM OSTAR Projection LED module or operation with one, two or our chips, in relation to the duty cycle D (D = t p /T). The various curves in the diagram represent dierent repetition rates. For a more precise, detailed view o the thermal behavior o the device during and between the pulses, one can resort to an electro-thermal simulation o the corresponding equivalent thermal circuit diagram. This allows the time dependent temperature behavior to be precisely modeled and visualized. As can be seen in Figures 3 to 5, the transient thermal resistance Z * th decreases at lower duty cycles and at higher requencies. When designing the thermal management, it should be determined whether an improved perormance level can be achieved by increasing the switching requency or decreasing the duty cycle while maintaining the system parameters. January, 2014 Page 4 o 16

5 Z*th (Frequency,Dutycycle) [K/W] Z*th (Frequency,Dutycycle) [K/W] 11,0 10,0 9,0 8,0 7,0 6,0 5,0 4,0 3,0 2,0 1,0 0,0 120Hz 180Hz 240Hz 360Hz 0 0,1 0,2 0,3 0,4 0,5 0,6 Dutycycle D Figure 3: Thermal resistance or OSRAM OSTAR Projection module, pulsed operation o 1 chip 6,0 5,0 4,0 3,0 2,0 1,0 120Hz 180Hz 240Hz 360Hz 0,0 0 0,1 0,2 0,3 0,4 0,5 0,6 Dutycycle D Figure 4: Thermal resistance or OSRAM OSTAR Projection module, pulsed operation o 2 chips January, 2014 Page 5 o 16

6 Z*th (Frequency,Dutycycle) [K/W] 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 120Hz 180Hz 240Hz 360Hz 0 0,1 0,2 0,3 0,4 0,5 0,6 Dutycycle D Figure 5: Thermal resistance or OSRAM OSTAR Projection module, pulsed operation o all 4 Chips Inluence and Consequences o Thermal Management To achieve reliability and optimal perormance o LED light sources like the OSRAM OSTAR Projection, proper thermal management is necessary. Basically, there are two principal limitations or the maximum allowable temperature, both speciied in the datasheet. Firstly, the maximum allowed board temperature may not be exceeded, i.e. 85 C or the OSRAM OSTAR Projection. Secondly, the junction temperature may not rise above the allowed maximum, i.e. 125 C or the OSRAM OSTAR Projection. In general, the heating up o the OSRAM OSTAR Projection results rom two sources; one has an external origin (existing ambient temperature) and the other an internal one (current dependent power dissipation). From this, it ollows that not every mode o operation is allowed at a given ambient temperature. In the datasheets, the January, 2014 Page 6 o 16 maximum allowed currents or continuous and pulsed operation are given at two ambient temperatures. For all cases in between, the maximum driving conditions can be estimated rom interpolation o the curves or by the use o thermal replacement circuits as described in the sections above. Inluence o Junction Temperature Basically, the maximum allowable junction temperature should not be exceeded, since this can lead to irreversible damage to the LED and spontaneous ailure. Due to the undamental physical interdependencies which arise during the operation o light emitting diodes, changes in the junction temperature T J within the allowable temperature range have an eect on several LED parameters. These eects are reversible in nature. The orward voltage, luminous lux, wavelength (color) and lietime o the LED

7 are all inluenced by the junction temperature. Depending on the given requirements, this can inally aect the application. means o the respective temperature coeicients (Table 1). Inluence o Forward Voltage V and Luminous Flux Φ v For LEDs, an increase in junction temperature leads to a decrease in the orward voltage V F (Figure 6), as well as a reduction in luminous lux Φ v (Figure 7). Figure 7: Relative Luminous Flux vs Junction Temperature (e.g. OSRAM OSTAR Projection LE RTB-A2A) Figure 6: Relative Forward Voltage vs. Junction Temperature (e.g. OSRAM OSTAR Projection LE RTB-A2A) In this case, the resulting changes are reversible. That is, the output values return to their original level when the temperature change is reversed. For the application, this means that the light output increases with a decrease in the junction temperature T j. Wavelength (λ peak and λ dom. ) The inluence on the wavelength by a change in junction temperature appears as a reversible shit in the output values. The amount o the shit can be calculated by An increase in temperature to 40 C, or example, leads to a shit o the peak wavelengths in the red range o nearly 6 nm rom 634 nm to 640 nm and a shit o the dominant wavelengths o 3.2 nm (625 nm 628 nm). This shit leads to a change in the color settings and appearance, and can have an inluence on the application, depending on the given requirements. Temperature coeicient [nm/k] TC λ peak I= 750mA (R), 500mA (G,B) -10<T<100 C TC λ dom I= 750mA (R), 500mA (G,B) -10<T<100 C R G B Table 1: Typical Temperature Coeicient o Peak Wavelength λ peak and Dominant January, 2014 Page 7 o 16

8 Wavelength λ dom (e.g. OSRAM OSTAR Projection LE RTB-A2A) Due to the various temperature dependencies o the luminous lux and the dierent wavelengths or red, green and blue light, the white point generally shits with a change in temperature. Depending on the application, it must be determined i this shit can be tolerated, or that appropriate measures can be taken to avoid or at least compensate or temperature related eects. OSRAM OSTAR Projection Light Source with a Heat Sink in DC Operation The simplest and least susceptible cooling method is the use o a heat sink (Figure 8). Here, the OSRAM OSTAR Projection LED is mounted to the heat sink with screws. In order to achieve good heat transer between the ceramic and the heat sink, a heat transer oil or paste is most oten used in practice. For the simulation, an optimal heat transer rom the module to the heat sink was assumed. Reliability and Lietime Moreover, regarding aging, reliability and perormance, driving an LED at its maximum allowed junction temperature is not recommended. With increasing temperature, the lietime o the module decreases and the probability o spontaneous ailures increases. Practical Application Examples The ollowing section describes various possibilities or thermal management o the OSRAM OSTAR Projection light source. The primary goal o the development was to minimize the size o the cooling element. The appropriateness o each design was veriied by a thermal simulation (Flowtherm). As a basis or the simulation, the ollowing drive parameters were chosen or the semiconductor (Table 2), with a total power dissipation or the OSRAM OSTAR Projection light source o 9.45 W, a thermal resistance o 5 K/W and an ambient temperature o 50 C. Chip I F [ma] U F F P Chip [W] Red Green Blue Table 2: Basic data or the chips used Figure 8: Construction o OSRAM OSTAR Projection with heat sink Since the heat transer only occurs by ree convection, the heat sink must have a certain minimal size. Figure 9 shows the results o the thermal simulation or a simple heat sink with minimal dimensions. As the temperature distribution diagram shows, a maximum junction temperature o 125 C was reached or the semiconductor chips. In this case, operation within the given parameters is still allowed. In order to achieve a lower junction temperature, a more eicient heat sink must be used, or in general, a heat sink with lower thermal resistance must be used. For the application, this would lead to increased weight and/or costs. January, 2014 Page 8 o 16

9 Figure 9: Simulation results or OSRAM OSTAR Projection with minimal heat sink OSRAM OSTAR Projection Light Source with Heat Sink and Forced Convection in DC Operation Another possibility is the operation o the light source with a heat sink and orced convection by means o an additional an, respectively. To permit a comparison with passive cooling and to show the inluence o air low, the same heat sink was used as in the irst simulation. The results show that even a low air low velocity leads to a signiicant reduction in the junction temperature. In practice, this means that through the use o an additional an, the junction temperature can be stably maintained at a signiicantly reduced level. This also means that in conjunction with a an, the size o the heat sink can be reduced. OSRAM OSTAR Projection Light Source with Minimized Heat Sink in DC Operation In a urther step, it was determined how small the entire heat sink could be reduced with the use o optimal standard components. Figure 11 shows the simulation results or a heat sink with a length and width o 30 mm and a total height o 20 mm. The unit consists o a heat sink and cooling an. With the given parameters, a junction temperature o 115 C was reached, lying signiicantly under the maximum allowable value o 125 C. January, 2014 Page 9 o 16

10 Figure 10: Simulation results o OSRAM OSTAR Projection with heat sink and an Figure 11: Simulation results o OSRAM OSTAR Projection with a minimized cooling element January, 2014 Page 10 o 16

11 OSRAM OSTAR Projection Light Source with Minimized Cooling Element in Pulse Mode Since the light source is operated in pulse mode in a majority o applications, it was urther determined whether the minimal cooling element was also suitably dimensioned or typical operation in pulse mode. As a irst step, the average temperature and temperature ield o the cooling element was calculated by means o average heat dissipation. Figure 12 shows the simulation results or the temperature ield o the cooling element. For the pulse parameters used, the aluminum base plate yields a temperature o 65 C. Chip I F [A] U F F Duty cycle Avg. P Chip [W] Max. P Chip [W] Red Green 1.2 (two chips parallel driven) Blue Table 3: Basic semiconductor chip data or transient simulation As a basis or the transient simulation, the semiconductor chip pulse parameters were selected at an ambient temperature o 50 C and a requency o 180 Hz (Table 3). The transient simulation or calculation o the individual junction temperatures occurred in two steps. From the temperature o the base plate, electro-thermal and transient thermal simulations were carried out, in which the second step showed the temperature dierence and the time dependent course o the individual junction temperatures (see Application Note: Transient Thermal Simulation o OSRAM OSTAR Projection using PSpice ). Figure 12: Simulation results o OSRAM OSTAR Projection in pulse operation, average temperature level January, 2014 Page 11 o 16

12 Temperature [ C] Red TrueGreen 1 TrueGreen 2 Blue Board Aluminium Plate Time [ms] Figure 13: Simulation results o OSRAM OSTAR Projection in pulse operation, transient temperature curve For the semiconductor chips, a junction temperature o T J,red = 113 C T J,green = 121 C T J, blue = 123 C and an individual time dependent temperature curve was obtained (Figure 13). As the simulation shows, no chip exceeded the maximum allowable junction temperature limit o 125 C. Thus, the previously designed minimal cooling element is also suitably dimensioned or this pulse operation. Based on these results, the minimal cooling element can also be easily adapted to applications with lower pulse loads. Since real suraces always exhibit a roughness at the micro level and thereore are never perectly smooth, physical contact between two suraces only occurs at isolated points. In between, air pockets are ormed. Approximately 99% o the surace is separated by a layer o interstitial air. Since air is a poor conductor o heat, a more conductive material should be used in order to signiicantly reduce the thermal resistance and to increase the heat low between the two suraces. Transer Module - Heat sink Figure 14: Heat low without/with thermal An important aspect during development o interace material an appropriate thermal management is the transer o heat rom the heat source to the Without an appropriate optimal interace, heat sink. only a certain amount o heat transer occurs January, 2014 Page 12 o 16

13 between the components, leading to overheating o the light source. In order to improve the heat transer and reduce the thermal contact resistance, several materials are suitable. Thermal greases and compounds provide the lowest interace resistance, but require care in handling. Elastomers and tapes eliminate handling problems but mostly require compressive loads even with well prepared suraces. The success o a particular thermal interace material is dependent on the quality and processing o the material and the thoroughness o the design. Table 4 provides a summary o the most commonly used thermal interace materials along with their advantages and disadvantages. Heat Sink In addition to the geometric dimensions, the thermal resistance value is particularly important when selecting a suitable heat sink. To estimate the thermal resistance o the heat sink, the ollowing equation or DC operation can be used. Description Material Advantages Disadvantages Thermal Grease Thermal Conductive Compound Typically silicone based with thermal conductive particles Improvement o thermal grease. Compounds are converted to a cured rubber ilm ater application at the thermal interace. Thinnest joint with minimal pressure High thermal conductivity No delamination issues Flow out beyond the edges Messy or high volume manuacturing Greased joints can pump out and separate with time Compounds requires curing process Phase Change Materials Typically polyester or acrylic materials with low glass transition temperature illed with conductive particles. Easy handling and installation No delamination issues No curing Requires attachment pressure Pre-treatment heating necessary Thermal Conductive Elastomers Thermal Conductive Adhesive Tapes Typically silicone elastomer pads illed with thermally conductive particles. Oten reinorced with glass iber or dielectric ilms. Typically double sided pressure adhesive ilms illed with suicient particles to balance their thermal and adhesive Table 4: Thermal Interace Materials January, 2014 Page 13 o 16 No pump out or migration concerns No curing required Delamination issues Thermal conductivity is moderate Requires attachment pressure

14 P T Diss, Modul R th, Interace R th, JB R th, Heat sink R R th, Heat sin k th, Heat sin k ( 0.1 5) K / W K / W where T [ K] T P Diss, Modul J ( unction) [ W ] U T A( mbient) [ V ] I [ A] T Saety Factor T Junction = Junction Temperature T Ambient = Ambient Temperature T Saety-Factor = Saety actor (10 C - 20 ) U = orward voltage I = orward current R th,interace = thermal resistance o interace material R th,jb = thermal resistance o OSRAM OSTAR Projection LED R th,heatsink = thermal resistance o heat sink Using this thermal resistance value, an appropriate heat sink can now be selected rom the manuacturer. In order to saeguard the design and to veriy the board or base plate temperature, the resistance value can be obtained rom the NTC. Typical characteristics or the NTC are provided in the respective datasheets or the OSRAM OSTAR Projection LED. In general, it can be said that the lower the thermal resistance must be to ulill the given requirements, the more diicult and costly the construction o the thermal management becomes when only a heat sink is used. For more stringent requirements, the use o an additional an should be considered, in order to positively aect the behavior o the heat sink In general, the presence o air low or the use o a an can help to reduce the thermal resistance o the heat sink. Example Using the OSRAM OSTAR Projection Light Source LE B A2A: Driving the light source with constant current I = 500mA; Figure 15: Typical thermistor graph (e.g. OSRAM OSTAR Projection LE B-A2A, NTC EPCOS 8502) U, typ. = 3.5 V typ. P Diss, Modul = 7 W T Junction = 125 C (max.) T Ambient = 25 C T Saety-Factor = 10 C R th,jb = 5 K/W R th,interace = 0.1K/W (e.g. thermal paste) January, 2014 Page 14 o 16

15 Summary Designed or high power operation with currents o up to two Amperes, the OSRAM OSTAR Projection LED light source delivers a power dissipation o 10 W and more, depending on the operational parameters chosen. Appropriate thermal management is imperative in order to dissipate the heat which arises and guarantee optimal perormance and reliability o the module. As the simulations have shown, the OSRAM OSTAR Projection can be operated with a simple heat sink o minimal dimensions. With air low or through the use o an additional an, the heat sink dimensions and/or the junction temperature can be signiicantly reduced. Based on generally available standard components and the speciic operational conditions mentioned above, a minimally sized cooling element (heat sink and an) was developed with dimension amounting to just a ew centimeters (3 cm x 3 cm x 2cm). In general, it is necessary and strongly recommended that in addition to a thermal simulation, a prototype o the design should be veriied by means o thermal measurements. Appendix Heatsinks Aavid Thermalloy Advanced Thermal Solutions Alutronic Kühlkörper Fischer Elektronik NTC EPCOS (NTC 8502) Thermal Interace Materials 3M Aavid Thermalloy Arctic Silver Bergquist Chomerics Fujipoly Gratech Intl. Keraol Thermagon Universal-Science January, 2014 Page 15 o 16

16 Authors: Andreas Stich, Dr. Nicole Breidenassel, Rainer Huber ABOUT OSRAM OPTO SEMICONDUCTORS OSRAM, Munich, Germany is one o the two leading light manuacturers in the world. Its subsidiary, OSRAM Opto Semiconductors GmbH in Regensburg (Germany), oers its customers solutions based on semiconductor technology or lighting, sensor and visualization applications. Osram Opto Semiconductors has production sites in Regensburg (Germany), Penang (Malaysia) and Wuxi (China). Its headquarters or North America is in Sunnyvale (USA), and or Asia in Hong Kong. Osram Opto Semiconductors also has sales oices throughout the world. For more inormation go to DISCLAIMER PLEASE CAREFULLY READ THE BELOW TERMS AND CONDITIONS BEFORE USING THE INFORMATION SHOWN HEREIN. IF YOU DO NOT AGREE WITH ANY OF THESE TERMS AND CONDITIONS, DO NOT USE THE INFORMATION. The inormation shown in this document is provided by OSRAM Opto Semiconductors GmbH on an as is basis and without OSRAM Opto Semiconductors GmbH assuming, express or implied, any warranty or liability whatsoever, including, but not limited to the warranties o correctness, completeness, merchantability, itness or a particular purpose, title or non-inringement o rights. In no event shall OSRAM Opto Semiconductors GmbH be liable - regardless o the legal theory - or any direct, indirect, special, incidental, exemplary, consequential, or punitive damages related to the use o the inormation. This limitation shall apply even i OSRAM Opto Semiconductors GmbH has been advised o possible damages. As some jurisdictions do not allow the exclusion o certain warranties or limitations o liability, the above limitations or exclusions might not apply. The liability o OSRAM Opto Semiconductors GmbH would in such case be limited to the greatest extent permitted by law. OSRAM Opto Semiconductors GmbH may change the inormation shown herein at anytime without notice to users and is not obligated to provide any maintenance (including updates or notiications upon changes) or support related to the inormation. Any rights not expressly granted herein are reserved. Except or the right to use the inormation shown herein, no other rights are granted nor shall any obligation be implied requiring the grant o urther rights. Any and all rights or licenses or or regarding patents or patent applications are expressly excluded. January, 2014 Page 16 o 16