Cree® XLamp® LED Thermal Management XLamp LEDs lead the solid-state lighting industry in brightness while providing a reflow-solderable design that is optimized for ease of use and thermal management. Lighting applications featuring XLamp LEDs maximize light output and increase design flexibility, while minimizing environmental impact. They are used in a broad range of both specialized and general lighting applications. When designing lighting systems using LEDs, one of the most critical design parameters should be the system’s ability to draw heat away from the LED junction. High operating temperatures at the LED junction adversely affect the performance of LEDs, resulting in decreased output and lifetime. This application note serves as a guide to understanding thermal management of XLamp LEDs and minimizing the effects of elevated junction temperatures. Introduction The majority of LED failure mechanisms are temperature-dependent. Elevated junction temperatures cause light output reduction and accelerated chip degradation. The maximum junction temperature for each product line is specified in the product data sheet. Junction temperature is primarily affected by three parameters: • • • Ambient temperature of the LED’s immediate surroundings Thermal path between the LED junction and ambient conditions Power dissipated by the LED 5.002 ote: CLD-AP0 Application N When designing lighting systems using high-power LEDs, the following general guidelines should be followed: • The most important consideration for successful thermal design is to minimize the amount of heat that needs to be removed. It is important to separate the LED drive circuitry from the LED board so that the heat generated by the driver will not contribute to the LED junction temperature. • The next most effective strategy is to minimize the ambient temperature inside the fixture. This goal is achieved by paying attention to several design parameters such as a conservative packaging design that does not allow the upper limit on overall system power density to be reached. Maintaining clear and clean airflow paths for natural convection cooling is vital as well. • Enhancing thermal conductivity between the heat sinks and the LED is very preferable for thermal management. Even though the heat removal from the heat sink is via convection, the path from the LED to the heat sink depends upon conduction. • Finally, the orientation of the LED PCB/heat sink should be considered carefully. It is important to position the board/heat sink so that the plane is vertical. If the board plane is horizontal, it will block the formation of air convection currents and substantially reduce the cooling capability of the system. Subject to change without notice. www.cree.com/xlamp 1 Thermal Management Technology Thermal resistance The thermal resistance between two points is defined as the ratio of the difference in temperature to the power dissipated. For calculations in this document the units used are °C/W. In the case of LEDs, the resistance of two important thermal paths affects the junction temperature: • • From the LED junction to the thermal contact at the bottom of the package. This thermal resistance is governed by the package design. It is referred to as the thermal resistance between junction and solder point (Rth j-sp) From the thermal contact to ambient conditions. This thermal resistance is defined by the path between the solder point and ambient. It is referred to as the thermal resistance between solder point and ambient (Rth sp-a) The overall thermal resistance between the LED junction and ambient (Rth j-a) can be modeled as the sum of the series resistances Rth j-sp and Rth sp-a. LED Chip Solder Point Rth j-sp Ambient Rth sp-a Tj Ta Tsp Figure 1: Thermal Resistance Model Power dissipated The total power dissipated by the LED (Pd) is the product of the forward voltage (Vf) and the forward current (If) of the LED. Junction temperature The temperature of the LED junction (Tj) is the sum of the ambient temperature (Ta) and the product of the thermal resistance from junction to ambient and the power dissipated. Tj = Ta + (Rth j-a x Pd) Calculations In the most cases, power LEDs will be mounted on metal-core printed circuit boards (MCPCB), which will be attached to a heat sink. Heat flows from the LED junction through the MCPCB to the heat sink by way of conduction. The heat sink diffuses heat to the ambient surroundings by convection. In most LED applications, the contact thermal resistance between LED and MCPCB and/or heat sink is small with respect to the thermal resistance between the junction and thermal pad and thermal pad to ambient. LED Chip Solder Point Rth j-sp Tj Heatsink Rth sp-h Tsp Ambient Rth h-a Th Ta Figure 2: Thermal Resistance Model Including Heat Sink When a heat sink is used, the total thermal resistance is the series resistances from the junction to the solder point (Rth ), from the solder point to the heat sink (Rth sp-h) and from the heat sink to ambient (Rth h-a). j-sp Rth j-a = Rth j-sp + Rth sp-h + Rth h-a Copyright © 2004-2006 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo, and XLamp are registered trademarks of Cree, Inc. Other trademarks, product and company names are the property of their respective owners and do not imply specific product and/or vendor endorsement, sponsorship or association. 2 CLD-AP05.002 Cree, Inc. 4600 Silicon Drive Durham, NC 27703 USA Tel: +1.919.313.5300 www.cree.com/xlamp Note that the direct heat loss from the LED package to ambient is small enough to be neglected for calculations. The overall design goal in determining the size and nature of the required heat sink is to calculate the maximum heat sink thermal resistance (Rth h-a) that will maintain the junction temperature below the maximum value at worst-case operation conditions. Example 1: Heat Sink Thermal Resistance In this example, six white 7090 XLamp LEDs are used in an application that sees a maximum ambient temperature (Ta) of 55°C. Assuming a typical forward voltage (Vf) of 3.25 V at 350 mA and that the power supply is outside of the fixture, the total power dissipated is: Ptotal = 6 x 0.350 A x 3.25 V = 6.825 W LED Chip Rth j-sp Tj LED Chip Rth j-sp Tj LED Chip Rth j-sp Heat Sink Tj Rth sp-h Solder Point LED Chip Tsp Ambient Rth h-a Th Ta Rth j-sp Tj LED Chip Rth j-sp Tj LED Chip Rth j-sp Tj Figure 3: Thermal Resistance Model for Six (6) XLamp LEDs Copyright © 2004-2006 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo, and XLamp are registered trademarks of Cree, Inc. Other trademarks, product and company names are the property of their respective owners and do not imply specific product and/or vendor endorsement, sponsorship or association. 3 CLD-AP05.002 Cree, Inc. 4600 Silicon Drive Durham, NC 27703 USA Tel: +1.919.313.5300 www.cree.com/xlamp The )) is The thermal thermal resistance resistance from from junction junction to to solder solder point point (R (Rthth j-sp is listed listed in in the the data data sheet sheet as as 8°C/W. 8°C/W. The The maximum maximum LED LED j-sp junction (Tj) provided the data 145°C. junction temperature temperature (Tj)2: provided in the calculate data sheet sheet is isthermal 145°C. Therefore: Therefore: > Example Test in and resistance Cree does not recommend operating XLamp without heatsink. This example demonstrates the TTjj = + P totalLEDs (R /6 + )) = TT (Rthth j-sp /6 + +R Ra +R Rthth h-a aa + Ptotal th j-sp th sp-h sp-h h-a procedure for calculating the thermal resistance and maximum operating temperature of one XLamp 7090 ,, depends The between The thermal thermal between the the LED LED solder solder point point and and heat heat sink, sink, R Rthth sp-h depends on on the the surface surface finish, finish, flatness, flatness, on a resistance 1resistance inch2 MCPCB. sp-h applied applied mounting mounting pressure, pressure, contact contact area, area, and and the the type type of of interface interface material material and and its its thickness. thickness. With With good good design, design, it it can can be be minimized minimized to to less less than than 1°C/W. 1°C/W. The )) can The maximum maximum thermal thermal resistance resistance from from the the heatsink heatsink to to ambient ambient (R (Rthth h-a can be be calculated. calculated. Using Using the the previous previous equation equation h-a and : and solving solving for for R Rthth h-a : h-a R = Rthth h-a = (145°C (145°C -- 55°C 55°C –8°C/W –8°C/W xx 6.825 6.825 W/6 W/6 –– 1°C/W 1°C/W xx 6.825 6.825 W)/6.825 W)/6.825 W W= = 10.85°C/W 10.85°C/W h-a In In order order to to keep keep the the junction junction temperature temperature below below 145°C 145°C in in worst-case worst-case conditions, conditions, aa heat heat sink sink with with thermal thermal resistance resistance from )) less from heat heat sink sink to to air air (R (Rthth h-a less than than 10.85°C/W 10.85°C/W must must be be chosen. chosen. A A heat heat sink sink with with the the required required characteristics characteristics may may be be h-a selected selected using using figures figures published published by by heat heat sink sink manufacturers manufacturers or or through through modeling modeling and and testing. testing. Example Example 2: 2: Test Test and and Calculate Calculate Thermal Thermal Resistance Resistance Cree Cree does does not not recommend recommend operating operating XLamp XLamp LEDs LEDs without without aa heat heat sink. sink. This This example example demonstrates the thermal resistance and maximum demonstrates the the procedure procedure for for calculating calculating the thermal resistance and maximum Figure 4: XLamp 7090 White on MCPCB operating operating temperature temperature of of one one XLamp XLamp 7090 7090 on on aa 1-inch 1-inch22 MCPCB. MCPCB. Since there is no additional heatsink in this example, the MCPCB serves as the heatsink and thermal Since Since there there is is no no additional additional heat heat sink sink in in this this example, example, the the MCPCB MCPCB serves serves as as the the heat heat ambient.to order to calculate the thermal resistance from junction to ambient the temperature sink thermal interface In the resistance sink and andinterface thermalto interface toInambient. ambient. In order order to to calculate calculate the thermal thermal resistance on theto back of thethe LEDtemperature must be measured. In this case, the LEDbe is measured. reflow soldered on MCPCB. The from ambient, on of LED must from junction junction to ambient, the temperature on the the back back of the the LED must be measured. boardthe temperature can be measured applying thermocouple directly to the In LED soldered on MCPCB. The can be In this this case, case, the LED is is reflow reflow soldered on by MCPCB. Thea board board temperature temperature can be back of the MCPCB. In measured by applying to back In most most it is impossibledirectly to attach thermocouple toMCPCB. the solder measured by applications, applying aa thermocouple thermocouple directly toathe the back of of the the MCPCB. Inpoint mostof the LED. The test point applications, it to aa thermocouple to the solder of LED. applications, it is is impossible impossible to attach attach thermocouple tothe theback solder point of the the LED. should be the point that is as close as possible to of point the LED or the hottest point on the back of The point should be the point that is as close as possible to the back of the LED The test testthe point should be the point that is as close as possible to the back of the LED MCPCB. or or the the hottest hottest point point on on the the back back of of the the MCPCB. MCPCB. Figure 6 shows the MCPCB temperature at different forward currents. The room temperature was con- Figure Figure 55 shows shows the the MCPCB MCPCB temperature temperature at at different different forward forward currents. currents. The The room room Figure 7090 White Figure 4: 4: XLamp XLamp 7090 trolled to 23ºC. Due to the low thermal conductance of the MCPCB to air, temperatures quickly reach a White temperature temperature was was controlled controlled to to 23°C. 23°C. Due Due to to the the low low thermal thermal conductance conductance of of the the on on MCPCB MCPCB steady-state. MCPCB air, MCPCB to to air, temperatures temperatures quickly quickly reach reach aa steady-state. steady-state. Figure 5: MCPCB temperature Figure 4: Temperature Figure 5: MCPCB MCPCB Temperature Page 7 Subject to change without notice. www.xlamp.com L I G H T I N G Copyright Copyright © © 2004-2006 2004-2006 Cree, Cree, Inc. Inc. All All rights rights reserved. reserved. The The information information in in this this document document is is subject subject to to change change without without notice. notice. Cree, Cree, the the Cree Cree logo, logo, and and XLamp XLamp are are registered registered trademarks trademarks of of Cree, Cree, Inc. Inc. Other Other trademarks, trademarks, product product and and company company names names are are the the property property of their respective owners and do not imply specific product and/or vendor endorsement, sponsorship or association. of their respective owners and do not imply specific product and/or vendor endorsement, sponsorship or association. 4 CLD-AP05.002 CLD-AP05.002 Cree, Cree, Inc. Inc. 4600 4600 Silicon Silicon Drive Drive Durham, Durham, NC NC 27703 27703 USA USA Tel: Tel: +1.919.313.5300 +1.919.313.5300 www.cree.com/xlamp www.cree.com/xlamp The board temperature (Tb) can be measured at different forward currents. Using a junction-to-solder-pad resistance of 8°C/W and assuming a thermal resistance from solder point to MCPCB of 1°C/W (this will be on the high end based on the mounting method), the junction temperature at different currents can be calculated by: Tj = Ta + Ptotal x Rth j-a Where, Rth j-a = Rth j-b + Rth b-a Rth j-b = Rth j-sp + Rth sp-b =8°C/W + 1°C/W = 9°C/W Rth b-a = (Tb - Ta) / Ptotal Ptotal = If x Vf Ta = 23°C Table 1 shows the test and calculated results. If (mA) Vf (V) Tb (°C) Rth b-a (°C/W) Rth j-a (°C/W) Tj (°C) 200 2.95 51 47 56 56 350 3.11 74 47 56 84 450 3.21 90 46 55 102 Table 1: Junction-Temperature Calculations From the test results we can conclude that with a 1-inch2 MCPCB as a heat sink, the XLamp 7090 can be safely operated at room temperature when driven at 350 mA. The maximum allowable ambient temperature is 84°C. However, at roomtemperature conditions, the LED junction will quickly reach a temperature of 84°C. For better results, the designer should use a secondary heat sink. Heat Sink Design and Selection In order to design and select the heat sink, it is useful to know how heat sinks work. Transmission of heat from a heat source (e.g. the junction of a LED) via the heat sink into the surrounding medium takes place in four successive steps: 1. 2. 3. 4. Transfer from heat source to the heat sink Conduction from within the heat sink to its surface Transfer from surface into the surrounding medium by convection Radiation depending on the nature of the heat sink’s surface The efficiency and capability of a heat sink are a function of the heat transfer modes utilized. Heat sinks provide a path for heat from the LED to flow through conduction. The heat “trapped” in the heatsink must be dissipated in order for the power from the source to continually flow. If the heat remains trapped in the sink, the temperature will rise and eventually overheat the source. Heat sinks can dissipate power in three ways: conduction (heat transfer from one solid to another), convection (heat transfer from a solid to a moving fluid, for most LED applications the fluid will be air), or radiation (heat transfer from two bodies of different surface temperatures through electromagnetic waves). There are three common varieties of heat sinks: flat plates, die-cast finned heatsinks, and extruded finned heat sinks. The material normally used for heat sink construction is aluminum, although copper may be used with an advantage for flat-sheet heat sinks. Heat sink thermal radiation is a function of surface finish, especially when the heat sink is at higher temperatures. A painted surface will have a greater emissivity than a bright, unpainted one. The effect is most remarkable with flat- Copyright © 2004-2006 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo, and XLamp are registered trademarks of Cree, Inc. Other trademarks, product and company names are the property of their respective owners and do not imply specific product and/or vendor endorsement, sponsorship or association. 5 CLD-AP05.002 Cree, Inc. 4600 Silicon Drive Durham, NC 27703 USA Tel: +1.919.313.5300 www.cree.com/xlamp plate heat sinks, where about one-third of the heat is dissipated by radiation. The color of the paint used is relatively unimportant. The thermal resistance of a flat-plate heatsink painted gloss white will be only about 3% higher than that of the same heat sink painted matte black. With finned heat sinks, painting is less effective since heat radiated from most fins will fall on adjacent fins, but it is still worthwhile. Both anodizing and etching will decrease the thermal resistance. A number of important factors need to be considered when selecting a heat sink: • Surface area: Thermal transfer takes place at the surface of the heat sink. Therefore, heat sinks should be designed to have a large surface area. This goal can be reached by using a large number of fine fins or by increasing the size of the heat sink itself. • Aerodynamics: Heat sinks must be designed in a way that air can flow through easily and quickly. Heat sinks with a large number of fine fins with short distances between the fins may not allow good air flow. A compromise between high surface area (many fins with small gaps between them) and good aerodynamics must be found. • Thermal transfer within the heat sink: Large cooling fins are ineffective if the heat can’t reach them. The heat sink must be designed to allow adequate thermal transfer from the heat source to the fins. Thicker fins have better thermal conductivity; so again, a compromise between large surface area (many thin fins) and good thermal transfer (thicker fins) must be found. The material used has a major influence on thermal transfer within the heat sink. • Flatness of the contact area: The portion of the heat sink that is in contact with the LED or MCPCB must be perfectly flat. A flat contact area allows the use of a thinner layer of thermal compound, which will reduce the thermal resistance between the heat sink and LED source. • Mounting method: For good thermal transfer, the pressure between the heat sink and the heat source must be high. Heat sink clips must be designed to provide high pressure, while still being reasonably easy to install. Heatsink mountings with screws or springs are often better than regular clips. Thermoconductive glue or sticky tape should only be used in situations where mounting with clips or screws is not possible. For more information regarding to heat sink design and selection, please contact your heat sink manufacturer and explore these links for further information: http://www.electronics-cooling.com/ http://www.r-theta.com/ http://www.aavidthermalloy.com http://www.electronics-cooling.com/html/consultants.html http://www.coolingzone.com/ The formulas and diagrams given in this application note should be considered as a guide for thermal management of XLamp LEDs. The thermal resistance of a heat sink depends on numerous parameters that cannot be predetermined. These parameters include but are not limited to the position of the LED on the heat sink, the extent to which air can flow unhindered, the screening effect of nearby components, and heating from other components in the fixture. It is always advisable to check important temperatures in the finished fixture under the worst possible operating conditions and calculate the LED junction temperature. The probe points should be as close as possible to the back of the LED. Copyright © 2004-2006 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo, and XLamp are registered trademarks of Cree, Inc. Other trademarks, product and company names are the property of their respective owners and do not imply specific product and/or vendor endorsement, sponsorship or association. 6 CLD-AP05.002 Cree, Inc. 4600 Silicon Drive Durham, NC 27703 USA Tel: +1.919.313.5300 www.cree.com/xlamp