Effective Thermal Management Of LEDs

Transcription

1 Application Note: Effective Thermal Management Of LEDs This application note describes the thermal management concepts and guidelines for the proper use of Crystal IS LEDs. Included is a basic thermal model, best practices in heat dissipation, an example on selecting the heat sink for an application and additional resources for thermal analysis.

2 Table of Contents Introduction 3 Heat Generation 3 Impact on Junction Temperature 3 Best Practices in Heat Dissipation 4 Heat Sink Configuration 5 Heat Sink Selection 6 Design Validation 6 Example 7 Thermal Management Resources 8 Heat Sink Manufacturers 8 Thermal Interface Material Manufacturers 8 Additional Links 8 2

3 application note: EFFECTIVE TO-39 soldering THERMAL notes MANAGEMENT OF LEDS Introduction Crystal IS LEDs deliver the most efficient and reliable LEDs in the UVC wavelengths from 250 nm 280 nm. We achieve this first, by using low defect aluminum nitride (AlN) substrates, for LED growth, that are manufactured using Crystal IS proprietary technology. Next, the LED layers are grown in a way which preserves the low defect densities of the substrate. The result is higher efficiencies in converting the input electrical energy into UV radiation and longer lifetimes in the UVC spectrum than other commercially available LEDs. To realize and maintain the high performance and reliability of Crystal IS LEDs, thermal management is critical. Excess heat directly affects both short-term and long-term LED performance. The short-term (reversible) effects are reduced light output and voltage while the long-term effect is an accelerated depreciation of light output and thus, shortened useful life. The goal in the development of thermal management is to keep the junction temperature as low as required for the given application. The word junction refers to the p-n junction within the LED chip, where the photons are generated and emitted. This application note will assist design engineers with thermal management strategies for the proper use of Crystal IS LEDs. Heat Generation When voltage or current is applied across the junction of an LED, some of the input electrical energy is converted into light and the rest, into heat. The amount of electrical energy that is converted to heat (P D ) can be estimated by multiplying the forward voltage (V f ) times the current (I f ) times the LED wall plug efficiency (WPE). This is described in Equation 1. Equation 1 P D = V f * I f * (1-WPE) Wall plug efficiency is typically 1% for UVC LEDs. For pulsed driving, the heat dissipated varies with duty factor as follows. Equation 2 P D = V f * I f * (Duty factor) * (1-WPE) Impact on Junction Temperature The generation of heat in the junction of the LED causes the temperature to rise, with the increase in temperature controlled by the amount of heat that is dissipated to the ambient. The heat transfers from the junction to the ambient via all elements that make up the thermal management solution. For a simple thermal management solution that consists of the Crystal IS LED mounted to a heat sink, the heat from the LED junction is typically transferred to the ambient through the following thermal path. 3

4 > Heat is conducted from the semiconductor chip to the submount and then to the LED package. > Heat is then conducted from the LED package through a thermal interface material to the heat sink. > Finally, heat is conducted through the heat sink, transferred via convection to the air around the heat sink, and then radiated to the ambient. The junction temperature (T J ) is related to the thermal resistance of the entire path (Θ JA ) where T A is the ambient temperature. Equation 3 T J = T A + RΘ JA * P D The overall thermal resistance (RΘ JA ) can be expressed as the sum of the individual resistances of the thermal path from junction to ambient. For the Crystal IS UVC LEDs, the thermal resistance can be simply represented in the formula below, where RΘ JC is the thermal resistance from junction to the LED surface, RΘ CS is the thermal resistance from the surface to the heat sink, and RΘ SA is the thermal resistance from the heat sink to the ambient. Equation 4 RΘ JA = RΘ JC + RΘ CS + RΘ SA This model is analogous to an electrical circuit where heat flow is represented by current, voltages represent temperatures and resistors represent thermal resistances. Best Practices in Heat Dissipation While the thermal resistance from the junction to the LED surface is fixed by the materials used in the package, application designers have flexibility in the selection of the heat sink and the thermal interface material between the package and the heat sink. > Some simple rules in heat sink design or selection: > Minimize the thickness of the heat sink (i.e. the distance the heat must travel). > Use a heat sink with a large surface area. > Use materials that have a high thermal conductivity (k). Although copper is a better thermal conductor, aluminum is frequently the material of choice for heat sinks due to cost and weight considerations. Black anodized Al is used to combine good thermal conductivity and good radiative heat transfer to the ambient. 4

5 FIGURE 1: A SIMPLIFIED DESCRIPTION OF THE THERMAL MODEL FOR A CRYSTAL IS LED PACKAGED ATTACHED TO A HEAT SINK T J LED Junction RΘ JC Case RΘ CS Heat Sink RΘ SA T A Ambient Heat Sink Configurations The most common designs of heat sinks incorporate fins, to increase the surface area and reduce the thermal resistance without increasing the footprint. This type of heat sink is also called as a passive heat sink. To further improve the transfer of thermal energy from the heat sink to the air, fans can be used to force convection by moving cooler air between the fins, and is referred to as active heat sinking. In addition, fluids can be used to increase the transfer of heat to the ambient. When mounting LED packages to heat sinks, it is best to use mechanical fasteners to minimize resistance. If mechanical attachment is not possible, tapes and adhesives can aid in thermal contact. The addition of thermal grease can also minimize air gaps and improve thermal contact to uneven surfaces. Heat Sink Selection As a starting point, you can get an idea of the minimum requirements for the heatsink needed for your application by considering the following. 1. The amount of heat to be dissipated in watts (P D ): P D is estimated using either Equation 1 or 2 as described in the Heat Generation section of this document, depending on whether the LED is driven continuously or in the pulsed mode. 2. The ambient air temperature (T A ) in which the LED will be operated: For estimating the ambient temperature during device operation, please include other sources of heat such as electronics in the consideration. Refer to the product data sheet of the Crystal IS LED to ensure that the operating temperature does not exceed the maximum rated temperature of operation. 3. The maximum junction temperature, T J (max), of the LED during operation: LEDs experience a reversible loss of light output as the junction temperature (T J ) 5

6 increases, as shown below. Knowing the minimum light output requirement for your application, the maximum junction temperature that can be tolerated in your application can be determined from the graph (and the LED light output at 20 C). FIGURE 2: TYPICAL LIGHT OUTPUT CHARACTERISTICS OVER TEMPERATURE 100 RELATIVE LIGHT OUTPUT, % JUNCTION TEMPERATURE (T J ), C Using the information from Equations 1, 2 and 3, RΘ JA, the total thermal resistance between the LED junction and the ambient air, can be estimated as: Equation 5 RΘ JA (max) = (T J (max) T A )/P D Knowing the thermal path from the LED junction to the ambient, the thermal resistance of the heat sink (RΘ SA ) can be reverse calculated from the overall thermal resistance (RΘ JA ) as: Equation 6 RΘ SA (max) = RΘ JA (max) -(RΘ JC + RΘ CS ) Thus, in order to keep the junction temperature below T J (max), we need a heat sink with a thermal resistance lower than RΘ SA (max). A heat sink with the required characteristics may be selected from one of the heat sink manufacturers listed in the Resources section of this document. Design Validation The thermal resistance of a heat sink depends on numerous parameters that cannot be predetermined such as the position of the LED on the heat sink, the extent to which air flow is unhindered, the screening effect of nearby components, and heating from other components in the fixture. It is, therefore, important to monitor the temperature of the package and compare with the results from the thermal model, when physical prototypes of the application are available. 6

7 The design validation should be performed at the expected ambient temperature range, ambient air flow and with any additional heat loads. The temperature measurement should be done with the thermocouple in contact with the Cu stud at the base of the package. Example A Crystal IS Optan TO-39 LED is to be operated at 0.5 W in an application at an ambient temperature of 30 C. The minimum light intensity requirement in this application is 0.8 mw. The LED product datasheet states that the LED light output at 20 C is 2.0 mw. The TO-39 package is to be attached to a heat sink with a thermal paste, with a thermal resistance of 1 C/W. Select a heat sink that meets the above design requirements. 1. P D = 0.5 W 2. From Figure 2 (and the LED light output at 20 C), the LED light output will be greater than 0.8 mw as long as the junction temperature, T J (max), does not exceed 60 C. 3. Therefore, from Equation 3, the maximum allowable thermal resistance from the junction to the ambient. RΘ JA (max) = (60-30)/0.5 = 60 C/W 4. The thermal resistance from the junction to the ambient, RΘ JA, is the sum of the thermal resistances from the junction to the case, case to heat sink and from the heat sink to the ambient. RΘ SA (max) = RΘ JA (max) -(RΘ JC + RΘ CS ) 5. To estimate the maximum allowable thermal resistance from the heat sink to the ambient, we need to know the thermal resistance from the junction to the case, RΘ JC and from the case to the sink, RΘ CS. 6. The thermal resistance from the junction to the case, RΘ JC, can be found in the Crystal IS LED product datasheet. For Crystal IS Optan TO-39 package, the thermal resistance, RΘ JC is 37 C/W. 7. The thermal resistance from the case to the sink, RΘ CS, is stated in this example as 1 C/W. 8. The maximum allowable thermal resistance from the heat sink to the ambient. RΘ SA (max) = 60 (37+1) = 22 C/W 9. Depending on the space requirements of the application, the thermal resistance target (RΘ SA = 37 C/W) could be met with several different heat sink designs. For example, this can be achieved with a flat, horizontal heat sink with only one free convection surface or with a finned heat sink with a reduced footprint. Contact one of the manufacturers listed in the Resources section of this document for a heat sink with a thermal resistance less than 22 C/W. 7

8 Thermal Management Resources You may look for an appropriate heat sink from one of the heat sink manufacturers listed below. For attachment of the LED package to the heat sink, select the appropriate thermal interface material to minimize the thermal resistance from the package to the heat sink. A few manufacturers of thermal interface materials are also listed. Heat Sink Manufacturers Aavid Thermalloy Future Electronics Mersen Wakefield Thermal Interface Material Manufacturers 3M Aavid Thermalloy Bergquist Dow Corning Shin-Etsu Indium Corp. of America Omega Additional Links

9 Disclaimer The information in this document has been compiled from reference materials and other sources believed to be reliable, and given in good faith. No warranty, either expressed or implied, is made, however, to the accuracy and completeness of the information, nor is any responsibility assumed or implied for any loss or damage resulting from inaccuracies or omissions. Each user bears full responsibility for making their own determination as to the suitability of Crystal IS products, recommendations or advice for its own particular use. Crystal IS makes no warranty or guarantee, express or implied, as to results obtained in end-use, nor of any design incorporating its Products, recommendation or advice. Each user must identify and perform all tests and analyses necessary to assure that its finished application incorporating Crystal IS Products will be safe and suitable for use under end-use conditions. Each user of devices assumes full responsibility to become educated in and to protect from harmful irradiation. Crystal IS specifically disclaims any and all liability for harm arising from buyer s use or misuse of UVC devices either in development or in end-use. We invite you to learn more about our UVC LEDs. 70 Cohoes Avenue Green Island, NY U.S.A. sales@cisuvc.com 2014 Crystal IS, Inc. All rights reserved. Crystal IS and the Crystal IS logo are trademarks of Crystal IS, Inc. and/or its affiliates. All other trademarks are the property of their respective owners