Thermal Consideration of Flash LEDs Application Note

Transcription

1 Thermal Consideration of Flash LEs Application Note Abstract This application note focuses on how to develop an adequate ermal management for LEs in camera flash applications. Introduction LEs are very common in modern mobile devices, such as mobile phones and tablet Cs and are used for features like camera flash, torch or beacon light. epending on e function, e LEs eier operate continuously in e milliampere range (torch) or pulsed in e ampere range (flash). Generally speaking, e operation of LEs is restricted by a number of different factors based on e technology and e material used. Key parameter here is e temperature at e active semiconductor layer (junction temperature), as it influences e function of e LE. Thus, e primary objective in e application is to ensure at e maximum value recommended by e LE manufacturer is not exceeded during operation. This generally means at a suitable ermal management system is absolutely essential in e application. A key consideration during e development of a ermal management system is e ermal resistance of e overall system. In pulse mode, however, e usual description of e ermal performance of a LE - e ermal resistance R, JA (junction to ambient) used for C operation - is not useful since e pulse wid is often limited to milliseconds. The usual approach would result here in a redundant and expensive ermal design. Therefore for pulse operation it is more useful to consider e transient ermal behavior of e LE system setup. Its adequate design can limit e junction temperature to wiin e ratings of e LE device even in e presence of high dissipation peaks. The following document provides information on critical factors and e ermal properties of LEs during a range of operation modes as well as information on how to develop an adequate ermal management in flashlight applications. Basics of LE Systems LE systems are typically designed according to a basic structure and comprise one or more LEs affixed to a CB for control and operation purposes. epending on e application and e related requirements and conditions, is setup is generally mounted on a heat sink (such as cooler, enclosure cover, carrier, etc.) for enhanced heat dissipation. In e case of flash applications, it is usually necessary to drive e LE wiout extended heat sinks due to space restrictions. Instead, e board should be connected to shielding integrated in e device or to e cover in order to achieve a degree of heat distribution. Figure 1 shows a diagram of e basic configuration of an LE system. ecember, 2014 age 1 of 12

2 temperature (T O ) in e application and e maximum temperature when handling e LE (assembly, mounting). Figure 1: Schematic layout of an LE system wi heat sink The degree to which e LE system influences operating conditions depends on a range of factors, based on e technology and e material used. The critical factors can generally be grouped into two categories/levels relative to e system e LE itself and e remaining configuration of e application. Critical factors - LE On e LE side, e operating conditions are primarily influenced/restricted by e semiconductor and chip technology used and e design of e chip. The semiconductor technology determines e maximum permitted junction temperature (T J, max ). The chip technology and design are relevant for high current capability and decisive for e minimum and maximum forward current (I F, min / I F, max ), as well as for e pulse current (I F, plus ) wi which e LE can be operated. Furermore, package and lens material influence e maximum operating Togeer, e material and construction of e LE determine e ermal behavior of e LE during operation, and us, e ermal resistance (R, JS ) of e LE (Figure 2). The ermal resistance is usually specified in e data sheet of e respective component. For e application, e ermal resistance of an LE represents a predefined value, which cannot be modified by e customer. Critical factors - application On e application side, e operating conditions are largely influenced by e system setup, starting wi LE connection, solder pad design and CB type and rough to e use/non-use of a heat sink. This influence is primarily based on e ermal properties of e individual components. The main objective is to keep e junction temperature during operation below e maximum permitted value. The component wi e greatest potential influence in is instance is e CB. The type, material and setup of e CB determines its ermal resistance, which in turn has considerable impact on e ermal properties of e application. Figure 2: iagram of a simplified, general resistor network for static operation ecember, 2014 age 2 of 12

3 In relation to is, e design of e solder pad also contributes to e ermal character of e system. The way in which e LE is connected to e CB (solder, adhesive, clamps, contact pins, standard or low-temperature solder, etc.) also has a certain influence on e operating parameters. The ermal resistance of contact pins, for example, is considerably higher an at of a bonded or soldered joint. The application-specific ermal resistance (R, SA ), which comprises e values of e individual components of e system (Figure 2), generally serves as system-relevant characteristic. It generally applies at e smaller e ermal resistance R, SA, e better e ermal conductivity of e overall setup, and e higher e operating conditions/parameters under which e LE can be operated. The data sheets of e LEs show ese influences in e C derating diagram, which describes e dependence of e maximum forward current of an LE in a system on e ambient temperature. Figure 3 shows a general representation of a C derating wi limiting/critical factors and eir effect. By way of an example, e diagram shows e influence of e application-specific resistance (Setup 1, Setup 2, ) on e maximum permitted forward current and e limitations caused by e respective LE. Figure 3: General representation of LE and application-specific influences on e C derating (max. permitted forward current) ecember, 2014 age 3 of 12

4 Static Operation Assuming at constant power is dissipated in e semiconductor wiin e LE, e junction temperature reaches a value determined by e ermal resistance from e junction where e power is generated to e lower side of e LE package respectively e contacts or e substrate. For is e static equivalent circuit diagram for one dimensional heat flow is shown in Figure 4. The power dissipation is symbolized by a current source. R T 1 T 2 Heat Flow Figure 4: Static Equivalent Circuit for one dimensional heat flow In accordance wi e analogy, e ermal current = Q/t can now be calculated from e ermal Ohm s law. For an overview of e used parameter a nomenclature is shown at e end of e paper. T T R When e amount of generated heat in e junction equals e heat transferred away, a steady state condition is reached and e junction temperature of a LE (for multichip LEs it will be assumed at all chips have e same power dissipation level) can be calculated by e simple equation: T j T R ; S, JS real el In is case of ermal steady state behavior, a LE can be characterized wi e ermal resistance value R, JS real specified in e data sheet. opt For an analysis of LE systems and e development of a suitable ermal management system for C operation, e equation must be expanded to include system-related parameters. T j T A ( R, JS real R, SA Based on e assumed LE values from e data sheet (forward current & voltage at e requested luminance) and e expected ambient temperature of e application, e necessary requirements for e system setup (cooling) can be determined by e following formula: TJ T A, LE R, JS real R, SA In terms of an adequate ermal management system for e C operation of an LE, is is e minimum value at e setup must achieve. If e setup has a lower value, ere is additional potential for operation at a higher current or ambient temperature. If e setup has a higher value, ere needs to be an improvement in e ermal properties of e setup or, if applicable, a reduction of operating conditions. ynamic Operation Single ulse (Z ) While e ermal characteristics for stationary states can be described by e ermal resistance R, dynamic, pulsed processes, in e region of micro- to milliseconds, additionally require at e ermal capacitance of all components has be taken into account. This behavior can be described by terms of ermal capacitance C which is directly proportional to e relevant volume V, to e density of e material and to a proportionality factor of e specific heat c p. The applicable equation is: ) ecember, 2014 age 4 of 12

5 C c V m c p p The ermal capacitance of a body of mass m = x V corresponds to e quantity of heat needed to heat e body by 1 C. To calculate e temperature change T, it is necessary to use e quantity-of-charge equation for a capacitance C. Z T ( Tmax Tmin ) For any pulse leng t p, a transient ermal resistance Z (t p ) is defined as e ratio between e rise of temperature at e end of e pulse and e dissipated power. T C t Q The power dissipation represents e transport of ermal energy per unit of time. Consequently: T t C Figure 6: Effect of a single power pulse The equivalent circuit wi e ermal capacitances added, is shown in Figure 5. R C t Figure 5: Transient Equivalent Circuit for one dimensional heat flow The added ermal capacitances are shown in parallel wi e ermal resistances. The product of e ermal resistance and capacitance gives e time constant t: t R C This value determines how fast e temperature rise will occur wi respect to time. The junction temperature transients can be represented in form of a function if e dynamic ermal impedance Z is shown versus e pulse wid. Figure 7 shows e ermal impedance in single power pulse operation using e example of an OSLUX LUW FQEL LE. This function shows e regions of dominance of e various time constants. For an infinite pulse leng, e transient ermal resistance reaches a constant value. This is e steady state ermal resistance (R ). As described above, e dynamic ermal impedance of a LE is defined as e ratio of e temperature difference T = T J -Ts and e power dissipation. Z ( T T ) J S, LEreal (LE) where [ W ] el opt power dissipation The described Z function characterizes e transient ermal behavior of a LE and is always of interest in applications where pulse operation ermal resistances are required which are below e steady state value. ecember, 2014 age 5 of 12

6 Figure 7: Transient Thermal Resistance Z of OSLUX LUW FQEL (example curve) For e analysis of pulsed LE systems, e application-specific parameters also need to be taken into account and e equation adapted accordingly. For e calculation of e junction temperature in e system e following formula applies: T J T A Z, LE System (LE system) Continued ulse (Z *) From a ermal standpoint, is operating mode represents a mixture of pulsed and continuous operation, during which a selfadjusting mean base temperature is superimposed wi temperature peaks. For e characterization of e ermal behavior e so-called Z * is used here, which describes e transient ermal resistance dependent on e repetition rate and e duty cycle. Z * ( t ; ) (1 ) Z ( t ) R wi t and R Z ( for t ) T ue to e complex interrelations and transient behavior of continued pulsed systems, one usually considers e ermal state at e end of a pulse in order to obtain an impression of e maximal change in temperature. ecember, 2014 age 6 of 12

7 If Z * is multiplied by e maximum power dissipation during e pulse, an upper estimate for e junction temperature can be obtained. To calculate e maximum junction layer temperature at is operation mode, a first approximation can be obtained by: T Junction T Z A *, real The pulse derating diagrams generally included in e LE data sheets provide initial information on e maximum permitted current (Figure 8) for a sample application wi a total ermal resistance (system R, JA real ). The current pa in e diagrams generally refers to an assumed total ermal resistance and usually applies to two different ambient temperatures, as well as being dependent on pulse wid and duty cycle. Figure 8: Example for permitted pulse handling capability diagram (e.g. LUW FQEL) ecember, 2014 age 7 of 12

8 Simulation of various LE systems for Flash application Based on mobile flash applications, a closed enclosure made of plastic and dimensions of 50mm x 50mm x 10mm was defined as base and entry model for e ermal analysis. It includes e LE in pulse mode wi varying degrees of system complexity. For e CB, we used common types, such as FR4, Flex- C and IMS, and contact pins on FR4 wi a common size of 20mm x 20mm. For e LE we used an OSLUX LUW FQEL. Wiin e plastic enclosure, e LE system is defined as levitating. Figure 9 shows an overview of e models and additional marginal conditions. The shown system setups were used for calculating e respective Z curves (Figure 10). As expected, e system setup wi IMS board displayed e lowest ermal resistance and e best behavior when subjected to pulse loading. When compared to e two FR4 setups, it can be observed at even an expansion of e copper area of e solder pad (standard solder pad -> max. area) effects in a considerable improvement. A similar effect, if to a lesser degree, can also be seen wi e different FC systems. Starting wi a pure Flex C rough to FC on metal stiffeners and shielding, ere is a clear improvement in ermal properties for pulse mode. The setup wi contact pins displayed e highest Z values. uring e ermal analysis, e flash systems were loaded wi a typical pulse sequence and pulse quantity (10x) and e resulting temperature curve simulated for e junction layer of e LE OSLUX LUW FQEL. Figure 9: Overview of e various simulation models and marginal conditions ecember, 2014 age 8 of 12

9 Figure 11 shows a typical pulse sequence and compares e results of e various flash systems. Figure 10: Overview of e Z curves for various simulation models Figure 11: ulse conditions and results of transient simulations ecember, 2014 age 9 of 12

10 Conclusion The operating parameters of an LE system are influenced and limited by a wide range of direct and indirect factors. The operating parameters at enable operation of a LE at a maximum level in an application are primarily determined by its ermal properties and e specific requirements. A system-relevant, and erefore crucial parameter is e junction temperature in e semiconductor chip of e LE, as is influences e function. In practice, e junction temperature cannot be measured directly but has to be calculated indirectly via R based on e solderpoint temperature or measured Z progression. Because e LE wi its properties is generally ermally optimized by e manufacturer, e only way to influence e operating parameters from e system side is via e system setup or its components. For e development of a ermal management system, is means at e better e ermal properties of e setup, e greater e degree of flexibility offered by e operating parameters Customers should erefore question simple, wholesale statements concerning e maximum operating current of an LE, or such statements should refer directly to or provide concrete information on a specific system. OSRAM Opto Semiconductors supports customers in eir development and design process to help em find e best solution for specific applications. For furer information or queries, please contact your sales representative or OSRAM Opto Semiconductors. NOMENCLATURE c p ρ V m λ T t t p R τ T A T S T Safety C Z specific heat at constant pressure density volume mass ermal conductivity Temperature time pulse leng ermal resistance time constant Ambient temperature Solderpoint Temperature Thermal Safety factor ermal capacity transient ermal resistance heat generation f Q C G L Solder Solder pad CB type TIM Heat sink duty cycle frequency heat capacitance per unit leng conductivity per unit leng inductance per unit leng e.g. Glue, SAC Solder, etc. recommended solder pad design (e.g. datasheet) FR4, IMS, FC, etc. Thermal Interface Material (e.g. Foil, Adhesive, Glue, etc.) e.g. cooler, fan, stiffener, shielding, cover, etc. ecember, 2014 age 10 of 12

11 Appendix on't forget: LE Light for you is your place to be whenever you are looking for information or worldwide partners for your LE Lighting project. Revision History ate ec Revision History Release application note Auors: Andreas Stich, Rainer Huber ABOUT OSRAM OTO SEMICONUCTORS OSRAM, Munich, Germany is one of e two leading light manufacturers in e world. Its subsidiary, OSRAM Opto Semiconductors GmbH in Regensburg (Germany), offers its customers solutions based on semiconductor technology for lighting, sensor and visualization applications. Osram Opto Semiconductors has production sites in Regensburg (Germany), enang (Malaysia) and Wuxi (China). Its headquarters for Nor America is in Sunnyvale (USA), and for Asia in Hong Kong. Osram Opto Semiconductors also has sales offices roughout e world. For more information go to ecember, 2014 age 11 of 12

12 ISCLAIMER LEASE CAREFULLY REA THE BELOW TERMS AN CONITIONS BEFORE USING THE INFORMATION SHOWN HEREIN. IF YOU O NOT AGREE WITH ANY OF THESE TERMS AN CONITIONS, O NOT USE THE INFORMATION. The information shown in is document is provided by OSRAM Opto Semiconductors GmbH on an as is basis and wiout OSRAM Opto Semiconductors GmbH assuming, express or implied, any warranty or liability whatsoever, including, but not limited to e warranties of correctness, completeness, merchantability, fitness for a particular purpose, title or non-infringement of rights. In no event shall OSRAM Opto Semiconductors GmbH be liable - regardless of e legal eory - for any direct, indirect, special, incidental, exemplary, consequential, or punitive damages related to e use of e information. This limitation shall apply even if OSRAM Opto Semiconductors GmbH has been advised of possible damages. As some jurisdictions do not allow e exclusion of certain warranties or limitations of liability, e above limitations or exclusions might not apply. The liability of OSRAM Opto Semiconductors GmbH would in such case be limited to e greatest extent permitted by law. OSRAM Opto Semiconductors GmbH may change e information shown herein at anytime wiout notice to users and is not obligated to provide any maintenance (including updates or notifications upon changes) or support related to e information. Any rights not expressly granted herein are reserved. Except for e right to use e information shown herein, no oer rights are granted nor shall any obligation be implied requiring e grant of furer rights. Any and all rights or licenses for or regarding patents or patent applications are expressly excluded. ecember, 2014 age 12 of 12