# Understanding the thermal impedances in power LED applications

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### Overview

Thermal impedance is dynamic property of any semiconductor package. In simple words, it describes how long does it takes for an encapsulated chip to heat up to the steady-state value of the junction temperature when the power is abruptly switched on or how long it takes for the chip to cool down to the ambient temperature when the power is switched on. Also, the steady-state value of the thermal impedance determines the temperature change of the junction due to the applied power step. The thermal impedance is usually given by means of a time function where the junction temperature elevation due to a nominal 1W dissipation is shown in time. Because the time-constants of the major elements of the junction-to-ambient heat-flow path span over many orders of magnitude, this time function is always presented in logarithmic time and is called Zth-curve. The semi-logarithmic plot of the junction temperature change is not the only representation of the thermal impedance. The most obvious alternate representation of the thermal impedance is a thermal RC model whose time response to step-wise power change is the same as the measured Zth curve. If such a model consists of a few RC-stages only, it is called a dynamic compact thermal model of the thermal impedance (approximate). A very precise model would be a structure function – corresponding to the fact that along the junction-to-ambient heat-flow path there is a continuous and smooth change of the distribution of different materials – resulting in changes of the ratio of the actual thermal capacitance and thermal resistance of a given slice of the path.

### What You Will Learn

• Basic concept of the Zth curve as a time function
• Concepts of alternate representations of the thermal impedance, including complex loci, compact thermal models, structure functions, pulsed thermal resistance
• Issues specific to thermal measurement of LEDs: concept of the combined thermal and radiometric measurements, problems of the AC driven LEDs

## About the Presenter

AndrĂˇs Poppe

András POPPE obtained his MSc degree in electrical engineering in 1986 from the Budapest University of Tecnology (BME), Faculty of Electrical Engineering.In 1996 he obtained a Cand.Sci. degree from the Hungarian Academy of Sciences and his PhD from TUB. Between 1986 and 1989 he was a researcher at BME Department of Electron Devices with scholarship of the Hungarian Academy of Sciences. His research filed was circuit simulation and semiconductor device modeling. In the academic year 1989-1990 he was a guest researcher at IMEC (Leuven, Belgium) where he was dealing with mobility modeling for the purpose of device simulation, postgraduate studies at KUL (Katholike Universiteit Leuven). Since 1990 he is with the Budapest University of Tecnology, Department of Electron Devices. In 1991/94 has been active in the Monte Carlo simulation of submicron MOS devices. Since 1996 he has been working at BME as an associate professor. In 1997 he was one of the co-founders of MicReD, now Mentor Graphics MicReD Division. At Mentor Graphics today he supports marketing of the MicReD products T3Ster TeraLED. Besides his academic activities he is involved in various national and international research projects (e.g. EU FW7 Fast2Light, KÖZLED, EU FW7 NANOPACK). He is actively involved in the JEDEC JC15 and CIE TC2-63 and TC2-64 standardization committees. His fields of interest include thermal transient testing of packaged semiconductor devices, characterization of LEDs and OLEDs, electro-thermal simulation.

### Who Should Attend

• Thermal engineers dealing thermal testing
• Engineers dealing with thermal compact modeling
• LED luminaire designers
• Engineers dealing with thermal management in solid-state lighting

## What do I need to watch and hear this web seminar?

Mentor Graphics’ web seminars are delivered using Adobe Connect. You will be able to login to the seminar room 15 minutes prior to the start time on the day of the presentation. You can hear the audio using your computer’s speakers via VoIP (Voice over IP) and background music will play prior to the beginning of the presentation.

## Detailed system requirements

### Microsoft® Windows

• Windows XP, Windows Vista, Windows 7, Windows 8
• Microsoft Internet Explorer 7, 8, 9, 10; Mozilla Firefox; Google Chrome
• Adobe® Flash® Player 10.3 or later
• 1.4GHz Intel® Pentium® 4 or faster processor and 512MB of RAM

### Mac OS X, 10.5, 10.6, 10.7.4, 10.8

• Mozilla Firefox; Apple Safari; Google Chrome
• Adobe Flash Player 10.3
• 1.83GHz Intel Core™ Duo or faster processor and 512MB of RAM

### Linux

• Ubuntu 10.04, 11.04; Red Hat Enterprise Linux 6; OpenSuSE 11.3
• Mozilla Firefox
• Adobe Flash Player 10.3

### Mobile

• Apple supported devices: iPad, iPad2, iPad3; iPhone 4 and 4 S, iPod touch (3rd generation minimum recommended)
• Apple supported OS versions summary: iOS 4.3.x, 5.x, or 6.x (5.x or higher recommended)
• Android supported devices: Samsung Galaxy Tab 2 (10.1), Samsung Galaxy Tab (10.1), ASUS Transformer, Samsung Galaxy Tab (7”) , Motorola Xoom, Motorola Xoom 2, Nexus 7
• Android supported OS versions summary: 2.2 and higher
• Android AIR Runtime required: 3.2 or higher