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Understanding the thermal impedances in power LED applications



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.

Simple mathematical transformation allows us to calculate the frequency-domain representation of the impedance. This representation can be used to calculate the junction temperature when a sinusoidal change in the heating power takes place, provided, we know the frequency of the heating waveform. This property is important for the characterization of power semiconductor devices which are driven by sinusoidal signals. In the frequency domain impedances at a given frequency are given by a complex number. Showing the real and imaginary part of the impedance values for different values results in the so called complex locus – also known as Nyquist diagrams in electrical engineering. If a power semiconductor is driven by square wave signal with a given duty cycle, the so called pulsed thermal resistance diagrams can be used to predict the effective chip temperature. The value of the pulsed thermal resistance depends on the frequency and the duty cycle of the applied square wave. This is such an important property of power semiconductor device packages that it is provided on product data sheets. In case of LEDs the pulsed thermal resistance diagrams provide essential information for the thermal effect of PWM (pulse width modulated) based dimming. Again, pulsed thermal resistance can be calculated from the Zth-functions.

In the case of LEDs measurement of the real Zth requires considering the emitted optical energy. Therefore identification of the real thermal resistance or thermal impedance of LEDs requires light output measurements (total flux measurements) performed simultaneously with the thermal measurements. LEDs driven directly from the AC mains pose other problems. Due to the highly non-linear electrical characteristics the heating power waveform contains many harmonics of the base frequency of the AC mains, therefore calculating "a" single Zth value poses some problems.

In the webinar an overview of the thermal impedance and its alternate representations will be explained, along with LED related issues. We shall also highlight why the AC thermal impedance of LEDs shrinks with increasing frequency. Suggestions for measurement of this property using existing, standard test methods will also be given.

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

Presenter Image AndrĂ¡s 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 View

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

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