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CFD Events Calendar, Event Record #14006

Understand the Thermal Impedances in Power LED Applications
In this complimentary web presentation from Mentor Graphics, an overview of 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.
Date: May 11, 2011
Contact Email:
Organizer: Mentor Graphics
Special Fields: Measurement Techniques
Type of Event: Online Event, International

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.

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Event record first posted on April 4, 2011, last modified on April 10, 2011

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