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Chronicles from the Frequency Domain: Benefits and Applications of AC Thermal Modeling, Simulation and Measurement
Keywords: AC analysis, thermal impedance, frequency domain
AC analysis, i.e. the study of sinusoidal regime, is not widely spread in the field of thermal analysis of electronic devices. Still this technique, as commonly used in other branches of electrical engineering, can be favorably adopted to thermal problems. The method involves the investigation of periodic oscillations of the temperature and heat flux induced by a sinusoidal power dissipation (with angular frequency omega=2*pi*f). The thermal fields can be mathematically described using a complex phasor notation. A major role is played by the thermal impedance Zth(j*omega), i.e. the ratio of the temperature and power phasor in the junction. /// Thermal frequency domain analysis is directly applicable to components that are typically operated in pulsed mode, such as power electronics and electrothermal filters. More generically, the use of phasors often simplifies both analytical modeling and numerical simulation. Electrothermal analogies can be beneficially exploited as well, e.g. RC ladder networks can be elegantly analysed in the frequency domain. This has lead to the development of a transient fixed-angle heat spreading model. In addition, the Nyquist plot of Zth(j*omega) shows some interesting properties not revealed in the time domain. Such an impedance curve offers a compact yet complete thermal blueprint of the device and can be used for quality inspection purposes. Finally, AC measurements provide phase information, which acts as a sensitive heat detector hardly affected by calibration errors or noise. Frequency domain experiments also enable separation of Joule and Peltier effects. AC temperature fields on the IC surface can be visualised with high spatial resolution by means of CCD-based thermoreflectance. As an example, some results obtained with a novel heterodyne technique on thermoelectric microcoolers are presented.
Bjorn Vermeersch, Research Assistant of the Research Foundation
Ghent University - ELIS Dept.
Gent, Eastern Flanders 9000,

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