Tomasz Torzewicz

Estimation of heat transfer coefficient temperature dependence from cooling curve measurements

This paper discusses on the practical example of a power diode attached to a heat sink the problem of heat transfer coefficient temperature dependence and its importance for free convection cooled electronic systems. The average values of this coefficient are estimated from multiple measurements of device cooling curves performed for different device heating currents. These values are used then for transient simulations of the device with compact thermal models.

Thermal analysis of hybrid circuits with variable heat transfer coefficient

This paper, based on the practical example of a hybrid test circuit, illustrates the importance of proper modeling of the heat transfer coefficient dependence on surface temperature rise and fluid velocity in air cooled electronic systems. The presented experiments show that for the considered circuit the value of the heat transfer coefficient strongly depends on both these factors, thus its changes have to be taken into account in thermal simulations. A simple empirical relation proposed here by the authors allows accurate assessment of local heat transfer coefficient values in different cooling conditions and significant increase of thermal simulation accuracy.

Modelling of average radiation and convection heat transfer coefficient value in electronic systems

This paper discusses, based on a practical example, the problem of estimating the average radiation and convection heat transfer coefficient value in thermal models of electronic systems. The proposed empirical formulas are fitted to the results of temperature measurements taken for a power diode attached to a heat sink. The measurements were repeated in natural and forced air cooling conditions. These empirical relations were used subsequently in device simulation with compact thermal models.

Accuracy and boundary condition independence of Cauer RC ladder compact thermal models

Abstract This paper presents an experimental study aimed at the investigation of the influence of variable boundary conditions on the element values of compact thermal models. These models are generated from the time constant spectra of thermal responses in the form Cauer RC ladders in such a way that their individual components might have some physical interpretation and correspond to individual stages in the heat flow path. The accuracy of thermal simulation results obtained using the proposed models is validated with temperature measurements of discrete silicon carbide power devices.

Investigation of heat transfer coefficient variation in air cooled hybrid electronic circuits

This paper discusses the problem of heat transfer coefficient variation in air cooled hybrid circuits. The investigations are based on a practical example of a circuit containing a bipolar transistor heat source. Its temperature is measured using the base-emitter junction. Additionally, infrared measurements of circuit surface temperature are taken. The measurements are carried out in a wind tunnel for different values of dissipated power and with variable cooling air speed. The measurement results are analysed allowing the assessment of heat transfer coefficient variation with surface temperature and cooling air velocity. Based on the analyses, compact thermal models are generated for the circuit allowing fast and accurate simulation of circuit temperature in various cooling conditions.

Analysis of nonlinear heat exchange phenomena in natural convection cooled electronic systems

Abstract Thermal simulations of electronic systems usually are carried out employing linear models whose element values do not depend on temperature. The hypothesis on system linearity can be verified in practice by the summation of measured heating and cooling curves. When a considered problem is linear, at all time instants the curves always add up to the straight line corresponding to the steady state temperature value. However, the experiments carried out for a natural convection cooled power device with a heat sink showed that the temperature dependence of the heat transfer coefficient results in the appearance of a characteristic hump in this curve. A simple analytical model was developed here to predict the location of this extremum. The analyses presented in this paper are valid when the thermal time constant reflecting the thermal capacitance of a heat sink and the heat exchange with ambient is dominant.

Identification of nonlinearities based on comparison of device heating and cooling curves

This paper discusses the problem of nonlinear effects occurring in thermal modelling of electronic systems. In particular, the influence of heat transfer coefficient temperature dependence is studied based on a practical example of free convection cooled power device. The nonlinearity is identified by comparing the recorded heating and cooling curves. Additionally, the analyses are supported also by the presentation of the cumulative thermal structure functions and the time constant spectra.

Experimental identification of LED compact thermal model element values

This paper discusses, based on a practical example, the problem of power LED thermal modelling. The precise determination of thermal resistance is crucial for accurate computation of junction temperature, which influences both device lifetime and reliability as well as its operating parameters. Here, diode heating curves are recorded at different levels of dissipated power and in various cooling conditions. Moreover, the devices are soldered to the substrate in different ways, what renders possible the determination of their junction-to-case thermal resistances. For each case, an adequate compact thermal model is generated using the Network Identification by Deconvolution method and validated against the measurements.

Comparative analysis of compact thermal models generated from measured thermal responses and detailed thermal models

This paper compares different compact thermal models generated for a test hybrid circuit using the network identification by deconvolution method. The compact models are obtained both from measured device dynamic thermal responses and from heating curves simulated with a numerical FDM solver. The paper discusses also the influence of different simplifying assumptions on resulting thermal models and their accuracy.

Experimental investigation of discrete air cooled device thermal resistance dependence on cooling conditions

Abstract Temperature is an important factor influencing the operation of electronic devices, thus adequate thermal models should be used for their accurate simulations. Unfortunately, the information on device thermal characteristics provided by their manufacturers is very scarce and usually it is limited only to the specification of the junction-to-case thermal resistance, which is not sufficient for the prediction of device dynamic thermal behaviour. This problem is discussed here based on the practical example of an air cooled power device operating at different power dissipation levels and in variable cooling conditions. The experiments presented in this paper demonstrate that the notion of junction-to-case thermal resistance is ambiguous since the cooling conditions affect the heat diffusion processes inside the package and consequently influence the thermal resistance value. For the analysed device, a simple four-stage RC Foster ladder model is proposed here to simulate the device dynamic thermal behaviour. Owing to the fact that this model is generated in a structure preserving way, its element values can be assigned some physical meaning.