Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyIII. THERMAL INVESTIGATIONThis section will demonstrate how the information gainedfrom the transient thermal measurements can be employedfor the analysis of the internal package structure. First, themeasurement results obtained for both silicon carbide diodeswill be presented. Then, a short theoretical discussion on theinfluence of the contact thermal resistance on the responsetime constant spectra will be provided. Finally, the previoustheoretical considerations will be compared to the processedmeasurement results.Fig. 1. Schematic of the CCM-PFC boost converter.B. Electrical CharacteristicsThe operation of the converter was visualised using digitaloscilloscopes and its performance was compared for bothdiode types; i.e. Diode 1 and Diode 2. The captured currentcurves over one period are presented in Figs. 2-3, where thetransistor T1 and the diode D1 currents are the upper and thelower waveforms respectively. The numbers 1 and 2 on theleft side indicate the ground levels for the curves.As can be seen in the figures, both silicon carbide diodetypes exhibit similar electrical behaviour, but the Diode 1produced long lasting oscillations after each switching. Thisphenomenon, reproducible for all the available diodes of thistype, can have serious impact on the converter performancebecause in order to avoid unexpected transients in the circuitthe diode cannot be switched on again before the oscillationsdie out, hence limiting the switching frequency. Presumably,these current oscillations can be attributed to the presenceof some parasitic impedance inside the package of Diode 1.This hypothesis will be verified in the next section basedon the analysis of the diode thermal response time spectrum.In particular, the influence of the air voids in the die attachon this spectrum will be considered.A. MeasurementsInitially, the diodes were carefully calibrated. After somethermal grease was applied, the devices were firmly attachedto a copper plate through which the heating liquid flow wasforced by means of a thermostat. The dependence of thejunction voltage on temperature was measured by steppingtemperature first up and then down. The same procedure wasrepeated twice for each device. The average measured slopewas -1.75 mV/K.The next stage was to carry out the measurements of thediode cooling curves. The measurements were taken for thedevices attached to a large heat sink and placed in a windtunnel. The wind speed was set to the maximum attainablein this tunnel, i.e. 4.15 m/s. Such measurement conditionsassured a low value of the thermal impedance to the ambientand emphasized the heat flow path within the package, thusallowing more detailed insight into the internal parts of thediode packages.The measurements were taken with the thermal transienttester T3Ster manufactured by the MicRed company. First,the diodes were heated with the constant current of 2 A untilthe thermal steady state was reached. Then, the power wasswitched off and the device cooling process was registered.The diode forward current used during the measurement wasthe same as during the calibration.hor 1µs/div vert 2A/divFig. 2. Measured active device current curves over one period- Diode 1.hor 1µs/div vert 2A/divFig.3. Measured active device current curves over one period- Diode 2.©EDA Publishing/THERMINIC 2008 44ISBN: 978-2-35500-008-9