By using LPN simulations based on the copper and aluminum cooling plates, the tempe-rature is recorded throughout the power cycle at the top of the thermal paste, representing
6.2. Cooling system investigation for thermal stressing mitigation the level directly below the baseplate of the IGBT and at the junction of IGBT switch Q1.
The focus is on the temperature profile at junction level that is considered to be more crit-ical than the temperature profile at baseplate level, as it is going to be mentioned later in this section. For all figures the focus is on the first fifteen seconds of the cycle where the most interesting part of the response takes place. For every case, within the 100s cycle, the junction temperature returns to the initial level that is assumed to be the water inlet temperature of 25◦C.
Figure 6.3a and Figure 6.3b illustrate the junction temperature for the different values of thickness between 0.5cm and 5cm for the aluminum and the copper cooling plate. The heat convection coefficient used in both cases is 5kW/(◦Cm2). The temperature response is almost identical for all the thickness values at the first second. According to the IGBT datasheet, this time period represents, approximately, the time until thermal steady state for the module itself, therefore for pulses shorter than one second, the cooling plate makes no difference. For the 0.5cm cooling plate, the thermal capacitance is very low leading to a high maximum temperature of approximately 85◦C and, also, to a high temperature swing from the maximum temperature to the ambient temperature. Similar observations are valid for the 1cm cooling plate, with a maximum temperature of 78◦C.
For the two thick cooling plates the difference is small, although the increase in the thick-ness is significant. The 3cm cooling plate limits the junction temperature to a maximum of 68◦C and the temperature swing to 43.5◦C. In the 5cm cooling plate case, the increased thermal resistance obstructs the heat extraction and the thermal capacitance is not high enough to further filter the temperature swing. For the 5cm cooling plate the tempera-ture swing is 40.1◦C and the maximum temperature around 65◦C. Independently of the application, the 3cm cooling plate would be selected for the aluminum case, due to its dimensions and weight advantage over the 5cm.
In the copper cooling plate case, see Figure 6.3b, the observations are the same as for the aluminum. Despite the better thermal characteristics of copper, its increased cost out-weighs the benefit in the temperature variation. If the 3cm cooling plate is compared for aluminum and copper, the lead of copper is only 2◦C for the maximum temperature and for the temperature swing.
Figure 6.3: (a) Junction temperature response for different thickness values of aluminum cooling plate, (b) Junction temperature response for different thickness values of copper cooling plate, both cases with a convection coefficient of 5kW/(◦Cm2).
Moreover, a second model is built to run an AC sweep analysis for the heat produced at the most stressed device of the H-bridge. This set-up can be represented again by the Cauer network of Figure 6.1c. In this way the Bode plot magnitude diagram for the power loss extracted to the ambient in relation to the power losses produced by the device is obtained covering the range of frequencies included in the application. The Bode plot diagram of
6.2. Cooling system investigation for thermal stressing mitigation Figure 6.4 illustrates the heat extraction ability for the different thickness values of the aluminum case by presenting the output heat to the ambient with respect to the input heat at chip level. The focus is on the frequency range of the power magnet supply current pulses. The 0.5cm case demonstrates the highest ratio of output to input heat, due to its low thermal resistance. This is in accordance with Figure 6.3a. This heat extraction ability is not the selection criterion because it implies a low heat storage capability, a disadvantage for medium and long pulse applications. This Bode plot could, mostly, be of interest for short pulses. The final choice would be the aluminum 3cm cooling plate.
Figure 6.4: Magnitude bode diagram for the heat extracted to the ambient with respect to the heat dissipated by the chip.
The next step is to test the aluminum cooling plate with a highly effective water-cooling system, in order to identify a possible gain. Figure 6.5 shows the junction temperature re-sponse for the aluminum cooling plate, if the convection coefficient increases to 10kW/(◦Cm2).
Figure 6.5: Junction temperature response for different thickness values of copper cooling plate with a convection coefficient of 10kW/(◦Cm2).
For the 0.5cm the temperature swing and the maximum temperature are remarkably lim-ited compared to the 5kW/(◦Cm2) case. For both the 3cm and the 5cm case, the enhanced convection does not offer a considerable advantage for the thermal stressing at chip level.
However, for the two cases with the smallest thickness values, the impact of the increased convection coefficient is important due to the reduced thermal capacitance of the cool-ing plate. The selected coolcool-ing system would be the one with a convection coefficient of 5kW/(◦Cm2), because it could, potentially, reduce the energy consumption and the cost for the increased water flow in the cooling system.
Table 6.3 summarizes the temperature swing levels for the three cases; aluminum-5kW/(◦Cm2), copper-5kW/(◦Cm2) and aluminum-10kW/(◦Cm2). According to Chapter 2, the junc-tion temperature variajunc-tion is considered as the main thermal stressing parameter compared to maximum junction temperature, since the heating time is the same for all cases.
6.2. Cooling system investigation for thermal stressing mitigation
TABLE 6.3: Junction temperature swing for different materials, cooling plate thickness and convection coefficient