Page 103 - NEW Armstrong Book - 2

FIGURE 2: VALIDATED ZTH CURVES FOR THE ROADPAK MODULE
their high coefficient of thermal expansion (CTE). However, use of a mold compound that matches the effective CTE of the overall power module design compensates for the nega- tive impact of high CTE of the Cu base plate in the RoadPak design.
SiC MOSFETs should be selected based on their conduction losses, switching losses at different frequencies, and reliability.
The power module layout requires particular attention to the electromagnetic design. To reach a high current rating and account for the small size of a SiC MOSFET, several SiC devices must be paralleled. The non-uniform switching that arises can lead to increased oscillations and parasitic turn-on of the device. These undesired effects may also lead to current derating or limit the switching speed, thereby reducing the low-loss advantages of SiC MOSFETs. For Formula E, Hitachi ABB Power Grids optimized the RoadPak
1.2-kV design and module manufacture to achieve the maxi- mum possible power density. The validated Zth curve (tran- sient temperature response to a unit power step) for the opti- mized 6Pak design is shown in Figure 2.
Thanks to the improvements in the PM thermal management and overall cooling system, an Rthmax of less than 83 K/kW was achieved for the conditions shown in Figure 2. The minor difference between the average Z and the hottest chip Z
th th indicates the homogeneous cooling of the parallel devices. The aforementioned low Rth allows an operating point of Irms > 900 A at the operating conditions (VDC = 800 V, fsw = 10 kHz, Tin = 45˚C, CosΦ = 0.825, m = 0.95) with clean switching, which makes RoadPak a benchmark SiC power
module for demanding applications.
This article was originally published on Power Electronics News on June 22, 2021.
Maurizio Di Paolo Emilio
is editor-in-chief of Power Electronics News and EEWeb.
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