Page 102 - NEW Armstrong Book - 2

88
ASPENCORE GUIDE TO SILICON CARBIDE
during races; therefore, requirements such as long-term reliability and cost have less importance for Formula E cars than for commercial EVs. Power density (or current per unit surface), weight-to-power ratio, and immediate full-power availability are all mandatory require- ments in Formula E. One advantage is that there is more flex- ibility in the inverter’s design, thanks to more relaxed vol- ume production requirements and a more manual inverter assembly process. Moreover, cooling fluid selection and cooling characteristics can be more freely chosen, and there is a higher allowed pressure drop in the cooling system.
Design considerations
As in many other applications, Formula E silicon IGBT mod- ules have been progressively replaced with silicon carbide MOSFET devices. As an application in which low weight and enhanced efficiency are key factors, Formula E can benefit from the low losses of SiC MOSFETs in the design of inverters. The Hitachi ABB Power Grids RoadPak 1.2-kV SiC MOSFET half-bridge module, shown in Figure 1, includes in a small footprint (<70 × 75 mm) the latest-generation SiC MOSFET featuring low losses and high reliability to meet the increasing demands of the e-mobility market.
Conduction losses play the most relevant role in limiting the maximum current, which is a main function of the channel on-state resistance (RDS(on)) of the MOSFETs during conduc- tion according to the formula Pcond ~ RDS(on) × Irms2. As a result, selection of a SiC MOSFET with the lowest possible RDS(on) is mandatory, although the lifetime tradeoff must be consid- ered. Another advantage offered by SiC-based PMs is their low switching losses, including turn-on (Eon), turn-off (Eoff), and reverse-recovery losses (Erec). However, it should be noted that the higher the switching frequency, the higher the total switching losses in the power module. Besides RDS(on), maximum current depends on the maximum junc- tion temperature (Tjmax). Therefore, it is necessary to select devices with higher Tjmax and prioritize thermal management to decrease the Rth of the module. Whereas most of the SiC MOSFETs in the market are rated at a Tjmax of 150˚C to 175˚C, the current RoadPak module is rated at 175˚C, and the trend is to increase the Tjmax to 200˚C.
Unlike commercial EVs, Formula E cars are not designed to operate in very low ambient temperatures; therefore, the coolant can include less glycol in a water-glycol mix- ture, with the benefit of lower viscosity and better cooling
performance. Lower-viscosity cool- ants decrease the pressure drop in the system and thus require a smaller and lighter pump. After a careful study, serial one-sided cooling with an opti- mized pin-fin approach was chosen for the RoadPak power module, a solution that provides high cooling performance and easy integration in the inverter.
In addition to the cooling scheme, ther- mal resistance is affected by design choices such as die-attach materi- als and thickness, substrate ceramic type, and base plate design. For the
FIGURE 1: THE HITACHI ABB POWER GRIDS ROADPAK 1.2-KV SiC MOSFET HALF-BRIDGE MODULE
The RoadPak baseline design for the EV market is based on eight parallel SiC chips, which can be increased to 10 without changing the outline. This is the most practical solution, as increasing the active surface of individual SiC chips is limited because of yield challenges in their manufacturing (the active area of SiC MOSFETs currently in the market is less than 30 mm2). As confirmed by thermal simulation, connecting several SiC chips in parallel results in worse heat spreading and in an increase in thermal resistance (Rth) due to the ther- mal crosstalk between neighboring chips.
SiC MOSFETs should be selected based on their conduc- tion losses (Pcond), switching losses (Psw) at different switch- ing frequencies, and, of course, reliability considerations.
RoadPak, Hitachi ABB Power Grids went with a sinter die- attach material that provides considerably higher reliabil- ity during power cycling, lower electrical resistance, and higher thermal conductivity. A thin and dense silver sin- tered layer for die attach (featuring a thermal conductivity of 320 W/mK) achieves a reduction of about 5% in thermal resistance in comparison with the soldering. Regarding the heatsink/base plate for the RoadPak design, copper was preferred to aluminum–silicon carbide (AlSiC) because of its considerably higher thermal conductivity (385 W/mK for Cu, versus 180 W/mK for AlSiC) and better heat-spread effect.
Modules with a copper base plate are usually expected to have a lower lifetime in passive power cycling because of