Issue link: https://resources.mouser.com/i/1484091
31 Qorvo 2022 A surface-mount alternative would seem attractive, but at the 22kW level? Actually, yes... It's useful to look at some examples of the package performance comparisons in the different stages of a typical on-board charger using the UnitedSiC online FET-Jet Calculator™. A 'Totem Pole PFC' stage is common, and an example rated at 6.6kW with 400V output, 75kHz, continuous conduction mode (CCM) was evaluated with a range of TO-247-4L and D2PAK-7L SiC FETs for the 'fast switching' leg, for a heatsink/fluid temperature of 80°C. The junction temperature differences between the two packages ranged from 3°C to 8°C depending on the class of on-resistance. At higher power and with a three-phase AC supply, a 'Vienna rectifier' might be used with an 800V DC-link at, say, 40kHz (Figure 1). 750V SiC FETs can be used, and if 18-milliohm TO-247-4L and D2PAK-7L parts are compared again, the difference in junction temperature is just 3°C with 0.1% difference in 'semiconductor' efficiency. Higher on-resistance parts in this application inevitably show a bigger difference with an unworkable temperature rise for single devices, but at 22kW in a high-value product, the cost of the lower resistance parts is not a large overhead for the benefits gained. D2PAK-7Ls Can Replace TO-247-4Ls in the DC/DC Stage Effectively The Totem-Pole PFC and Vienna rectifier stages just considered are 'hard' switching, and frequency is kept relatively low to minimize dynamic losses. The DC/DC stage in an OBC can be a resonant or 'soft' switched converter, such as the CLLC topology with a much higher frequency for small magnetics and low loss, typically 300kHz. For example, at 6.6kW with a 400V DC-link and using 18-milliohm SiC FETS, losses according to FET-Jet Calculator™ per device are 4.1W and 4.2W for TO-247-4L and D2PAK-7L respectively, and the Figure 1: The image illustrates the Vienna rectifier front-end. (Source: Qorvo) lower inductance of the SMT package makes it a natural choice for the higher frequency used. Moving to SMT D2PAK-7L packages is a natural progression from TO-247-4L types when total system cost is considered with minimal or no difference in temperature rise or system efficiency, especially if the electrical and mechanical ease of paralleling is factored in. As SMT devices, along with their class-leading Figures of Merit (FoM) and easy gate drive, SiC FETs are inching closer to the ideal switch choice for EV on- board charger applications. Conclusion With standard 1700V ratings and better efficiency than IGBTs, SiC FETs are becoming more attractive than super-junction MOSFETs, firmly establishing themselves as contenders in all stages of EV on-board charging. While SiC FETs in a TO-247-4L package provide excellent thermal performance, a downside is that they require mechanical fixing and through-hole soldering. So, migrating to an SMT device like the UnitedSiC D2PAK-7L package is a natural evolution when total system cost is considered with minimal or no impact on temperature rise or efficiency. These SMT SiC FETs not only offer designers significant savings on circuit assembly but also class-leading FoM and an easy gate drive solution, making them the ideal switch choice for on-board chargers for EVs.