Using Wide BandGap (WBG) devices, like SiC and GaN, enable EV chargers to operate with lower losses and higher efficiency than Si, enabling less generated heat which leads to less substantial and less costly thermal management; WBG devices have higher breakdown voltage as well (See Figure 1). Discrete SiC MOSFETs are commercially available 650V-700V, 900V, 1000V, 1200V, 1700V while GaN transistors operate up to 650V. Since WBG materials will withstand higher operating temperatures, and have lower leakage currents as well as lower thermal resistance, these devices will handle more power in a given footprint than Si.
Today’s battery voltages in Electric Vehicles are typically 200V to 800V+, so GaN will capably handle less than 500V for safe design practice taking into account switching transients and SiC will easily handle the higher end voltages.
In addition to the lower heat generated by the reduced switching losses, SiC devices have a higher thermal conductivity than Si (4.9 W/cm-K for SiC, as opposed to 1.5 W/cm-K for Si). Therefore, heat is more easily transferred out of the SiC device, and thus the device temperature increase is slower; GaN has a lower thermal conductivity than Si; however, the superior switching loss performance of GaN far outweigh that of Si in this fast charging application as we will see in the paragraphs below.
Fast switching capabilities result in lower switching losses. Lower switching losses reduce heat generated during the switching process and increase efficiency. Less heat generated can lead to smaller, less expensive thermal management solutions. Operating at higher frequencies can also enable the use of smaller, less expensive passive energy storage components like inductors and capacitors. GaN is far superior than SiC or Si regarding switching losses.
However, switching losses don’t tell the full story. Total losses include switching and conduction losses. Switching losses are those generated when the device “switches” from the off-to-on or on-to-off state. Conduction losses occur when the device is fully on; the primary contributor to conduction losses is Rds(on) or (Resistance from drain to source in the “on” state). In general, SiC devices have a lower, more stable Rds(on) over temperature than GaN devices. This can lead to lower conduction losses for SiC compared to GaN.
Gauging overall losses
Figure of Merit (FOM) is used as a general comparison of both switching and conduction losses and is calculated by (gate charge * on-resistance) or (Qg * Rds(on)). The gate charge (Qg) is an indicator of switching losses while the Rds(on) represents conduction losses. Comparing the product of the two parameters gives an indication of overall performance capabilities.
GaN devices have a FOM 13x better than the best Si super junction MOSFETs primarily due to the very low gate charge.