Design Considerations for Silicon Carbide Power
Article
Silicon carbide (SiC) is a well-established device technology with clear advantages over silicon (Si) technologies, including Si superjunction (SJ) and insulated-gate bipolar transistors (IGBTs), in the 900 V to over 1,200 V high-voltage, high-switching-frequency applications.1 The recent introduction of the 650 V SiC MOSFET products has further broadened SiC use by easily replacing IGBTs, taking a bite out of the Si SJ application share and offering an alternative to gallium nitride (GaN) in the mid-voltage range.
When replacing Si devices with SiC or designing anew with the latter, engineers must consider the different characteristics, capabilities, and advantages of SiC to ensure success. Here is a list of SiC design tips from the power experts at Wolfspeed.
RDS(ON) variance with temperature
A key advantage of SiC is a low RDS(ON) that changes as little as 1.3× to 1.4× over a wide temperature range, whereas in Si or GaN devices, RDS(ON) may double to triple from that rated at 25°C to the practical junction temperatures in the 120°C to 140°C range (Figure 1). It is therefore important to carefully check the datasheet and specify the correct I2R or conduction loss.
![A 60 mΩ Si or GaN device could be >120 mΩ hot, while a 90 mΩ SiC device would be 120 mΩ hot.](/static/7f45519eba9f92996d54bb95f391504c/360aa/design-considerations-1.jpg)
No knee voltage
IGBTs are optimized for a thermal design point at the full rated current. Below that point lies the VCE(sat)exponential “knee” voltage curve (Figure 2). SiC MOSFETs’ VDS characteristics are linear, offering lower conduction loss at any point lower than the full rated current.
This is particularly useful to bear in mind when designing EV drivetrains, in which the drive cycle is mostly below the full rated power. When used in parallel, the IGBT VCE(sat) curve exacerbates the problem.
Designers must therefore carefully consider where lies their thermal design point and mission profile.
![Comparison of 50 A IGBT with 50 A SiC MOSFET in module at Tj = 150](/static/16d01d2ef43015a8d6e577fd0093f5ec/c4d2c/design-considerations-2.jpg)