October / November 2021

www.unitedsic.com POWER SEMICONDUCTORS 25 www.power-mag.com Issue 4 2021 Power Electronics Europe with 1200 V withstand and 300 A rating. These can be packaged in a SOT-227 module, comparable with an IGBT solution with similar performance. UnitedSiC have trialed this arrangement and demonstrated the device interrupting a peak fault current of 1950 A safely (Figure 4). SiC JFETs have an inherent current limiting characteristic from an internal channel ‘pinch-off’ effect and the positive temperature coefficient of their on- resistance helps with current sharing between the paralleled devices, in contrast to IGBTs which do not naturally share current when paralleled. Future SSCBs Mechanical circuit breakers, with their speed, arcing and lifetime problems will surely give way to solid state versions at higher and higher power levels as semiconductor performance advances. Even today, an automotive-grade breaker is expensive and as the market expands, particularly in EVs, the solid-state version costs will drop quickly, making it a compelling alternative given the added benefits in speed and reliability. SiC devices are still in their relative infancy and it can be expected that prices will anyway fall over the coming years, with wafer costs predicted to halve. On- resistance performance will also improve, as the technology is some way from its theoretical best, so for a given voltage rating and die size, SiC RDS(ON) should drop to less than a third of its current value, with overall resistance approaching that of a mechanical solution, with all associated connector and cable resistances included. When the lifetime costs of mechanical breaker maintenance and regular replacement are factored in, the SiC version looks more attractive still. LEFT Figure 4: A SiC FET SSCB interrupts nearly 2000 A safely The company has responded to the power designer’s requests for higher-performance, higher-efficient SiC FETs with the announcement of the 750 V 7 6 m Ω device which provides a robust short-circuit withstand time rating of 5?s. The announcement of September 14 includes nine new device/package options in the 750 V SiC FET series, rated at 6, 9, 11, 23, 33, and 44 m Ω . The contribution of wiring in TO-247 package amounts to 1m Ω in the 6 m Ω version. All devices are available in the TO-247-4L package while the 18, 23, 33, 44, and 60 m Ω 7devices also come in the TO- 247-3L. Complemented by the already available 18 and 60 m Ω devices, this 750 V expanded series provides designers with more device options, enabling more design flexibility to achieve an optimum cost/efficiency trade-off while maintaining generous design margins and circuit robustness. Gen 4 SiC FETs are a ‘cascode’ of a SiC JFET and a co-packaged Silicon MOSFET. These together provide high speed and low losses with high temperature operation, while retaining an easy, stable, and robust gate drive with integral ESD protection. The advantages are quantified by Figures of Merit (FoM) such as RDS(on) x A, a measure of conduction losses per unit die area. Gen 4 SiC FETs achieve the lowest values in the market at both high and low die temperatures. FoM RDS(on) x EOSS/QOSS is important in hard-switching applications. FoM RDS(on) x COSS(tr) is critical in soft-switching applications. “With SiC FETs the switching losses are controlled by the SiC JFET and not directly the the gate charge of the low voltage MOSFET, thus the typical FoM = Ron x Qg is not useful with SiC FETs,” Anup Bhalla explains. For hard switching applications, the integral body diode of SiC FETs is superior in recovery speed and forward voltage drop to competing Si MOSFET or SiC MOSFET technologies. Other advantages incorporated into the Gen 4 technology are reduced thermal resistance from die to case by advanced wafer thinning techniques and silver-sinter die-attach. “Sintering is the mechanism by which the SiC JFET chip is attached to the Cu leadframe of the package, and by which the low voltage MOSFET is attached to the top of the Sic JFET”, Bhalla underlines. With their latest improvements in switching efficiency and on-resistance, the new UnitedSiC SiC FETs are ideal for challenging, emerging applications. These include traction drives and on- and off-board chargers in electric vehicles and all stages of uni- and bi-directional power conversion in renewable energy inverters, power factor correction, telecoms converters and AC/DC or DC/DC power conversion generally. Established applications also benefit from use of the devices for an easy boost in efficiency with their backwards compatibility with Si MOSFET and IGBT gate drives and established TO-247 packaging. Pricing (1000-up, FOB USA) for the new 750 V Gen 4 SiC FETs range from $4.15 for the UJ4C075044K3S, to $23.46 for the UJ4SC075006K4S. All devices are available from authorized distributors. UnitedSiC Announces 6 m Ω SiC FET Gen 4 SiC FETs are a ‘cascode’ of a SiC JFET and a co-packaged Silicon MOSFET in TO-247-4L or -3L package LEFT: “Sintering is the mechanism by which the SiC JFET chip is attached to the Cu leadframe of the package, thus reducing thermal resistance and avoiding solder cracking,” Anup Bhalla explains