The advantages of SiC devices for use in power electronics are driven by the wide-bandgap semiconductor’s high material performance, high breakdown voltage, and thermal conductivity...
The advantages of silicon carbide (SiC) devices for use in power electronics are driven by the wide-bandgap (WBG) semiconductor’s high material performance, high breakdown voltage, and thermal conductivity. The strong market momentum of automotive inverter companies developing SiC solutions is projected to help drive revenue growth past the US$1 billion threshold over the 2020–2024 period (Figure 1).
In 2019, SiC became a key factor in growth markets such as electric vehicles (EVs), battery chargers, and IT infrastructure. SiC FETs are a major contributor to performance and reliability improvements, with overall system cost reduction in next-generation data centers. UnitedSiC has delivered 14 new SiC FET products with the industry’s lowest drain-source on-resistance (RDS(on)), at <10 mΩ, and a high-performance, four-lead Kelvin package.
The third-generation SiC products in UnitedSiC’s lineup have found use in a wide range of applications. UnitedSiC now is building on the foundation of its Gen 3 technology to move toward even higher voltages and lower RDS(on) in Gen 4. That next-generation technology is in development, and products will be launched later this year.
Major advantages of power switches based on WBG semiconductors such as SiC and gallium nitride (GaN) include high current density, faster switching, and lower RDS(on). These device performance improvements lead to significant system-level benefits from an end-customer perspective. In real-life applications, customers can achieve high-temperature operation, along with overall system size and weight reductions.
A tangible difference between SiC and GaN is in their packaging. SiC parts are commonly available in the TO-247 and T0-220 styles; as a result, they can replace silicon MOSFETs in existing projects. Another difference between the WBG options is that SiC has simply been around longer; it matured with diodes in power factor correction (PFC) circuits very rapidly.
UnitedSiC is focusing its SiC efforts on applications including automotive traction inverters, on-board chargers, and DC/DC converters. “We are still doing all the battery-charging business; we are pushing very hard with our 650-V products in the computer server business and the 5G business,” said Anup Bhalla, vice president of engineering at UnitedSiC. “Although 5G did see some slowdown last year, hopefully it will recover strongly this year. We have been pushing in the energy storage business even more than solar for renewables.”
The push toward solutions with ever-lower RDS(on) is in response to strong market demand. The thermal conductivity of SiC is about 3.5× that of silicon, allowing the material to support high-temperature operation with high voltage and power levels. Because SiC has breakdown field strength 10× higher than that of silicon, high-breakdown–voltage devices can be achieved through a thin drift layer with high doping concentration. This means that at the same breakdown voltage, SiC devices have quite low specific on-resistance (on-resistance per unit area).
When selecting semiconductor switches, it pays to investigate the detail of datasheet specifications — especially how critical parameters like RDS(on) change with temperature. RDS(on) is the total resistance in the path from source to drain. It is made up of a series of resistances that traverse the path of current flow. The Gen 4 technology maintains the natural strength that distinguishes SiC from GaN and silicon devices.
“Every time we make a new generation, we think about what we can improve from the past generation,” said Bhalla. “The first thing we observed was that there was a large market developing at 750 V, especially in the EV market. And we were not able to participate, because we had 650-V devices and then we had 1,200 V. [So] people would use our 1,200-V solutions there, and that’s not cost-effective. We thus decided in Gen 4 that the first thing that we would make is 750 V.”
The most significant improvement from Gen 3 to Gen 4 is the even lower resistance per unit area. Chip size can be reduced by 40% if the resistance decreases by a factor of two per unit area. The reduction in size also reduces the capacitance, which in turn lowers switching losses. These improvements will meet several requirements in industrial motor drives (Figures 2 and 3).
“Since our chips are so much smaller, the total output capacitance of this technology is lower than GaN devices,” said Bhalla. High reverse-recovery charge (Qrr) and high forward-voltage drop correspond to high losses in circuits; these force or require the body diode to conduct, for example, in hard-switched bridges with inductive loads. An extra parallel SiC Schottky diode can be added to bypass the body diode but at high cost and with limited benefit. Gen 4 improves Qrr to 462 nC from Gen 3’s 840 nC.
“The total switching energy loss here for the new-generation device is better than the old one,” said Bhalla. “You can see in Figure 4 that the turnoff is very good in both cases. All these devices are very fast for inductive turn-off. But EON [switching energy] loss improvement is what really helps when you’re doing hard switching.”
The switching frequency of a silicon carbide MOSFET can be up to 5× that of a silicon device, with the consequent possibility of using smaller passive components. The higher switching frequency enables some extra requirements in the design phase of the gate driver, which not only has to generate a larger gate-source voltage (VGS) to bring the device into conduction state (minimizing RDS(on)) but also must provide a very fast output slew rate so as to charge and discharge the capacitance of the gate circuit very quickly.