Gen 4 SiC FETs to Achieve Breakthrough Performance

Article By : Maurizio Di Paolo Emilio

UnitedSiC has launched the first four devices based on its Gen 4 SiC FET technology platform. Compared to Gen 3, the new SiC FETs offer reduced on-resistance (Rds(on)) per unit area from 18 to 60 milliohm...

UnitedSiC has launched the first four devices based on its Gen 4 SiC FET technology platform. The devices are 750V and are suited for applications ranging from industrial charging to renewable energy solutions.

Compared to Gen 3, the new SiC FETs offer reduced on-resistance (Rds(on)) per unit area from 18 to 60 milliohm, and low intrinsic capacitance. In switching applications, they demonstrate low conduction losses as shown in the data sheets by the RDS(on) x Coss(tr) value.

These FETs also have application advantages for 400/500V bus voltage applications.  With a standard gate drive, all devices can be driven with gate voltages from 0 to +12V using existing SiC MOSFET, Si IGBT and Si MOSFET drivers.

As Anup Bhalla, VP engineering at UnitedSiC, pointed out: “In addition to the already released Gen3, we have released Gen4 to accelerate the deployment of wide bandgaps (WBG) in the automotive sector. These devices help address the challenges faced by engineers working in the areas with high voltage and power demands — from DC-DC conversion and on-board charging, but also to power factor correction and solar inverters in industrial applications.”

A new wave of SiC
The new power market demands a lot of energy, higher power density and efficiency. All without compromising reliability. Silicon carbide (SiC) provides several advantages over silicon for making power switching devices extremely hard. SiC has 10x the breakdown electric field strength, 3x the bandgap, and enables a varied range of p- and n-type control required for device construction. SiC also has 3x the thermal conductivity, meaning 3x the cooling capability of silicon.

Silicon solutions drove the power conversion ecosystem until the advent of WBG devices became commercially available in 2008 with SiC JFETs and then SiC MOSFETs in 2011. The new technology promised higher efficiency and faster switching, with the consequent benefits of energy savings and improved power density.

However, to overcome the difficulties of driving SiC switch devices, new engineering was required with the introduction of SiC FETs by combining a SiC JFET with a low-voltage Si MOSFET as a ‘cascode’ in a single package. That is, combining the speed and efficiency of WBG technology with the easy gate actuation of a Si MOSFET. JFETs are less prone to failure compared to traditional MOSFET devices and suit circuit breaker and current limiting applications. SiC FETs have also found their way into the broader 400V/500V bus application market for battery charging, solar and Electric Vehicles (EVs) inverter markets.

Figure 1: SIC JFET and SiC cascode arrangement FET from UnitedSiC (source UnitedSiC)
Figure 1: SIC JFET and SiC cascode arrangement FET from UnitedSiC (source UnitedSiC)

SiC JFETs offer robustness for long and repetitive short-circuit cycles, and the sinter process technology used achieves a low thermal resistance, which for some liquid-cooled designs, such as automotive, is very beneficial.

Gen 4 SiC
To accelerate the adoption of SiC in the automotive market, UnitedSiC has introduced the 750V Gen 4 as improvement over alternative 650V SiC MOSFETs.

The Gen 4 devices employ a higher material density than the previous generation (Gen 3) in order to reduce RDS(on) per unit area (UJ4C075018K3S and UJ4C075018K4S have a resistance of 18mohm). Despite the die’s small size, the high current values it can handle are achieved through process technology that offers better thermal capacity.

In fixed-switching circuits such as Totem-Pole PFC or standard 2-level circuits, the low turn-on resistance per unit area and low output capacitance together with the stored charge offer interesting performance in terms of reverse recovery charge (Qrr) and low Eoss/Qoss.

Figure 2: features of GeN 4 (Source: UnitedSiC)
Figure 2: features of GeN 4 (Source: UnitedSiC)


Figure 3: UnitedSiC Gen4 750V vs. Si superjunction (Source: UnitedSiC)
Figure 3: UnitedSiC Gen4 750V vs. Si superjunction (Source: UnitedSiC)

“400 volt bus applications are very mainstream. So everybody has 650-volt transistors, but when the bus voltage goes up to 500, many people are using 900-volt parts, which is far too much. So you don’t really need 900 for this, you need 750. In this way, you can take care of the impact on performance and cost,” said Bhalla.

He added, “from the third to the fourth generation, we have made the chip even thinner, going from 150 microns to 100 microns to reduce thermal and electrical resistance with an even better die attach material at the copper lead frame to offer better thermal management”.

In energy storage systems with solar panels, there is a need for bidirectional circuits to manage the flow of energy from the panel to the battery and from the battery to the grid.

“So you necessarily need a bidirectional circuit. Although even if it is a PFC totem pole or an active front-end rectifier, it works in both directions as a rectifier or as an inverter. SiC FETs are suited for that, because you can hard switch them, you can soft switch them, it doesn’t matter which direction you flow the current, they work great.”

For renewable energy equipment, such as solar inverters and energy storage, SiC devices keep the amount of heat to be dissipated to a minimum.

The simplicity of low-voltage driving ensures maximum safety of these devices. The short-circuit current of the device is controlled by the JFET channel, which has a threshold that is largely independent of temperature and therefore independent of the gate drive voltage above 12V. “The Miller effect is effectively absent, avoiding the possibility of spurious turn-on and the solutions offered include Kelvin connections to avoid spurious coupling of the inductance with transients in the gate drive,” said Bhalla.

Therefore, the evolution of the new SiC technology has enabled the use of 750V solutions as opposed to the traditional 650V Si and SiC MOSFETs.  Gen 4 is set to enable new power density standards in several power conversion and storage applications.

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