UnitedSiC has announced the addition of four new SiC FETs with what they claim to be the lowest RDS(on) in the industry.

At an RDS(on) as low as 7mΩ, the SiC FETs are designed for high-powered applications, including electric vehicle (EV) converters, high-powered DC/DC converters, high-current battery chargers, and circuit breakers.

The UF3C/SC SiC FET series is available in a single device rated at 650V with an RDS(on) at 7mΩ or three devices rated at 1200V with an RDS(on)​​​​​​​ at 9mΩ and 16mΩ. All four of the devices are available in a TO247 package.

UnitedSiC CEO Chris Dries distills the three main things that power engineers care about in a FET: on-resistance, switching loss, and price. The anatomy and specifications of the new SiC FETs, as discussed below, are geared to meet each of these priorities.  

The new SiC FETs were created by combining third-generation JFETs (Junction Gate Field-Effect Transistor) with a cascode-optimized silicon MOSFET (Metal Oxide Semiconductor Field-Effect Transistor). By combining the two, UnitedSiC developers intend to increase the speed and efficiency of circuit designs while still allowing compatibility with such gate voltages as silicon IGBTs, silicon MOSFETS, and silicon carbide MOSFETs.

“We're unique in the industry in that we're the only ones that make the JFET-based cascodes in silicon carbide," states Dries. “They're providing better performance for our customers compared to silicon carbide MOSFETs.” 

To optimize the high-temperature operation of all four devices, UnitedSiC used silver sintering for mounting the TO247 package to the ICs. 

UnitedSiC VP of engineering, Anup Bhalla, states what sets this product apart from others like it: “We have achieved the industry’s lowest RDS(on)​​​​​​​ for any device in this class." 

Dries likewise describes the new units as having "the lowest specific on-resistance of a silicon carbide power device on the planet.”

According to the company, the low RDS(on)​​​​​​​ characteristics of both SiC FETs can achieve efficiencies of more than 99% in inverter designs due to their reverse recovery performance, as well as low conduction drop-off while in freewheeling mode.

Because the RDS(on)​​​​​​​ is said to be so low, designers might use discretes in high-powered applications, where they might typically think about switching to a module. 

Together, these characteristics may allow inverter designers to extract power from their existing designs without needing to overhaul the circuit architecture.

If the UF3C series are used to create a synchronous rectifier to replace the secondary-side diodes, they can dramatically cut losses, thus reducing the cooling load on that charger.

UnitedSiC offers the contrasting example of a “100A operating current with a 50% duty cycle," in which "a JBS diode will have conduction losses of nearly 100W." The UF3SC065007K4S, on the other hand, can function "as a synchronous rectifier [which] will have conduction losses of 45W.”

By replacing the diodes with a FET, designers are expected to see voltage drop by nearly a volt, which can, in turn, save the engineer nearly half the losses. For fast-charging stations, this entails battery charging instead of heat generation.

This also makes it a useful option for bi-directional designs—allowing for both grid-to-vehicle designs and vehicle-to-grid designs.

Dries shared that the predominant feedback from engineers on the new SiC FETs is that they are easy to implement—particularly, in upgrading silicon IGBTs, silicon FETs, silicon carbide MOSFETs or silicon super-junction devices.

He explains, "When I ask engineers, 'Why did you choose our device over competitors?” they almost universally come back and say, “Because it was easy. I didn’t have to change my gate driver. I just took out my old FET and put in your FET and I was able to upgrade my design from X number of kilowatts to Y number of kilowatts.” 

Bhalla backs up this point, adding that "the standard drive characteristics and versatile packaging mean these SiC FETs can be used as drop-in replacements for less efficient parts in a wide variety of applications with little or no additional design effort.”

Including many die in parallel can be an obstacle to designing compact inverters. UnitedSiC claims that with these new devices, engineers can experience the same level of performance in one discrete that they might ordinarily achieve with many die in a module.

UnitedSiC states, “Careful loss calculations show that the combined switching and conduction losses of a 200kW, 8kHz inverter built using six UF3SC120009K4S SiC FETs in parallel will be about one third those of a similar inverter built using state of the art IGBT/diode modules.”

While it's clear that silicon carbide could never displace the value of silicon in a 500W power converter supply, silicon carbide is most effective in power converters that are typically a kilowatt and up. Yet, in the past, the high price of silicon carbide ruled it out as a reasonable option for many designers.

Dries posits, however, that the four new SiC FETs are driving price parity to silicon. For instance, UnitedSiC points out that the new SiC FET at 650 volts seems comparable in price with many silicon competitors, including Infineon's 650V CoolMOS.

With their ultra-low conduction losses, the SiC FETS can be utilized for several applications, the most obvious being for electric vehicles.

“We've been actively engaged with inverter manufacturers, OEMs, and others on designing these parts for traction inverters," said Dries. "Now with these super-low RDS(on)​​​​​​​ devices, we can build inverters in the 200- to 300-kilowatt range with simple discrete solutions."

Because the SiC FETs are designed to quickly turn off high currents, they can be used as a circuit breaker with a self-limiting feature that can control peak currents. This means they can also be used to limit inrush currents flowing into inverters and motors.

The UF3C series is also built for high-voltage battery chargers; the UF3SC65007K4S is said to provide increased efficiency in charging circuits over IGBT-based platforms for lower-voltage systems.

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As seen above, the device might also be useful to engineers in the aerospace industry with its ability to handle high surge currents and safely turn off in a few microseconds. 

What's your order of priorities when it comes to on-resistance, switching loss, and price? Does it vary for specific applications or designs? Share your thoughts in the comments below.

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