Unusually for SIC transistors, the gate is fully compatible with existing IGBT drivers, and has a 5V gate threshold – avoiding accidental turn-on issues associated with the lower thresholds of SiC mosfets.

Called UJ3C120040K3S, its gate characteristics come from the cascode-connected pair inside the TO-247 package – a technology initially common with early SiC power transistors, before SiC mosfets became more popular.

In this kind of cascode, a high-voltage SiC JFET is operated by a low-voltage silicon mosfet (see diagram) – it is the conventional silicon mosfet gate that is connected to the outside world.

UnitedSiC, a spin-out from Rutgers University with years of SiC research behind it, is championing SiC JFETs because the technology needs far less SiC area than an equivalent SiC mosfet, and needs no special drivers. Its arguments are presented here.

Unlike some other cascode devices, the firm has not integrated an off-the-shelf Si mosfet die, but designed a custom device to match the needs of its SiG JFET – also a custom design. In the JFET, source-drain capacitance is designed to be very low, to prevent over-voltage on the mosfet drain during switching – a possibility with poorly-paired cascodes.

Compared with its earlier devices, company v-p engineering Anup Bhalla told Electronics Weekly, package thermal resistance has been halved – junction to case resistance is now typically 0.27°C/W – up to 65A can be handled at 25°C, at which temperature 175A pulses are also possible.

Cascode switches have the disadvantage that they switch fast, sometimes causing EMC issues through high dV/dt and dI/dt figures.

In this case, said Bhalla, the cascode pair has been designed to switch at a speed range that matches the characteristics of the package and the intended applications: power factor correction (PFC), active front-end rectifiers, LLC converters and phase-shift full bridge converters.

A range of speed adjustment is available by altering the gate drive resistor, he added, although not as much as with a SiC mosfet or Si IGBT.

For other applications, as they occur, UltraSiC can design faster or slower devices – anywhere from something to switch motor windings at 10kHz to devices that can complete with GaN power HEMTs, said Bhalla.

The firm has seen its earlier devices designed into on-board electric vehicle chargers and photovoltaic inverters, amongst other things, he said, and the gate characteristics have made them popular as drop-in replacements for Si IGBTs, Si mosfets and SiC mosfets – for evaluation and production.

No external inverse parallel diode is needed, and reverse voltage drop across the in-built structures, which are fast and rated for full current, is ~1.5V – lower than with SiC Schottlys, added Bhalla.

A second member of UJ3C1200 series, also new, is the UJ3C120080K3S, which is broadly similar to the above …40K3S, but with 80mΩ on-resistance and lower current handling.

UnitedSiC will show the 1,200V devices at PCIM 2018 on the Ecomal Europe booth (7-406), and will take part in two panel discussions in booth 155 hall 6.

• ‘Challenges and opportunities facing power supply manufacturers over the next 5 years’
  12:00-13:00 Tue 5 June,
• ‘SiC – devices for the future design’
  Wed 6 June