Responding to positive feedback with more choice

  • Sep 13, 2021
  • UnitedSiC

Feedback is a good thing – control engineers want it to be negative with a decent gain and phase margin, but marketeers prefer positive, such as customer requests for more options in the 750V class of UnitedSiC Gen 4 SiC FETS.  The company has led the market with its 18 and 60 milliohm devices, but there are applications where yet lower resistance parts and higher intermediate values would be beneficial. It depends on the application and budget – a single premium SiC FET with the lowest on-resistance occupies the smallest space, requires minimal assembly but paralleled, lower cost parts with the same resulting on-resistance will often be as effective, especially when there is space to spread the heat out. However, this requires paralleling of devices and more design effort.

More parts added to the Gen 4 SiC FET range

To give this flexibility, UnitedSiC has made additional Gen 4 750V SiC FETs available with on-resistances of 23, 33 and 44 milliohms, along with parts at 6, 9 and 11 milliohms, a major improvement on the 18 and 60 milliohm devices already launched. Now, customers can ‘mix and match’ to get the optimum combination of price and performance for their particular thermal and operating conditions, perhaps leveraging volume pricing of the lower cost parts by making them common to different designs, with more or fewer in parallel as the application demands.

Paralleling SiC FETs is easy, gate drive power is so low that the extra from paralleling is normally inconsequential. Devices naturally share current due to their on-resistance positive temperature coefficient and there is the added, related advantage that, compared with say a single 9 milliohm device, two at 18 milliohms in parallel will have less conduction loss in total. This is because half the current in each of the two devices produces half the power and a lower temperature rise in each than a single device, and on-resistance rises proportionally less. Reliability naturally also improves with the lower junction temperature, giving even more headroom below the already high SiC maximum. The UnitedSiC FET-Jet web-based calculator, now at version 2, can be used to see the effect and put some numbers on actual losses and temperature rises for any number of paralleled SiC FETs in a variety of applications and operating conditions.

Gen 4 SiC FETS are still class-leading

The range additions have the same class-leading figures of merit as their stablemates and have the same advanced Gen 4 fabrication features, such as wafer thinning to reduce substrate conduction loss and silver sinter die-attach for lowest junction-to-case thermal resistance and consequential low junction operating temperature. The parts are available in TO-247 3-lead and additionally 4-lead packages, when a source ‘Kelvin’ connection is required. Figure 1 summarises where the new 750V SiC FETs sit, compared with the SiC MOSFET competition, remembering that these only have 650V rating.

Figure 1: The new UnitedSiC 750V SiC FET family additions in a competitive landscape

The flexibility of having a wider selection of parts in the UnitedSiC 750V SiC FET range means that they can be applied in many applications – with automotive AEC-Q101 qualification, they can be used in traction inverters, on- and off-board chargers, DC-DC converters and in wireless charging for a boost in efficiency and an improved voltage margin compared with 650V rated parts. The ultra-low on resistance now available is also valuable in AC and DC solid state circuit breaker applications where low conduction loss is critical for efficiency and small size. Generally, in industry and IT power conversion products, Gen 4 SiC FETS also improve performance in both hard- and soft-switched topologies.

So, keep the feedback coming – based on your needs, UnitedSiC have optimized their loop compensation to give a rapid response with even more options to come.