Page 8 - Power Electronics News - December 2020

Semiconductors
Figure 1: UnitedSiC 750V Gen 4 SiC FET on-resistance per
unit area compared to 650V rated SiC MOSFET competitors. compared to 650V rated SiC MOSFET competitors.
ful comparison. The same UnitedSiC 18 milliohm
device as in Figure 1 is compared in Figure 2 with
SiC MOSFET alternatives and the Gen 4 SiC FET
advantages are clear, 50% better FOM at 25°C and
40% at 125°C. In hard-switching applications such
as the ‘Totem-Pole PFC’ or in standard 2-level
inverters, stored charge in the body diode is also
important and is very low in SiC FETs, with just a
small contribution from the integral low voltage Si
MOSFET. Although there is no parasitic body diode
as such in the integrated JFET, there is a reverse
conduction path through the JFET channel and
the body diode of the co-packaged Si MOSFET.
The body diode drop appears as the sum of a 0.7V
knee voltage from the Silicon MOSFET, and the
ohmic drop of the JFET channel, amounting to
1.3V or less. This compares with a typical 650V SiC
MOSFET body diode which can drop more than 3V
with hundreds of nanocoulombs of stored charge.
As shown in Figure 3, soft-switching applications
such as in LLC and PSFB converters also benefit
from using SiC FETs – while output capacitance
C OSS(TR ) is not discharged rapidly in these circuits,
it does introduce a delay at the switch turn-off
edge as it charges, which can limit maximum
useable frequency. As one of the attractions of
8 DECEMBER 2020 | www.powerelectronicsnews.com
Figure 2: UnitedSiC 750V Gen 4 SiC FET R DS(ON) , EOSS FOM
WBG device switching is to operate at higher
frequencies to reduce size, cost and weight of
associated components, particularly magnetics,
the extra low C OSS(TR) of Gen 4 SiC FETs is a dis-
tinct advantage.
Gen 4 SiC FETs from UnitedSiC retain the ma-
jor advantage that they can be safely driven
from fully off to saturation using a 0V-12V gate
drive with a maximum of +/20V, clamped by
ESD protection diodes. The threshold voltage of
5V allows unipolar operation. The device short
circuit current is controlled by the JFET channel,
which has a largely temperature-independent
threshold. Therefore, the short-circuit capabil-
ity becomes independent of gate drive voltage
above 12V, a behavior quite unlike SiC MOSFET
and IGBTs. The Miller effect is effectively absent,
avoiding the possibility of spurious turn-on and
packages offered include Kelvin connections to
avoid source inductance being included in the
gate drive loop which might otherwise couple
transients into the gate drive. As with earlier SiC
FET generations, the high maximum gate drive
voltage allows parts to be retrofitted into exist-
ing circuits to change out IGBTs, Si-MOSFETs and
SiC-MOSFETs with little or no modification to
Semiconductors
ing established Si MOSFETs and IGBTs. This is
achieved with parts in standard discrete pack-
ages making them easy to engineer into new and
existing designs.
The excellent efficiency gains realized with Gen
3 SiC FETs from UnitedSiC promoted widespread
adoption of the technology and now Gen 4 is
delivering an entirely new level of performance,
enabled by advancing fabrication and packaging
technology. With parts rated at 750V, designers
now have additional headroom over traditional
650V rated Si and SiC MOSFETs typically used in
Figure 3: UnitedSiC 750V Gen 4 SiC FET R DS(ON) , C OSS(tr) FOM bus applications at low and medium power lev-
compared to 650V rated SiC MOSFET competitors. els. With figures of merit that excel in all areas
the gate drive arrangement, for an instant boost and convenient, thermally enhanced packages,
in performance. As a bonus, total gate charge of Gen 4 is set to enable new standards of efficien-
SiC FETs is a few tens of nanocoulombs, so that cy and power density from chargers, rectifiers,
even when switched at high frequency, gate drive PFC stages and DC-DC conversion in all power
power required is minimal. Also, as in previous conversion and storage applications.
generations, Gen 4 SiC FETs have robust ava-
lanche and short circuit ratings.
The advantages of Gen 4 750V
SiC FETs are summarized and
compared with competing SiC
MOSFETs in the radar plot
of Figure 4. Parameters that
affect hard and soft switching
efficiency are shown at low
and high temperatures with the
UnitedSiC parts winning over
SiC MOSFETs in all respects
Figure 4: UnitedSiC 750V SiC FETs comparative advantages with key parameters
and certainly out-perform- normalized (note: lower values are superior).
For More Information
▶ UnitedSiC
▶ UnitedSiC Design Resources
DECEMBER 2020 | www.powerelectronicsnews.com 9