Page 7 - PowerUp 2021 Conference Proceedings for PEN

WIDE BANDGAP

Efficiency Gains in Traction

Applications Using

UnitedSiC FETs and JFETs

By Anup Bhalla, vice president of engineering at UnitedSiC

Semiconductor switches used in traction applications are critical components for achieving high
efficiency. The latest semiconductor technology allows high switching frequency, and in most
other power-conversion applications, this results in the benefit of smaller magnetics, traded
off against some extra switching losses. However, in drive inverters, the magnetic component
is the motor, which is sized for torque and power. Higher efficiency of low switching frequency
generally outweighs other factors. As a result, IGBTs switching at about 10 kHz have been widely
used, with their track record of ruggedness, low conduction losses, and reasonably low dynamic

losses.

MOVING ON FROM IGBTs

Efficiency targets get progressively tougher, and understandably so — every fraction of a percent
gain forms part of a virtuous circle of smaller size, weight, and cost, as well as longer driving range.
Figures achieved of about 98% with IGBTs are creditable, but there is pressure to improve further.
The residual switching losses can be reduced by using silicon or silicon carbide MOSFETs, but
historically, they have had higher conduction losses than IGBTs at high power levels. This stems
from their on-resistance, dissipating power that scales with the square of the current. IGBTs have

a relatively fixed saturation voltage, so there is a crossover point at high power where IGBTs still
win. To put it in perspective, at 500 A, a MOSFET would need an on-resistance of about 3 mΩ
at the operating junction temperature for conduction loss comparable to an IGBT. In EV traction
applications, devices need ratings of 650 to 750 V, and that on-resistance is not yet achieved at
those voltages with single Si or SiC MOSFET devices. Paralleled MOSFETs are a solution, but then
costs and complexity spiral.

To achieve the lowest conduction loss, UnitedSiC FETs can be considered — a cascode combination
of a SiC JFET and a Si MOSFET. Given that conduction loss dominates in a motor drive, and the
price of power semiconductors is generally proportional to total chip area, a useful comparative
measure of switch performance is the figure of merit R × A. This is the product of on-resistance
DS
and chip area for a particular voltage class. UnitedSiC FETs have the best R × A across current
DS
technologies, including wide-bandgap types. As with UnitedSiC MOSFETs or gallium nitride FETs,
the low reverse-recovery energy of the intrinsic diode eliminates the need for an external anti-
parallel diode, as in IGBT-based motor drives, thus further saving cost. UnitedSiC FETs also compete

12 FEBRUARY 2022 | www.eetimes.com