IEEE Electrification Magazine - December 2013 - 60

Tek Stop
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higher-voltage GaN HEmT. However,
the result of the cascode configuration demonstrates inferior characteristics both in efficiency and switching
times as compared to GaN HEmT.
Recently, the development of
enhanced-mode GaN (e-GaN) FETs
revolutionized applications of GaN
technology in dc-dc power converters. These switches provide high performance while simply replacing
conventional normally off si switches. If these e-GaNs are scaled up in
power, they will be attractive alternatives for traction motor drives.

4
Gate Voltage

Drain Voltage
2.50 GS/s
4 50.0 V 40.0 ns
129.200 ns 10 k Points
T

Gate drivers

Another additional cost that must be
considered to migrate to widebandgap technologies in automotive
motor drives is the cost associated
Figure 1. The measured signals from a low-speed gate driver for SiC MOSFETs.
with replacing the gate driver circuits.
For commercially available siC mOswide-bandgap devices an attractive replacement for tracFETs, the recommended on-voltage is 20 V, which is higher
tion motor drives.
than that for the si technology. Conventional gate driver
auxiliary power supplies must be redesigned, and new gate
economics/manufacturing
drivers with higher voltage (and thus peak current) tolerCurrently, economic concerns are the main roadblock in
ances must be used. Gate drivers that fit these requirethe integration of wide-bandgap technologies in automoments exist; however, they are more expensive than
tive motor drives. siC technology is not compatible with
conventional si gate drivers.
conventional fabrication equipment in semiconductor
e-GaN switches require 5 V for optimal operations of the
production companies. Hence, to compensate for installswitch. To benefit from the fast switching times of this teching the new equipment, these switches carry a high price
nology, fast gate drivers are required. because of the fast
tag. The production processes for epitaxial-ready GaN subswitching times of GaN switches (i.e., <10 ns), conventional
strates with low defects are less mature than those for
low-speed optocoupler-based gate drivers cannot provide
siC. Therefore, the heteroepitaxy approach is common for
sufficient charge and discharge slew rates for maximum
commercial GaN switches. GaN-on-si switches can be
e-GaN speed utilization. moreover, because of the low
manufactured using the existing fabrication equipment.
threshold voltage of these switches, a robust gate driver is
The recent introduction of low-cost GaN-on-si switches
required to prevent any mistrigger due to common-mode
resulted in a leap in integrating wide-bandgap technolonoise and electromagnetic interferences. For these reasons,
gies for commercial products.
GaN-based converters must also use specialized gate drivsiC technology provides a straightforward replacement
ers at potentially greater cost than their si counterparts.
for conventional si switches in automotive motor drives
To demonstrate the challenges regarding the gate driver
with siC junction gate field-effect transistors (JFETs) and
of wide-bandgap technologies, a test scenario using siC
enhancement-mode mOsFETs. This is not the case for
mOsFETs is demonstrated in Figure 1. In this scenario, the
GaN technology. These switches were developed as high
gate driver is not capable of charging the gate capacitor
electron mobility transistors (HEmTs). The HEmT structure
with an acceptable slew rate. This is due to the large interwas first described in 1975 by mimura et al., who presentnal impedance of the gate driver at high frequencies. This
ed high electron mobility near the interface between an
impedance has resulted in the gate voltage sag due to the
aluminum GaN (AlGaN) and GaN heterostructure interface.
voltage variations of the drain. The capacitive current of Cgd
in combination with the voltage spike over the source stray
GaN HEmTs are normally on transistors. To provide a
inductance has resulted in momentary voltage sag of the
replacement for enhancement-mode mOsFETs, these
gate. This process introduces increased switching losses
switches are cascaded with an si mOsFET. This topology,
over the switch. It should be noted that the same gate drivwhich is known as cascode, enables the switch to benefit
er is capable of handling conventional low-speed si-IGbTs.
from the normally off characteristics of the si mOsFET as
This experiment verifies that high-performance gate
well as the high switching performances of the
1 2.50 V Ω

I E E E E l e c t r i f i c ati o n M agaz ine / december 2013

-800 mV

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2013