IEEE Power Electronics Magazine - September 2016 - 17

7,000

14,000

6,000

12,000

5,000

10,000

4,000

8,000

3,000

6,000

2,000

4,000

1,000

2,000

0

SJ Si
(IPL60R199CP)
(600 V)

Cascode GaN
(TPH3206LD)
(650 V)

RDS(ON) × COSS (tr) (mohm.pF)

RDS(ON) × QG (mohm.nC)

and internal overvoltage stress due to ac and dc mismatch
electrons in a sheet of charged carriers that are free to move
of the GaN device and the Si device, as well as additional
laterally with extremely low resistance. The lateral device
internal passives to mitigate these effects. This results in a
structure achieves extremely low gate charge (Qg) due to the
reduced overall yield and ultimately a higher cost.
very small fraction of the area covered by the gate. The output charge (QOSS) determines the amount of energy required
to switch the output state of the transistor and is also very
Device Performance
low, due to the small size of the transistor, the insulating
What does the physical device structure mean for power
GaN epi layer, and the short drain drift region enabled by
semiconductor performance? At a basic level, let's look
the high critical electric field. In addition, the lateral strucat a 650-V switch used for the primary of a 400-V input
ture, the isolated substrate, and the ability to electrically
dc-dc converter, for example, LLC. Two normalized figisolate one 2DEG region from another
ures of merit are considered: one
by shallow etching or implantation all
for drive input losses (R DS(ON) ×
work together to allow integration of
QG), the other for output switching
Lateral GaN offers
multiple power devices on a chip, as
losses (R DS(ON) × C OSS (tr)). As
the highest switching
well as drivers and other functions.
shown in Figure 3, eMode GaN techNormally on dMode devices renology shows a significant improvespeed and lowest
quire a negative VGS to turn off-not
ment over best-in-class superjuncswitching losses.
tion Si cascoded dMode GaN.
a practical solution for off-line appliSome eMode devices are offered in
cations. This unwanted characteristic
traditional through-hole packages such
was mitigated by the addition of a
as TO220 or TO247. Such devices can be difficult to drive due
second, low-voltage cascode Si FET [5], [6] used to turn the
to high inductance in the package, both in the gate loop and
GaN power device on and off. This essentially converts the
the drain loop. A high level of sensitivity exists at the gate due
dMode device into an eMode device, required to block high
to a low threshold voltage and limited VGS(MAX) for overdrive
bus voltages when a power converter is first turned on. The
cascode FET allows for a standard gate drive signal to be
above the target on-state gate voltage, so a very clean, low
used (0 V = Off). However, the Si FET frequency characovershoot gate voltage is required. These devices are very
teristics are inferior to GaN and compromise the switchsusceptible to voltage spikes that can easily occur due to
ing performance of the combined device. Another cashigh switching frequency and high-voltage slew-rate (dV/dt).
code variant uses the low-voltage FET simply as an on/off
This results in additional circuitry such as Zener clamps, ferswitch for the circuit, and then it drives the GaN transistor
rite beads, and negative gate drive, which increases compodirectly with a negative voltage drive, leading to a complex
nent count, printed circuit board (PCB) area, and cost, while
double gate-drive solution with two supply rails. Cascoding
also limiting the switching frequency of the circuit.
requires at least a two-chip package that presents a variety of other concerns including complex (multiple and/or
Monolithic: The GaN Power IC
stacked die) packaging, high parasitic inductance [7], [8],
The key to improved performance (reduced loss and higher
ceramic interposers for isolation, tendency for oscillation,
switching frequency) and rapid market adoption (simple,

eMode GaN
(Navitas)
(650 V)

FIG 3 Hard-switching (RDS(ON) × QG)- and soft-switching (RDS(ON) × COSS(tr)-related figures of merit for Si, cascoded SiC, cascoded
dMode GaN, and eMode GaN.

September 2016

z	IEEE PowEr ElEctronIcs MagazInE

17
http://www.mohm.pF http://www.mohm.nC
Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - September 2016
