IEEE Power Electronics Magazine - March 2015 - 39

inductance and the total winding loss of transformers optimized for GaN and silicon as well as an ungapped baseline
case, each evaluated at 80% load.
It is clear that the fringe-field losses are a significant
component of the loss in an LLC converter and can be very
difficult to estimate without numerical simulation tools
such as FEMM. GaN can help to improve the efficiency of
an LLC by requiring a significantly smaller core gap in an
otherwise-identical design.

Device-Specific Advantages of GaN
in a Resonant LLC Converter
While we see that the reduced output capacitance of
fig 4 The FEMM simulation of EQ25/LP estimation using the
GaN allows for lower circulating current losses and
gap size for GaN. Note the fringing flux extending toward the
lower fringe-field losses in LLC converters, GaN also exwindings near the gap. The colors represent the magnitude of
hibits other practical advantages in an LLC converter.
the magnetic flux density.
These advantages, which depend on the implementation
of the GaN device, include lack of reverse recovery and a
of the devices' output capacitance. The performance
low-hysteresis, near-lossless output capacitance.
advantage of soft-switching converters assumes that
One of the failure modes of LLC converters is losing
all the energy used to charge a device's output capaciZVS over some fraction of the line and load range. LLC
tance can be recovered by dischargconverters are thermally designed
ing that capacitance using a current
for soft-switching operation, and
source. There are two known effects
if operated out of ZVS, they will
The faster the switching
that can prohibit recovering all the
quickly exceed their thermal limits.
frequency, the lower the
energy from the output capacitance.
This failure results from the reverse
Many superjunction transistors
recovery of the primary-side transisoptimal magnetizing
suffer from output charge hystertors at light-load conditions [7].
inductance, and the
esis [9], which causes the amount of
During each on time, first the
energy recovered from the output
primary-side conducts current in the
larger the gap. GaN can
capacitance to be less than the energy
third-quadrant mode while the load
help to improve the
used to charge that capacitance. The
current builds and the magnetizing
device complexity of these modern
current reverses. At light load, the folefficiency of an LLC by
superjunction transistors, to achieve
lowing first-quadrant current may not
requiring a significantly
superior on-resistance to area metric,
be able to fully reset the body diode
of the primary-side FETs. If the body
seems
to worsen the output charge
smaller core gap in an
diode is not fully reset, the following
hysteresis. To eliminate this loss, a
otherwise-identical
transition time is extended to allow
simple structure, such as a planar
design.
the diode to reset. If the controller
MOSFET or a lateral GaN device,
does not adapt for this load-depenmust be used. While planar MOSFETs
dent dead time, the opposite transishave inferior figures of merit comtor can turn on with partial or no ZVS. This can contribpared with superjunction MOSFETs, today's GaN devices
have superior figures of merit with relatively simple device
ute a significant loss and cause failure. In the case of full
structures.
hard switching, a switch turning on against a nonrecovered
Another consideration when using GaN in an LLC conbody diode can be detrimental.
verter has to do with the GaN device structure itself. Many
For silicon-based LLC converters, the use of the ultramanufacturers [2]-[4] are utilizing depletion-mode (d-mode)
fast body diode transistors is recommended to prevent this
devices in a cascode structure to create a normally off
failure mode [8]. However, these transistors typically have
inferior output capacitance and are more expensive than
structure. There are a number of potential issues with
standard superjunction transistors. Because GaN devices
switching a cascode structure, one relating to the charge
have little to no reverse recovery charge, they will not cause
sharing between the GaN FET and the low-voltage silicon
the dead time to expand during light-load conditions. This
FET [10]-[12]. When the device is turned off and its output
will allow for a simpler control algorithm and will be more
capacitance charged by a current source, that charge must
tolerant of occasional hard-switching events.
be handled equally by the C OSS of the silicon device and
An additional loss mechanism for soft-switching
the C DS of the GaN device. In the case of the C OSS of the
converters, such as the LLC, is related to nonidealities
silicon device, integrated between the GaN's threshold and
March 2015

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Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - March 2015