IEEE Power Electronics Magazine - March 2015 - 40

QDS, GaN
Qav
VDS, GaN
VDS, Si

GaN Self-Discharge

Silicon Device Avalanche

Charge
Voltage

Qav

QOSS, Si

Time

Time
fig 5 The charge balance in a soft-switched cascoded GaN device.

fig 6 The scope plot of LLC voltage waveforms (using directdrive d-mode GaN).

this method increases the C OSS of the cascode device and
the silicon's VDSS^bdh , is less than the GaN device's C DS , integrated from zero to the total blocking voltage, the silicon
negates some of the performance advantages. The best
device will avalanche.
method is to directly switch the GaN device. By directly
The avalanche of the silicon device can create a reliabilswitching a GaN device, whether an enhancement-mode
ity issue as well as contributing to an additional loss: charge
device or a d-mode device controlled using an intelligent
imbalance loss at ZVS turn on. During avalanche, the differgate driver, this avalanche loss can be eliminated while
preserving the switching performance advantages of GaN.
ence between the two output charges, denoted Q av, is lost
from the lower device's Q OSS . At this point, the GaN device
has Q av more charge than the charge stored on the silicon
Experimental Results
device above the GaN's threshold voltage, as shown in FigThe performance advantages of GaN discussed in this
ure 5. On the turn on transition, where the resonant inducarticle are validated with an LLC converter prototype. The
design of this LLC converter is discussed in [11]. The
tor discharges the output capacitance prior to the device
specifications are shown in Table 2.
turning on, the silicon device's output
The target application for this concapacitance will now be discharged to
verter is high-voltage dc distribution
the threshold voltage prior to the GaN
The performance
in server and telecom environments.
device being totally discharged. Now,
advantages of GaN
The converter was built using
the GaN device will turn on to disboth
superjunction silicon MOSFETs
charge its output capacitance to regdiscussed in this article
and a proprietary depletion-mode
ulate the VDS of the silicon device to
are validated with an
GaN device. The GaN device was
approximately equal the threshold of
driven using a discrete driver implethe GaN. While the charge dissipated
LLC converter
mentation. Besides the primary-side
in the GaN device is equal to Q av , the
prototype.
transistor and gate driver, the only
energy will be significantly higher due
difference between the two impleto the higher VDS of the GaN device.
mentations is the size of the gap in
This avalanche loss can be elimithe LLC transformer.
nated in cascode-connected GaN by adding an additional
A switching frequency of approximately 350 kHz is chocapacitor in parallel with the silicon device [10]. However,
sen to optimize the efficiency-size tradeoff. While GaN is
capable of higher switching frequencies, the circuit losses
Table 2. The parameters of the
are dominated by the magnetics and circulating currents
LLC converter prototype.
-
in the circuit. Increasing switching frequency beyond 350
Parameter
silicon
gan
kHz in this design will yield lower efficiency. The converter
Primary FETs
199 mX IPL60R199CP 140 mX d-mode GaN
design between the GaN-based converter and silicon-based
Secondary FETs
1.2 mX 30 V CSD17335 NexFET (three paralleled)
converter remained the same except for the modification
Transformer
16:1 turns, eight layers, 4 oz copper
of the gate-drive circuit in addition to reducing the size of
EQ25+PLT 3F35 core
the magnetic core's gap.
Switching frequency
352 kHz
327 kHz
The waveform from the LLC converter under full load is
Resonant inductor
1.8 nH SER1360
shown in Figure 6. The transition period is much sharper
Resonant capacitor
50 nF
50 nF
than that from the silicon-based converter, as the output
Magnetizing
82 nH
128 nH
capacitance of the GaN device varies much less over VDS .
inductance
Due to parasitics in the circuit (e.g., secondary leakage

IEEE PowEr ElEctronIcs MagazInE

z March 2015

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