September 2016
26
P
ower
E
lectronics
Improving power supply density and
efficiency with GaN power transistors
By Eric Persson,
Infineon
For power supply applications
requiring higher density, GaN HEMTs
enable much higher operating
frequency without increasing
switching loss. This article describes
an LLC circuit which, compared to Si
superjunction FETs, operates at 3X
higher frequency with no increase
in power loss. This combination of
factors is necessary to drive power
density to higher levels for the next
generation of power supplies.
Power supplies today are quite efficient
compared to their counterparts 10 or 20 years
ago. It is not uncommon for power supplies
to feature energy efficiency of >90% over a
wide operating range. Premium performance
power supplies with 12 or 48 V outputs for
server and telecom applications commonly
have peak efficiency of 94%, and 96% is also
available. And just emerging onto the market
are ultra-premium power supplies with peak
efficiency of 98%. Clearly the efficiency can-
not continue to increase at a linear pace, as it
must ultimately asymptote to 100%, a value
we will never achieve.
But efficiency is only one aspect of power sup-
ply figure-of-merit. The other two key attri-
butes are power density and cost. These three
attributes – efficiency, density and cost – bal-
ance each other for a particular power supply
technology. For example, improving either
efficiency or density will come at a price. Or,
for a given price-point, increasing efficiency
generally comes at the expense of density.
Every once in a while, a new technology
comes along and shifts this balance, enabling
new levels of performance and cost previously
not possible. Thirty years ago, it was the sili-
con power MOSFET that emerged to enable
switchmode power supplies with far greater
performance than the previous bipolar junc-
tion transistors of the day could achieve. Sil-
icon carbide Schottky diodes began to make
an impact around 15 years ago, and are now
widely used as high performance rectifiers in
efficient power supplies.
In 2016 it is now gallium nitride (GaN) mak-
ing high-performance transistors that are
again changing the landscape. Specifically, it is
enhancement-mode lateral GaN HEMTs (high
electron mobility transistors) that are making
the biggest difference. Compared to the near-
est silicon alternative, the superjunction MOS-
FET, GaN transistors have far lower charge on
the gate and output for a given on-resistance.
Moreover, GaN HEMTs have no parasitic body
diode. The poor reverse recovery performance
of high voltage FET body diodes limits their
applicability to topologies that do not require
body-diode commutation. GaN HEMTs on
the other hand have a diode-like behavior in
the reverse direction with zero reverse recovery
behavior, so they are ideally suited for high fre-
quency applications, even those requiring body
diode commutations.
To begin with, let’s look at the general goals
of power supply manufacturers. For all but
the most demanding applications, efficiency
in the range of 95% is already good enough
using actual silicon FETs. The next goal is
density, how to shrink the envelope for exist-
ing power levels, or more commonly – how
to pack more power into the existing size
box. The key to improving density is to sub-
stantially increase the operating frequency
of the power converter. At higher frequency,
the magnetic components need fewer turns
of copper, less magnetic core volume, and are
therefore smaller. Even the capacitors that
manage the high frequency ripple current
shrink in size. The only capacitors that do not
shrink are those required for hold-up time.
But how is it possible to increase frequency
without additional switching loss? The answer
lies in the control strategy for a particular
topology. Regardless of which transistor tech-
nology is used, zero voltage switching (ZVS) is
one of the keys to minimizing switching loss
and enabling higher frequency operation. The
majority of power supply topologies are based
on the concept of using transistors to switch a
voltage source into an inductive load. The goal
Figure 1. Power MOSFET
turn-on switching loss during
hard-switching in a typical
CCM boost PFC circuit
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