By Salil Chellappan, Systems Manager, Power Delivery Industrial Systems, Texas Instruments
What You Need To Know About
GaN for Power-Conversion
Main property considerations for selecting a Gallium Nitride device for power converter performance.
The availability of Gallium Nitride (GaN) devices for power conversion is improving, as many
manufacturers are coming out with catalog parts. The
application of these devices has gained the keen attention
of power-system designers from all over the world.
Although a lot of study has gone into understanding
their properties, the selection of GaN devices for various
power-conversion applications based on their properties
remains very misunderstood.
After much study dedicated to GaN as an alternative
to silicon in power switching, multiple manufacturers
now offer GaN switching devices for power-conversion
applications. However, you must look at the properties
of GaN devices in detail before assessing their suitability
GaN switching devices come in two different types
based on their internal architecture: enhancement mode
(e-GaN) and cascoded depletion mode (d-GaN). An
e-GaN switch operates like a normal silicon metal-oxide semiconductor field-effect transistor (MOSFET),
although it has reduced gate-to-source voltage levels.
An e-GaN device also has simpler architecture and
packaging, low on-resistance, and zero body-diode reverse
recovery (there is no body diode, but the channel itself is
bidirectional in nature and behaves like a body diode).
The first (and main) concern with this type of device is
the critical nature of its gate-drive design. The problem
is that the device’s fully enhanced gate-drive voltage is
very close to its breakdown voltage—the safety margin is
typically only about 1 V. This might cause a device failure
in the event of a voltage spike or parasitic ringing. Second,
the comparatively lower gate threshold voltage could
reduce noise margins. A third concern for these devices—
although not very serious—is the higher gate-leakage
current, which could increase gate-driver dissipation.
The depletion mode GaN device offers both
performance and manufacturing advantages. However,
its normally “on” nature may be a problem during power-up and other abnormal operating conditions. It also
requires the use of a negative supply. You can overcome
this problem by connecting the depletion mode GaN
high-electron mobility transistor (HEMT) in series with
a low-voltage silicon MOSFET in the cascoded d-GaN
structure. The gate of the HEMT is shorted to the source
of the MOSFET, while the HEMT source connects to the
drain of the MOSFET. As Figure 1 shows, the gate-to-source voltage of the HEMT is the source-to-drain voltage
of the MOSFET. This is so the silicon MOSFET can
control the turning on and off of the GaN HEMT.
Figure 1: Symbols of e-GaN and cascode d-GaN devices
highlight structural difference.
The main advantage of this structure is that the
complete cascoded d-GaN switch has the gate
characteristics of a low-voltage silicon MOSFET.