IEEE Power Electronics Magazine - June 2021 - 76

Expert View
by Balu Balakrishnan
G aN Outperforms Silicon,
Enters Mainstream
H
ow does one define 'mainstream'?
In the case of gallium
nitride (GaN) power
semiconductor technology, we
believe that since you can now buy a
sleek, 65 W mobile charger using GaN
for under $25 on Amazon, it is reasonable
to assume that it has been fully
accepted by the consumer markets,
which-almost by definition-makes
it a mainstream product.
While the benefits of GaN have
been discussed regularly at conferences
such as IEEE APEC and PCIM for
well over a decade, until relatively
recently it was positioned as a solution
for an exotic corner of the market that
leveraged the speed of GaN devices.
As size and efficiency demands
become mainstream, GaN is now
becoming an excellent solution for
power applications. For reasons of
efficiency, size and robustness, GaN is
simply a better switch than silicon, so
whether the requirement is for ultracompact
mobile device chargers or a
power supply for an appliance that
removes the need for bulky heatsinks,
or even within electric vehicles (EVs),
GaN is an optimal solution.
GaN Outperforms Silicon
Why do we say that GaN is 'a better
switch'? If we consider a regular MOSFET
using silicon technology, we see
that the total resistance between the
Digital Object Identifier 10.1109/MPEL.2021.3075805
Date of current version: 16 June 2021
drain and source, Rds(on) and output
capacitance, Coss, are inversely proportional
to each other. As the die size
increases, Rds(on) goes down, but Coss
increases. Although the same trend
is true for GaN, device characteristics
yield dramatically lower Coss
and Rds(on) per unit die size. Table 1
normalizes these characteristics and
compares them to an equivalent GaN
switch, Power Integrations' PowiGaN
technology. The table shows that the
Rds(on) for a given size switch is much
lower, as is the Coss, when compared
to a conventional switch. Switch transition
times can also be much faster.
Since switching loss is caused by
parasitic capacitance, Coss directly
influences the amount of power lost
during switching. The bigger the
MOSFET, the more the switching loss.
However, as we have already noted,
the bigger the MOSFET, the smaller
the Rds(on) loss. Therefore, these two
parameters are working in opposition.
Figure 1 plots the graph of conduction
losses and switching losses
against device size, showing that as
the device gets larger, conduction loss
reduces and switching loss increases.
The total power loss is a sum of those
two elements.
The grey curve represents the losses
in a silicon transistor, and the losses
for a GaN FET are shown in red.
Both are optimized at 65 watts across
line voltage and full load. We can see
that the total losses for a GaN switch
are significantly lower than for a silicon
transistor of equivalent size-perhaps
as much as 50% at highline operation.
That's a significant reduction in
the amount of heat generated from
the design. This enables designers to
switch much more efficiently in a
given package size and reduce the
size of their power supplies by eliminating
heatsinks.
Why Is Efficiency Important?
A one percent increase in efficiency
is of huge importance. In a typical
adapter that meets EU efficiency
rules, every one percent increase in
efficiency represents more than 10%
reduction in heat generation, eliminating
the need for a heatsink or heat
spreader. It allows the designer to
Table 1. Comparing performance characteristics of silicon MOSFET
and PowiGaN.
Technology
MOSFET
PowiGaN Switch
76 IEEE POWER ELECTRONICS MAGAZINE z June 2021
RDS(ON)
1
0.07
Normalized Per Unit Area
COSS
1
< 0.18
Switch Transition Time
1
0.1 to 0.05
Gate Voltage
VGS(ON)
~5 V
~5 V
IEEE Power Electronics Magazine - June 2021

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