Evaluation Engineering - 24

POWER SUPPLIES

THE EVOLUTION OF AC POWER SUPPLIES
by Takemi Iguchi
Recently, the applications in which
AC power supplies are used have
changed. Until now, because it was mainly
used for testing home appliances and office equipment, a high power capacity of
several kW was required. However, with
the progress of electrification of automobiles and aircraft power systems, and the
increase in capacity of air conditioners
and power conditioners, electric power
of 20 kW or more is required. The trend
to increase the capacity of AC power supplies is expected to continue through the
near future.
To meet this trend, AC power supplies
essential for compliance test or endurance test are required to support the
demands for high capacity and multifunction. The performance of the large
capacity AC power supplies required to
test these applications should be the
same performance as the small capacity AC power supplies.

Problems with previous high
capacity AC power supplies
There are two different types of AC power
supplies, but each has problems when
used for evaluation tests such as EMC.
Linear type AC power supply
* Larger size
* Higher weight
* Lowest amplifier efficiency
* De-rating by low power factor
* Higher cost
PWM switched-type AC power supply
* Larger noise
* Slower response

Problems with lineartype power supplies
Low noise and high response speed are required for testing power supplies. Lineartype AC power supplies are widely used
in anechoic chamber and shielded rooms
as test power supplies. Due to the large
size of the equipment, a large footprint
is required. Furthermore, because of its

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heavy weight, portability is poor, as it is
difficult to relocate it once installed.
In the currently required applications,
the power capacity is increasing year-byyear, and it is necessary to increase the
power capacity by operating in parallel
and the like. And in that case, size and
weight become big restrictions.
Linear type is inefficient because of the
dropper method, where the loss of the amplifier part is large in the form of heat. As
a result, power consumption increases,
and exhaust heat treatment is required.
Therefore, in order to cool the installation
site, it is necessary to manage air conditioning, which requires cooling costs.
In the case of an AC power supply for
an anechoic chamber or a shield room, a
large line filter is inserted at the output to
remove noise. Linear-type power supplies
are inefficient at low power factor, and
equipment must be prepared according
to the efficiency. Therefore, it was necessary to prepare a model of power that was
larger than the power actually needed.
This further increases installation site issues, exhaust heat treatment issues, and
cost issues.

Problem with PWM switched type
On low-volume models, high-speed, lownoise, low-ripple products have been
present. But on high-volume
models, there are still problems such as high output
noise (conductive noise) and
slow response speed. Also,
output impedance was high,
and load fluctuation performance was poor. However,
because PWM switched-type
AC power supplies can clear many of the
problems that linear AC power supplies
have, these performance improvements
have been sought.

Performance improvement
of PWM switched type
Presently, even with a large capacity
PWM switched-type AC power supply

exceeding 10 kW, performance has been
at the same level as the linear type power
supply due to evolution of switching devices and the new topology:
* Low output distortion rate
* Low output impedance
* Fast response (<50us)

Evolution of power semiconductors
With the evolution of low-loss MOSFETs,
SiC, GaN, and other power semiconductors, faster switching has become possible. Faster switching frequency contributes to the miniaturization of LC filters
for making output waveforms sinusoidal.
Along with this, cutoff frequency can be
lowered, the amount of attenuation in
the attenuation region can be increased,
and noise at the switching frequency and
its harmonic noise can be reduced. This
achieves a low output distortion rate.
Moreover, low output impedance is able to
be realized by increasing amplifier gain.

Adopting a new topology
Multilevel-type inverters represented by
3-level inverters have many advantages
over conventional 2-level inverters. Since
the output waveform of a 3-level inverter
becomes closer to a sine wave, the LC filter for making output waveform sinusoidal can be miniaturized. In addition, since

the voltage fluctuation range per switch
operation is half that of a two-level inverter, switching loss generated in the
switch element is approximately halved,
and noise generated from the device is
reduced.
By interleave control of the inverter,
power supply is divided into two power
lines and the current phase has a phase

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