Selecting a Linear of PWM Power Source

Selecting a Linear of PWM Power Source
POWER SOURCES
Selecting a Linear or
PWM Power Source
by Mitchel Orr, Pacific Power Source
U
nderstanding the capabilities and differences of linear and
pulse-width modulated (PWM) AC
power sources is especially helpful
when determining which models meet
your requirements. And, the selection
of an AC power source should be based
on more than just three catalog specifications of rated voltage, frequency,
and power.
Critical operational capabilities include bandwidth, current, and regulation. Other application-specific requirements are size, weight, operating
temperature, and cost.
For many midrange applications,
either linear or PWM switchmode
technology will provide equally satisfactory performance. In other more
stringent applications, only one of the
two technologies will best meet an
operator’s particular needs.
Within the United States, commercial supplied AC power is convenient,
reliable, and usually stable. By intent,
supplied AC is inflexible. Commercial AC is neither flexible enough nor
noise-free enough to allow precision
measurements. However, many tests
require a wide range of controlled
voltages and frequencies. Other tests
must support startup surges and the
measurement of harmonic currents.
Unpredictable variations make repeatable controlled tests impossible.
As a result, a precision AC power
source becomes an obvious solution.
A well-instrumented AC power
source can deliver precisely managed
power to fully characterize a UUT.
• E E • November 2008
It may be used to present a range of
voltages and currents to determine
steady-state power needs. In addition, transients, harmonic waveforms,
and other voltage perturbations may
be added.
These features support limit testing and verification of operational
extremes for a UUT. Using the AC
power source’s built-in measurement
features, the loading characteristics of
the UUT can be analyzed.
Load and Performance Testing
Requirements
A rugged power source is essential
for use in the production environment.
A production source must be convenient to use and deliver the prescribed
wave shapes, frequencies, and power
to the UUTs.
For general-purpose testing, ease of
reconfiguration is important. Reconfiguration capabilities include voltage
range, frequency range, number of
output phases, and execution of preestablished voltage limit tests.
Many loads are not resistive. As a
result, an applied voltage and load
current may not be in phase. Out-ofphase operation is identified here as
low power factor. By definition, power
factor is the ratio of real to apparent
power but most often observed as the
phase angle difference between voltage
and current waveforms.
Additionally, a load may demand
peak current many times greater than
that predicted by average power con-
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POWER SOURCES
Noisy
Unregulated
AC Input
Single-Phase
or
Three-Phase
AC/DC
and
Energy Storage
Well-Defined
Well-Regulated
A
Waveform
and Timing
B
C
Linear
Amp(s)
AC Output
Single-Phase
or
Three-Phase
Figure 1. AC to AC Linear Amplification Power Conversion
sumption. A high peak current condition is identified by the crest factor,
which is the ratio of peak current to
an AC waveform’s rms current.
Finally, in many applications, the selected AC power source must provide
a reservoir of energy. Reserve energy
is necessary to meet startup surge current requirements without distorting
the voltage waveform.
Overview of Power Conversion
Electrical power converters transform electrical energy in one form into
electrical energy in another form. An
AC power converter can be thought
of as a unit with only input and output terminals. From this simplistic
perspective, the input-to-output process appears to be simply AC to AC
power conversion.
However, from the implementation
perspective, internal power management involves AC to DC conversion
at the input. The output requires DC
to AC conversion. Any wide-range AC
to AC power source includes intermediate processes of AC to DC and DC to
AC conversion. These are not academic
considerations. The efficiency of each
conversion process has implications
for weight, size, temperature rise,
and cost.
More efficient conversion processes including amplification facilitate
smaller, lighter weight, and cooler
running units. Less efficient conversion
processes result in larger, heavier, and
hotter running units. It follows that the
efficiency of each type of amplifier is a
significant factor in determining size,
weight, and temperature rise.
AC to AC power conversion provides a reliable source of operator-controlled AC power. The AC
mains provide basic power. The
converter delivers a tightly controlled synthesized waveform at the
prescribed voltage, frequency, and
power level.
• E E • November 2008
Basic AC to AC Conversion
The reason for using either linear
or switching technology depends on
the specific application. For example,
consider a nonsinusoidal unity power
factor load with a high crest factor. This
load requirement is best satisfied by
a linear amplifier. High-power linear
amplifiers can deliver peak currents
with undistorted voltage waveforms.
For another example, consider a
load with a very low power factor.
Low power-factor requirements are
more easily satisfied by PWM switchmode amplifiers. Switchmode amplifiers can deliver full current in all
four quadrants.
Technological Considerations
Consider the individual testing requirements to determine whether linear
amplification or PWM switchmode operation provides superior performance.
In addition, careful evaluation of requirements will determine which of the
two technologies provides the more
cost-effective solution. Operational
requirements include the following:
• Fast transient response
• High crest factor
• Low output impedance
• Low power factor loads, both nonlinear and reactive
• Startup surge current
• Size and weight
Noisy
Unregulated
AC Input
Single-Phase
or
Three-Phase
AC/DC
and
Energy Storage
Pulse Width
Modulator
advantage of linear amplification
is faithful reproduction of the oscillator waveforms.
However, linear amplifiers have the
disadvantage of being very inefficient.
The types of linear amplification include Class A, B, and AB. The letters
refer to the signal conduction angle.
Class A linear amplifiers typically operate at less than 50% efficiency. Class
B or AB can achieve peak efficiencies
greater than 50%.
As a consequence of low operating efficiency, size and weight can
become major issues for linear power
sources. Keep in mind that linear AC
power sources feature full power
wide bandwidth, excellent transient
response, and the lowest possible
output impedance.
The characteristics of linear-amplifier technology include the following:
• Very low output distortion
• Wide output bandwidth
• High crest factor handling for a
wide range of loads without waveform
distortion
• Wide range of active output impedance control (optional)
• Higher temperature operation due
to Class A, B, and AB amplifier inefficiencies
• Larger size due to increased component count
• Higher weight due to increased
component count
Figure 1 illustrates the operation of
an AC to AC linear power source. At
the input, single-phase or three-phase
AC is converted to DC. Following
rectification, filtering removes AC
ripple, broadband noise, and intermittent transients.
A
B
C
Bipolar
Power
Amp(s)
LPF
Well-Defined
Well-Regulated
LPF
AC Output
LPF
Single-Phase
or
Three-Phase
Waveform
and Timing
Figure 2. AC to AC Switchmode Power Conversion
Linear AC Power Source
Linear AC power sources produce
low-distortion output waveforms.
Linear amplification is achieved by
using nonsaturating methods. The
Energy storage overcomes the effects of dropouts and line sags. The
stored energy is subsequently used
for powering the output amplifier.
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off. Consequently, less power is
dissipated in the amplifier than
DC Supply ATE Tests
Best
for linear amplifiers with full400 Hz, Synchronous ATE System
Best
range conduction cycles.
R&D Power Line Disturbance Tests
Best
Switchmode amplification
Watt-Hour Meter Testing
Best
is called Class D. Class D
Power Line Disturbance Tests
Best
amplifiers provide an output
with high harmonic content.
Production Life Tests (frequency conversion)
Best
An output-stage low-pass filCircuit Breaker Tests
Best
ter removes high-frequency
Safety Compliance Tests
Best
distortion. The output of the
Commercial Appliance Test and Burn-In
Best
low-pass filter is an amplified
version of the input signal.
Motor Performance and Safety Tests
Best
Figure 2 illustrates the opTable 1. Typical AC to AC Power Source Applications
eration
of an AC to AC switchIn a particular situation, either amplifier type may be more
appropriate.
mode power source. Input
power processing and oscillator
Feature/Capability
Linear Switchmode
signal generation are identical
Highest Amplifier Efficiency
Best
to those of the linear AC power
Lowest Operating Temperature
Best
source. However, for switchmode conversion, the lowLowest Weight
Best
level analog signal is not sent
Smallest Size
Best
directly to a linear amplifier.
Lowest Cost
Best
Rather, it is sent to the input of
Low-Power Factor Handling
Best
a pulse-width modulator, which
Lowest Harmonic Distortion
Best
operates at a free-running freHighest Small-Signal Bandwidth
Best
quency many times greater
than the highest frequency in
Highest Large-Signal Bandwidth
Best
the input waveform.
Table 2. Feature/Capability of Each Technology
With zero analog signal
amplitude input, the output of the
Simultaneously, a low-level controlled
modulator is a rectangular wave with
waveform is generated by an oscillator
equal positive and negative periods.
in the power source.
Consequently, for zero input, the output
As a practical matter, typical waveof the low-pass output filter is exactly
forms are stored as digital samples.
zero volts.
Consequently, synthesized wave
As the width of the modulated pulses
shapes are identical regardless of outfollows the polarity and amplitude
put frequency. Finally, the waveform
of the input signal, the filter’s output
is amplified to the required level of
becomes an amplified version of the osvoltage and power. When required
cillator input signal. Because the output
to manage complex loads, the linear
is derived from pulse-width samples,
amplifier’s output impedance can be
there is a larger fractional percentage
fixed or managed through controlled
feedback. The linear amplifier’s wide
200
bandwidth is ideally suited to faithfully
delivering complex waveforms.
150
Application
Linear
Switchmode AC Power Source
Switchmode AC power sources use
a combination of linear and nonlinear
methods to achieve waveform amplification, including PWM, nonlinear amplification, and low-pass filtering. Switchmode amplifiers are highly efficient
because they are either fully on or fully
Switchmode
distortion than with a linear Class A,
B, or AB amplifier. But, the output
stage efficiency is much greater than
for linear amplifiers, typically 80% or
more. Lower power loss in the output
stage results in cooler operation and
smaller size components.
The characteristics of switchmode
technology include the following:
• Moderately low output distortion
• Capability to provide full current into
very low power factor reactive loads
• Capability to provide full current over
full voltage range without derating
• Moderately wide output bandwidth
• Moderate range for active output
impedance control
• Lower weight due to higher amplification efficiencies
• Smaller size due to smaller/fewer
components
• Lower temperature operation due to
higher amplifier efficiencies
• Limited ability to reproduce complex transient waveforms
Comparison of Features and
Capabilities
Table 1 lists typical applications
for AC to AC power sources. Linear
technologies provide superior performance for some testing applications;
PWM switchmode technology excels
for other applications.
Table 2 lists generic benefits of each
of the two technologies. No single
technology performs well in all areas.
And remember that over-specifying
may lead to avoidable cost, weight,
and environmental concerns.
Figure 3 demonstrates the peak current capability of a typical linear amplifier driving a nonlinear load. It depicts
100
10
8
6
4
50
2
0
0
-2
-50
-4
-100
-6
-150
-8
-200
-10
Volts Phase 1
Nonlinear Load Driven
With Linear Amplifier
Vrms
Irms
Ipeak
Icrest
kW
kVA
PF
124.20
2.77
7.71
2.78
0.23
0.34
0.66
Amps Phase 1
Figure 3. Peak Current Capability of Linear Power Source
• E E • November 2008
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POWER SOURCES
200
8
150
6
100
4
50
2
0
0
-50
-2
-100
-4
-150
-6
-200
-8
Volts Phase 1
Nonlinear Load Driven
With Switchmode
Amplifier
Vrms
Irms
Ipeak
Icrest
kW
kVA
PF
122.46
2.58
6.70
2.59
0.21
0.31
0.68
Amps Phase 1
Figure 4. Peak Current Capability of Switchmode Power Source
the output voltage and current waveforms of a typical nonlinear load.
Of particular note is the peak current
demand of 7.71 A pk creating a 2.78:1
crest factor even though the rms current measures at less than 3 A rms. By
driving this particular load with a linear
amplifier, the output voltage waveform
remains undistorted, allowing unrestricted evaluation of the UUT.
In Figure 4, the same load was
connected to a switchmode AC power
source. Here we see clipping on the
peak of the voltage waveform as the
current demanded by the load exceeds
the crest factor capability of the amplifier. This clipping is a direct result
of the increased output impedance of
the power source.
The increased impedance served
to reduce both the peak current and
the current crest factor. Had this power amplifier been used to evaluate
the UUT, the rms and peak current
demand would be underreported and
the load-induced voltage distortion
overstated.
• High peak current for nonlinear
(high crest factor) loads
• Phase angle of output current (power
factor)
• Accurate replication of custom or
high harmonic waveforms or both
• Fast transient capability
• Amplifier output voltage distortion
• Amplifier output impedance and
control
• Size, weight, and efficiency limits
• Environmental needs and limits
• Performance vs. price considerations
Given a set of realistic requirements,
you can make an objective decision
about which technology provides the
appropriate solution. When the exact
requirements are not obvious, knowledgeable vendor application support
often is the best path to success.
About the Author
Mitchel Orr is the sales manager
for Pacific Power Source. Mr. Orr, a
U.S. Navy veteran, began his career
at the company in 1986 and since then
has worked as a test technician, an
in-house and field service technician,
an applications engineer, and product
marketing manager. He earned a business degree from National University.
Pacific Power Source, 17692 Fitch,
Irvine, CA 92614, 949-251-1800, email: [email protected]
Conclusion
Under conditions of midrange performance, switchmode sources are
more cost-effective. For the absolute highest midrange performance,
linear amplifiers are more capable.
There is no single-parameter, right
or wrong solution when it comes to
selecting a linear or a switchmode AC
power source. The appropriate solution
depends on the full set of require­ments
including:
• Output voltage range
• Output current, including inrush
and overload
• E E • November 2008
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