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Keeping It Clean
by Robert R. Close, Teal Electronics
Powering ATE is a dirty job, and don’t let anyone tell you to ignore it.
You’ve just won a program to replace an older weapons system with the latest
upgrades. But when it comes to support, you find marketing plans to update only
the ATE’s programmable supplies, computer, and instrumentation. The older system
had linear power supplies and data acquisition rates that really are in the
cellar.
New processors, programmable switching supplies, and high-speed Ethernet
controls will save you a bundle on throughput. But what will they do to noise
susceptibility?
Starting with a foundation of properly conditioned power will eliminate the
phantom failures that haunt most new projects. Providing the appropriate
conditioning, along with proper grounding, will effectively mitigate a future
breakdown of system components and shelter you from those beyond-your-control
environmental hazards that occur long after the system has been installed.
Present-day electronic systems and components are far more sensitive to noise
than their predecessors. To begin with, switching logic levels are down around 3
V on most of the new semiconductors, and it doesn’t take much to mistake a stray
harmonic for a timing pulse.
The vulnerability problem becomes more complex when you consider higher data
rates and DAC resolutions that have moved from 12 b to 20 b. High-speed
microprocessors manage PID loops with update rates that are multiples of those
used in the 1990s, and the high-speed switching circuits used in programmable DC
and AC supplies create nonlinear loading on your power bus at frequencies that
were never considered during the original ATE design.
These elements come together in ways that may bypass or even resonate with your
old power conditioners, making the test system even more susceptible to
electrical noise and transients. Noise and transients can trigger semiconductor
switching at the wrong time, interrupt digital clock timing or sequencing, or
cause errors in your acquired data. Best case, these things are merely
disruptive. Worst case, they are destructive.
Operating environments also are changing. ATE systems must frequently be
transportable, so weight is a major issue. Shelter-mounted systems like
clean-room systems are starved for space.
In many combat or Third World operations, your only power sources come from
Vietnam era gen-sets or local power of questionable quality. The
208-V three-phase power often looks more like 200 V, and the 60-cycle generator
you were promised turns out to be 50 Hz from the British battalion across the
field. What’s worse, about the time you run your lines across the ground to your
test site, some high powered transmitter turns on, radiating every inch of the
power lines each time it sweeps.
Facilities for ATE support everything from PCBs to jet-engine test cells. In
today’s world, more and more of them are being remotely located, and stability
of the power source often is an afterthought.
What are the symptoms? Phantom failures. You chase them from instrument to
controller and back again, replacing boards in your VXI chassis, switching power
supplies, and rewiring the backplane only to find the problem resurfaces on
another run.
System errors may become prevalent on your monitor. Warning signs, like
discolored capacitors or melted insulation on a neutral leg, point strongly
toward common-mode noise and high-impedance returns. Over time, you notice
unexpected performance degradation, a sure sign of transient problems, and
likely the loss of what transient voltage surge suppression (TVSS) protection
you may have had, a fault that generally is transparent to the system users.
The Problems
Where do power problems originate? Problems addressed by conditioners consist
primarily of transients in the form of ring pulses or unidirectional transients,
harmonics, filter resonance, high-impedance grounding, and brownouts. There are
four sources of the noise:
• Noise generated from the power lines as they distribute power to your
facility.
• Noise generated by your neighbors on the power grid, be they down the street
or in the next room.
• Load-side noises generated by test equipment within your system or even the
units under test and then fed back across the ATE power bus or ground to the
controller and displays that orchestrate your test program sets.
• The connections intended to provide clean, low-impedance power; the grounds;
and neutral returns can create problems that relate to wiring practices as well
as proximity of the power conditioner to what it is protecting.
Here are just a few types of power-related problems that can occur in your
application along with some commonly applied solutions:
• High-Frequency Electrical Noise—typically addressed with low-pass filters and
clean grounding
• Spikes and Transients—typically addressed with low-pass filters, TVSS, and
clean grounding
• Voltage Fluctuations—typically addressed with voltage regulation and
uninterruptible power supplies (UPS)
• Incorrect Line Voltage—typically addressed with step-up or step-down
transformers or voltage regulation
• AC Line Sags and Swells—typically addressed with voltage regulation and UPS
• Generally Unstable Power—typically addressed with filtering, suppression,
transformers, isolation, clean grounding, voltage regulation, and UPS
Managing these power problems is critical. You must know enough about the issues
presented by every new piece of hardware in your system to provide your specific
power subsystem with the right mix of solutions.
Source Saboteurs
Line-side noise generators like lightning strikes, peak loading due to regional
air-conditioning demands, and power-factor capacitor bank switching are examples
of sources of electrical noise on the line that can infiltrate your sensitive
electronics. Figure 1a shows severe voltage sags. Even if your regional
electric utility guarantees a level of power quality, it often has no control
over these transient issues.
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Figure 1a. Severe Voltage Sag on the AC Line Causing Random System
Dropouts or Brownouts |
Noisy Neighbors
Noise generated by co-users of incoming power can be frustrating because of the
social and technical problems it raises. The printing press next door, the
elevators in your building, the welders in the shop out back are examples of
sources of electrical noise that may come from co-users of your incoming power
lines, and they can all affect your sensitive electronic systems (Figure 1b).
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Figure 1b. Intermediate Frequency Noise Riding on the 60-Hz AC Power |
Productive Polluters
Productive polluters are noise sources found within the ATE systems and their
test loads. Switching power supplies, high-speed microprocessors, and high-power
test equipment are examples of sources of electrical noise that may be generated
by the equipment you are using to excite your test object and measure its
performance.
Although you have some control over these systems, you still must deal with the
disruptive and potentially damaging effects of the noise they create. Even if
you use a dual-conversion, online UPS to eliminate all of the line-side noise,
you have to live with these load-side perturbations (Figure 1c).
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Figure 1c. High-Frequency Noise and Transients Riding on the 60-Hz
AC Voltages, Often Load-Side Generated |
Since you cannot completely eliminate the source of these noises, the only
choice you have is to reduce them to the point that they are no longer
troublesome.
A Clean Sweep
Modern power conditioning and distribution units (PCDUs) can mitigate your power
problems with a single-box solution. All you do is connect the power you’re
dealt to the input and plug in the ATE.
The modern PCDU can contain one or more of the following power solutions in a
single package:
• Isolation transformers provide line-to-load isolation; a clean, single-point
ground; and common-mode noise rejection and often include voltage conversion and
world input voltage capability.
• Fast-response low-pass filters improve power quality by reducing electrical
noise spikes and transients (Figure 2).
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Figure 2. Teal Three-Phase Transformer Filtering Performance With
and Without TEALwave™ High Energy Filter |
• TVSS reduces large voltage spikes to a safer level. Table 1 lists
transient suppression performance.
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Table 1. Typical PCU Data Including Filtering and Metal Oxide
Varistors
ANSI/IEEE C62.41 and EN61000-4-5
Evaluation Baseline |
• Voltage regulation or stabilization helps to eliminate a slowly fluctuating AC
line. Modern regulation techniques include continuous subcycle voltage
regulation with full dual-conversion for the most complete method of AC power
stabilization.
• Fully integrated UPS are a cost-effective solution for the most critical parts
of your system.
 •
High power conversion from AC to DC for applications requiring regulated bulk DC
power can drive many DC-to-DC converters, replacing expensive programmable
supplies.
• Custom power distribution, specific for your equipment needs, includes circuit
breakers sized for each load.
• Internet-based, remote power monitoring enables you to quickly correlate a
system problem with a power problem. It also can settle issues of who’s at fault
when a system goes down.
What’s New
The advent of tougher agency controls, concerns about product liability, demand
for higher resolution, international applications, and the move toward
distributed DC programmable power have driven the power conditioner
manufacturers to offer many new options. Some of them enhance performance, and
others provide new levels of customer service and product support: remote
Ethernet monitors, redundant TVSS, regulated AC, low-impedance transformers, and
pulse power accommodation just to name a few.
Many of these new features are designed to improve overall system reliability
and uptime and assist in overall system diagnosis if problems do occur. Even
though AC power is a mature industry, modern technology improvements can be
tapped to improve your overall power quality and reduce the impact of an
otherwise dirty power source. Starting with a known clean power line will
shorten your development time, minimize your customer support costs, and add to
the overall success of your program.
About the Author
Robert Close is the military aerospace product manager at Teal Electronics. He
has spent 30 years in the military and aerospace power and controls market where
he has worked in engineering, quality, sales, and marketing. Teal Electronics,
10350 Sorento Valley Rd., San Diego, CA 92121, 858-558-9000, e-mail:
RClose@teal.com
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