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Buck-boost transformer and PWM topology help OEMs operate equipment in
countries with mains fluctuating outside design limits
Background
Automatic voltage regulators (AVR) are used in countries with insufficient infrastructure
for the generation, transmission, and distribution of electricity; in industrialized countries
where grids are weak, and inside facilities with inadequate electrical wiring. Conventional
AVR technology has not kept pace with the requirements of the sophisticated electronics
it protects. The high-speed AVR recently developed by TSI Power Corporation and
Romarsh Ltd. addresses the new reality.
The electrical mains were designed to provide power to linear loads such as light bulbs
and heaters. The power they draw from the mains decreases with supply voltage,
mitigating some of the consequences of low supply voltage. Modern power converters
used in computing, telecommunications, and industrial equipment are based on the
principle of constant power. Where the current draw increases when the supply voltage
decreases, thus compounding problems within the distribution systems. Problems with
transmission and distribution are primarily found in developing countries, which tend to
have an inadequate electrical infrastructure. The problem also exists in industrialized
countries, but in a different form; namely, weak local distribution systems and/or
inadequate electrical wiring in some buildings create mains voltage stability problems.
Modern electricity systems are based on high-inertia generation, stiff transmission
backbone, and adequately sized distribution systems to permit very quick clearing of
local faults in order to prevent interruptions upstream. Various regulatory bodies and
standards organizations assume that the mains supply voltage is fairly stable since
specific standards are based on conditions within EU and the USA. Constant power
loads, whether linear (if power-factor corrected) or non-linear (such as power
converters), therefore, are designed to perform within set limits, i.e., 184 to 264V and
with permanent operation allowed between normalized 208 to 240V. The same is true
for inductive loads (e.g., air conditioners and other AC motors). Manufacturers of these
devices all operate in a competitive international market and thus will not over-engineer
power supplies using magnetic and semi-conductor components with higher voltage
ratings than actually required by the standard input voltage envelope. OEMs integrate
these products into their systems so as to be subject to the same limitations. Hence the
need for added mains voltage regulation when a system operates from an inadequate
mains supply.
One economical way to mitigate such problems is using a modern AVR. Unfortunately,
current AVR technology was developed years ago and has not kept pace with today’s
reality. The most prevalent technologies currently in use are of the servo and tap-
switching types. It is the end user’s responsibility to assess whether the mains supply is
of sufficient quality to provide power to sensitive equipment. The burden of selecting a
proper solution that meets technical requirements also falls on the end user, who may or
may not be qualified to deal with this--sometimes a consultant is hired to provide
recommendations. In order to shed some light on the relevant issues, this article will
discuss the advantages and drawbacks of these established technologies while
describing the newly developed, high-speed AVR.
AVR System Requirements
Regardless of technology, an AVR for today’s sophisticated equipment must operate
without causing disruption or additional problems to the user’s connected load. Such
problems may occur due to the inherent design of an AVR. The following performance
characteristics are deemed essential:
• Voltage correction must begin in 20ms as power converters typically have a hold-
up time of 20ms.
• Output regulation as a percentage of nominal supply voltage should be precise
• Low impedance to minimize load induced voltage swings.
• No breaking of power path during switching
• High efficiency
• Fail safe
• Automatic bypass in case of failure
• Reliability
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