[pcbforum] IsolationTransformers
- From: "vishwa" <vishwa@xxxxxxxxxxxx>
- To: <pcbforum@xxxxxxxxxxxxx>
- Date: Wed, 18 Sep 2002 11:49:30 +0530
IsolationTransformers Increase Safety of Electronic Systems
Adequate isolation between a power source and a user of electronic equipment
ensures the safety of that equipment. Given the high voltages that exist in
modern electronic equipment, proper isolation protects an operator from contact
with excessive electrical energy should a short circuit occur in the equipment.
Isolation transformers have represented a traditional solution for providing
high isolation in electronic circuitry. Even with the increased use of
efficient, switched-mode power supplies (SMPS), isolation transformers can
improve the overall isolation of an electronic design without severe penalties
in added size, weight, and cost.
Isolation transformers offer an effective means of meeting the requirements of
domestic and international safety standards for electronic equipment. In the
United States, for example, such standards are set by the Occupational Safety
and Health Administration (OSHA), with product testing performed according to
appointed laboratories, such as Underwriters Laboratories (UL). Throughout
Europe, safety standards are established by the International Electrotechnical
Commission (IEC), with testing performed by the laboratories of individual
member nations, such as the Verband Deutscher Electrotechniker (VDE) in
Germany.
Isolation transformers enable a variety of electronic systems to meet safety
requirements. Such systems include medical diagnostic equipment, computer
systems, and telecommunications equipment. The systems may incorporate linear
power supplies, SMPS, and sometimes a combination of both. A single isolation
transformer can help an electronic design meet all of its isolation
requirements. With proper system design, an isolation transformer can also help
reduce the size and cost of the power-electronics components following it in a
design.
Understanding UL
Several techniques commonly provide isolation when designing electronic
equipment. Fuses or circuit breakers, for example, can protect both the
equipment and its operator from overvoltage conditions or surges of
high-voltage energy. Careful component placement and printed-circuit-board
(PCB) layout can provide adequate room for creepage and clearance of components
in close proximity of high voltages.
Creepage is defined as the shortest distance between two conductors, measured
along the surface of the insulators. Clearance is the shortest path through the
air between two conductors that must be isolated. Each component subject to
creepage and clearance must meet the requirements in UL, CSA, VDE, or other
applicable standards. An isolation transformer can reduce the impact of meeting
these requirements by reducing the line voltage from hazardous to nonhazardous
levels.
Decreasing the need to consider creepage and clearance in an electronic design
can improve a product's time to market, simplify its circuit layout, and reduce
its cost. An isolation transformer is considerably more effective than a
full-wave bridge rectifier in screening electronic equipment from high input
voltages. Unfortunately, an isolation transformer can add cost, weight, size,
and increased cooling requirements to a design. But it represents a reliable
solution for increased isolation, even for systems employing switching power
supplies.
Switching power supplies convert AC voltage to DC voltage directly in an
off-line rectifier followed by a capacitive filter. The converted high voltage
is switched at frequencies from thousands of times per second (kilohertz rates)
to millions of times per second (megahertz rates). Usually, semiconductor
devices, such as silicon bipolar transistors or silicon
metal-oxide-semiconductor field-effect transistors (MOSFETs) are used to switch
the voltage waveforms on and off. The output voltage of a switching power
supply is proportional to the pulse width of the switched or chopped waveform
and the duty cycle of the pulse wavetrain. By varying the pulse width of the
output waveform, the output voltage can be automatically adjusted.
A large transformer is not needed in a SMPS to achieve the same levels of
isolation and voltage step-down functions compared to a lower-frequency
50/60-Hz linear power supply with the same power rating. As a result, switching
power supplies are smaller, lighter, and dissipate less power than equivalent
linear regulated power supplies. Because of this, SMPS have long been used in
airborne, military, and space applications where weight and size were key
design requirements.
When used with a switching power supply, an isolation transformer can prevent
higher-order harmonic signals from degrading the performance of adjoining
circuitry. This is especially important in computers or other equipment
incorporating microprocessors, which rely on harmonically rich, high-frequency
clock signals for their timing. Improperly isolated, these harmonic signals can
appear as interference to other functions in the system, even resulting in
excessive output-voltage ripple in the power supply.
Isolation transformers are specified in terms of the amount of isolation that
they provide, usually given as the root-mean-square (RMS) voltage, as well as
the power rating, in terms of volts-amperes (VA). Additional specifications
include efficiency (in percent) and the tolerance of the voltage regulation (in
percent).
Switching power supplies can be designed with either internal or external
isolation transformers, although greater isolation is achieved by means of the
latter approach. When an external isolation transformer is placed within an
electronic design in series with a switching power supply, the output voltage
from the transformer is reduced to a level that is no longer hazardous to the
operator of that equipment. Because there is no hazardous voltage after the
transformer, the subsequent circuitry is below the voltage threshold (30.0 V
RMS or 42.4 VDC peak open circuit for Class 2 circuits) required for circuits
to meet creepage and clearance electronic safety requirements. Because the
transformer provides adequate isolation within a single component, there is no
longer a need to achieve distributed isolation throughout the circuitry of a
product. Because the voltage following the transformer is low, smaller
components (such as inductors and capacitors) can be used throughout the
remaining circuitry. In many cases, standard off-the-shelf switching power
supplies can be used in the design because of the relaxed creepage and
clearance requirements of lower-voltage circuitry. Isolation transformers that
meet international safety standards can be specified for use with both linear
and switching power supplies, with a variety of power ratings. For example,
low-profile isolation transformers in Signal Transformer's International
Flathead series meet a wide range of United States and international standards,
including UL 506, IEC 950, as well as German VDE standards and Canadian CSA
standards. The transformers, with heights as low as 0.69 in., have standard
isolation of 4000 V RMS and can be supplied with ratings from 2 to 30 VA with
dual primaries of 115/230-V, 50/60-Hz operation. These compact transformers are
ideal for applications on densely packed PCBs.
At higher power levels, the company's MultiPurpose Isolation (MPI) and High
Power International (HPI) isolation transformers operate at line frequencies of
50/60 Hz with a power range of 200 to 3500 VA. Designed for use with UPS
designs and linear power supplies, these transformers comply with UL, CSA, VDE,
and IEC safety specifications. A typical 1-kVA unit features 3-percent voltage
regulation and 96-percent efficiency.
Isolation transformers represent an effective means of achieving high isolation
in distributed-power systems, such as computers and telecommunications systems.
In a typical distributed power system, multiple DC-to-DC converters, rather
than a single, centralized power source, provide voltage and current to the
system's subsystems and circuits. Small, efficient converters can typically
generate 200 W or more at a specific location, helping to overcome voltage
drops common when power is transmitted over a distance within a system. By
locating converters on each of the system's circuit boards, the system can be
assembled in a modular fashion, speeding and simplifying manufacturing and
testing processes. Thermal design is simplified in the same way, since heat is
distributed throughout the system, rather than concentrated in one location.
Even though isolated DC-to-DC converters can be used to achieve high isolation
in such a modular, distributed-power architecture, they are expensive compared
to nonisolated converters. A better approach is the use of nonisolated DC-to-DC
converters where necessary in a distributed-power system, with a single
isolation transformer providing the necessary high-voltage isolation. In this
way, each DC-to-DC converter need not meet the high-voltage isolation,
creepage, and clearance safety requirements for a particular United States or
international electronic safety standard. A single isolation transformer can
provide the isolation and the low-voltage transformation to simplify the safety
requirements of subsequent circuitry.
Otherwise, each converter or separate power supply must be specified to
applicable United States and international safety requirements, greatly
increasing the overall cost of the equipment.
Isolation transformers are commonly used with linear power supplies to improve
the amount of isolation in the overall circuit. But such transformers can also
pay huge dividends when incorporated into high-frequency switching power
supplies. They can improve the isolation of a design, as well as enable the
overall power-supply circuitry to be made smaller, lighter, less complicated,
and less expensive.
In Europe, the IEC either directly or indirectly sets the electrical safety
standards for a great many individual nations. The IEC's chief standard for
Safety Isolation and Safety Isolating Transformers is the IEC 1558 (recently
replacing the IEC 742). In contrast, the European Community (EC) version of the
IEC 1558 is EN61558. An additional IEC standard, IEC-601-1, is generally
accepted throughout Europe as the standard by which medical electronic
equipment must comply (such as UL 544 in the United States and C22.2 No. 125 in
Canada).
As with the UL requirements, IEC 950 specifies the amount of leakage current
that can be allowed while still gaining certification. Per IEC 950, leakage
current should not exceed 3.5 mA for Class I machines and 0.25 mA for Class II
machines. Class I electronic products that are designed for hand-held use must
be limited to 0.75 mA or less leakage current.
In order to simplify the design and manufacture of electronic products in
Europe, a great deal of consolidation has taken place in the European
electronic safety standards. The EC standard EN 60950 is an example of this
trend. It is designed to provide an umbrella standard that safety agencies in
various countries can use either as is or with modifications that satisfy local
needs.
Consolidation of standards is also affecting countries such as Germany, well
known and respected for its own safety standards per the Verband Deutscher
Electrotechniker (VDE). Even though the organization has seen many other
nations adopt many of its reference standards, including VDE 0805 and VDE 0750,
even Germany is moving toward adoption of the IEC and EN standards as
consolidation continues.
The expected end result is a set of electrical safety standards which is
uniformly adopted and recognized throughout all Europe.
Worst-case conditions should always be considered when trying to comply with
one or more international standards. The following recommendations can be
applied to the selection of a transformer for working voltages of 250 V or
less. For example, a transformer should be specified with high dielectric
strength of 4 kV or more. This ensures that the level of isolation will meet
general as well as specific medical standards. Transformers should also meet
minimum requirements for creepage and clearance, say 10 mm. The transformer's
minimum insulation temperature should be at least +130 ºC, and the
primary-to-secondary current leakage should be no more than 30 µA.
By meeting these minimum provisions, and evaluating related requirements, such
as the type of operating environment (indoors, outdoors, surrounded by
hazardous materials, etc.) and the number of fuses and circuit breakers in the
remaining circuitry, product developers can ensure compliance with a large
number of international standards for operating voltages of 250 V or less.
Strategies can be applied similarly to higher or lower operating voltages.
Meeting minimum requirements may add some expense to a design. But failure to
achieve minimum requirements for a safety standard can be costly in terms of
redesign time and lost time to market.
Other related posts:
- » [pcbforum] IsolationTransformers
