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Technical Article
Surging Across the Barrier:
Digital Isolators Set the
Standard for Reinforced
There are a variety of reasons for requiring galvanic isolation
in electrical systems. The most obvious—and most critical—
is the protection of human operators from potentially lethal
shocks. Shock hazards can come from the mains power that
the equipment is plugged into, or from high voltages
generated within an enclosure. A reinforced insulation rating
is required when humans are being protected from
potentially lethal shock.
In large industrial settings there may be potential differences
between physically separated ground points that may create
unwanted current flow. Ground loops can also lead to noise
and hum in a system.
Optocouplers have traditionally been used to isolate
potentially dangerous voltages in many types of electrical
equipment. Today, more modern digital isolators based on
transformer or capacitive coupling are widely used.
Transformer-based digital isolators offer many benefits such
as improved performance, integrated functionality, lower
power dissipation, better longer-term reliability, and
improved ease of use.
Isolation can be required for several reasons. It allows
functional circuit operation when disparate ground
references or supply voltages are used. Equipment operators
or medical patients must be protected from shocks or
dangerous currents during long-term system operation.
Damage to sensitive and/or costly systems due to electrical
surges—such as lightning strikes—must be prevented.
Over time, national and international standards were
developed with the goal of providing uniform specification
and testing of isolators and electrical systems employing
isolation. Safety certifications can be achieved both at the
component level and for the end system, and certification
requirements can vary in different regions of the world even
when the same base safety standards are referenced. Isolators
that meet a superset of safety standard specifications provide
maximum flexibility to equipment vendors in meeting these
varying requirements.
High reliability or high availability systems often require that
individual circuit faults be contained such that overall
system operation is not impaired beyond an acceptable level.
Isolation can be used in these systems to contain faults so
neighboring circuitry remains operational.
Different portions of an electrical system may have
unreferenced grounds or be referenced to unrelated highside supply voltages, again leading to a need for isolation.
Finally, noise in a particular portion of a circuit may need to
be contained to eliminate interference with sensitive
The International Electrotechnical Commission (IEC)
publishes a number of international standards related to
electrical safety, while national bodies such as Underwriters
Laboratory (UL) in the US and Verband der Elektrotechnik
(VDE) in Germany publish regional specifications. Testing
and certification to the standards is provided by several
entities, including UL, VDE, the Canadian Standards
Association (CSA), and Technischer Überwachungs-Verein
(TÜV) in Germany. The choice of which certification to
obtain depends on the region where the component or
Table 1. System-Level Standards Relevant to Applications Requiring Isolation by Market and Region
IEC 60065
IEC 60204
VDE 860
UL 60065
IEC 60950
Measurement and
IEC 61010-1
IEC 60601
IEC 60950
EN 60950
VDE 410/0411
VDE 0750
VDE 0804
UL 508, UL 60947
UL 60950
UL 61010
UL 60601
UL 60950
CSA. 14-10
CSA 60950
CSA 61010
CSA 601
CSA 60950
June 2012 | Page 1 of 5
©2012 Analog Devices, Inc. All rights reserved.
Technical Article
system will be sold, as well as the end user. In some cases an
equipment vendor may require certification to an IEC
standard from a specific company (for example, TÜV) even
if certification to that same standard has already been
obtained through another agency (such as CSA). This is
sometimes based on preference or previous experience, but
there can also be differences in the certification levels that
drive these requirements.
Standards bodies have been working to harmonize the
international and regional documents to simplify the
certification process. This is a slow process due to the
number and complexity of the standards. Until this is
achieved, component suppliers are best served in meeting a
superset of the various requirements to provide maximum
flexibility in terms of worldwide sale of the end equipment.
Further complicating the situation is the fact that there are
system-level and component-level standards for isolation.
The system-level standards most relevant to applications
requiring isolation are:
This approach provides a system isolation specification that
flows down to the component level. System requirements are
used to define the isolation characteristics of the individual
parts that comprise the system. Following the system
standard should result in a known level of safety in the final
system design.
Standards that apply to specific isolation components are
IEC 60747: Semiconductor Devices—Part 1: General
UL 1577: Standard for Optical Isolators
VDE 0884-10: Semiconductor Devices—Magnetic and
Capacitive Coupler for Safe Isolation
These standards certify that a digital isolator component
meets particular safety requirements, but will not guarantee
the isolation level of the overall system. Determining
suitability of individual digital isolators is left to the system
designer based on the overall safety requirements.
Another consideration is how the various parameters are
handled by each agency. For example, UL 1577 documents
the creepage and clearance achieved and verifies survival to
the specified withstand voltage. There is no specific
requirement on creepage or clearance so long as the
component passes the test. On the other hand, IEC 60950
mandates specific creepage and clearance requirements
based on the working voltage. The component must meet
©2012 Analog Devices, Inc. All rights reserved.
the creepage and clearance for a given voltage regardless of
the test results.
Some of the component-level standards are written
specifically for products utilizing optical communication
across an isolation barrier. These products, known as
optocouplers, have been in use for some time and are
governed by the IEC 60747 and UL 1577 standards. Modern
digital isolators that use transformer or capacitive coupling
across an isolation barrier are now widely used as an
alternative to optocouplers. The standards bodies have
begun to catch up to technology changes in isolation
products. For example, VDE 0884-10 was released in 2006 to
address components utilizing transformer-based and
capacitive isolation. The IEC is currently working to
harmonize these standards.
Several physical and electrical isolator characteristics are
specified within the various safety standards. The insulating
properties of the component, as well as the physical
dimensions of the packaging and insulating barrier, are
specified in order to withstand specific voltage stresses.
Different stresses are defined based on magnitude and
Working Voltage
Working Voltage is a continuous dc or ac voltage that the
component is specified to endure over its lifetime. IEC
60950 specifies three levels for working voltage: 250 VRMS,
320 VRMS and 400 VRMS.
Withstand Voltage
Withstand Voltage—also called Isolation Voltage—is an
overvoltage condition that the component will survive for up
to one minute. UL 1577 withstand voltage ratings of 1 kVRMS,
2.5 kVRMS, 3.5 kVRMS and 5 kVRMS are common.
Surge Voltage
Surge Voltage defines survivability after a repetitive series of
short duration high voltage pulses (see Figure 1). An isolator
must pass 10 kV surge voltage testing to achieve a reinforced
insulation rating to the VDE 0884-10 specification. The
ability to pass this test is primarily determined by the
insulation thickness (also known as Distance Through
Insulation or DTI) and the quality of the insulating material.
The applied electric field tends to concentrate at defect
points within the insulator, so lower defect densities
generally lead to higher breakdown ratings. Thicker
materials are more resistant to breakdown since the field
June 2012 | Page 2 of 5
Technical Article
Figure 1. Surge Voltage Waveform
strength is inversely proportional to the distance between
the conductors on either side of the insulation.
Optocouplers commonly meet this requirement because
DTI is typically 400 µm, which reduces the impact of
insulation quality on the breakdown characteristics. Simply
put, the insulation is so thick that a high quality material is
not required to pass the 10 kV test. Transformer-based
digital isolators use a high quality 20 µm polyimide layer
deposited in a clean room environment. Since this material
has a much lower defect level than the injection molded
epoxies used in optocouplers, a much thinner layer can still
meet the 10 kV requirement. Capacitive isolators also use a
high-quality insulating layer, in this case silicon dioxide
(SiO2), deposited during wafer fabrication. Silicon dioxide
has a high dielectric strength, but typically can’t be deposited
in very thick layers without creating mechanical stress
within the film. Thicker SiO2 also reduces the capacitance,
which in turn reduces the coupling efficiency across the
barrier. For this reason, capacitive isolators typically will not
pass the 10 kV surge test and, therefore, can’t be certified by
VDE as reinforced insulation.
Voltage levels may be specified as a peak voltage or in rms
terms, so careful attention must be paid to the details within
each standard.
June 2012 | Page 3 of 5
©2012 Analog Devices, Inc. All rights reserved.
Technical Article
Clearance (through air)
Creepage (across package body)
Figure 2. Creepage and Clearance
There are two basic levels of insulation specified within the
safety standards. Functional insulation is that which is
required to allow a circuit to operate properly. An example
would be isolating different ground potentials between two
circuits to avoid an overvoltage situation. Functional
insulation does not provide protection from shock.
Insulation that does provide protection from shock is
referred to as basic insulation or reinforced insulation in the
IEC standards. Basic insulation provides protection from
shock for users of the end equipment. Reinforced insulation
is a single insulation system that provides protection equal to
two redundant single insulation systems. Safety regulations
frequently require either redundant single insulation or
reinforced insulation to protect users from lethal shocks.
Using reinforced insulation is more practical since
redundant single insulation systems require power between
the two isolation boundaries.
The required physical distance across the isolation barrier is
determined primarily by the desired voltage ratings, but the
characteristics of the environment play a part as well. Two
dimensions—creepage and clearance—are used to describe
the isolation distance. Creepage is the shortest distance
along a solid surface across the isolation barrier, while
clearance is the shortest line-of-sight path through air across
the barrier.
©2012 Analog Devices, Inc. All rights reserved.
Figure 2 shows how creepage and clearance are defined for a
gull-wing style package such as a SOIC (small outline
integrated circuit). An important consideration is that
metallic stabilizing bars visible on the package ends must be
subtracted from the creepage measurement.
Conductive paths will develop along a surface in the
presence of an electric field and electrolytic contaminants.
This process is referred to as tracking and is characterized by
the Comparative Tracking Index (CTI). The CTI for a given
material is the voltage that causes tracking with a specific
quantity of an electrolyte on the surface. Higher CTI values
indicate that a material has more resistance to tracking, and
will allow a lower creepage value.
The final characteristic that determines creepage
requirements is the level of contamination in the
environment, referred to as Pollution Degree. Environments
are classified into four groups based on the amount of dry
pollutants and condensation present. Higher levels of
contamination and condensation result in higher creepage
requirements. Tables within the standards are used to
determine the creepage requirements once the voltage
ratings, CTI, and pollution degree are known.
June 2012 | Page 4 of 5
Technical Article
Galvanic isolation is essential in many electrical system
designs. International and regional standards have been
developed to ensure uniform specification and testing of
isolator components and systems. There are several
technologies available to achieve the necessary isolation for a
particular system requirement, and each has its own
strengths. Isolators that meet the superset of international
and regional standards—including 10 kV surge testing—
provide maximum flexibility to equipment vendors in
meeting these varying requirements.
For more information on digital isolators, please visit
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©2012 Analog Devices, Inc. All rights reserved.