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Topic 1
Safety Considerations in Power Supply Design
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Safety Considerations in Power Supply Design
Bob Mammano, Texas Instruments
Lal Bahra, Underwriters Laboratories
A
BSTRACT
Increasingly, the responsibilities of a power supply designer extend beyond merely meeting a functional
specification, with designing to meet safety standards an important collateral task. Since all commercial
and home-use supplies must eventually be certified as to safety, knowledge of the requirements should
be a part of every designer’s repertoire. This simplified overview has been prepared with the
collaboration of Underwriters Laboratories, Inc. to provide a basic introduction to the issues and design
solutions implicit in assuring the safety for both the user and service personnel of your power supply
products, as well as easing the certification process.
I. I
NTRODUCTION
It should come as no surprise that safety is an
important issue in the design of any electrical
equipment which is liable to come into contact
with a human operator or servicing individual.
And this issue should be even more obvious
when the equipment is designed to operate from a
source of power which could experience or
deliver voltage levels that could be hazardous to
the human body. Recognizing this, a large
collection of design standards and certification
processes have been developed to define the
requirements for insuring the safety of power
supplies. A thorough treatment of this
information takes much more space and time than
available within the context of this seminar
program – it is a subject more commonly taught
with the dedication of one to two full days of
presentation – however it is hoped that the brief
overview provided herein will be useful in
describing the basics both for designers who may
have in-house resources with more detailed
expertise, or who plan to follow up with
attendance at a more in-depth program as those
presented by Underwriters Laboratories, Inc.
Note that UL60950-1 (including Annexes P
and Q) was used as the source for all numbers
quoted here but many of the conditions and
contingencies have been excluded in the interests
of simplicity. Reference to the complete and
latest revision of the appropriate standard should
be made for any and all design decisions.
1-1
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II. P
RINCIPLES OF
S
AFETY
While one would expect that a safety standard
for power supplies would be dominated by
consideration of electrical hazards, this is not the
only aspect of power supply design affected. A
more complete listing of safety issues could
include the following:
•
Electric Shock:
This is the shock hazard
resulting from the passage of electric current
through the human body. The physiological
effects can range from perception or a startle
involuntary movement, all the way to
ventricular fibrillation or, ultimately, death.
•
Energy Hazards:
Even at voltages too low
to produce a shock, burns can be caused when
metallic objects such as tools, jewelry, etc.
get very hot or melt and splash when they
bridge sources with high VA potential
(typically 240 VA or more).
•
Fire:
Fire is normally considered as a
secondary effect from overload, abnormal
operating conditions, or fault in some system
component. However induced, it should not
spread to adjacent components or equipment.
•
Heat Related Hazards:
High temperatures
on accessible surfaces or components under
normal operating conditions.
•
Mechanical:
Injury or damage resulting
from contact with sharp edges or corners,
moving parts, or physical instability.
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While not usually associated with power
supplies, other hazards that might need
consideration could include the effects of
radiation, chemicals, or hazardous vapors.
There are at least two types of persons whose
safety needs must be considered: users (or
operators) of the equipment, and service
personnel. Users are not expected to identify
hazards, and must not be allowed contact with
hazardous parts. This is normally accomplished
through the use of such means as enclosures or
other protective shielding. Service personnel, on
the other hand, are assumed to have access to all
parts of the system and for their safety, the
requirement is to identify the hazardous
components or areas and to ensure against
inadvertent contact with a hazardous surface, or
bridging a tool between parts with high energy
levels while working in another part of the
equipment.
In addition to the types of personnel coming
into contact with the equipment, there is an
additional consideration as to the end use for a
power supply. Power supplies can typically be
divided into two categories:
•
•
whether the supply is to be sold as a stand-
alone item, or
as a component to be installed into a specific
system or equipment.
III. S
AFETY
S
TANDARDS FOR
P
OWER
S
UPPLIES
Safety standards, like most standards
affecting electrical equipment, were originally
very specialized and unique to a given country.
The driving force for a unified standard was
primarily the information technology industry
whose efforts led to the first international
standard for safety, IEC950, prepared by the
International Electrotechnical Commission (IEC).
With the release in the late 1980s of UL1950, UL
expanded the scope of IEC950 to include
electrical business equipment along with ITE, but
this standard excluded telecommunication
equipment. In the meantime, however, a working
group of the IEC (TC-74) had generated a
harmonized standard, IEC60950 (third edition),
to cover products from all three industries and,
upon its release in 1999, it was quickly adopted
by most countries and is today the primary
standard for safety for most, but certainly not all,
users of power supplies. In addition to IEC,
designations of this standard can be found as EN
(European Union), UL (United States), and CSA
(Canada). In the USA, the plan is to withdraw
approvals to all earlier standards by July, 2006.
The US National Standard, as of this writing, is
UL60950-1, first edition, published in November,
2003.
While UL60950-1 is the most widely applied
standard for power supplies today, it is intended
for use with information technology, business,
and telecom equipment. Other standards exist for
other industries, such as IEC 60065 for audio and
video, IEC 60601 for medical, IEC 61010 for
laboratory supplies, and others. Further efforts at
harmonization are under way with a sub-
committee of the IEC (SC22E) proposing a new
standard, IEC 61204-7, which is intended for use
with power supplies sold into multiple industries.
This standard is currently under development.
The point to remember here is that safety
standards, like most things high-tech, represent
an evolving field. While UL 60950-1 has been
used to prepare this subject, one of the first tasks
in any new design activity should be to identify
the standards, including recent revisions, which
applies to the intended end use.
In either case, however, it is the end use
conditions that apply and it is the end use
standards that must be considered with respect to
safety.
In additional principle of safety is that
designers must consider not only normal
operating conditions, but also likely faults,
foreseeable misuse, external influences and
environments, and overvoltages that might occur
on input or output lines.
1-2
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IV. E
LEMENTS OF A
P
OWER
S
UPPLY
A block diagram for a typical power supply is
shown in Fig. 1 where the blocks have been
defined in a way to ease the consideration of
safety implications. While this figure illustrates
an AC-line powered unit which, of course, is
clearly an application where hazardous voltage
levels could be present or internally generated, a
similar blocking of functions could be derived for
other designs, i.e., battery chargers or dc-to-dc
converters.
The task of certifying a design for safety is
made easier by the identification and use of as
many components which, in themselves, are
already qualified to an appropriate IEC safety
standard. Many such power supply components
are available, including:
•
Power cords and/or input terminal
assemblies
•
Protective devices (fuses, clamps, etc.)
•
EMI filters
•
Power switches
•
Wiring, PWBs, chassis
•
Isolators (optocouplers)
•
Transformers
•
Rectifier assemblies
•
Output connectors or terminals
•
Cooling devices
•
And many others…
Components which are already certified as
conforming to the applicable standards need be
evaluated only individually as to their application
within their ratings, and then indirectly as a part
of the complete power supply or end use
application. Non-qualified components may need
additional testing to the appropriate standard at
the component level. Since this can add a
significant amount of time and cost in the
qualification process, it is highly advantageous to
pick components which have already achieved
prior approval.
V. C
ONSIDERATIONS FOR
E
LECTRICAL
S
AFETY
The prevention of electric shock is clearly a
major safety goal. The impact on a human body
is defined by the flow of current which, in turn, is
affected by the body’s resistance. The accepted
value for the body’s resistance is approximately
2000
Ω
at a voltage of 110 Vdc; however this
value decreases with increasing voltage. Another
factor that affects resistance is the amount of
surface area of contact. This has been quantized
by defining two classifications of skin contact as:
•
Full contact, meaning full contact by hand
which has a typical area of about 8000 mm
2
,
but is simulated by a metal contact surface of
20 cm by 10 cm.
RF Noise
Reduction
L
Appliance
Inlet
Wiring
Terminals
EMI Filter
ON/OFF
Switch
Rectifier
Fuse
+
DC Filter
Rectifier
Regulator
Isolator
Switch
Mode
Control
Circuit
Switch
Mode
Regulator
Linear
Regulator
N
Primary
Wiring
Protective
Earthing
PE
-
Feedback
Enclosure (optional)
Fig. 1. The elements of a typical power supply.
1-3
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•
Limited contact is described as contact by
finger tip and simulated by a metal contact
surface of 10 mm
2
. A standardized “finger
probe” is defined to test for contact in tight
regions or through enclosure openings.
•
In any case, it is current that affects the body
and the effects have been categorized according
to the table of threshold values in Table 1.
T
ABLE
1. T
HRESHOLD
V
ALUES
Current
(mA)
0.0 to 0.5
0.5 to 3.5
3.5 to 10
10 to 50
Effect
Perception, minimal reaction
Startle reaction, but ability to tolerate
Muscles contract, inability to let go
Fibrillation, cell damage
Telecommunication Network Voltage
(TNV)
voltages may exceed SELV limits but
are constrained by either accessibility or
duration. The normal operating voltage can
be up to 71 V peak ac or 120 V dc where the
accessible contact area is limited to that of a
connector pin. The voltages under a single
fault can be higher for a short duration but
must return to normal limits within 200 ms.
Higher transient levels (up to 1500 V, but of
short duration) are possible from the public
switching telecom network.
The acceptable limit for current is 2.0 mA dc,
0.7 mA pk ac and 0.5 mArms at frequencies up to
60 Hz. High-frequency current is less harmful to
the human body and the permitted current is
calculated by multiplying the 50/60 Hz threshold
by the frequency in kHz, but with a maximum of
70 mA at all frequencies above 100 kHz.
However, these limits can still cause burns if one
touches a sharp edge or corner, as current density
may be high.
Since it is the circuit voltage that drives this
current, UL60950-1 categorizes circuits within a
power supply as either hazardous or safe
according to the maximum voltage or the
maximum current possible at all points within the
circuit, during both normal operating conditions
and under any single fault. Within this criterion,
there are three classifications of safe circuits
•
Limited Current Circuits (LCC)
where the
maximum available current cannot exceed
2.0 mA dc, 0.7 mA peak ac, or 0.5 mA rms
under both normal and single-fault
conditions. There are also limits on
allowable capacitance.
Safety Extra Low Voltage (SELV)
circuits
where voltage levels cannot exceed 42.4 V pk
ac or 60 Vdc, under both normal and single-
fault conditions.
1-4
Note that although all three of the above
designations are considered as safe, only SELV
and LCC circuits allow the operator unrestricted
access to bare circuit components.
Classifications for circuits which are
considered as unsafe and which must be
protected against operator contact include:
•
Hazardous Voltage
circuits, where voltages
above SELV limits can appear on bare
components, or which contain components
without adequate insulation from a potential
high voltage source.
Extra Low Voltage (ELV)
circuits, which
defines a circuit that meets SELV voltage
limits under normal operating conditions but
is not safe with a single fault.
•
Two other circuit classifications are defined
by their location within a power supply’s
architecture:
•
Primary circuits,
where there is a direct
connection to the ac mains voltage and
clearly have the potential to reach hazardous
voltage levels, and
Secondary circuits,
which have no direct
connection to the primary circuits but may
experience hazardous voltage levels.
•
•
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