Selecting Fuses Article

Nabil Sadiq, Senior Field Application Engineer
Trina Noor, Field Applications/New Product Development Engineer,
Cooper Bussmann
Fuses protect against overcurrent events by melting
their elements and opening
the circuit. Fuses must be
applied at or below their
specified voltage rating,
which differs between ac and
dc current
Simple Procedures to Get the
Right Overcurrent Protection
for DC-DC Converters
lthough features and functionality attract the most attention for
new electronic products, whether consumer, industrial, or medical, their reliability depends on protecting their power systems
from overcurrent events. Internal, external, and nuisance threats
can affect circuit and system reliability. Through proper fuse
selection, you can minimize risks and failures so that an electronic
product retains its competitive edge.
Fuses are overcurrent devices that protect electrical and electronic devices by
melting and opening a circuit to prevent excessive current from causing damage or
starting fires. Fuses serve two main purposes:
1. To protect components, equipment and people from risk of fire and electric
2. To isolate sub systems from the main system.
The fusing action begins when the circuit current is high enough to heat the fuse
element and starts it to melt. Once melting begins, a gap is created that the current
will “arc” across. Melting continues and the gap grows wider until it is too wide to
10 Power Electronics Technology | August 2010
Percentage of rating
Temperature in Degrees C
Fig. 1: Typical fuse derating curves
sustain the
arc. At that point, current ceases to flow and
the overcurrent event is “cleared,” opening and making the
circuit safe.
1. There are two types of overcurrent events:
1) Overload - simply drawing excessive current beyond
the designed capacity of the circuit,
2) Short-circuit, or fault current.
Regardless of the overcurrent event, fuses are designed
and specified to be a circuit’s “weakest link.” These “thermally operated” devices typically employ a metal wire or
strip element in their construction.
Fuse Types
Fast-acting fuses open very quickly when their current
rating is exceeded. This action is needed when speed is
important for sensitive electronics and for many dc power
applications. They are generally used in resistive loads
with low inrush current levels.
Time-delay fuses have a time-delay mechanism. They
are designed to open only on an excessive current draw
for a defined period of time and are typically used to
protect inductive and capacitive loads that experience
heavy current draws upon initial powering. The time delay
action prevents the fuse from needlessly blowing during a
temporary heavy current draw or surge. Time-delay fuses
tolerate higher inrush currents than fast-acting fuses and
are often ideal for dc-dc converter input protection, as
Fig. 2. Fuse locations in a typical dc-dc converter
most converters have an input capacitor that draws a large
amount of current when initially charged.
Selecting the right fuse is critical in all electronic and
electrical system designs. Catastrophic system failure can
be prevented with the proper fuse on the dc-dc converter
input. In the event the converter’s internal circuitry can
no longer withstand an overload condition, the fuse will
prevent fire or further damage to the board, the converter,
or neighboring components. Most dc-dc converters are
protected from short-circuits on their outputs by either
circuit-sensing current limit and/or thermal overload circuits. Fuses are required to protect against a catastrophic
component failure (e.g., MOSFET failure) or if a component failure creates a short-circuit on the input side of the
dc-dc converter.
Proper selection of an input fuse for a dc-dc converter
involves understanding and consideration of the following
1. Voltage Rating
2. Current Rating
3. Interrupting Rating
4. Temperature Derating
5. Melting Integral (I2t)
6. Maximum Circuit Fault Current
7. Required Agency Approvals
8. Mechanical Considerations
Voltage Rating
Fuses are first rated by the ac and/or dc circuit voltage
into which they can be safely applied. A fuse installed in
an AC circuit performs differently than when installed in
a DC circuit. With AC circuits, the current is crossing the
zero potential at 60 or 50 cycles a second. This helps in
breaking the arc that forms when the fuse element melts
and creates a gap. In dc circuits, the voltage does not go to
a zero potential, making it more difficult to suppress the
arc in the melting element’s gap.
Generally, fuse ac voltage ratings coincide with the
utility supply, e.g., 110V, 240V, 415V, etc. This means
that a fuse is suitable for use with these nominal voltages
and is tested for voltage levels at least
3.3 VOUT
15% higher than the nominal rating.
This is not true with dc voltage ratDC/DC
ings, which are normally maximum
ratings and should not be exceeded.
More specifically, the voltage rating of
a fuse must be equal to or greater than
the maximum voltage expected in the
Fuses are insensitive to voltage
changes within their ratings so selecting the proper voltage rating is strictly
a safety issue. Fuses can operate at any
SADIQ_F2 August 2010 | Power Electronics Technology
voltage below or equal to their rated voltage.
Current Rating
Although some power supplies are designed for constant
current output regulation, most typical DC-DC converters are designed as constant power devices. This means
that as the input voltage drops, the input current must
increase to uphold the constant output power relationship
of P= V*I.
The fuse’s minimum current rating is determined by the
maximum input current of a DC-DC converter. Typically,
the maximum current consumption occurs at the maximum output load and the minimum input voltage. The
magnitude of the input current can be determined from:
VIN (MIN ) × Efficiency
POUT(MAX) = Maximum dc-dc converter output
VIN(MIN) = Minimum input
voltage on the dc-dc converter input.
Efficiency = Efficiency
I2 t (FUSE) = Iof
and VIN(MIN); can be determined from the dc-dc converter’s datasheet.
To prevent damage to converter components, the fuse
current rating is selected with a large enough current
capability so that the fuse will not open under steady state
conditions, yet will open during an abnormal (excessive)
overload or short-circuit condition. Usually this results in
selecting a fuse to be 150% to 200% percent of the maximum steady state input current at maximum load and
minimum line input voltage.
Interrupting Rating
The fuse interrupting rating is the maximum amperage
at rated voltage the fuse can safely interrupt. This rating
must exceed the maximum fault (short-circuit) current
the circuit can produce. Interrupting ratings for AC and
DC currents are different and the fuse data sheet should
be consulted before selection.
Melting Integral
The DC-DC converter peak inrush current is usually significantly greater than the steady state current.
Additionally, periodic inrush currents can be sufficiently
powerful to warm the fuse element. Though not large
enough to melt the element, it can still cause significant
thermal stress to the element. Cyclical expansions and
contractions of the fuse element can lead to mechanical
fatigue and premature failure.
Selecting the appropriate fuse involves choosing the
appropriate melting integral. The melting integral of a
fuse, termed melting I2t, is the thermal energy required to
melt a specific fuse element. The fuse element construction, materials and cross sectional
area will determine this
VIN (MIN ) × Efficiency
The task of a system designer
is to select a fuse with the
minimum I2t greater than the energy of the inrush current
pulse. This rating ensures that the fuse will not cause a
(MAX) conditions. For relinuisance openingI during
able system operation for the
required number of turn-on
cycles, the following condition must be met:
I2 t (FUSE) = I2 t (PULSE) × FP
I2t (PULSE) = Energy of a current pulse
I2t (FUSE) = Melting integral of a fuse
FP = the pulse factor (dependent on fuse element construction in Table 1)
I2t (FUSE) can be found in fuse datasheets. Do not use
Table 1. pulse factor for
solid matrix construction
Pulse Factor for Solid Matrix Construction
Temperature Derating
When a fuse is applied in an ambient temperature
exceeding the standard 23°C, the fuse current rating
should be derated (a higher amp rating with higher temPOUT(MAX)
peratures).I Conversely,
at an ambient tempera(1)
VIN (MINstandard
ture lower than the 23°C
allows using a lower
fuse amp rating. Fig.1 shows a typical fuse derating curve.
The fuse rating is determined by:
I INPUT(MAX) = Current determined from Equation (1)
or a dc-dc converter datasheet
KTEMP = Temperature derating factor determined from
Fig. 1.
The lowest suitable fuse rating is obtained by rounding
up the calculated value to the next higher current rating
shown in the fuse datasheet.
I2 t (FUSE) = I2 t (PULSE) × FP
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Number of Surge Pulses
Pulse Factor, FP
1 to 100,000
Table 2. Fuse Pulse Factor
Pulse Factor for Wire-in-Air Construction
Number of Surge Pulses
Pulse Factor, FP
the fuse’s maximum melting integral
in Equation (3), and use either the
minimum or nominal melting integral
of the fuse.
a fuse to open at lower current levels.
Wire-in-air construction, as in the
3216TD and new S505H series, and
many traditional ferrule fuses, provides high inrush withstand. Wire-inMaximum Circuit Fault Current
air technology makes a smaller fuse
Other selection considerations include
possible without sacrificing I2t, temstart-up (inrush) currents and transient Fig. 3. Compact C310T Series 3.6mm x 10mm time- perature or operating voltage range.
load conditions. When a dc-dc con- delay fuse for off-line protection
Using a fuse with high surge-withstand
verter is initially powered, the input
capability means fewer open fuses durbulk capacitors of dc-dc must be charged. Current flowing momentary overloads.
ing into the input terminals of a dc-dc converter is
approximately I = V/R for typical power supplies
with charge times less than 10 milliseconds. When
North American UL/CSA and IEC standards
V is the input voltage change, and R is a combinafor overcurrent protective devices require
tion of wiring resistance, your source’s resistance under
significantly different Time-versus-Current
start-up, and the Equivalent Series Rating (ESR) of the
characteristics. UL rated fuses are tested
converter’s input bulk capacitors.
to open at 135% of rated current while
Larger dc-dc converters often use a large Fig. 4. PC-Tron® Fuses
IEC fuse ratings are tested to carry 150%
capacitor with very low ESR inside the con- operate up to 5A
of rated current. Be aware of these difverter. This inrush current can have a sigferences as the fuses are tested and specified
nificant effect on the fuse’s life. Size the fuse properly to
differently between these standards for products sold in
allow these inrush current pulses to pass without nuisance
different parts of the world.
openings or degrading the fuse element as discussed in
The physical dimensions and materials for both UL and
melting integral.
IEC fuses are similar. However, fuses made to different
To calculate current pulse energy, one must first deterstandards are not interchangeable. Their element melting
mine the magnitude and duration of the current pulse.
and opening times will differ when subjected to the same
The most accurate way to determine parameters of a
magnitude of current. The circuit designer must consider
current pulse is to measure this current in the application
that different world markets may require different fuse
under minimum and maximum voltage conditions.
agency standards.
Note that the melting I2t values of the fuse must be
To select a fuse that ensures system and agency complicalculated at the condition where the product of the peak
ance the following conditions must be met:
current squared and time the peak occurs is maximum.
• Fuse current rating does not exceed the rating of the
For example, the steady state current is maximum at low
fuse used for safety testing of the dc-dc converter it is
line so a transient load surge needs to be added to the low
intended to protect.
line current to establish the maximum peak current for
• Fuse is installed on the ungrounded side of the circuit
an operating condition. But the inrush current is usually
to ensure uninterrupted ground connection in case the
maximum at the highest input voltage. The fuse’s melting
fuse opens.
I2t must be evaluated at the condition with the highest
• The input traces and chassis ground trace (if used) are
calculated I2t to ensure that the fuse will not open during
capable of conducting a current of 1.5 times the fuse
these “normal” operating conditions.
current rating.
The pulse factor is dependent on the construction of
the fuse element (see pulse factor tables under Melting
Mechanical Considerations
There are numerous physical sizes of fuses for electronThe patented solid matrix construction used in the
ics, including subminiature fuses. The most common ferrule
Cooper Bussmann® 0603FA, 3216FF, CC12H and CC06
designs are 5x15mm, 5x20mm and 6.3x32mm (¼ in. x
fuse series provides excellent cycling and temperature
1¼ in.). Ferrule fuses are generally mounted in fuse clips
performance while significantly reducing nuisance openor holders with some available with axial leads for solderings from high inrush currents. It also provides protection
ing directly onto a PCB. Subminiature fuses are often used
against unanticipated current surges from the system. The
when board space is limited. For applications of this type,
small physical size allows maximum protection without
there are through-hole and surface mount devices available.
oversizing the fuse rating. Solid matrix construction reducStandard package sizes for surface mount fuses are 0402
es heating from repeated surges that would normally cause
(1005), 0603 (1608), 1206 (3216), 6125, and 1025.
13 Power Electronics Technology | August 2010
These sizes are standard throughout the electronic industry. Throughhole axial and radial leaded products
allow fuses to be PCB mounted. For
example, Cooper Bussmann offers
electronic fuses ranging from 32V
to 450V. Voltage ratings can and do
vary inside a fuse family or series, as
well as interrupting ratings, I2t and
agency approvals. Always consult
data sheets for the ratings that apply
to the desired voltage and amp rating
of the application.
Typical Fuse Locations
in Power Supplies
Product safety standards require fuses
for primary ac power protection and
secondary protection against any catastrophic failure in the input filter
capacitors, Power Factor Correction
(PFC) boost module, output capacitors, or within the dc-dc converters
where fuse F1 in Fig. 2 is a typical
ac fuse location. The fuse is placed
near the input connector so that all
other components are downstream
and protected.
The PFC boost module usually
does not contain overcurrent protection. If a short-circuit is applied
across the PFC output terminals,
there is no internal circuit opening
device to safely interrupt the power.
The fuse in the AC input line (Fuse
F1 in Fig. 2) protects the PFC boost
Although the primary input line
fuse will eventually open, dc fuses
positioned right at the input to the
dc-dc converters will limit the energy
delivered by the hold-up capacitors
and prevent failure to the PFC boost
module. DC fuses between the PFC
and dc-dc converters protect against
a catastrophic failure in the dc-dc
converter (Fuses F2 and F3 in Fig.2).
Fusing each dc-dc converter will
allow the converter not subject to a
fault to continue operating by isolating the failed converter.
Fuses F2 and F3 have an added
benefit during product development. Fig. 5. The S505H Series of time delay fuses
By selectively removing these fuses,
the various converters can be powered separately, or the PFC operated
with an external load. In addition
to facilitating testing of the different power sections during product
development, the fuses can aid troubleshooting in production and in
the event the product needs to be
Fuses applied to overcurrent protection points of Fig. 2 include F1
providing primary overcurrent protection. Use ac line voltage rated
fuses located on the transformer primary side (typically 125Vac / 250Vac
line voltage)
• SR-5 / SS-5 radial fuses
• S501-2-R fast-acting fuse
• C310T Series (coming soon)
3.6x10mm axial-leaded, timedelay, ceramic tube fuse (Fig. 3)
• 5mm or ¼ in. ferrule fuses
Fuses F2 and F3 that provide secondary overcurrent protection. Use
400Vdc or higher rated fuses on the
secondary side of the transformer or
on battery powered applications (ac
or dc, typically lower voltages, but
not always).
• PC-Tron® (up to 2.5A) (Fig. 4)
• S505H Series (coming soon)
400Vdc/500-600Vac, time-delay,
5x20mm (Fig. 5)
August 2010 | Power Electronics Technology