Consumer Electronics TFF Training
Training Agenda
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Thin Film Fuse (TFF) Definition and Circuit Protection
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TFF Basic Characteristics and Related Physics
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Typical TFF Application Examples
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TFF Selection Concepts
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Littelfuse TFF Brands and Future Development
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Frontiers of TFF Technology
Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved.
1
Consumer Electronics TFF Training
Section 1
TFF Definition and Circuit Protection
TFF Definition
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In an electrical circuit, a Thin Film Fuse (TFF) is a calibrated, intentionally weak fuse-link
that provides overcurrent protection.
TFF Circuit Protection Concepts
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Potential safety threats that require circuit protection
•
Protect abnormal operation of a circuit caused by component failure
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Regulatory requirements related to circuit protection
•
IEC/UL requirements where faults are expected to occur frequently and changing
fuses or resetting breakers is undesirable
Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved.
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TFF Definition and Consumer Electronics Circuit Protection
TFF Definition
–
In an electrical circuit, a Thin Film Fuse (TFF) is a calibrated, intentionally weak fuse-link that
provides overcurrent protection, protecting sensitive electrical circuitry against damaging
overload and short circuit conditions.
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The Thin Film Fuse element is a patented bimetal design. The fusing mechanism is a
thermally driven event. When current is carried by the element, the element undergoes Joule
Heating (l2R). During an over current condition the elements heats up to a point where its
capability to carry current diminishes and the element melts, vaporizes and the conduction
path is opened.
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A TFF fuse in the circuit will operate before a catastrophic event occurs and prevent a fire.
Electrical components such as ICs, capacitors, transformers, resistors, etc. can fail due to
manufacturing defects, old age, or extreme environmental conditions (aging), and if this
happens, the circuit could draw an excessive amount of current and possibly catch fire.
–
A TFF Fuse does not prevent a fault occurring, but will operate quickly to prevent further
damage from occurring or the equipment from becoming a safety hazard. The fuse will also
limit the extent of the damage to a small part of the equipment making repairs less costly.
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TFF Definition and Consumer Electronics Circuit Protection
Circuit Protection Needs in Consumer TFF Segment
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Secondary protection of electrical circuits particularly those in consumer and general electronic
systems such as laptops, mobile phones, PDAs, MP3 players, digital cameras, hard disc drives,
modem cards, LCD inverters
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Terminals that might get grounded or shorted
• Cell phone batteries, DC power ports
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Process controllers and monitors
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Load and power management ICs
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Consumer Electronics TFF Training
Section 2
TFF Characteristics and Device Physics
Basic TFF Characteristics
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Current rating
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Voltage rating
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Temperature rating
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Time-current characteristic
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Interrupt rating
TFF Construction and how it affects the related TFF characteristics
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TFF Element vs. electrical characteristics
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TFF Package vs. maximum ratings
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TFF De-rating
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TFF Characteristics and Device Physics
TFF Electrical Characteristics
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TC Curve
• Defines the fuse opening time
• Directly denotes the fuse rating
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Slo-Blo vs Fast Acting
The structure of the fuse element determines the
fuse opening characteristics. Generally, the fast
acting fuse has the straight fusing element while
the Slo Blo TFF fuse has a serpentine shape fusing
element.
Fast Acting
Slo-Blo
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TFF Characteristics and Device Physics
TFF Electrical Characteristics
Thin Film Fuses react to overloads
or short circuit conditions by melting
or vaporizing the fusing element.
Usually the overload condition
should be more than 150% of the
rated current.
The opening time is determined by
measuring the melting time of the
TFF with a constant current power
source.
A current shunt is used in the
current measurement for accurate
opening time of the TTF fuse.
SMD TFF Test Scheme
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TFF Characteristics and Device Physics
TFF Electrical Characteristics
Thin Film Fuses function by reacting
to the heat generated due to
excessive current flow. Once the
fuses I2t rating is exceeded, the
center conductor opens.
Exceeding the fuses I2t is an Adiabatic
process: no heat is transferred to the
surroundings. All energy is concentrated
into a specific area for a split second.
The I2t is a measure of the energy required to
open the fuse when subjected to an
adiabatic current pulse.
Due to [adiabatic] current pulses the fusing
element is stressed (thermally and
mechanically). Depending on the energy
and number of pulses it will suffer from
aging from fatigue, pre-diffusion and
oxidation.
Fuse opening time measurement
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TFF Characteristics and Device Physics
TFF Fuse Construction
There are mainly four components of the TFF fuse
construction: The substrate, fusing element,
protective polymeric coating, and termination.
Terminations
The substrate is made of high performance glass /
polymer, the fuse element is formed by Cu/Sn,
and the termination is by Cu/Ni/Sn.
The available footprint of the TFF fuses are as
following:
1206 Fast Acting
1206 Slo Blo
0603 Fast Acting
0402 Fast Acting
Lead-Free & RoHS compliant TFF series are
available.
Substrate
Fusing Element
Polymeric Coating
The TFF is UL recognized and CSA certified.
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Consumer Electronics TFF Training
Section 3
TFF Consumer Electronics Applications Examples

Personal Computers (PCs) And Peripherals

PC Cards

Printers

Network Interfaces

Battery Protection

Portable Devices (Cell Phones, Laptops, PDAs, & Palm Computers)
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TFF Consumer Electronics Applications Examples
Portable Device DC/Power Line Protection
For portable devices, the power bus of the battery pack should be protected
against an overcurrent runaway condition of the Li-ion cell. A TFF should be
used for short circuit and over current protection.
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TFF Consumer Electronics Applications Examples
DC Power Secondary Protection
For products that include an external AC adapter, a DC voltage (typically in the range of
5VDC to 18VDC) will be supplied. As shown above, a TFF can be used for short circuit
and overload current conditions.
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TFF Consumer Electronics Applications Examples
Battery System Protection
There are many instances where a short circuit condition could happen to a battery.
Also, a battery could exposed to a high temperature during normal operation. A TFF
can be used to protect battery from explosion due and extreme overtemp causing a
runaway thermal condition and unacceptably high discharge rate.
Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved.
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TFF Consumer Electronics Applications Examples
Global Lab Capabilities
•
•
•
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Qualification of all LF products
UL-Approved Customer Testing in ISO 17025 Lab (Des Plaines)
– High power (AC/DC up to 1KV/50KA) UL approvals available in DP
– Telcordia approvals in DP planned (2008)
Verification of Telcordia, ITU, IEC, FCC, and other industry, regulatory, and safety standards
– Verification to various OC and OV standards
• Insure application meets standards before submitting for approval
Customer Application testing
– Assistance with design-in and performance verification
• Help with selection of appropriate technology and rating
– Application troubleshooting
• Assistance insuring proper OV/OC and primary/secondary protection coordination
– Competitive evaluations
• Competitive or technology performance comparisons
– Reliability & Tin Whisker data/testing
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Consumer Electronics TFF Training
Section 4
TFF Consumer Electronics Application Product Selection

Identify normal operating current of circuit to be protected

Identify ambient working temperature to which the fuse will be exposed


Estimate magnitude of the In-rush current, peak, pulse shape, duration, and number
of pulses
Select maximum operating voltage
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TFF Consumer Electronics Applications Product Selection
TFF Rating Selection and TC Curve
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—
—
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Time Current (TC) Curves
present the average values of the
trip time at a given current for
every part number.
TFF opening times will be
distributed above and below the
curve.
Lower percentage overloads
result in a greater variation in
opening time.
Verification testing needs to be
done for actual application.
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TFF Consumer Electronics Applications Product Selection
TFF De-Rating Curve
Resistance of the TFF device
changes directly with temperature.
The rating of the TFF is influenced
by the ambient temperature as
shown at the temperature de-rating
chart for the A line.
The heat required to open the
device may come from several
sources. Some common sources
are:
—
resistive heating from the
electrical current
—
ambient environment
—
adjacent components
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TFF Consumer Electronics Applications Product Selection
Some TFF Board Application Guide Lines
The I2t must be considered for the selection of the TTF fuse to compare with the pulsing I2t
of the signal.
The fuse voltage rating is the maximum allowable applied voltage at which the fuse can
open safely (arcing occurs when the fuse element melts, the voltage rating defines the
maximum suppressible arc). Standard voltage ratings are 16, 32, and 63V DC.
Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved.
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Consumer Electronics TFF Training
Section 5 Littelfuse TFF Product Road Map
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TFF Road Map
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New 0402 Ratings
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Higher Operating Temperature 1206 TFF Fuses (Rated to 135C)
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Higher Operating Temperature 0603 TFF Fuses (Rated to 135C)
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New 0201 Very Fast-Acting Fuse – Smallest Ever
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Consumer Electronics TFF Training
Section 6 Consumer Electronics TFF Technology Challenges
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Multi Elements in Single Package
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TFF Technology Combined with other Technologies in a Single Package
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Improved De-rating Characteristics
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Higher Operating Temperatures
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Hi-Rel TFF Development
Confidential and Proprietary to Littelfuse, Inc. © 2007 Littelfuse, Inc. All rights reserved.
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