Application Note:
Use of Low Resistivity Surface Mount PPTC
in Li-ion Polymer Battery Packs
General Safety Standard that Applies to
Smartphones and Tablets
IEC/UL/EN 60950-1 - Information Technology Equip-
ment Safety, Part 1: General Requirement
● The standard applies to battery operated devices that
can be charged from AC mains supply.
● Sec 2.5 – Limited Power Source
– Fire enclosure requirements in 4.7.2 are reduced or
not required if the components/connectors are
connected to a Limited Power Source.
– This allows designer to reduce cost, use thinner
materials, etc.
– Limited Power Source spec has two tables:
● Table 2B – no OC protective device (so using PTC or
electronic fuse)
– Must limit current to 8A within 5 sec if using PTC
● Table 2C – OC protective device is used (fuse)
– Fuse rating 5A or less (210% / 120sec overload
gate)
– Limit Short ckt current to less than 1000 / Vmax
and 250VA within 60sec
● Where an overcurrent protective device is used, it
shall be a fuse or a non-adjustable, non-autorest,
electromechanical device
Agency Approvals: Littelfuse PPTCs are recognized
under the Component Program of Underwriters
Laboratories to UL Standard 1434 for Thermistors. The
devices have also been certi ed under the CSA Com-
ponent Acceptance Program.
Introduction to PPTC Technology
PTC stands for Positive Temperature Coef cient,
which means the resistance of the device increases as
its temperature goes up. PTCs increase in resistance
as temperature increases due to increased current
ow. Polymer PPTC (PPTC) devices are made of a
polymer plastic material. Unlike a typical “one-time”
fuse, a PPTC device (see Figure 4) will reset when
cooled.
How a PPTC Works
Carbon Crystalline Polymer
Under Normal Operation
■ At operating current
■ Many conductive paths
■ Very low resistance
Carbon Amorphous
Under Fault Condition
■ Excessive current causes
device to heat up
■ Fewer conductive paths
■ Result is high resistance
■ Cools down and resets
when fault is removed
Figure 4. How a PPTC works
I
Voltage
Source
V
RPTC
Load
RL Resistance
Figure 5. PPTC resistors
©2012 Littelfuse, Inc
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