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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 certied under the CSA Com-

ponent Acceptance Program.

Introduction to PPTC Technology

PTC stands for Positive Temperature Coefcient,

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.

Figure 4. How a PPTC works

Figure 5. PPTC resistors

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

How a PPTC Works

I

V

Voltage

Source

Load

Resistance