POLYFUSE

A NEW STANDARD FOR CIRCUIT PROTECTION

INTRODUCTION

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Polyfuses are resettable fuse.These are generally positive-temperature

coefficient thermistors.

The device resistance rises with temperature.

These comprise thin sheets of conductive plastic with electrodes attached to either side.

These fuses provide over-current protection and automatic restoration.

Voltage rating-----3o to 250 volts.Current rating-----20mA to 100A.

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Polyfuses are usually packaged in radial, axial, surface-mount, chip, disk, or washer form.The conductive plastic is basically a non- conductive crystalline polymer loaded with a highly conductive carbon to make it conductive.PPTC devices limit the flow of dangerously high current during fault conditions.PPTC devices reset after the fault is cleared and the power to the circuit is removed.

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CONSTRUCTION AND OPERATION• PPTC fuses are constructed with a non-conductive polymer plastic film that exhibits two phases.• The first phase is a crystalline or semi-crystalline statewhere the molecules form long chains and arrange in a regular structure.• As the temperature increases the polymer maintains this structure but eventually transitions to an amorphous phase where the molecules are aligned randomly and there is an increase in volume.

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• The polymer is combined with highly conductive carbon.• In the crystalline phase the carbon particles are packed into the crystalline boundaries and form many conductive paths, and the polymer-carbon combination has a low resistance.• When the temperature falls, the polymer returns to its crystalline structure, the volume decreases, and the carbon particles touch and form conductive paths.

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WORKING PRINCIPLEAt room temperature numerous carbon chains form conductive paths through the plastic material. In case an excessive current flows through the device, the temperature of the conductive plastic material rises. When the temperature exceeds its phase-transformation temperature, the crystalline structure suddenly changes into an expanded amorphous state. Now the conductive particles move apart from each other. A dramatic increase in resistance stops the flow of current through the device. The critical current level at which this happens is known as the trip current.

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APPLICATIONSThe equipment powered by a transformer get overheated due to excessive current or short-circuit. A PolyFuse on the secondary side of the transformer will protect the equipment against overload.

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Nowadays, in speakers, there are possibilities of damage due to mismatches; for example, high-power amplifiers coupled with low-power speakers or a speaker coil driven with a high volume. The protection choices for loudspeaker systems are limited. Using a PolyFuse in series with the speaker will protect it fromover-current/over-heating damage. Choosing a correct trip -current rated PolyFuse is important to match the power level of the speaker.

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Batteries are constantly charged and discharged over their life-cycle. Over-charge results in an increase in the temperature of the electrolyte. This could cause either a fire or an explosion. PolyFuse can play a vital role in the charging and discharging cycles of batteries.

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OPERATING PARAMETERS

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ADVANTAGES

IprogmA

VprogV

tprog 0µs 1µs 2µs 3µs

Imax

Imelt

Iheat

Ilinear

Imin

Iosc

Ialloy

Ialloy: No autonomous

current pinch off

Ilinear: Linear resistor characteristicsIheat: Temp. is raisingImelt: Tungsten Silicide is meltingImax: Maximum current of

minimum resistanceImin: Local current min.Iosc: Oscillation because of

break

PROGRAMMING CHARACTERISTICS

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TYPES OF POLYFUSES Surface Mount Resettable FusesThis surface mount polyfuse family of polymer of polymer basedresettable fuses provides reliable over current protection for a wide range of  products such as computer motherboards, USB hubs and ports, CD/DVD drives , digital cameras and battery packs. Each of these polyfuse series features low voltage drops and fast trip times while offering full resettability. This makes each anideal choice for protection in data com and battery powered applications where momentary surges may occur during interchange of batteries or plug and play operations.Radial-Leaded Resettable FusesDue to the automatic resetting of the polyfuse, these components are idealfor applications, where temporary fault conditions (eg: during hot plugging) canoccur. The radial-leaded RLD-USB-series 709 is specifically designed for universalserial bus (USB) applications with lower resistance, faster trip times and lower voltage drop.

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POLYFUSE IN MARKET

Low Current Programmed Poly Fuse- High energy is forcing the

Tungsten seperation- Break before Tungsten

completely removed

- Relatively high ohmic resistor

- Lifetime drift to lower resistor values possible

Substrate

Field Oxide

Poly Silicon

Tungsten Silicide

Tungsten Plug Field Oxide Tungsten Plug

Poly Silicon

Tungsten Silicide

Substrate

TungstenHALO

Tungsten

CROSS SECTION

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THANKING EVERYONE FOR YOUR KIND PERUSAL

THIS PRESENTATION IS PREPARED BY:ASANSOL ENGINEERING COLLEGE

ELECTRICAL DEPERTMENT