Polyfuse

POLYFUSE

SATISH SUKUMARAN1DS08EC407

INTRODUCTION

Basically a Thermistor

Polymeric Positive Temperature Coefficient

(PPTC)

A new standard for circuit protection

Self Resettable

A series element in a circuit

Over Current Protection

Polyfuse is a series element in a circuit. The PPTC device protects the circuit by going from a low-resistance to a high-resistance state in response to an over-current condition

In normal operation the device has a resistance that is much lower than the remainder of the circuit. In response to an over-current condition, the device increases in resistance (trips), reducing the current in the circuit to a value that can be safely carried by any of the circuit elements.

Construction

Polyfuse circuit protection devices are made from a compositeof semi-crystalline polymer and conductive particles. At normaltemperature, the conductive particles form low-resistancenetworks in the polymer. However, if the temperature rises above the device’s switching temperature (Tsw) either from high current through the part or from an increase in the ambient temperature, the crystallites in the polymer melt and become amorphous. The increase in volume during melting of the crystalline phase separates the conductive particles resulting in a large non-linear increase in the resistance of the device.

Principle of Operation

Operating CharacteristicsTypical pair of operating

curves for a PolySwitchdevice in still air at 0°C and 75°C. The curves are different because the heat required to trip the device comes both from electrical I2R heating and from the device environment.

At 75°C the heat input from the environment is substantially greater than it is at 0°C, so the additional I2R needed to trip the device iscorrespondingly less, resulting in a lower trip current at a giventrip time (or a faster trip at given trip current).

Typical Resistance Recovery after a Trip Event

Figure shows typical behavior of a PolySwitch device that istripped and then allowed to cool. This figure illustrates how, even after a no.of hours, the device resistance is still greater than the initial resistance. Over an extended period of time, device resistance will continue to fall and will eventually approach initial resistance.

when PolySwitch devices are chosen R1MAX should be taken into consideration when determining hold current. R1MAX is the resistance of the device one hour after the thermal event.

Operating Parameters

Initial Resistance: The resistance of the device as received

from the factory

Operating Voltage: The maximum voltage a device can

withstand without damage at rated current

Holding Current: Safe current through the device.

Trip Current: The current at which the interrupts the current

Time to Trip: The time it takes for the device to trip at a given

temperature and current

Tripped State: Transition from low resistance state to high

resistance state due to an overload

Leakage Current: A small value of stray current flowing

through the device after it has switched to high resistance mode.

Operating Parameters

Trip Cycle: The number of trip cycles the device sustains

without failure.

Trip Endurance: The duration of time the device sustains its

maximum rated voltage in the tripped state without failure.

Power Dissipation: Power dissipated by the device in the

tripped state.

Thermal Duration: Influence of ambient temperature.

Hysteresis: The period between the actual beginning of the

signaling of the device to trip and the actual tripping of the

device.

Hold and Trip Current as a Function of Temperature

Hold and Trip Current as a Function of Temperature

Region A describes the combinations of current and

temperature at which the PolySwitch device will trip (go into

the high-resistance state) and protect the circuit.

Region B describes the combinations of current and

temperature at which the PolySwitch device will allow for

normal operation of the circuit.

Region C, it is possible for the device to either trip or

remain in the low-resistance state.

Types of Polyfuses

Surface Mount Resettable Fuses

Radial-Leaded Resettable Fuses

Battery Strap Resettable Fuses

Edges Over Conventional Fuses

Over current protection

Low base resistance

Latching operation

Automatic resettability

Short time to trip

No arching during faulty situations

Small dimensions and compact designs

Internationally standardized and approved.

No accidental hot plugging

Withstand mechanical shocks and vibrations

Life time- up to 10 times longer

Item PPTC CPTC Bi-Metal Fuse

Resettable

Yes Yes Yes No

Size Small Medium Large Large

Warranty Cost

Low Low High High

Power Loss

Low High Low Low

Resistance

Low High Low Low

Comparison between PPTC and Other Types of Fuses

Applications of Polyfuse

Speaker Protection

Battery Protection

IC Protection

Protection of Electrical Equipments

Protection Device Inside Consumer Electronics

Protection of Computer Peripherals

Automobiles, Industrial Control

Medical Electronics

Lighting, Security & Fire Alarm System

Tele Communication Equipment

Conclusion

Self Resettable Device

Compact

Low Resistance

Fast Trip Time

Ideal for low cost over current protection

Ideal for Low Voltage A.C & D.C

References

Electronics For You, Edition-September, 2004

Raychem circuit protection products- Tyco Electronics

http://www.circuitprotection.com

http://www.wikipedia.com

http://www.inter-technical.com

Queries ?

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