Polyfuses is a new standard for circuit protection .It is re-settable by itself. Many manufactures also call it as Polyswitch or Multifuse. Polyfuses are not fuses but Polymeric Positive temperature Coefficient Thermistors (PPTC).We can use several circuit protection schemes in power supplies to provide protection against fault condition and the resultant over current and over temperature damage.

1. POLYFUSE
SATISH SUKUMARAN
1DS08EC407

2. INTRODUCTION
 Basically a Thermistor
Polymeric Positive Temperature Coefficient (PPTC)
 A new standard for circuit protection
 Self Resettable
A series element in a circuit

3. 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.

4. Construction
Polyfuse circuit protection devices are made from a composite
of semi-crystalline polymer and conductive particles. At normal
temperature, the conductive particles form low-resistance
networks 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.

5. Principle of Operation

6. Operating Characteristics
Typical pair of operating
curves for a PolySwitch
device 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 is
correspondingly less, resulting in a lower trip current at a given
trip time (or a faster trip at given trip current).

7. Typical Resistance Recovery after a
Trip Event
Figure shows typical behavior of
a PolySwitch device that is
tripped 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.

8. 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.

9. 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.

10. Hold and Trip Current as a Function of
Temperature

11. 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.

12. Types of Polyfuses
Surface Mount Resettable Fuses
Radial-Leaded Resettable Fuses
Battery Strap Resettable Fuses

13. 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

14. Comparison between PPTC and Other Types of Fuses
Item PPTC CPTC Bi-Metal Fuse
Resettable Yes Yes Yes No
Size Small Medium Large Large
Warranty Low Low High High
Cost
Power Loss Low High Low Low
Resistance Low High Low Low

15. 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

16. 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

17. 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

18. Queries ?

19. Thank You !!!