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Seminar Report, 2010

INTRODUCTION

Current flow in a conductor always generates heat. Excess heat is damaging to electrical

components. Overcurrent protection devices are used to protect conductors from

excessive current flow. Thus protective devices are designed to keep the flow of current

in a circuit at a safe level to prevent the circuit conductors from overheating.

A fuse is a one-time over-current protection device employing a fusible link that melts

(blows) after the current exceeds a certain level for a certain length of time. Typically, a

wire or chemical compound breaks the circuit when the current exceeds the rated value.

A fuse interrupts excessive current so that further damage by overheating or fire is

prevented. Wiring regulations often define a maximum fuse current rating for particular

circuits. Overcurrent protection devices are essential in electrical systems to limit threats

to human life and property damage. Fuses are selected to allow passage of normal

current and of excessive current only for short periods.

Polyfuse is a resettable fuse that doesn’t need to be replaced like the conventional fuse.

Many manufacturers also call it PolySwitch or MultiFuse. Polyfuse are designed and

made of PPTC material in thin chip form. It is placed in series to protect a circuit.

Polyfuse provide over-current protection and automatic restoration.

Like traditional fuses, PPTC devices limit the flow of dangerously high current during

fault condition. Unlike traditional fuses, PPTC devices reset after the fault is cleared and

the power to the circuit is removed. Because a PPTC device does not usually have to be

replaced after it trips and because it is small enough to be mounted directly into a motor

or on a circuit board, it can be located inside electronic modules, junction boxes and

power distribution center

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OVERCURRENT 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 overcurrent condition,

as shown in Figure-1. This is referred to as "tripping" the

Figure 1 - Overcurrent protection circuit using Polyfuse

device. In normal operation the device has a resistance that is much lower than the

remainder of the circuit. In response to an overcurrent 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. This change is the result of a rapid increase in the

temperature of the device, caused by I2R heating.

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PRINCIPLE OF OPERATIONTechnically these are not fuses but Polymeric Positive Temperature Coefficient (PPTC)

Thermistors. Polyfuse device operation is based on an overall energy balance. Under

normal operating conditions, the heat generated by the device and the heat lost by the

device to the environment are in balance at a relatively low temperature, as shown in

Point 1of Figure-2. If the current through the device is increased while the ambient

temperature is kept constant, the temperature of the device increases. Further increases

in either current, ambient temperature or both will cause the device to reach a

temperature where the resistance rapidly increases, as shown in Point 3 of Figure-2.

Figure 2 – Operating curve as resistance varies with temperature

Any further increase in current or ambient temperature will cause the device to

generate heat at a rate greater than the rate at which heat can be dissipated, thus causing

the device to heat up rapidly. At this stage, a very large increase in resistance occurs for

a very small change in temperature, between points 3 and 4 of Figure-2. This is the

normal operating region for a device in the tripped state. This large change in resistance

causes a corresponding decrease in the current flowing in the circuit. This relation holds

until the device resistance reaches the upper knee of the curve (Point 4 of Figure-2). As

long as the applied voltage remains at this level, the device will remain in the tripped

state (that is, the device will remain latched in its protective state). Once the voltage is

decreased and the power is removed the device will reset.

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CONSTRUCTION & 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 state where 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. 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.

Figure 3 - Polymer film in semi crystalline phase and conducting chains of carbon molecules.

A current flowing through the device generates heat (I2R losses). As long as the

temperature increase does not cause a phase change, nothing happens. However, if the

current increases enough so that corresponding temperature rise causes a phase change,

the polymer’s crystalline structure disappears, the volume expands, and the conducting

carbon chains are broken. The result is a dramatic increase in resistance. Whereas

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before the phase change a polymer-carbon combination may have a resistance measured

in milliohms or ohms, after the phase change the same structure’s resistance may be

measured in megaohms. Current flow is reduced accordingly, but the small residual

current and associated I2R loss is enough to latch the polymer in this state, and the fuse

will stay open until power is removed.

Figure 4 - Polymer film in amorphous phase and broken carbon chains

The process is almost reversible, in that when the temperature falls, the polymer returns

to its crystalline structure, the volume decreases, and the carbon particles touch and

form conductive paths. However, the exact same conductive paths never form so that

the resistance after reset is slightly different from before. The resistances of a PPTC

fuse may triple or quadruple after the first reset, but thereafter changes are relatively

unimportant.

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OPERATING PARAMETERS

Initial Resistance: It is the resistance of the device as received from the factory of

manufacturing.

Operating Voltage: The maximum voltage a device can withstand without damage

at the rated current.

Holding Current: Safe current passing through the device under normal operating

conditions.

Trip Current: It is the value of current at which the device interrupts the current.

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

Tripped State: Transition from the low resistance state to the 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.

Trip Cycle: The number of trip cycles (at rated voltage and current) 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 its tripped state.

Thermal Duration: Influence of ambient temperature.

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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

Figure 5 illustrates the hold- and trip-current behavior of Polyfuse devices as a function

of temperature. One such curve can be defined for each available device. Region A

describes the combinations of current and temperature at which the Polyfuse 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 Polyfuse device will allow for

normal operation of the circuit. In Region C, it is possible for the device to either trip or

remains in the low-resistance state (depending on individual device resistance).

Figure 5 – Hold current & Trip current variation with temperature

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OPERATING CHARACTERISTICS

Figure 6 – Operating characteristics of polyfuse as current increases with time

Figure-6 shows a typical pair of operating curves for a PPTC device in still air at 0oC

and 75oC. The curves are different because the heat required to trip the device comes

both from electrical I2R heating and from the device environment. At 75oC the heat

input from the environment is substantially greater than it is at 0oC, 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).

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Typical Resistance Recovery after a Trip Event

Figure-7 shows typical behavior of a Polyfuse device that is tripped and then

Figure 7 – Typical resistance recovery after a trip event

allowed to cool. Over an extended period of time, device resistance will continue to fall

and will eventually approach initial resistance. However, since this time can be days,

months, or years, it is not practical to expect that the device resistance will reach the

original value for operation purposes. Therefore, when Polyfuse 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.

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ADVANTAGES OVER TRADITIONAL FUSESConventional thermal fuses are not resettable and are therefore limited in their ability to

match the low temperature protection of PPTC devices. The selection of a low fusing

temperature in conventional thermal fuses is limited by the need to avoid nuisance

tripping in temporary high ambient temperature environments, such as car dashboards

on a hot day or high storage temperatures. Even thermal fuses with 94°C or higher

fusing temperatures often nuisance trip during normal operation or pack assembly.

Figure 8 – Table showing a comparison between a PPTC polyfuse and types of fuses

Hence, the major benefits of polyfuse are as-

Low base resistance

Latching (non-cycling) operation

Automatic reset ability

Short time to trip

No arcing during faulty situations

Small dimensions and compact design

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Internationally standardized and approved

No accidental hot plugging

Withstand mechanical shocks and vibrations and comply with the safety norms

APPLICATIONS

PolyFuses are used in automobiles, batteries, computers and peripherals, industrial

controls, consumer electronics, medical electronics, lighting, security and fire alarm

systems, telecommunication equipment and a host of other applications where circuit

protection is required.

Some of its applications in protecting various equipments are discussed as below-

TRANSFORMERS PROTECTION

Figure 9 – Transformer protection by Polyfuse

The equipment powered by a transformer gets overheated due to excessive current or

short-circuit. A Polyfuse on the secondary side of the transformer will protect the

equipment against overload as shown in Figure-9.

SPEAKER PROTECTION

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Figure 10 – Speaker protection by Polyfuse

Nowadays speakers are designed and sold independently of amplifiers. Therefore, there

are possibilities of damage due to mismatches. The protection choices for loudspeaker

systems are limited. Fuses protect the speaker, but a blown fuse is always a source of

frustration. Using a Polyfuse in series with the speaker as shown in Figure-10 will

protect it from over-current/over-heating damage. Choosing a correct trip-current rated

Polyfuse is important to match the power level of the speaker.

BATTERY PROTECTION

Figure 11 – Battery protection circuit for Li-ion batteries

The Figure-11 below shows a schematic of a typical single-cell Li-ion battery pack for

cellular phone applications, using a Polyfuse. 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 play a vital role

in the charging and discharging cycles of batteries. The Polyfuse low resistance

overcomes the additional series resistance introduced by the MOSFETs and the low trip

temperature can provide protection against thermal runaway in the case of an abusive

overcharge

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KEYBOARD/MOUSE PROTECTION

FIGURE 11 – Protection of keyboard/mouse through Polyfuse Device

The operating current of keyboard/ mouse is usually from 200 to 500 mA, but in a short

circuit the current will increase many times. Using PPTC in series between the

connector and host power supply will limit the current cut the keyboard/ mouse port to

the specified maximum.

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CONCLUSION

PPTC resettable fuses are designed for today’s demanding electronic and electrical

industries. The concept of a self-resetting fuse of course predates this technology.

Bimetal fuses, for example are widely used in appliances such as hairdryers, but these

are generally large current devices. PPTC resettable fuses compete with another

common overcurrent protection device, namely positive temperature coefficient (PTC)

ceramic thermistors. However, PPTC fuses offer several advantages. First, they have

lower resistance and therefore lower I2R heating, and can be rated for much higher

currents. Second, the ratio between open-resistance and close-resistance is much higher

than with ceramic PTC fuses. For example, the resistance change in PTC thermistors is

generally in the range of 1–2 orders of magnitude, but with PPTC fuses, the change may

be 6–7 orders of magnitude. However, ceramic PTC fuses don’t exhibit the increase in

resistance after a reset.

The vast majority PPTC fuses on the market have trip times in the range 1–10 seconds,

but there are PPTC fuses with trip times of a few milliseconds. Generally speaking,

however, these devices are considered slow-trip fuses. The blow time depends on the

overcurrent, so that a fuse that may open within a few milliseconds with a severe

overload, may take tens of seconds for a light overload. They are ideal for all low

voltage DC and AC application.

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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

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