Polyfuse

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A Polyfuse is a one-time-programmable memory component used in semiconductor circuits for storing unique data like chip identification numbers or memory repair data. Polyfuses were developed as a replacement of laser fuses. Initially, the flexibility of electrical programming was the main driver. In addition to adding flexibility, modern polyfuses are much smaller than laser fuses, since they do not require as much distance between the devices. Laser fuses need this large distance since a lot of debris is generated by blowing the fuses up with a laser. This debris can unprogram nearby programmed devices.

[edit] History

The first polyfuses consisted of a polysilicon line, which was programmed by applying a high (10V-15V) voltage across the device. The resultant current physically alters the device and results in an increase in electrical resistance. This change in resistance can be detected and registered as a logical one. An unprogrammed polyfuse would be registered as a logical zero. These early devices had severe drawbacks like a high programming voltage and unreliability of the programmed devices.

A Fuse is a one-time over-current protection device employing a fusible link that melts 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. Polyfuse is a resettable fuse that doesn’t need to be replaced like the conventional fuse. Resettable fuses provide over-current protection and automatic restoration. Fuses work between life and death of a circuitry. Choosing the right fuse is important for prolonged protection of the circuitry from over-current and voltage spikes. Conventional fuse is either ‘ok’ or ‘blown,’ but polyfuse is a resettable fuse by itself.

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. Current can be accomplished by using resistors, fuses, switches, circuit breakers or positive temperature coefficient devices.

Resistors are rarely an acceptable solution because the high power resistors required are expensive .One shot fuses can be used but they might fatigue and they must be replaced after a fault event. Another good solution available is the resettable Ceramic Positive Temperature Coefficient (CPTC) device. This technology is not widely used because of its high resistance and power dissipation characteristics. These devices are also relatively large and vulnerable to cracking as result of shock and vibration.

The preferred solution is the PPTC device, which has a very low resistance in normal operation and high resistance when exposed to fault. Electrical shorts and electrically overloaded circuits can cause over current and over temperature damage.

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 centers.

[edit] The Basics

Technically Polyfuses are not fuses but Polymeric Positive Temperature Coefficient Thermistors (PPTC). For thermistors characterized as positive temperature coefficient, the device resistance increases with temperature. The PPTC circuit protection devices are formed from thin sheets of conductive semi-crystalline plastic polymers with electrodes attached to either side. The conductive plastic is basically a non-conductive crystalline polymer loaded with a highly conductive carbon to make it conductive. The electrodes ensure the distribution of power through the circuit.

Polyfuses are usually packaged in radial, axial, surface mount, chip or washer form. These are available in voltage ratings of 30 to 250 volts and current ratings of 20 mA to 100A.

[edit] Principle of Operation

PPTC circuit protection devices are formed from a composite of semi-crystalline polymer and conductive carbon particles. At normal temperature the carbon chains form low resistance conductive network through the polymer. In case an excessive current flows through the device, the temperature of the conductive plastic material rises. When the temperature exceeds the device's switching temperature, the crystallides in the polymer suddenly melts and become amorphous. The increase in volume during melting of the crystalline phase cause separation of the conductive particles and results in a large non-linear increase in the resistance of the device. The resistance typically increases by 3 or orders of magnitude

Polyfuse is a new standard for circuit protection. It is resettable. Many manufacturers also call it polyswitch or multifuse. Polyfuses are not fuses but polymeric positive temperature co-efficient (PPTC) thermistors. Current limiting can be accomplished by using resistors , fuses , switches or positive temperature co-efficient devices. Resistors are rarely an acceptable solution because the high power resistors that are usually required are expensive. One-shot fuses can be used, but they might fatigue, and they must be replaced after a fault event. Ceramic PTC devices tends to have high resistance and power dissipation characteristics. The preferred solution is a PPTC device which has low resistance in normal operation and high resistance when exposed to a fault. Electrical shorts or electrically over-loaded circuits can cause over-current and over temperature damage. Like traditional fuses , PPTC devices limit the flow of dangerously high current during fault conditions. Unlike traditional fuses, PPTC devices reset after the fault is cleared and the power to the circuit is removed.

[edit] Operating Parameters

• Initial resistance: 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 through the device.
• Trip current: Where 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.
• Hysteresis: The period between the actual beginning of the signaling of the device to trip and the actual tripping of the device.

[edit] Modern polyfuses

Modern polyfuses consist of a siliced polysilicon line, which is also programmed by applying a voltage across the device. Again, the resultant current physically alters the device and results in an increase in resistance. The silicide layer covering the polysilicon line reduces its resistance (before programming), allowing the use of much lower programming voltages (1.8V-3.3V). Polyfuses have been shown to reliably store programmed data and can be programmed at high speed. Programming speeds of 100ns have been reported.