14-12-2012, 02:16 PM
POLYFUSE
POLYFUSE.docx (Size: 56.25 KB / Downloads: 436)
What is Fuse?
Fuses work between life and death of a circuitry.Choosing the right fuse is important for prolongedprotection of the circuitry from over-current andvoltage spikes. Conventional fuse is either ‘ok’ or‘blown,’ but PolyFuse is a resettable fuse by itself
Polyfuse
Poly fuse is a new technique for circuit protection. It is not only a fuse but also a Polymeric Positive Temperature Co-efficient (PPTC)thermistors. The main advantages of polf fuse is its resettable property, so it is also called resettable fuses. It provides better temperature stability , so pptc also called thermal fuse. Sometimes poly fuse takes the names like multi fuse and poly switch. surface mount poly fuse is the important category, which used in motherboard and usb hubs.
A polyfuse as the name suggests, is not exactly a fuse but it is a one time programmable semiconductor component which is a polymeric positive temperature coefficient (PPTC) device which is used in standard CMOS process. It being a one time programmable switch, these polyfuses are commonly used to store permanent data like chip identification number or some other data that does not need to be changed in future, it can also be used as a circuit protection devices similar to a fuse.
The polyfuses consist of siliced polysilicon which is a compound of silicon with an element like tungsten that is more electro-positive than silicon. These compounds being a positive temperature coefficient substance, they can be programmed by applying a high voltage across them. When a high voltage is applied, a high current flows through the polyfuse and this high current generates heat which alters the structure of the substance and results in an exponential increase of the resistance of the material used. Normally this ploy silicon line is a low resistive path. Since the resistance of the polysilicon line is increasing with increase in temperature this can also be considered as a thermistor characterized by positive thermal coefficient. The standard resistance of a polyfuse before programming is considered to be less than 100ohm and that after programming should be greater than 10K ohm. The schematic of a polyfuse is as shown in the figure.The programming of the polyfuse is done by firing up the NMOS transistor. When the gate of the nmos transistor is given a positive potential, a high current will begin to flow through the polyfuse. As high current flows through the material, heat is generated and this heat physically alters the structure of the polyfuse material and increases the resistance of the polyfuse. The major advantage of using a polyfuse in CMOS is its low programming time and high reliability.
Constructional details of poly fuse
PPTC fuses are constructed by a non-conductive polymer plastic film with conducting chains of carbon molecules. Non-conductive polymer plastic film exhibits either in Crystalline or Semi-crystalline (conduction) state or Amorphous phase (non conduction state).
Principle of operation of poly fuse
When very small current will flow through the circuit, heat generated will be equal to heat loss to atmosphere. But when very high current flow will leads to excess generation of heat that leads to state transition of poly fuse from crystalline to amorphous state. Amorphous state offer high resistance and circuit get isolated from power supply.