19-07-2011, 04:09 PM
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.
These polyfuses have also been used as a circuit protection devices because it is a PPTC thermistor. While it is being used as a circuit protection device, it is connected in series with the supply and circuit. As current through the polysilicon increases due to some faults in the circuit, the resistance increases and limits the current through the circuit. Such polyfuses are available in various packages and with various voltage and current ratings. Since the polyfuse require less time to vary its resistance, it very useful in the protection of sensitive and expensive circuit components.
These polyfuses have also been used as a circuit protection devices because it is a PPTC thermistor. While it is being used as a circuit protection device, it is connected in series with the supply and circuit. As current through the polysilicon increases due to some faults in the circuit, the resistance increases and limits the current through the circuit. Such polyfuses are available in various packages and with various voltage and current ratings. Since the polyfuse require less time to vary its resistance, it very useful in the protection of sensitive and expensive circuit components.
