For protection of semiconductor devices, the thermal energy (I2t) of the semiconductor fuse is selected less than the thermal energy (I2t) of the power device.
There is no semiconductor material used for construction of fuse, the special category of the HRC fuse used for protection of the semiconductor devices is called a semiconductor fuse.
Why Fast acting semiconductor fuses are required for protection of semiconductor devices?
The heavy current passing through the device during short circuit fault also raise the junction temperature and if the junction temperature of the semiconductor devices increases above its rated capacity,the device is prone to fail.
If the current flowing through the semiconductor devices is more than the rated current of the device, it can leads to rise in device temperature. If the short circuit current pass through the semiconductor device, the device is apt to fail if the fault current is not immediately interrupted by blowing off the fuse.
The resistance of the semiconductor devices gets halved with every rise in 10 degree- centigrade temperature. Thus the temperature rise caused by overloading or over-current during fault condition can permanently damage the semiconductor device.
The semiconductor devices are prone to failure with rise of device temperature caused by short circuit or over current through the device therefore a fast acting fuse is required to be installed for device protection.
The semiconductor fuses are widely used in the power electronics application like thyristor DC drives, Variable frequency drives, Uninterrupted Power Supplies (UPS), Rectifiers etc.
Semiconductor fuse construction:
A semiconductor fuse is a high rupturing capacity (HRC) fuse. The HRC fuse has a fuse element which is surrounded by a filler and enclosed by fuse body.
The current pass through the fuse element and heat is produced according to the element resistance. The heat generated in the fuse element is passed to the filler and transferred to the fuse body. The filler material like sand and quartz provides excellent heat transfer and thus the lower cross section area fuse element can be used. The small area of the fuse element make the fuse melt under short circuit condition.The filler also absorbs the arc energy when the fuse clears the fault.
Semiconductor Fuse Characteristics
The fuse has an inverse time current characteristic.The greater the over current, the lesser the fault clearing time. The semiconductor fuse exhibit super fast fault clearing characteristics. The metal fuse characteristics of the fuse is set in line with the thermal load carrying capacity of the semiconductor. The prearcing time of the semiconductor fuse is very less. The cut-off characteristics of the fuse is as given below.
The semiconductor devices has low thermal reserves andd function reliably if the junction temperature is below 125°C. The semiconductor devices have very low temperature limit margin between the operating and the limit temperature and super- fast-acting fuses are required for protection of the semiconductor devices. The semiconductor fuse has small limiter cross-sections and temperature reached very quickly when overcurrents are present.
The fuse element of the semiconductor fuse is made of oxidant resistant fine silver, with regions of reduced cross section area called ( often called notches). The silver has melting point of 960°C which can withstand the high operating temperature of the limiter. The fuse body is made of thermally stable aluminum oxide ceramic. The semiconductor fuse construction is as given below.
The semiconductor fuse is also called current limiting or high breaking capacity fuse. The semiconductor fuses are sometimes referred to as rectifier or ultra fast fuse. The semiconductor fuse characteristics is as given below.
The time taken by the fuse element for its melting is known as prearcing time.
The fuse element of the semiconductor fuse is made of oxidant resistant fine silver with melting point of 960°C which can withstand the high operating temperature of the limiter. The fuse body is made of thermally stable aluminum oxide ceramic. The semiconductor fuse construction is as given below.
Application of Semiconductor fuse:
Semiconductor fuses are widely used for protection of power semiconductor devices. The semiconductor fuses are used in following equipment.
- DC Drives
- AC Drives
- UPS system
- Soft starter
- Electronic equipment