Effects of Harmonic Currents on Semiconductor Fuse Ratings

In power electronic applications, proximity and skin effects cause an increase in the resistance of fuse elements and possible unequal sharing of the total current between multiple parallel elements. In this paper the current density distribution is calculated for typical arrangements of fuse strips as a function of frequency, and methods of de-rating the fuse are discussed. The calculations are done using an equivalent wiregrid array method. De-rating factors are calculated for sinusoidal and some non-sinusoidal waveforms commonly found in power electronic applications. The implications for the design of semiconductor fuses for h.f. applications are also considered.

The continuous current ratings of semiconductor fuses are determined in standard "type" tests with specified conditions. In real applications the conditions are always different, and it is well known that adjustments have to be made to allow for ambient temperature, size of connecting cables and any air cooling. Standard type tests are done at 50 or 60Hz, but in modern power electronic applications the frequency may be much higher. It is not widely recognized that this may require de-rating of the fuses. A recent paper described problems caused by premature opening of fuses in the a.c. line inputs to adjustable-speed drives which drew unusually high harmonic currents.