Fuses for the protection of circuits containing semiconductor devices

The energy let-through to a protected circuit during short-circuit conditions is related to the integral with respect to time of the square of the instantaneous current, i.e. 

i2dt. For any circuit and piece of equipment there is a maximum value of this quantity which must not be exceeded if healthy equipment is not to be damaged. This quantity is designated I2t and the withstand I2tvalues of circuits have to be known or determined before suitable fuses can be selected. Further information on this topic is provided later, in Section 7.8.

The I2t withstand values of semiconductor devices of given ratings are consid- erably lower than those of other components and circuits of corresponding ratings. Fuselinks used in circuits containing semiconductor devices must therefore be capable of operating more rapidly at given currents than fuselinks used in other applications. Cartridge fuselinks of the same basic constructions as those described in Section 4.1 were developed and have been available for some time to protect semi- conductor devices. These fuselinks, which are usually referred to as semiconductor

Figure 4.18 House service fuse

fuselinks, invariably have notched strip elements. The restricted sections have rela- tively small cross-sectional areas so that they operate at quite high temperatures at rated current, temperatures up to 250◦C being quite common. Such a mode of opera- tion limits the extra energy input needed to raise element temperatures to the melting point when short circuits occur on protected circuits and the durations of pre-arcing periods and the input I2tvalues are reduced.

Continuous operation of elements at high temperatures would cause rapid oxida- tion of materials such as copper. This would clearly be unacceptable and therefore silver elements are invariably used in semiconductor fuselinks.

To maintain stable high-temperature operation of the restricted sections of elements, heat energy must be conducted away from them. Two measures are taken to achieve this objective. First, the restrictions are made relatively short so that the energy to be conducted away from each of them is limited. Second, the widths of the elements, i.e. the unrestricted sections, are made relatively large to enable them to run at low temperatures and thus to conduct sufficient heat energy from the restrictions. As explained earlier, current interruption is effected in a fuselink when the total voltage across the arcs is sufficiently high relative to the system voltage. In practice the necessary total voltages are obtained by providing the required numbers of restricted sections.

The differences outlined above in the geometries of the elements provided in fuselinks used to protect semiconductor devices and those used in normal general- purpose fuselinks are illustrated in Figure 4.19.

To assist further in obtaining the performances required of semiconductor fuselinks, techniques are employed to ensure that their quartz filling material is very

M-effect alloy

a

b

Figure 4.19 Fuselink elements a Industrial fuselink

b fuselink (for semiconductor protection)

highly compacted. This constricts the arcs and reduces the I2tlet-through when short circuits are being cleared. This practice also increases the levels of direct current which can be interrupted by these fuselinks, an improvement which is important when semiconductor convertor equipment is to be protected.

To further reduce the I2_{t values required by fuselinks of given current ratings} when clearing short circuits, the grains of the quartz filling materials are often coated with an inorganic binder. This increases the contact areas between the grains and thereby increases the thermal conductivity of the filling material. This does not affect fuselink behaviour when short circuits occur, but increases the permissible rated currents.

Because the restrictions in the elements of semiconductor fuselinks operate at quite high temperatures when rated currents are flowing, it is essential that their bodies and terminations be able to dissipate considerable amounts of heat. They are, therefore, not normally mounted in enclosed fuse holders and it is recommended that they be placed in positions where there is adequate natural ventilation. In some cases, the elements are placed in two or more bodies mounted in parallel to increase the surface areas from which heat can be dissipated. All designs have very substantial terminations to enable heat to be conducted away from the elements.

Typical single- and double-body fuselinks are shown in Figure 4.20.

In some applications it is desirable to have a positive indication when a partic- ular fuselink has operated. Special trip-indicator devices, which are electrically and usually mechanically coupled in parallel with the fuselink, are produced for this pur- pose. Such an arrangement is shown in Figure 4.21. Operation is initiated when the element in the fuselink melts. The current then transfers to the fine element in the trip- indicator device and this element melts quickly to release the plunger which is driven out by a spring. The plunger gives local indication of operation and, if desired, it may be used to operate a microswitch, which can then in turn operate a remote-warning device or initiate additional protective equipment.

Figure 4.20 Typical British semiconductor fuselinks

Figure 4.21 Trip indicator for semiconductor fuselink

In the UK, the majority of power semiconductors are used in three-phase and single-phase equipments operating at 240 V RMS per phase and fuselinks, dimensionally standardised to comply with IEC 60269-4-1 Section IA, are available for single-phase applications with ratings up to 900 A at 240 V. They are also available for use in three-phase circuits operating up to 660 V (line) in current ratings up to 710 A. The standard dimensions are as small as practicable because many fuselinks are used in applications where space is at a premium.

Non-standard fuselinks have been produced for use in very high-power three- phase equipment with current and voltage ratings up to about 4000 A and 7·2 kV (line).

Figure 4.22 Types of fuse-holders

A recent development has been the deposition of metal fuse elements onto a high- thermal-conductivity ceramic substrate. This permits good thermal conduction from the fuse element to the substrate which gives a higher continuous-current rating and enhanced withstand to normal repetitive overloads.