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Posted on 20 April 2019

Semiconductor Fuses - General

 

Fuse arrangements

Figure 1. Possible arrangements for semiconductor fuses in a converter bridge (a1 and b1) and for an AC controller (a2 und b2); a) Arm fuse or b) Phase fuse

Diodes and thyristors do not limit short-circuit current. For this reason, external measures have to be taken to ensure short-circuit protection. The silicon chip of a thyristor or rectifier diode has a very low thermal capacity, which is why strong, fast rising overcurrents under short circuit conditions can destroy this part within just a few milliseconds. For this reason, normal low-voltage fuses, including quick-blow fuses, are not suitable for protecting rectifier diodes and thyristors from being destroyed by short circuits. Instead, purpose-developed fuses – which are available as fast-blow, super-fast blow and ultra-fast blow fuses – or simple semiconductor fuses have to be used.

The most common causes of short-circuits in converters are:

  • Short circuit in the load or the connecting lines between the converter unit and load
  • Short-circuit in a rectifier diode or thyristor due to loss of blocking ability (sudden failure)
  • Loss of stability in inverters or converters due to trigger error

If a semiconductor fuse is connected in series to each rectifier diode and each thyristor, where the diodes and thyristors are used in the main legs of the converter circuit, the diodes and thyristors can be protected in all of the above short-circuit cases. At the same time, all other components in the main circuit are also protected, since chokes, transformers, resistors, etc. are far less sensitive to overcurrents than the semiconductor components. In bridge circuits, two power semiconductors each have a common AC terminal. For this reason they can be protected by a common fuse in the AC supply line (Figure 1b). This is known as a phase or AC side fuse. This has the advantage that fewer fuses are needed and that the switching voltage that occurs when the fuse element melts does not put any load onto the semiconductor components.

For high current loads on thyristors or rectifier diodes, as may be the case in systems with forced cooling, and high operating voltage at the same time, it may well be difficult to find a fuse with sufficient rated current (effective current in the phase 2 times higher than in each of the two arms) and with an operating i²t, on the other hand, that is lower than that of each of the two semiconductor components. In this case, each bridge arm must be allocated its own fuse (Figure 1a). In AC controllers, a common fuse normally suffices  (Figure 1b2).

If rectifier diodes or thyristors are connected in parallel for high power densities, each semiconductor component is equipped with a fuse. The advantage of this is that if one component or fuse suddenly fails, the device will remain fully functional – with less power, accordingly. The internal resistances of the fuses will also compensate somewhat for the differences in the characteristics of the semiconductor components, meaning the current distribution will be more homogenous. For short-circuit protection using blow-out fuses, short circuits are assumed to seldom occur. There are, however, applications where load short-circuit happens more frequently. In this case, a DC side high-speed circuit breaker, a power circuit breaker, or another device to block the driver pulses of the thyristor (driver signal blocking) is needed on the load side. This has to be used in addition to the fuses to cater for the other possibilities of short circuiting.

 

For more information, please read:

Semiconductor Fuses: Terms and Explanations

Fuse Placement in Typical Converter Circuits

Dimensioning Semiconductor Fuses

General Voltage Surge Protection

 

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