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Posted on 23 April 2019

Basic Considerations for Semiconductor Protection with Fuses

 

 

 

 

 

 

 

Solid state devices have progressed through several generations of sophistication since their introduction in the 1940s. Fuse designs have changed to match solid state protection demands. The protection task seems simple - choose a fuse of correct voltage and ampere rating which will protect a solid state device (diode, silicon-controlled rectifier, triac, etc.) through a wide range of overcurrents, yet carry normal rated loads without deterioration through a long life.

Considerations when using fuses for protection

Solid state power devices operate at high current densities. Cooling is a prime consideration. The fuse should be cooled with the solid state device. Cycling conditions must be considered. The ability of solid state devices to switch high currents at high speed subjects fuses to thermal and mechanical stresses. Proper fuse selection is mandatory for long-term reliability.

Solid state devices have relatively short thermal time constants. An overcurrent which may not harm an electromechanical device can cause catastrophic failure of a solid state device. Many solid state devices have an overcurrent withstand rating which is termed “I2t for fusing”. These values are found in most power semiconductor application handbooks. Fuses intended for solid state device protection are rated in terms of total clearing I2t. Fuses and devices are matched so that the total clearing I2t of the fuse is less than the withstand I2t for the device.

Example of I²t curve for fuses

Figure 1. Example of I²t curve for fuses

The published fuse total clearing I2t values are derived from short-circuit test oscillograms of the fuse under controlled conditions. The end application can vary significantly from the tested conditions. The specifier must take these differences into account since they will affect fuse clearing I2t.

 

For more information, please read:

Semiconductor Fuses - General

Basic Fuse Construction and Operation

Semiconductor Fuses: Terms and Explanations

How To Read A Fuse Time-Current Curve

Introduction to Current Rating for Fuses

I²t Curves and Operating Times for Fuses

Let-Thru Current and l²t for Fuses

 

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