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Explain the concept of thermal runway in BJT
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• The maximum power that a transistor can dissipate without getting damaged, depends largely on the maximum temperature that a collector - base junction can withstand.

• The rise in the collector - base junction takes place due to two reasons:

Due to increase in the ambient temperature and

Due to the internal heating

Out of them the internal heating process is cumulative as explained below:

  1. An increase in collector current IC increases the powerdissipated in the collector-base junction of the transistor.

  2. This will increase the temperature of C-B junction.

  3. As the transistor has a negative temperature co-efficient of resistivity, increased junction temperature reduces the resistance.

  4. The reduced resistance will increase the collector current further.

  5. This becomes a cumulative process which will finally damage the transistor due to excessive internal heating. This process is known as "Thermal Runaway"

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How to avoid the thermal runaway?

Never exceed the collector current beyond a certain maximum value specified by the manufacturer

Never exceed the internal power dissipation above the maximum permissible value Use heat sink

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