30 Nov '16, 3pm

Why does power dissipation of a Darlington transistor tend higher than single bipolar transistor in switching apps?

Why does power dissipation of a Darlington transistor tend higher than single bipolar transistor in switching apps?

—In switching applications, a single transistor can saturate, resulting in a low VCE of 0.3 to 0.4 V. However, in a Darlington pair, the output transistor is prevented from saturating by the negative feedback provided by the driver transistor. If the collector voltage drops below the sum of the VBE of the output transistor (about 0.7 V) and the VCE(sat) of the driver transistor (about 0.3 V), the drive current to the output transistor is reduced, preventing it from going into saturation itself. Therefore, the effective VCE(sat) of the Darlington pair is 1 V or more, resulting in much higher dissipation at a given current level.

Full article: http://circuitcellar.com/test-your-eq/issue-272-eq-answers/

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