BJT Total Dissipation

Formula

Description

The total power dissipated in a bipolar junction transistor is the sum of collector-emitter power and base-emitter power. In most practical circuits, the collector-emitter term (VCE × IC) dominates because IC is much larger than IB. When the BJT is used as a switch in saturation, VCE_sat is typically 0.1-0.3V, making dissipation quite low. In the active (linear) region, VCE can be significant and the BJT acts as a variable resistor, dissipating potentially large amounts of power. The base-emitter term is usually small but should not be ignored in precision calculations.

Variables

• P — Total power dissipation (W)
• VCE — Collector-emitter voltage (V)
• IC — Collector current (A)
• VBE — Base-emitter voltage (V)
• IB — Base current (A)

Practical Notes

In saturation, VCE_sat increases with collector current and decreases with higher base overdrive. A common rule of thumb is to set IB = IC / (β × 0.5) to ensure deep saturation. Safe operating area (SOA) curves in the datasheet define the maximum allowable combinations of VCE and IC.