BJT Power Dissipation

Compute transistor heat quickly: P = VCE × IC. Optional thermal rise estimate and Show Work.

How to Use

  1. Enter VCE (collector-emitter voltage) and IC (collector current).
  2. Optionally enter θJA and ambient temperature to estimate junction temperature.
  3. Open “Show Work” for step-by-step math in base units.
  4. Use “Share Link” to generate a restorable URL (only on button click).
Transistor Lab View
Visual feedback: dissipation + estimated temperature rise.
VCE
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IC
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P
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TJ
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Risk: —
Collector Emitter Base Power: P = VCE × IC (plus optional switching & thermal estimate) VCE IC Visual heat overlay scales with dissipation; TJ uses θJA + ambient if provided.
Inputs & Settings
Enter VCE and IC. Thermal inputs are optional.
Sat: ~0.05–0.3V • Linear: several volts
Common: 20mA small-signal • 1–5A power BJT
Package-to-ambient thermal resistance (datasheet).
Used only if θJA is provided.
Optional Switching Estimate (advanced)
Set to 0 to disable switching estimate.
Approx rise+fall time (datasheet / measured).
Model: Psw ≈ 0.5 × VCE × IC × (tr+tf) × f (very simplified).
Show Work (step-by-step)
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Work is shown in base units (V, A, W, °C/W) for clarity and consistency.

Reference Formulas

Core: P = VCE × IC

  • Conduction dissipation: Pcond = VCE × IC
  • Thermal rise (estimate): ΔT = P × θJA
  • Junction temp (estimate): TJ = TA + ΔT
  • Switching (simplified): Psw ≈ 0.5 × VCE × IC × (tr+tf) × f
Always confirm limits with the device datasheet: SOA, max TJ, package θ values, and heat-sinking.

FAQ

What is “power dissipation” in a BJT?

It’s the electrical power turned into heat inside the transistor. In steady conduction, it’s approximated by P = VCE × IC.

Why is VCE(sat) so important for switches?

In saturation, VCE is low, so conduction loss is usually small. If the BJT isn’t driven hard enough, VCE rises and heat increases fast.

Is θJA accurate for real builds?

It’s a rough estimate. θJA depends on PCB copper, airflow, mounting, and heat-sinking. Use it for sanity-checking, not final qualification.

What about SOA (safe operating area)?

SOA limits can be stricter than raw power. A device can be “under Pd” but still fail if VCE and IC fall outside SOA for the time scale.

Tool Info

Last updated:

Updates may include UI improvements, unit support, and edge-case handling.