Transistor Power Dissipation Calculator
Estimate steady-state conduction loss, temperature rise, junction temperature, and thermal margins for a BJT or MOSFET.
The calculator compares nominal loss against device power and maximum junction-temperature ratings using the selected package and PCB junction-to-ambient thermal resistance.
Engineering tool
Transistor Power Dissipation Calculator
Estimate BJT or MOSFET conduction loss, thermal rise, junction temperature, and rating margins.
Continuous device conduction current.
Voltage across the conducting BJT.
Local board or air temperature around the package.
Junction-to-ambient resistance for the actual package and layout.
Absolute maximum Tj from the device datasheet.
Rated dissipation before application derating.
Thermal operating status
Safe
The nominal point is below the 80% warning thresholds. Verify datasheet derating and worst-case thermal conditions before release.
Result console
- Power dissipation
- 100mW
- Temperature rise
- 10°C
- Estimated junction temperature
- 35°C
- Power margin
- 900mW
- Junction temperature margin
- 115°C
- Power utilization
- 10%
Formula reference
Transistor Power and Thermal Formulas
The θJA model is a steady-state first-order estimate and must use thermal resistance representative of the real layout and mounting.
BJT loss: P = VCE × IcMOSFET loss: P = Id² × RDS(on)Temperature rise: ΔT = P × θJAJunction temperature: Tj = Ta + ΔTPower margin = Prated - PTemperature margin = Tjmax - TjVariable definitions
- Ic / Id
- BJT collector or MOSFET drain current
- VCE
- voltage across the conducting BJT
- RDS(on)
- MOSFET on resistance
- θJA
- junction-to-ambient thermal resistance
Worked Examples
BJT Example
Ic = 500 mA, VCE = 0.2 V, θJA = 100 °C/W, Ta = 25 °C
P = 0.2 × 0.5 = 0.1 W
ΔT = 0.1 × 100 = 10 °C
Tj ≈ 35 °C
MOSFET Example
Id = 2 A, RDS(on) = 50 mΩ, θJA = 80 °C/W, Ta = 25 °C
P = 2² × 0.05 = 0.2 W
ΔT = 0.2 × 80 = 16 °C
Tj ≈ 41 °C
Engineering Notes
BJT conduction loss
BJT conduction loss depends on collector-emitter voltage and collector current.
MOSFET I-squared loss
MOSFET conduction loss depends on RDS(on) and the square of drain current.
Use realistic thermal resistance
Thermal resistance depends on package, PCB copper, airflow, mounting, and heatsinking.
Always derate
Operate below absolute maximum power and junction-temperature ratings with worst-case margin.
Analyze pulses separately
Pulsed loads require transient thermal impedance, duty-cycle, and safe-operating-area analysis.
Support reference
FAQ
How do you calculate transistor power dissipation?
For a conducting BJT, estimate loss using P = VCE × Ic at the selected operating point. Switching circuits may also require switching-loss and transient analysis.
How do you calculate MOSFET conduction loss?
Use P = Id² × RDS(on). Use RDS(on) at the actual gate voltage and expected junction temperature, not only the room-temperature typical value.
What is junction temperature?
Junction temperature is the estimated semiconductor die temperature. In the steady-state θJA model, Tj = Ta + P × θJA.
What does θJA mean?
θJA is junction-to-ambient thermal resistance in °C/W. It represents the complete heat path through package, PCB, mounting, and surrounding air under specified test conditions.
How do you reduce transistor heating?
Reduce current or voltage loss, choose a lower RDS(on) MOSFET or lower VCE device, increase PCB copper, improve airflow, add a heatsink, or select a more thermally capable package.
Documentation
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