Thermal Design Power Calculator

Calculator Inputs

Responsive form: three columns on large screens, two on smaller, one on mobile.

Calculation Method

Please choose a method.

Devices (count)

Enter at least 1.

Utilization / Duty (%)

Enter 0–100.

Supply Voltage (V)

Typical core or rail voltage.

Supply Current (A)

Average current at the chosen load level.

Heat Fraction (%)

Use 100% when nearly all power becomes heat.

Output Power (W) (optional)

If blank, calculator uses Vout × Iout.

Output Voltage (V)

Output Current (A)

Efficiency (%)

Loss = Pin − Pout, with Pin = Pout/eta.

Switching Loss (W)

Conduction Loss (W)

Leakage Loss (W)

Other Losses (W)

Derating / Safety (%)

Adds margin to heat load for real conditions.

Ambient Temp (C)

Enter a valid ambient temperature.

Max Allowable Temp (C)

Enter a valid maximum temperature.

RthetaJC (C/W)

RthetaCS (C/W)

RthetaTIM (C/W)

Provided RthetaSA (C/W) (optional)

Used to estimate resulting junction/case temperature.

Reset

Note This calculator provides engineering estimates. Validate with datasheets, thermal testing, and worst-case conditions.

Example Data Table

Use these sample rows to understand typical input patterns.

Scenario	Heat Load (W)	Ambient (C)	Max (C)	Fixed Rtheta (C/W)	Required RthetaSA (C/W)
Compute device, 1.1 V × 35 A, 90% duty	34.650	30	95	0.700	1.175
Converter, 12 V × 4 A at 92% efficiency	4.174	25	90	0.450	15.135
Custom losses, 1.2+2.8+0.4+0.6 W at 100%	5.000	35	105	0.900	13.100
Two devices, 10% derating applied	11.000	30	95	0.700	5.209

The required RthetaSA values vary widely. This helps you compare passive versus forced-air cooling needs.

Formula Used

Heat load

Pin = V × I × Duty

Pheat = Pin × HeatFraction

Ploss = Pout × (1/eta − 1)

Ptotal = Pheat_or_loss × Devices × (1 + Derating)

Thermal resistance targets

DeltaT = Tmax − Tambient

Rtheta_total(required) = DeltaT / Ptotal

Rtheta_fixed = RthetaJC + RthetaCS + RthetaTIM

RthetaSA(required) = Rtheta_total(required) − Rtheta_fixed

Test = Tambient + Ptotal × (Rtheta_fixed + RthetaSA(provided))

How to Use This Calculator

Choose a method that matches your device type and loss model.
Enter device count, utilization, and a realistic derating margin.
Provide ambient and maximum allowable temperatures for your design limit.
Enter thermal path resistances from datasheets or measurements.
Optional: add a heatsink RthetaSA to estimate junction/case temperature.
Submit to view results, then export CSV or PDF for documentation.

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