Calculator Inputs
Use direct loss, resistive loss, or efficiency loss. The input area uses three columns on large screens, two on medium, and one on mobile.
Example Data Table
| Scenario | Method | Base Loss (W) | Total θ (°C/W) | Rise (°C) | Final Temp (°C) | Headroom (°C) |
|---|---|---|---|---|---|---|
| Power transistor on heatsink | Current and resistance | 11.52 | 4.80 | 45.62 | 75.62 | 49.38 |
| Enclosure heater strip | Direct power loss | 18.50 | 3.60 | 69.93 | 94.93 | 10.07 |
| DC converter stage | Voltage, current, and efficiency | 17.28 | 4.30 | 51.27 | 86.27 | 23.73 |
Formula Used
Direct loss mode: Ploss = Pdirect
Resistive mode: Ploss = I2 × R
Efficiency mode: Ploss = V × I × (1 − η), where efficiency uses decimal form.
Effective loss: Peffective = Ploss × Duty Cycle × (1 + Safety Margin)
Total thermal resistance: θtotal = θjc + θcs + θsa + θextra
Thermal rise: ΔT = Peffective × θtotal
Final temperature: Tfinal = Tambient + ΔT
Headroom: Headroom = Tmax − Tfinal
These equations estimate steady-state temperature rise. For fast transients or pulsed thermal impedance curves, use detailed thermal network models.
How to Use This Calculator
- Select the power method that best matches your data source.
- Enter loss inputs, then set duty cycle and safety margin.
- Fill ambient temperature and the highest allowable operating temperature.
- Enter each thermal resistance segment or leave unused segments as zero.
- Press the calculate button to show results above the form.
- Review rise, final temperature, headroom, and required thermal resistance.
- Export the calculation using the CSV or PDF buttons.
Frequently Asked Questions
1. What does thermal rise mean?
Thermal rise is the temperature increase above ambient caused by power dissipation moving through a thermal resistance path. It helps engineers compare design options and check if a device stays inside safe temperature limits.
2. When should I use direct power loss mode?
Use direct power loss mode when testing, simulation, or a datasheet already provides heat dissipation. It is the quickest method when electrical loss has been measured or validated elsewhere.
3. Why is duty cycle included?
Duty cycle adjusts the average thermal load for intermittent operation. A device that runs part time often produces less average heating than a continuously loaded device with the same instantaneous loss.
4. What does safety margin do?
Safety margin increases the calculated heat load to cover tolerance, aging, fouling, airflow uncertainty, or mounting variation. It gives a more conservative result when exact conditions are not guaranteed.
5. Can I enter zero for unused resistance sections?
Yes. If a path segment does not apply, enter zero. The calculator adds all thermal resistance terms, so unused fields can stay at zero without affecting the final estimate.
6. Is this suitable for transient heating analysis?
Not by itself. This page estimates steady-state rise. Short pulses, startup surges, and cycling loads may require transient thermal impedance curves or a time-domain thermal network model.
7. What if the final temperature exceeds the limit?
Reduce power loss, lower ambient temperature, improve airflow, use a better interface material, or decrease total thermal resistance. The required resistance output helps set a clearer cooling design target.
8. Why is required maximum total resistance useful?
It shows the highest total thermal resistance your design can tolerate while staying within the temperature limit. You can compare that number against heatsinks, enclosures, or package options quickly.