Calculator Inputs
Formula Used
Layer resistance: Rlayer = t / (k × A) + Rcontact
Series resistance: Rseries = R1 + R2 + R3 + ...
Parallel resistance: Rparallel = 1 / (1/R1 + 1/R2 + 1/R3 + ...)
Total resistance: Rtotal = Rfixed + Rlayers + Rparallel
Design power: Pdesign = P × (1 + safety factor / 100)
Temperature rise: ΔT = Pdesign × Rtotal
Junction temperature: Tj = Tambient + ΔT
Allowable power: Pallow = (Tmax - Tambient) / Rtotal
How to Use This Calculator
- Enter the electrical power loss in watts.
- Add ambient and maximum allowed temperature values.
- Enter device, interface, heat sink, and board resistances.
- Add detailed layer data for pads, spreaders, and insulators.
- Enter parallel branch resistances if heat has multiple paths.
- Press the submit button to calculate the network result.
- Review temperature rise, junction temperature, and margin.
- Use CSV or PDF export for records and reviews.
Example Data Table
| Case | Power W | Ambient °C | Fixed R °C/W | Layer R °C/W | Parallel R °C/W | Total R °C/W | Estimated Tj °C |
|---|---|---|---|---|---|---|---|
| MOSFET heat sink | 30 | 35 | 3.35 | 0.26 | 2.61 | 6.22 | 240.26 |
| LED module plate | 12 | 30 | 4.10 | 0.42 | 3.20 | 7.72 | 122.64 |
| Regulator copper plane | 5 | 45 | 6.00 | 0.18 | 0.00 | 6.18 | 75.90 |
Understanding Thermal Resistance Networks
Why Thermal Networks Matter
A thermal resistance network converts a heat problem into a clear circuit style model. Heat flow acts like current. Temperature difference acts like voltage. Thermal resistance acts like electrical resistance. This view helps designers compare packages, interfaces, heat sinks, copper planes, and airflow paths.
Series and Parallel Paths
A series path adds each resistance in order. The same heat must pass through every layer. This is common for a junction, case, pad, sink, and surrounding air. A parallel path divides heat into branches. Copper spreading, chassis contact, and heat sink fins may work together. The equivalent branch resistance becomes lower when useful paths are added.
Design Margin
The calculator uses power loss and total resistance to estimate temperature rise. It then adds ambient temperature to estimate junction or source temperature. A safety factor can increase design power. This gives a conservative result. The margin compares the maximum allowed temperature with the estimated operating temperature. A positive margin suggests the network is acceptable. A negative margin means more cooling is needed.
Layer Modeling
Layer inputs use thickness, conductivity, and area. Thin layers with high conductivity give low resistance. Small areas raise resistance. Contact resistance can be added for pads, grease, clips, screws, or imperfect surfaces. These values often dominate practical assemblies. Always use realistic interface data when available.
Electrical Design Use
Thermal checks are important in power electronics. Regulators, MOSFETs, rectifiers, LEDs, and motor drivers can fail when heat is ignored. A quick network model helps choose packages and heat sinks before layout is locked. It also supports review notes and component comparisons.
Interpreting Results
Use the calculated total resistance as a design estimate. Compare the junction temperature with device ratings. Check allowed power for the same network. Review the strongest resistance contributors first. Reducing the largest term usually gives the best improvement. Increase copper area, improve airflow, select a lower resistance interface, or use a larger heat sink. Repeat the calculation after each change. The exported report can document assumptions for later testing.
Best Practice
Treat the model as a planning tool, not final proof. Measure prototype temperatures with steady power. Record sensor location, airflow, mounting pressure, and ambient conditions during validation. Keep validation notes with exported results.
FAQs
What is thermal resistance?
Thermal resistance shows how strongly a material or path resists heat flow. It is commonly measured in °C/W. A lower value means heat moves more easily.
How is a series thermal network calculated?
Series thermal resistances are added directly. This applies when heat must pass through each layer one after another, such as junction, case, pad, sink, and air.
How is a parallel thermal network calculated?
Parallel paths are combined using reciprocal resistance. The calculator adds each reciprocal, then takes the inverse. More useful paths usually reduce total resistance.
What does θJC mean?
θJC means junction to case thermal resistance. It describes the heat path inside a device package, from the semiconductor junction to the outside case.
Why is contact resistance important?
Contact resistance accounts for imperfect surface contact. Pads, grease, screws, clips, and mounting pressure can change it. It can strongly affect real cooling results.
What does the safety factor do?
The safety factor increases the design power before temperature rise is calculated. This gives a more conservative result for tolerance, aging, dust, and airflow changes.
Can this calculator replace testing?
No. It is useful for design estimates and comparison. Final products should be checked with measured temperatures under expected load, airflow, and enclosure conditions.
Why export CSV or PDF results?
Exports help keep assumptions, inputs, and results together. They are useful for design reviews, project records, heat sink comparisons, and later prototype validation.