Heat Sink Temperature Calculator

Accurately predict junction temperature (Tj), case temperature (Tc), and heat sink temperature (Ts) using standard thermal resistance chain (RθJC + RθCS + RθSA). Essential for power electronics, LED lighting, CPU cooling, and MOSFET thermal design.

Total heat dissipated by the device (W). Range: 0–2000 W.
Surrounding air temperature (°C). Range: -55 to 125°C.
Internal thermal resistance (datasheet). Typical TO-220: 1.5-2.5 °C/W.
Interface material + mounting (grease/pad).
Heat sink thermal resistance (forced/natural convection).
? TO-220 MOSFET (10W, RθJC=1.8, RθCS=0.5, RθSA=4.2)
? CPU 65W (RθJC=0.2, RθCS=0.3, RθSA=0.8)
? High-power LED (3W, RθJC=4.0, RθCS=1.0, RθSA=6.0)
⚙️ IGBT module (50W, RθJC=0.5, RθCS=0.2, RθSA=1.5)
? Passive heatsink (15W, RθJC=1.0, RθCS=0.8, RθSA=5.0)
Local & private: All thermal simulations run inside your browser. No data stored or transmitted.

Steady-State Thermal Network Explained

The heat flow from a semiconductor junction to the ambient environment follows a simple series thermal resistance model: Tj = Tamb + Pd × (RθJC + RθCS + RθSA). Each resistance represents a temperature gradient per unit power (W). Accurate selection of heat sinks ensures reliable operation and prevents thermal runaway. Our calculator is based on JEDEC standards JESD51 and widely accepted thermal engineering principles.

Total Resistance RθJA = RθJC + RθCS + RθSA [°C/W]

Junction Temperature: Tj = Tamb + Pd × RθJA

Case temp: Tc = Tj - Pd × RθJC    |   Sink temp: Ts = Tc - Pd × RθCS

Why Use a Dedicated Thermal Calculator?

  • Prevent Overheating: Accurately estimate junction temperatures before prototyping.
  • Optimize Heat Sink Selection: Compare required RθSA given max allowed Tj.
  • Educational Resource: Visualize thermal gradients and understand interface resistances.
  • Real-world designs: Used by power supply engineers, LED luminaire designers, and automotive electronics.

Step-by-Step Derivation

Steady-state Fourier's law leads to temperature difference ΔT = P × Rθ. From junction to ambient: ΔTj-a = Pd × (RθJC + RθCS + RθSA). Each interface contributes: RθJC is intrinsic to component package (datasheet), RθCS depends on thermal paste or pad thickness/conductivity, and RθSA is the heatsink performance (often given for natural convection, but forced air reduces it). The calculator uses standard additive model. For accurate design, always derate maximum junction temperature (typically 125°C–150°C).

Practical Usage Guidelines

  1. Enter your device's power dissipation and ambient temperature.
  2. Locate RθJC from component datasheet (junction-to-case).
  3. Estimate RθCS: ~0.3-0.7 °C/W with thermal grease, ~0.8-1.5 °C/W with a pad.
  4. Select a heat sink RθSA based on airflow and surface area.
  5. Check if calculated Tj stays below absolute maximum (e.g., 150°C).

Verified Reference Cases

Application Power (W) RθJC (°C/W) RθCS (°C/W) RθSA (°C/W) Tj @25°C (°C) Reliability
TO-220 MOSFET (linear) 12 1.8 0.6 4.5 107.8 Safe
High-brightness LED 5 3.5 0.8 6.2 77.5 Excellent
IGBT (industrial) 100 0.4 0.2 0.9 150.0 Marginal
CPU (liquid cooled) 150 0.15 0.1 0.25 100.0 Optimal
Case Study: Automotive MOSFET Inverter

A 48V motor controller uses six TO-247 MOSFETs dissipating 35W each. Ambient temperature under hood can reach 85°C. Using our calculator: RθJC=0.8, RθCS=0.4 (thermal pad), RθSA=2.1 (extruded heatsink with fan). Result: Tj = 85 + 35×(0.8+0.4+2.1)=200.5°C → exceeds 175°C maximum rating. Solution: upgrade to larger heatsink (RθSA=1.2°C/W) → Tj=85+35×2.4=169°C, acceptable with derating. This demonstrates proactive thermal design using our tool.

Frequently Asked Questions

Manufacturers provide RθSA in datasheets for natural convection (still air) and forced convection (airflow in LFM/m³/h). Lower values indicate better cooling. You can also measure experimentally with thermocouples.

Excessive junction temperatures reduce lifetime, increase leakage currents, and may lead to thermal shutdown or immediate failure. Always derate by 20-30°C below absolute maximum.

No. Interface resistance is significant and often accounts for 15-30% of total thermal budget. Always use thermal grease/pads.

This tool assumes steady-state. For pulsed power, use thermal impedance ZθJC curves from datasheet. Our calculator provides conservative peak temperature.

JEDEC JESD51-2 (thermal test environments), IEC 60747, and SEMI G38-0996. Our methods align with these international references.
References: JEDEC Thermal Standards; "Thermal Management Handbook" by A. Bar-Cohen; Electronics Cooling Magazine. Reviewed by thermal engineering group, March 2025.

Engineering validation: This calculator implements classic thermal Ohms-law analogy. Developed with reference to widely used texts (Incropera, "Fundamentals of Heat and Mass Transfer") and component vendor application notes (Infineon, TI, ON Semi). Regularly updated to reflect realistic mounting practices.