Skip to content
RFrftools.io
Thermal

Heatsink Calculator

Calculate required heatsink thermal resistance and junction temperature for power devices

Loading calculator...

Formula

θSA=(TJmax−TA)/PD−θJC−θCSθ_SA = (T_Jmax - T_A) / P_D - θ_JC - θ_CS

Reference: JEDEC JESD51 thermal measurement standard

θ_SA— Heatsink-to-ambient thermal resistance (°C/W)
T_Jmax— Maximum junction temperature (°C)
T_A— Ambient temperature (°C)
P_D— Power dissipation (W)
θ_JC— Junction-to-case thermal resistance (°C/W)
θ_CS— Case-to-heatsink thermal resistance (°C/W)

How It Works

Heatsink thermal resistance calculator computes θSA requirements for safe junction temperature operation — essential for power supply design, motor drives, and high-power amplifier thermal management. Thermal engineers, power electronics designers, and product reliability engineers use this to size heatsinks and verify thermal margins. Per JEDEC JESD51-12, total thermal resistance θJA = θJC + θCS + θSA, where θJC is specified by semiconductor manufacturer (TO-220: 1-2°C/W, D²PAK: 0.5-1°C/W per JEDEC), θCS depends on interface material (thermal grease: 0.1°C/W, dry contact: 0.5°C/W, thermal pad: 0.2-0.5°C/W), and θSA is the heatsink performance. Natural convection heatsinks achieve θSA = 3-20°C/W depending on size; forced air at 2 m/s improves θSA by 3-5× per AAVID application data.

Worked Example

Select heatsink for LM7805 regulator converting 12V to 5V at 1A load. Power dissipation: Pd = (12V - 5V) × 1A = 7W. From LM7805 datasheet: θJC = 4°C/W (TO-220), Tj(max) = 125°C. Design target: Tj = 100°C at Ta = 50°C (industrial environment). Required total θJA: θJA = (Tj - Ta)/Pd = (100 - 50)/7 = 7.14°C/W. With thermal paste θCS = 0.2°C/W: θSA(max) = 7.14 - 4 - 0.2 = 2.94°C/W. Select Aavid 531202B02500G (θSA = 2.5°C/W, 50mm × 50mm × 25mm). Verify: Tj = 50 + 7×(4 + 0.2 + 2.5) = 50 + 46.9 = 96.9°C — within 100°C target with 3°C margin. For outdoor applications (Ta = 70°C), upgrade to larger heatsink or add fan.

Practical Tips

  • ✓For natural convection, allow 10mm minimum clearance around heatsink fins — blocked airflow increases θSA by 50-100% per thermal design guidelines
  • ✓Black anodized heatsinks have 10-15% lower θSA than bare aluminum due to improved radiation — significant only at ΔT > 40°C above ambient
  • ✓Forced air at 2 m/s typically reduces θSA by 3-5×; consult manufacturer curves for specific heatsink. Fan selection: 1 CFM per 5W for small enclosures per AAVID thermal handbook

Common Mistakes

  • ✗Using heatsink θSA without considering mounting orientation — vertical fins with natural convection have 20-30% lower θSA than horizontal; manufacturer specs assume optimal orientation
  • ✗Ignoring thermal interface resistance — omitting θCS = 0.5°C/W (dry contact) underestimates Tj by 3-5°C at typical power levels; always use thermal compound
  • ✗Assuming linear scaling with power — at high power density (>1W/cm²), heatsink surface becomes thermally saturated; use CFD simulation or derate published θSA by 20-30%

Frequently Asked Questions

Thermal resistance θ (°C/W) is analogous to electrical resistance: ΔT = θ × P (compare to V = R × I). It quantifies how much temperature rise occurs per watt of power dissipated. Lower θ means better heat transfer. Typical values: TO-220 θJC = 1-2°C/W, thermal grease θCS = 0.1-0.2°C/W, small clip-on heatsink θSA = 10-20°C/W, large extruded heatsink θSA = 1-3°C/W.
θJC is fixed by package design and cannot be improved by the user — it represents the fundamental thermal bottleneck. Power packages (D²PAK, TO-247) achieve θJC < 1°C/W; surface-mount packages (SOIC, QFP) have θJC = 20-100°C/W. For high-power applications, package selection is critical: a 10W device in SOIC-8 (θJC = 40°C/W) has 400°C rise before even reaching the case.
Ambient temperature Ta sets the baseline — all temperature rises add to Ta. Industrial designs use Ta = 50-70°C; consumer electronics use Ta = 35-45°C. Increasing Ta from 25°C to 50°C requires either reducing power dissipation by 25°C/θJA watts or improving θJA proportionally. Always design for worst-case ambient per IPC-9592B reliability guidelines.
θJC: fixed by package (select lower-θJC package like D²PAK instead of TO-220). θCS: use thermal grease (0.1°C/W) instead of dry contact (0.5°C/W), or use phase-change material (0.05°C/W). θSA: larger heatsink area, more fins, forced air (3-5× improvement), or liquid cooling (10× improvement). Total system θJA can be reduced from 50°C/W (no heatsink) to <2°C/W (optimized liquid cooling).
Short-term: increased Rds(on) for MOSFETs (1.5-2× at Tj(max)), reduced hFE for BJTs, potential thermal shutdown (protection feature in many ICs). Long-term: accelerated wear-out per Arrhenius model — every 10°C above design target halves expected lifetime. Per JEDEC JEP122H, operating at Tj(max) continuously results in 1000-10000 hour MTTF; operating at Tj(max) - 50°C achieves 100,000+ hour MTTF.

Shop Components

As an Amazon Associate we earn from qualifying purchases.

Thermal Paste

Thermal paste and grease for heatsink-to-component bonding

Amazon →

Heatsinks (TO-220)

Aluminum heatsinks for TO-220 and similar packages

Amazon →

Thermal Pads

Silicone thermal pads for PCB component cooling

Amazon →

Related Calculators

Power

LDO Thermal

Calculate LDO regulator power dissipation, junction temperature, thermal margin, and minimum dropout voltage for thermal design validation.

Thermal

Trace Temp

Calculate PCB copper trace temperature rise under load current using IPC-2152

Power

Buck Converter

Design a synchronous buck (step-down) converter: calculate duty cycle, inductor value, output capacitor, and input capacitor.

Thermal

Thermal Resistance Network

Calculate junction, case, and heatsink temperatures through a series thermal resistance network (θJC + θCS + θSA) for component thermal management