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Thermal

Heatsink Selection Calculator

Calculate the required heatsink thermal resistance (θSA) to keep a device junction below its maximum temperature. Use this to select an appropriate heatsink.

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Formula

θSA=TJ(max)−TAPD−θJC−θCS\theta_{SA} = \frac{T_{J(max)} - T_A}{P_D} - \theta_{JC} - \theta_{CS}
θ_SA— Required heatsink thermal resistance (°C/W)
T_J(max)— 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 selection calculator determines required thermal resistance and recommends heatsink sizing — essential for power electronics reliability, LED thermal management, and CPU/GPU cooling design. Thermal engineers, PCB designers, and product engineers use this to ensure components operate within safe temperature limits. Per JEDEC JESD51-14, the selection process involves: (1) calculate power dissipation Pd, (2) determine maximum allowable θJA from Tj(max), Ta(max), and design margin, (3) subtract θJC and θCS to find required θSA, (4) select heatsink meeting θSA requirement. Standard heatsinks range from θSA = 20°C/W (small clip-on, 15×15×10mm) to θSA = 0.5°C/W (large finned with fan, 100×100×50mm). A 10% derating on θSA accounts for manufacturing variation and aging per thermal design guidelines.

Worked Example

Select heatsink for MOSFET driver dissipating 15W in TO-247 package. Requirements: Tj(max) = 150°C, Ta(max) = 55°C, target Tj = Tj(max) - 25°C = 125°C for reliability margin. From MOSFET datasheet: θJC = 0.4°C/W (TO-247AC). Calculate required θJA: θJA(max) = (125°C - 55°C)/15W = 4.67°C/W. With Bergquist Sil-Pad 2000 thermal pad (θCS = 0.3°C/W): θSA(max) = 4.67 - 0.4 - 0.3 = 3.97°C/W. Apply 10% derating: θSA(design) = 3.97 × 0.9 = 3.57°C/W. Select Aavid 62700 (60mm extruded, θSA = 3.2°C/W). Verify with forced air: if fan provides 2 m/s airflow, θSA drops to 1.2°C/W, allowing 40W dissipation at same Tj — useful for overload conditions.

Practical Tips

  • ✓Use online heatsink selection tools (Aavid, Wakefield-Vette) — input power, Tj(max), Ta, package type; tool recommends compatible products with θSA curves
  • ✓For tight spaces, consider heat pipes or vapor chambers — achieve θSA < 0.5°C/W in 5mm height, enabling thin form factors for mobile devices
  • ✓Thermal pads simplify assembly vs. thermal grease but have 2-3× higher θCS — for critical applications, use dispensed thermal grease with controlled bondline thickness

Common Mistakes

  • ✗Ignoring ambient temperature variation — designing for 25°C lab conditions fails in 50°C industrial environments; always use worst-case Ta from product specification
  • ✗Neglecting component-specific thermal resistance — θJC varies 10× between packages (TO-220: 1°C/W vs. SOIC-8: 40°C/W); verify from device datasheet
  • ✗Failing to apply derating factor — published θSA assumes ideal mounting and airflow; apply 10-20% derating for real-world margin per MIL-HDBK-251

Frequently Asked Questions

Thermal resistance θ (°C/W or K/W) measures opposition to heat flow: Tj = Ta + Pd × θJA. Lower θ means better cooling. The thermal path from junction to ambient consists of θJC (package, 0.4-40°C/W), θCS (interface, 0.1-1°C/W), and θSA (heatsink, 0.5-20°C/W). These add in series: θJA = θJC + θCS + θSA.
Published θSA assumes: optimal mounting pressure (50-100 psi), clean thermal interface, specified airflow, and vertical orientation. Real installations have variations: loose screws (+10% θSA), dust accumulation (+5-15%), horizontal mounting (+20-30% for natural convection). The 10% derating provides minimum margin; use 20-30% for harsh environments per IPC-9592B.
When natural convection cannot achieve required θSA, typically above 5-10W per device. Forced air at 2-3 m/s improves θSA by 3-5× for finned heatsinks. Fan sizing: 1 CFM per 5-10W of total dissipation in enclosure. For power densities >10W/cm², liquid cooling provides 10× better performance than air. Cost tradeoff: larger heatsink ($5-20) vs. fan ($3-10) + smaller heatsink.
Rule of thumb for natural convection aluminum extrusions: θSA ≈ 50 / √(area_cm²). For θSA = 5°C/W: area = (50/5)² = 100 cm² = 10×10 cm footprint. Add fins to double effective area in same footprint. Forced air allows 3-5× smaller heatsink for same θSA. Accurate sizing requires manufacturer thermal curves or CFD simulation.

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