Free MOSFET Heatsink Thermal Calculator
Size heatsinks for power MOSFETs, IGBTs, and regulators (TO-220, TO-247, D2PAK). Calculate junction temperature, total thermal resistance, and heatsink extrusion area.
⚡ Power Dissipation & Thermal Limits
✓ Achievable with standard aluminum extruded heatsink
• Estimated Extrusion Surface Area: ~420 cm² (65 sq in)
• Typical Dimensions: Approx 75mm × 50mm × 35mm finned extrusion
Thermal Circuit Analogy: Temperature is voltage ($V = Delta T$), thermal resistance is electrical resistance ($R = R_{ heta}$), and heat flow is current ($I = P$). The junction temperature is simply $T_J = T_A + P_D imes (R_{ heta JC} + R_{ heta CS} + R_{ heta SA})$.
Recommended Tools & Equipment
Tested hardware and components for high reliability
The Semiconductor Thermal Circuit
Every Watt of power dissipated in a silicon die must conduct through a series of thermal resistances to reach the ambient air:
Allowable Heatsink: R_θSA = [ (T_J_max - T_A) / P_D ] - (R_θJC + R_θCS)
Where:
- $R_{ heta JC}$ (Junction-to-Case): Internal resistance from silicon die to the copper mounting tab, fixed by package design.
- $R_{ heta CS}$ (Case-to-Sink): Resistance across the thermal paste, mica washer, or silicone Sil-Pad.
- $R_{ heta SA}$ (Sink-to-Ambient): Convective and radiative resistance of the finned aluminum heatsink.
Natural Convection vs. Forced Air
In natural convection, buoyant hot air rises slowly (air velocity < 50 LFM). Adding a small chassis fan (150 to 300 LFM) reduces heatsink thermal resistance by 50% to 70%, allowing a heatsink less than half the size to dissipate the same wattage.
Frequently Asked Questions
Why shouldn’t I run my MOSFET right at its 150°C or 175°C datasheet rating?
The datasheet 150°C rating is an absolute maximum stress rating. Operating silicon near 150°C drastically degrades reliability, accelerates electromigration, causes thermal runaway due to rising Rds(on), and risks solder joint fatigue. Commercial designs target Tj ≤ 100°C to 115°C.
What happens if the calculated Rth_sa is negative?
A negative Rth_sa means that the combined internal package resistance (R_jc) and interface pad (R_cs) alone are so high that the junction will overheat even with an infinitely large heatsink at 0 °C/W. You must either reduce power dissipation, step up to a larger package (TO-247), or parallel multiple MOSFETs.
Does thermal paste ever dry out?
Yes! Standard silicone-based thermal grease dries out and pumps out over 3 to 5 years of thermal cycling. For high-reliability industrial and automotive equipment, engineers use phase-change thermal materials or graphite foil pads.