Calculate semiconductor junction temperature (Tj), required heatsink thermal resistance (θsa), and thermal interface losses for MOSFETs, IGBTs, and voltage regulators.
🔥 Power & Thermal Properties
Example: Linear regulator (Vin - Vout) × Iout = 12W
🌡️ Thermal Performance
83.6°C
Safe Operating Range (+41.4°C Headroom)
65.6°C
Case Temperature (Tc)
63.8°C
Heatsink Temp (Ts)
5.85°C/W
Max Allowable θsa
4.05°C/W
Total Rth (Junction to Air)
Thermal Stack BreakdownTamb → Ts → Tc → Tj
■ Amb (35°C)■ Heatsink (+28.8°C)■ TIM (+1.8°C)■ Silicon (+18°C)
💡 Engineering Assessment: Your junction operates at 83.6°C, well below the 125°C ceiling. Natural convection with a 4.0°C/W extruded aluminum heatsink is completely adequate.
Heat transfer through electronic packages follows a direct mathematical analogy to Ohm's Law:
Temperature Difference (ΔT in °C) = Voltage (V)
Heat Flow / Power Dissipation (P in Watts) = Current (I)
Thermal Resistance (θ in °C/Watt) = Electrical Resistance (R)
ΔT = P × θ
To prevent thermal runaway and catastrophic semiconductor failure, heat generated within the microscopic silicon die ($T_j$) must flow through three series thermal resistances before dispersing into the room ($T_{amb}$):
$\theta_{jc}$ (Junction to Case): Internal resistance from silicon die to the copper tab. Governed by manufacturer package physics.
$\theta_{cs}$ (Case to Sink): Resistance across the mounting interface. Minimized by thermal grease, mica, or phase-change pads.
$\theta_{sa}$ (Sink to Ambient): Heatsink surface area, fin spacing, and cooling airflow velocity.
Frequently Asked Questions
What happens if a heatsink is too small?
If heatsink thermal resistance ($\theta_{sa}$) is too high, the junction temperature ($T_j$) will exceed safe silicone limits ($150^\circ\text{C}$). The transistor will either trigger thermal shutdown protection or suffer permanent gate oxide breakdown and short-circuit failure.
Can you mount a TO-220 transistor to a heatsink without an electrical insulator?
On almost all TO-220 MOSFETs and linear regulators, the metal mounting tab is internally connected to the Drain or Collector pin. If multiple transistors share a common metal heatsink or chassis without mica/Sil-Pad isolators, you will create a dead short circuit between power rails.