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Thermal Analysis Examples

Power semiconductor thermal modeling and heatsink design for reliable operation.

Examples

Example Description Difficulty
Loss Calculation Conduction and switching losses Intermediate
Junction Temperature Thermal network modeling Intermediate
Thermal Networks Foster/Cauer models Advanced

Quick Reference

Thermal Resistance Chain

Tj ──[Rth,jc]── Tc ──[Rth,ch]── Ts ──[Rth,ha]── Ta
     Junction    Case    Interface   Heatsink   Ambient

Typical Values

Component Rth (K/W) Notes
Rth,jc (IGBT) 0.1-0.5 From datasheet
Rth,jc (MOSFET) 0.5-2.0 Depends on package
Rth,ch (thermal grease) 0.05-0.2 With good contact
Rth,ch (thermal pad) 0.2-0.5 Electrical isolation
Rth,ha (natural convection) 1-5 Small heatsink
Rth,ha (forced air) 0.1-1 With fan

Power Loss Equations

Conduction Loss (IGBT): 

Pcond = Vce(sat) × Ic × D + Ic² × Ron × D

Switching Loss: 

Psw = (Eon + Eoff) × fs

Design Flow

  1. Calculate power losses (conduction + switching)
  2. Determine maximum allowable Tj (with margin)
  3. Calculate required total Rth
  4. Select heatsink with Rth,ha meeting requirement
  5. Verify with thermal simulation