503 - Heatsink Design¶
Thermal resistance calculation and heatsink selection.
Overview¶
Heatsink design ensures junction temperature stays within safe limits under all operating conditions.
Thermal Budget¶
Required Heatsink Thermal Resistance¶
\[R_{th,ha} = \frac{T_{j,max} - T_a}{P_{loss}} - R_{th,jc} - R_{th,ch}\]
Example Calculation¶
Given: - Tj,max = 125°C (design limit) - Ta,max = 50°C (ambient) - Ploss = 50W - Rth,jc = 0.5 K/W - Rth,ch = 0.2 K/W (with thermal paste)
Required: $\(R_{th,ha} = \frac{125 - 50}{50} - 0.5 - 0.2 = 0.8 \text{ K/W}\)$
Heatsink Types¶
Natural Convection¶
| Type | Rth Range | Application |
|---|---|---|
| TO-220 clip | 10-30 K/W | <5W |
| Extruded profile | 1-10 K/W | 5-50W |
| Large finned | 0.3-2 K/W | 50-200W |
Forced Air Cooling¶
Thermal resistance with airflow: $\(R_{th}(v) = R_{th,natural} \cdot \sqrt{\frac{v_0}{v}}\)$
Where v₀ ≈ 0.5 m/s (natural convection equivalent)
Liquid Cooling¶
| Type | Rth Range |
|---|---|
| Cold plate | 0.05-0.2 K/W |
| Microchannel | 0.02-0.1 K/W |
| Jet impingement | 0.01-0.05 K/W |
Thermal Interface Materials¶
TIM Thermal Resistance¶
\[R_{th,ch} = \frac{t_{TIM}}{k_{TIM} \cdot A} + R_{contact}\]
Material Comparison¶
| TIM Type | k (W/m·K) | Rth (typical) |
|---|---|---|
| Thermal grease | 1-5 | 0.1-0.3 K/W |
| Phase change | 3-5 | 0.1-0.2 K/W |
| Thermal pad | 1-6 | 0.2-0.5 K/W |
| Solder | 50 | 0.01-0.05 K/W |
Multi-Device Heatsinks¶
Thermal Coupling¶
Devices share heatsink → thermal interaction:
\[\begin{bmatrix} T_{j1} \\ T_{j2} \end{bmatrix} = \begin{bmatrix} Z_{11} & Z_{12} \\ Z_{21} & Z_{22} \end{bmatrix} \begin{bmatrix} P_1 \\ P_2 \end{bmatrix} + \begin{bmatrix} T_a \\ T_a \end{bmatrix}\]
Self-heating: Z₁₁, Z₂₂ Cross-coupling: Z₁₂ = Z₂₁
Design Procedure¶
Step 1: Loss Budget¶
- Calculate losses at worst-case operating point
- Include all devices on heatsink
Step 2: Thermal Budget¶
- Determine maximum ambient temperature
- Set junction temperature target
- Calculate required Rth,ha
Step 3: Heatsink Selection¶
- Select heatsink meeting Rth requirement
- Add margin for:
- Manufacturing variation (10-20%)
- TIM degradation over life
- Altitude derating
Step 4: Verification¶
- Simulate thermal transient
- Check at startup (maximum losses)
- Check at overload conditions
GeckoCIRCUITS Thermal Setup¶
Single Device¶
- Enable thermal on device
- Set Rth,jc from datasheet
- Set Rth,ch (interface)
- Add thermal node for heatsink
- Set Rth,ha and Cth,ha
Multiple Devices¶
- Connect devices to common thermal node
- Set individual Rth,jc
- Single heatsink Rth,ha
- Enable thermal coupling if needed
Simulation Exercises¶
- Size heatsink for 100W dissipation
- Compare natural vs forced convection
- Analyze thermal coupling effects
- Design for transient overload