Thermal design is critical for the reliable operation and longevity of power semiconductor devices. Proper thermal management ensures that junction temperatures remain within safe operating limits, preventing degradation and failure of power modules. Semikron provides comprehensive thermal analysis tools and guidelines to help engineers calculate junction temperatures under various operating conditions and design effective cooling solutions. Our thermal models account for conduction, convection, and radiation heat transfer mechanisms, as well as the thermal characteristics of different package types and mounting methods.
Precise thermal models based on actual device characteristics
Analysis for steady-state, transient, and cyclic conditions
Tools to optimize cooling solution cost and performance
Fundamental concepts for thermal design
The junction temperature (TJ) is calculated using the thermal resistance model:
TJ = TA + Ptot × Rth(J-A)
Where:
The total thermal resistance consists of multiple series resistances:
Rth(J-A) = Rth(J-C) + Rth(C-H) + Rth(H-A)
Where:
Total power dissipation includes both switching and conduction losses:
Ptot = Pcond + Psw
Where:
Interactive tools for thermal design analysis
Calculate junction temperature based on power dissipation and thermal resistance values.
Junction Temperature: -- °C
Determine required heatsink thermal resistance based on power dissipation and maximum junction temperature.
Required Heatsink Resistance: -- K/W
Thermal resistance values for common Semikron packages
| Package | Configuration | Rth(J-C) [K/W] | Rth(J-H) [K/W] | Notes |
|---|---|---|---|---|
| SEMITRANS | Phase Leg | 0.07 - 0.12 | 0.25 - 0.40 | Standard industrial package |
| SEMiX | Phase Leg | 0.04 - 0.07 | 0.15 - 0.25 | High power density |
| SEMITOP | Phase Leg | 0.05 - 0.08 | 0.18 - 0.30 | Top-side cooling |
| MiniSKiiP | Phase Leg | 0.03 - 0.05 | 0.12 - 0.20 | Integrated driver |
| SKiiP | Phase Leg | 0.03 - 0.06 | 0.08 - 0.15 | Press-fit technology |
| Package | Configuration | Rth(J-C) [K/W] | Rth(J-H) [K/W] | Notes |
|---|---|---|---|---|
| SEMIPACK 4 | Single Thyristor | 0.08 - 0.10 | 0.28 - 0.35 | Standard generation |
| SEMIPACK 6 | Single Thyristor | 0.06 - 0.08 | 0.22 - 0.30 | Latest generation |
| Semipont | Single Diode | 0.07 - 0.12 | 0.25 - 0.40 | Rectifier applications |
Comparison of different cooling approaches
Passive cooling relying on natural air circulation:
Typical thermal resistance: 10-50 K/W depending on heatsink size and orientation.
Active cooling with fan-forced air circulation:
Active cooling with liquid heat transfer medium:
Characteristics and selection guidelines for TIMs
| Material Type | Thermal Conductivity [W/m·K] | Interface Resistance [K·cm²/W] | Applications |
|---|---|---|---|
| Thermal Grease (Standard) | 2.0 - 4.0 | 0.5 - 1.0 | General purpose |
| Thermal Grease (High Performance) | 4.0 - 7.0 | 0.3 - 0.6 | High power applications |
| Thermal Pad | 1.0 - 5.0 | 0.7 - 1.5 | Assembly convenience |
| Phase Change Material | 3.0 - 6.0 | 0.4 - 0.8 | Temperature cycling |
| Solder | 20 - 80 | 0.05 - 0.1 | Permanent applications |
Practical examples for common applications
Scenario: 10kW string inverter using SEMiX 1200V/200A module
Parameters:
Calculation:
Rth(H-A) required = (TJ - TA - P × Rth(J-C)) / P
Rth(H-A) = (150 - 50 - 300 × 0.05) / 300 = (150 - 50 - 15) / 300 = 0.28 K/W
Therefore, a heatsink with thermal resistance ≤ 0.28 K/W is required at the specified ambient temperature.
Scenario: 50kW industrial drive using SKiiP 3 system
Parameters:
Calculation:
TA required = TJ - P × Rth(J-A)
For Rth(H-A) = 0.05 K/W: TA = 125 - 1200 × (0.08 + 0.05) = 125 - 156 = -31°C
This is impossible, so we need forced cooling with Rth(H-A) ≤ 0.02 K/W
Our FAE team can perform detailed thermal analysis for your application
Contact Thermal Experts
FAE Technical Commentary
Expert insights on thermal design
For high-power applications, always calculate the required thermal resistance at the maximum expected ambient temperature and power dissipation. Many designs work well at room temperature but fail at maximum operating conditions. As a rule of thumb, derate the thermal performance by 20-30% for safety margins.
When designing for temperature cycling, consider the impact of thermomechanical stress on both the power module and the thermal interface material. Power cycling can cause degradation in thermal performance over time, particularly for thermal grease.
For press-fit power modules (like SKiiP), the mounting pressure significantly affects thermal performance. Always follow Semikron's recommended torque values for base plate mounting to ensure proper thermal contact.