Power Module Thermal Management and Heatsink Design Guide
Thermal Resistance Model and Calculation
Power module thermal resistance model includes: (1) Junction-to-case thermal resistance Rth(j-c): IGBT chip to module baseplate thermal resistance, BYD IGBT approximately 0.12-0.35K/W, SiC approximately 0.08-0.15K/W. (2) Case-to-heatsink thermal resistance Rth(c-s): Module baseplate to heatsink thermal resistance, depends on thermal grease/phase change material, approximately 0.05-0.2K/W. (3) Heatsink-to-ambient thermal resistance Rth(s-a): Heatsink thermal resistance, depends on heatsink design and cooling method. Total thermal resistance Rth(j-a) = Rth(j-c) + Rth(c-s) + Rth(s-a). Junction temperature Tj = Ta + P_loss × Rth(j-a).
Heatsink Selection Method
Heatsink selection steps: (1) Calculate total loss P_loss (conduction loss + switching loss). (2) Determine allowable maximum junction temperature Tj_max (IGBT typically 150°C, SiC 175°C). (3) Determine ambient temperature Ta_max. (4) Calculate allowable total thermal resistance Rth(j-a) = (Tj_max - Ta_max) / P_loss. (5) Subtract Rth(j-c) and Rth(c-s) to get heatsink thermal resistance Rth(s-a) requirement. (6) Select heatsink based on Rth(s-a). Example: P_loss=500W, Tj_max=150°C, Ta_max=50°C, then Rth(j-a)<0.2K/W. If Rth(j-c)=0.18K/W, Rth(c-s)=0.1K/W, then Rth(s-a)<0.12K/W, requires liquid cooling heatsink.
Thermal Interface Material Selection
Thermal Interface Material (TIM) types and selection: (1) Thermal grease: Thermal conductivity 3-8W/mK, low cost, requires uniform application, may dry out over long term. (2) Phase change material: Solid at room temperature, softens at operating temperature to fill gaps, thermal conductivity 3-5W/mK, good long-term stability, recommended for automotive applications. (3) Thermal pad: Easy to install, thermal conductivity 1-10W/mK, suitable for frequent maintenance. (4) Metal matrix composite: Thermal conductivity >10W/mK, high cost, for high-performance applications. BYD recommends phase change materials for automotive applications.
Cooling Method Comparison
Different cooling method characteristics: (1) Natural cooling: No fan, no noise, high thermal resistance (>1K/W), suitable for low power (<1kW). (2) Forced air cooling: Uses fan, thermal resistance 0.2-0.5K/W, suitable for medium power (1-10kW), low cost, noisy. (3) Liquid cooling: Uses coolant, thermal resistance <0.1K/W, suitable for high power (>10kW), no noise, high cost. (4) Phase change cooling: Uses phase change material for heat storage, suitable for pulse power applications. EV electric drives typically use liquid cooling, charging stations choose air or liquid cooling based on power.
Thermal Simulation and Verification
Thermal simulation methods: (1) Use thermal simulation software: FloTHERM, Icepak, SolidWorks Flow Simulation, etc. (2) Establish thermal model: Including module, heatsink, TIM, cooling medium. (3) Set boundary conditions: Ambient temperature, cooling conditions, power loss. (4) Run simulation to obtain temperature distribution. (5) Optimize design: Adjust heatsink structure, add heat pipes, optimize airflow, etc. Verification methods: (1) Use thermocouples or infrared thermal imager to measure case temperature. (2) Estimate junction temperature through thermally sensitive parameters (such as VCE(sat)). (3) Conduct temperature rise tests to verify thermal design.
💡 FAE Insights
📋 Customer Cases
Major NEV OEM
New Energy Vehicles
Challenge
Electric drive system junction temperature exceeded limits in high-temperature environment
Solution
Optimized liquid cooling system design, improved channel layout, used high-performance phase change material
Customer Feedback
"Proper thermal design was critical to meeting automotive reliability requirements"
Results
Junction temperature reduced by 25°C, meeting 85°C ambient temperature requirements, passed all thermal tests
Frequently Asked Questions
1. How to calculate power module junction temperature?
Junction temperature calculation formula: Tj = Ta + P_loss × Rth(j-a). Where Ta is ambient temperature, P_loss is total loss, Rth(j-a) is total thermal resistance (Rth(j-c)+Rth(c-s)+Rth(s-a)). Example: Ta=50°C, P_loss=300W, Rth(j-c)=0.18K/W, Rth(c-s)=0.1K/W, Rth(s-a)=0.2K/W, then Tj=50+300×(0.18+0.1+0.2)=194°C, exceeding 150°C limit, requires improved cooling.
2. How to choose between thermal grease and phase change material?
Selection recommendations: (1) Thermal grease: Higher thermal conductivity (3-8W/mK), low cost, suitable for R&D and small batch, but may dry out over long term. (2) Phase change material: Good long-term stability, automatically fills gaps, suitable for automotive and other long-life applications, thermal conductivity 3-5W/mK. Automotive applications recommend phase change materials, industrial applications can choose based on cost.
3. What are the heatsink surface roughness requirements?
Surface roughness requirements: (1) Heatsink surface roughness Ra<3.2μm, flatness <50μm/100mm. (2) Module mounting surface must be clean, no oxide layer. (3) Use appropriate mounting torque (typically 2-4N·m) to ensure good contact. (4) Excessive roughness increases contact thermal resistance, reducing cooling effectiveness.
4. How to select air velocity for air-cooled heatsinks?
Air velocity selection recommendations: (1) General applications: 2-5m/s. (2) High-power applications: 3-8m/s. (3) Air velocity >10m/s provides limited cooling efficiency improvement, noise increases significantly. (4) Select appropriate fan based on heatsink thermal resistance requirements and airflow resistance curve. (5) Consider airflow duct design to ensure uniform airflow through heatsink.
5. What are the design points for liquid cooling systems?
Liquid cooling design points: (1) Coolant flow rate: Calculate based on heat load, typically 10-20L/min. (2) Coolant temperature: Inlet temperature typically 40-60°C. (3) Channel design: Ensure uniform flow velocity, avoid dead zones. (4) Material compatibility: Avoid electrochemical corrosion. (5) Sealing design: Prevent leakage. (6) Coolant selection: Ethylene glycol solution or dedicated coolant. Liquid cooling can achieve thermal resistance <0.1K/W, suitable for high-power applications.
6. How to perform thermal design verification?
Verification methods: (1) Temperature measurement: Use thermocouples to measure case temperature, infrared thermal imager to view temperature distribution. (2) Thermal sensitive parameter method: Use relationship between VCE(sat) or Vf and temperature to estimate junction temperature. (3) Temperature rise test: Run at rated conditions, monitor temperature stabilization. (4) Overload test: Verify short-term overload capability. (5) Thermal cycling test: Verify long-term reliability. Ensure junction temperature remains within safe limits under worst-case conditions.