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Optimum Cooling Solutions for Power Electronics Robert Skuriat PhD Student Nottingham University July 4th 2008 Optimum Cooling Solutions for Power Electronics Project outline Reducing the package thermal resistance by reducing the number of thermal layers Jet impingement cooling Experimental testing and results Improving package design and layout Optimising the cooling system Efficiency analysis of the complete system Optimum Cooling Solutions for Power Electronics Project outline Reducing the package thermal resistance by reducing the number of thermal layers Jet impingement cooling Experimental testing and results Improving package design and layout Optimising the cooling system Efficiency analysis of the complete system Reducing the number of thermal layers Power Module Assembly Standard Cooling System 1. Die 2. Solder 3. Direct Bonded Copper 4. Ceramic 5. Direct Bonded Copper 6. Solder 7. Heat spreader plate 8. Thermal Paste 9. Cooler Typical water cooled system 9 thermal layers and interfaces between electronic die and coolant fluid High package thermal resistance Low heat transfer coefficient generated by coldplate cooler Direct Baseplate Cooling Power Module Assembly Integrated Standard Baseplate Cooling Cooler System 1. Die 2. Solder 3. Direct Bonded Copper 4. Ceramic 5. Direct Bonded Copper 6. Solder 7. Heat spreader plate 8. Thermal Paste 9. Cooler The baseplate (heat spreader) can be cooled directly by jet impingement 7 thermal layers Shorter thermal path Lower thermal resistance High heat transfer coefficient Direct Substrate Cooling Substrate Power Tile Module Assembly Jet impingement can generate heat transfer coefficients in excess of 20kW/m2K Heat spreader plate no longer required Package reduced to 5 thermal layers Thermal path reduced further 1. Die 2. Solder 3. Direct Bonded Copper 4. Ceramic 5. Direct Bonded Copper 6. Solder 7. Heat spreader plate Direct Cooling Summary Substrate Tile 1. Die 2. Solder 3. Direct Bonded Copper 4. Ceramic 5. Direct Bonded Copper Removal of the baseplate results in a shorter thermal path Lower thermal resistance Fewer thermal layers fewer interfaces Reduced thermal stresses induced by differences in CTE Lower surface area of impingement cells required smaller cooler Less pumping power required Improved component reliability Smaller package Reduced weight Optimum Cooling Solutions for Power Electronics Project outline Reducing the package thermal resistance by reducing the number of thermal layers Jet impingement cooling Experimental testing and results Improving package design and layout Optimising the cooling system Efficiency analysis of the complete system Impingement Cooling 1. 2. 3. Jet impingement Heat transfer Mixing of working fluid 2 Heat from Electronics 1 3 Jet Impingement Cooling Arrays of jets of diameter 1mm Water jets sprayed onto flat surface Jet impingement reduces thermal gradient and thermal resistance High heat transfer coefficients can be generated Cells arranged in a serpentine pattern To minimise negative effect of downstream crossflow Jet Impingement Cooling Jet impingement coolers can generate high heat transfer coefficients Two jet impingement coolers were built with differing flow arrangements Baseplate Cooler has a 6 x 8 array of jets in 12 cells Direct Substrate Cooler has a 4 x 14 jet array in 6 cells Direct Baseplate Cooler Direct Substrate Cooler Optimum Cooling Solutions for Power Electronics Project outline Reducing the package thermal resistance by reducing the number of thermal layers Jet impingement cooling Experimental testing and results Improving package design and layout Optimising the cooling system Efficiency analysis of the complete system Cooler Testing Three coolers tested Baseplate cooler Die temperature accurately measured using diode forward voltage Heat transfer coefficient Electrical power input Calorimetry Thermal impedance Commercial Coldplate Jet impingement – Baseplate Jet impingement – Substrate tile Power transferred Coldplate Thermocouples near to fluid swept heat transfer surface Cartridge heaters embedded in copper Pressure drop across the cooler Fluid flow rate through the cooler Direct Substrate Cooler Thermal Impedance Results Measure of the ability of the cooler to cope with step inputs and thermal transients Better performance is indicated by a low die to coolant temperature difference Thermal Step Response - IGBT Die Temperature 50 45 Die to Coolant Temperature Difference 40 35 30 25 20 15 10 5 0 0.001 0.01 0.1 1 10 Time (seconds) COLDPLATE BASEPLATE SUBSTRATE 100 Pumping Power Energy required to pass coolant fluid through the cooler Both impingement coolers are significantly more efficient than the coldplate cooler Die To Coolant Temperature Difference vs Pumping Power 100 90 Die to Coolant Temperature Difference (K) 80 70 60 50 40 0.00 0.01 0.10 30 1.00 Pumping Power Required (Watts) SUBSTRATE BASEPLATE COLDPLATE 10.00 100.00 Optimum Cooling Solutions for Power Electronics Project outline Reducing the package thermal resistance by reducing the number of thermal layers Jet impingement cooling Experimental testing and results Improving package design and layout Optimising the cooling system Efficiency analysis of the complete system Custom designed substrate tile Previous direct substrate tile cooler testing was performed with substrate tiles intended to be soldered onto a heatspreader plate The component layout was not optimised for direct cooling of the substrate tile A substrate tile has been designed specifically to be cooled directly Half-bridge: 2 x IGBTs, 2 x diodes Good EMC (electromagnetic compatibility) Low inductance for high-frequency operation Half-bridge on a tile to reduce loop area Aluminium Nitride substrate good thermal conductivity Jet Impingement Optimisation Test Rig Test rig for optimising a jet impingement cooling array for direct cooling of a substrate tile The test rig is designed to allow a number of the parameters affecting the performance of a jet impingement array to be varied Open layout to allow a clear view of the components on the tile for thermal imaging All signal and power connections are located around the edge of the tile All parts are easily interchanged Jet Impingement Optimisation Test Rig Flexible design Allows a number of features and parameters to be varied 5 Inlets / Outlets – can be used in any combination O-ring seals Jet impingement plates are easily interchangeable Direct substrate tile cooling Various sizes of substrate tiles can be accommodated Optimum Cooling Solutions for Power Electronics Project outline Reducing the package thermal resistance by reducing the number of thermal layers Jet impingement cooling Experimental testing and results Improving package design and layout Optimising the cooling system Efficiency analysis of the complete system Jet Impingement Study Simulation Thermal modelling of substrate tile to determine the amount of heat spreading Temperature and heat flux profile for the heat transfer surface Jet impingement arrays are designed to match the cooling requirement rather than cooling the complete surface area of the tile Reduce redundancy in the system Design Arrangement of jets to match the cooling requirement Optimised for efficiency Trade-off between heat transfer and pumping power required Reducing temperature rise of the electronics Mathematical model and CFD simulation Mathematical model of an impingement array Using heat transfer theory and experimental results Design of experiments for optimising the cooling array CFD simulation of the impingement array at Greenwich Parametric optimisation verified by experiment Thermal model of the substrate tile Substrate tile is cooled with an even heat transfer coefficient of 10,000 W/m2K over its surface area: 40mm x 40mm Coolant at 40°C IGBTs dissipating 200 Watts each Hot spots located beneath IGBTs Hottest 50% of the temperature range accounts for 22% of the tile surface area Top view Underside Thermal model of the substrate tile Coolant at 40°C with a heat transfer coefficient of 10000 W/m2K Peak IGBT temperature 146°C Hot Top view Underside Thermal model of the substrate tile No baseplate to spread the heat to a larger surface area Rather than cool the complete surface area of the tile it is more efficient to direct the cooling below the hotspots with a higher heat transfer coefficient An impingement array can be optimised to match the cooling requirement over the reduced surface area Top view Underside Reducing hotspots Two jet impingement arrays are located directly beneath the hotspots Modelled as generating heat transfer coefficients of 20,000 W/m2K Remaining surface area of the tile is cooled by the spent fluid Peak IGBT temperature reduced from 146°C to 104°C Optimum Cooling Solutions for Power Electronics Project outline Reducing the package thermal resistance by reducing the number of thermal layers Jet impingement cooling Experimental testing and results Improving package design and layout Optimising the cooling system Efficiency analysis of the complete system Efficiency analysis of the complete system The impact of the cooler on the system as a whole varies depending on the application Automotive Aerospace Traction Industry The same power electronic devices will be cooled differently depending on the requirements of the application Standard duty cycles should be used if known Cooling requirement coolant flow rates Constant coolant flow rate or increase coolant flow rate to match cooling demand? Control system Real-time temperature monitoring to prevent large amplitude thermal cycles Complete system optimisation Finding the most efficient solution for the specific task A number of trade-offs and compromises are found once the complete system is analysed: A very compact cooler producing a high heat transfer coefficient may require a large pump with filter and complex control system in order to operate, increased complexity e.g. spray cooling A cooler may produce a very high heat transfer coefficient at the expense of producing a high pressure drop or being very bulky A slightly less efficient cooler may be very small and have a low mass but require a high flow rate The performance of a cooler may drop off if not working at its intended operating point Another cooler may have a wider range of operation Trade-off between performance and efficiency Efficient Cooler Design Summary Increase the built-in efficiency of the package Increases reliability and cooling performance Maintain electronics at constant temperature with minimal thermal cycles Reducing unnecessary redundancy in the cooler Looking at the impact of the cooler on the complete system Reducing Irreversibility Minimize ratio of heat transfer irreversibility to irreversibility due to fluid friction Thank you for your attention Questions??