Download Optimum cooling solutions for power electronics, Robert Skuriat

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??