Optimum Cooling Solutions for Power Electronics - lboro.ac.uk · Optimum Cooling Solutions for Power Electronics July 4 th 2008 Robert Skuriat PhD Student Nottingham University. Optimum

Optimum Cooling Solutions

for Power Electronics

July 4th 2008

Robert Skuriat

PhD Student

Nottingham University

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


� Project outline


thermal layers






� 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

Reducing the number of thermal layers

Standard

Cooling

System

Power

Module

Assembly

1. Die

2. Solder

3. Direct Bonded Copper

4. Ceramic


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 Baseplate Cooling

Standard

Cooling

System

Power

Module

Assembly

1. Die

2. Solder


4. Ceramic


6. Solder


8. Thermal Paste

9. CoolerIntegrated

Baseplate

Cooler

� 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

Direct Substrate Cooling

Power

Module

Assembly

1. Die

2. Solder


4. Ceramic


6. Solder


Substrate

Tile

Direct Cooling Summary

� 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

1. Die

2. Solder


4. Ceramic


Substrate

Tile


� Project outline


thermal layers






Impingement Cooling1. Jet impingement

2. Heat transfer

3. Mixing of working fluid

1

3

Heat from Electronics

2

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 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

Jet Impingement Cooling


� Project outline


thermal layers






Cooler Testing

� Three coolers tested� Commercial Coldplate

� Jet impingement – Baseplate

� Jet impingement – Substrate tile

� Power transferred� Electrical power input

� Calorimetry

� Thermal impedance� Die temperature accurately measured using

diode forward voltage

� Heat transfer coefficient� Thermocouples near to fluid swept heat transfer

surface

� Cartridge heaters embedded in copper

� Pressure drop across the cooler

� Fluid flow rate through the cooler

Coldplate

Baseplate 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

0

5

10

15

20

25

30

35

40

45

50

0.001 0.01 0.1 1 10 100

Time (seconds)

Die to Coolant Temperature

Difference

COLDPLATE BASEPLATE SUBSTRATE

� 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

30

40

50

60

70

80

90

100

0.00 0.01 0.10 1.00 10.00 100.00

Pumping Power Required (Watts)

Die to Coolant Temperature

Difference (K)

SUBSTRATE BASEPLATE COLDPLATE

Pumping Power


� Project outline


thermal layers






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

� 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

Jet Impingement Optimisation Test Rig


� Project outline


thermal layers






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


� Coolant at 40°C with a heat transfer coefficient of 10000 W/m2K

� Peak IGBT temperature 146°C � Hot

Top view Underside


� 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


� Project outline


thermal layers






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??

Documents

Optimum Cooling Solutions for Power Electronics - lboro.ac.uk · Optimum Cooling Solutions for Power Electronics July 4 th 2008 Robert Skuriat PhD Student Nottingham University. Optimum