NREL is a national laboratory of the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy operated by the Alliance for Sustainable Energy, LLC

Power Electronics Thermal Control

Sreekant Narumanchi

Center for Transportation Technologies and Systems

Presented at the

Interagency Advanced 

Power Group Meeting

Golden, Colorado 

May 5, 2010

NREL/PR-540-48053
NREL’s Center for Transportation Technologies and Systems

National Renewable Energy Laboratory 2 Innovation for Our Energy Future

Energy Storage 

Thermal Control

Biofuels and Engine 

Optimization

Powertrain Engine Tradeoffs

(Perf., Size, Cost, Life)

DIRECT

Vehicle Thermal and Energy

Management Strategies

Real World

Drive Cycle Analysis

Thermal Impacts on

Tailpipe Emissions

Cabin Thermal Comfort

and Waste Heat Utilization

WinDs

HOMER

Utility Interface/

Renewable Energy

Power Electronics 

Thermal Control
DOE’s Advanced Power Electronics and 

Electric Machines (APEEM)

National Renewable Energy Laboratory 3 Innovation for Our Energy Future

Industry and Academia

Oak Ridge National Laboratory (ORNL)

Lead: Power Electronics & Electric Machines R&D

National Renewable Energy Laboratory (NREL)

Lead: APEEM Thermal Control R&D

+ Other Industry Partners

and Universities
DOE’s Advanced Power Electronics and 

Electric Machines (APEEM)

National Renewable Energy Laboratory 4 Innovation for Our Energy Future

Research Focus Areas

Power Electronics

($/kW) (kW/kg) (kW/l)

7.9 10.8 8.7

5 12 12

3.3 14.1 13.4

Motors

($/kW) (kW/kg) (kW/l)

11.1 1.2 3.7

7 1.3 5

4.7 1.6 5.7

Traction Drive System

($/kW) (kW/kg) (kW/l) Efficiency

19 1.06 2.6 >90%

12 1.2 3.5 >93%

8 1.4 4 >94%

Power 

Electronics

Integrated 

Traction Drive 

System

PEEM Thermal Motors Control

 inverters and converters

 innovative topologies

 packaging

 temperature-tolerant devices

 capacitors

 permanent magnet (PM) motors

 high performance non-PM motors

 permanent magnets

 heat transfer techniques

 materials

 area enhancement

 alternative coolants

 benchmarking technologies

 innovative system designs

Challenges

size cost weight

Year

2010

2015

2020

Reduce Dependence on Oil

Via Electrification of Vehicle Drives

Technology Targets

Requirements: 55 kW peak for 18 sec; 30 kW continuous; 15-year life; coolant (air or 105oC WEG)
5

Technical targets

Coolant System

Material Selection

Number of Devices

Heat 

Generation

Cooling 

Requirements

Heat Exchanger Materials

Heat Flux

Heat 

Exchanger

Volume
FY10 – Thermal Management projectsPE Packaging

Thermal Systems 

Integration

Heat Transfer 

Technologies

Thermal Stress and 

Reliability

Thermal System Performance and Integration

Electical and Thermal Characterization, Modeling, and 

Reliability Assessment

Thermal Stress and Reliability

Thermal performance and reliability of bonded interfaces

Characterization and Development of Advanced Heat 

Transfer Technologies 

Air Cooling Technology

Thermal Control of PHEV / EV Charging Systems

Electric Motor Thermal Control

Thermal Assessment

PROJECT

FOCUS

AREA
Approach

National Renewable Energy Laboratory 7 Innovation for Our Energy Future

( ) Q = h A TB − TC

Improve PE device efficiency (ORNL)

Enhance heat transfer coefficient

- jet / spray cooling

- self-oscillating jets

- phase change

Coolant temperature

- reduce coolant temperature

- evaluate alternatives

Increase surface area

- fin shape optimization

- double sided cooling

- surface enhancements

- thermal spreading

Maximize base plate temperature

- PE materials selection

- reduce thermal resistance

Integration and Reliability
National Renewable Energy Laboratory 8 Innovation for Our Energy Future

Enhanced surfaces in conjunction with 

single-phase and two-phase flows 
Approach

Surface enhancement: Single-phase channel 

flow, jet impingement

• Further increase the heat transfer rates in conjunction 

with single-phase channel flows and jet impingement.

• Prior studies have shown that surface roughening can:

• Increase h-values by as much as 32% [Gabour & 

Lienhard (1994)],

• Reduce Rth by as much as 60% [Sullivan et al. (1992)].

• Limited, if any, studies exist on the use of microporous and nano-structures as a means of enhancing 

jet impingement and channel flow heat transfer.

National Renewable Energy Laboratory 9 Innovation for Our Energy Future

Procedure:

1. Fundamental study on the effect of enhanced surfaces 

on channel flow and jet impingement (free and 

submerged jets) heat transfer.

2. Conduct tests at various channel flow and jet velocities.
Approach

Surface enhancement: Two-phase 

heat transfer

• High heat transfer rates. 

• Direct cooling using dielectric fluids 

can eliminate thermal bottlenecks.

• There are very few published studies 

(if any) investigating the effect of 

microporous and/or nano-structures 

on spray impingement boiling 

performance.

National Renewable Energy Laboratory 10 Innovation for Our Energy Future

Procedure:

1. Fundamental study on the effect of 

enhanced surfaces on pool boiling, flow 

boiling, spray and jet impingement boiling.

2. Conduct tests at various fluid flow rates.
Approach

National Renewable Energy Laboratory 11 Innovation for Our Energy Future

Strategy

•Current Work: Fundamental study to characterize the thermal performance of 

the enhanced surfaces. 

•Future Work: Implement technology on an actual power electronics module & 

evaluate the surface enhancement’s reliability.

Enhanced Surfaces

Copper 

Microporous (3M)

Copper Nanowire 

(CU)

Spray Pyrolysis 

(NREL)

Skived Surface 

(Wolverine)

Finned Surfaces
Single-phase channel 

flow and jet impingement 

in conjunction with 

enhanced surfaces

National Renewable Energy Laboratory 12 Innovation for Our Energy Future

• Water @ 25°C inlet temperature.

• Channel flow, free and submerged jet 

configurations.

• 11 different surfaces tested.

• Channel flow tests for reference.
Enhanced surfaces: single-phase channel flow 

results 

National Renewable Energy Laboratory 13 Innovation for Our Energy Future

• Reference configuration, typical 

of existing cooling configurations.

• Roughened and microporous 

surfaces have no effect on 

performance.

HEATER

5 mm

Channel, Dh ≈ 8 mm

Entrance length ≈ 30×Dh

L=12.7 mm

Twall

U∞, Tl

0 0.5 1 1.5 2

0

1,000

2,000

3,000

0 5,000 10,000 15,000 20,000

U∞ (m/s)

Nu L

ReDh

Baseline (Ra=0.3 um) Channel Flow

Sandblasted (Ra= 4.16 um)

Pyr. Fins (140% increased area)

Microporous (3M)

Spray Pyrolysis

Skived (Wolverine)
Enhanced surfaces: single-phase submerged jet 

results

National Renewable Energy Laboratory 14 Innovation for Our Energy Future

• Microporous/roughened surfaces 

had minimal effect on performance.

• Skived (Wolverine) produced 

highest h-value enhancement 

(~100%).

• Finned structures outperformed 

microporous/roughened surfaces 

(increased area effect).

HEATER

Stagnation 

Zone

Wall Jet Zone

d =1.24 mm

Ud ,Tl

L=12.7 mm (heater diameter)

S = 6 mm 

(~4.8×d)

Twall 0 4 8 12

0

1,000

2,000

3,000

0 5,000 10,000 15,000 20,000

Ud (m/s)

Nu L

Red 

Submerged Jet

Baseline (Ra=0.3 um)

Sandblasted (Ra=4.16 um)

Pyr. Fins (140% increased area)

Microporous (3M)

Spray Pyrolysis

Skived (Wolverine)

Nanowire (CU)
Enhanced surfaces: single-phase free jet results

National Renewable Energy Laboratory 15 Innovation for Our Energy Future

• Microporous coating (3M) 

produced highest h-value 

enhancement (~130%).

• Greater enhancement than that 

reported in literature.

• Microporous/roughened surfaces 

outperformed finned surfaces at 

higher velocities.

HEATER

Stagnation 

Zone

Wall Jet Zone

d =1.24 mm

Ud ,Tl

L=12.7 mm (heater diameter)

S = 6 mm 

(~4.8×d)

Twall 0 4 8 12

0

1,000

2,000

3,000

0 5,000 10,000 15,000 20,000

Ud (m/s)

Nu L

Red 

Free Jet

Baseline (Ra=0.3 um)

Sandblasted (Ra=4.13 um)

Pyr. Fins (140% increased area) 

Microporous (3M)

Spray Pyrolysis

Skived (Wolverine)

Nanowire (CU)
Two-phase

cooling (pool boiling and spray 

impingement boiling) in 

conjunction with enhanced 

surfaces

National Renewable Energy Laboratory 16 Innovation for Our Energy Future

• HFE-7100 dielectric.

• Saturated and subcooled conditions.

• Pressurized, full cone spray nozzle.

• Three different enhanced surfaces tested.

• Pool boiling tests for reference.
Enhanced surfaces: pool boiling (saturated HFE 

7100)

National Renewable Energy Laboratory 17 Innovation for Our Energy Future

Microporous Coating and Nanowires

• Both the enhanced surfaces show lower incipience 

temperatures than the plain surface. 

• The microporous surface showed ~500% increase 

in the heat transfer coefficient at the same heater 

power, and ~10% increase in the critical heat flux 

(CHF) in comparison to the plain surface.

• The nanowire surface showed about 60% increase 

in the h near the CHF, while the CHF itself was 

considerably lower.

Copper 

Microporous (3M)

Copper Nanowire 

(CU)
Enhanced surfaces: spray impingement boiling 

(saturated HFE 7100)

National Renewable Energy Laboratory 18 Innovation for Our Energy Future

Microporous Coating 

• 100-300% increase in 

nucleate boiling (N.B.) heat 

transfer with respect to plain 

surface.

• 7-20% increase in the CHF 

with respect to plain surface.

• Coating structure (micro 

cavities of various sizes) 

enhances boiling heat 

transfer.

HFE-7100 dielectric Fluid 

Full cone spray @15.8 cm3/s 

(corresponds to 7 m/s velocity)
Enhanced surfaces: spray impingement boiling 

(saturated HFE 7100)

National Renewable Energy Laboratory 19 Innovation for Our Energy Future

Flow Rate Effect

• Increasing flow rate has 

minimal effect on nucleate 

boiling heat transfer for the 

microporous surface.

• Boiling is the dominant heat 

transfer mechanism on 

coated surface, less sensitive 

to convective effects.

HFE-7100 dielectric fluid 

Saturated conditions
0.1

1.0

1 10 100 1,000 10,000

R th, j-a (K/W)

R"th, h-a (mm2 -K/W)

Semikron SKM

 

System level implication

Jet impingement

•Submerged jet w/ skived surface 

decreases Rth-j-a by:

• 11% (Entire Package), 

• 39% (DCD).

Two-phase

•Pool boiling or spray cooling w/ 

microporous coating decreases Rth-ja by:

• 16% (Entire Package), 

• 61% (DCD).

Decrease in Rth-j-a will vary with 

different package configuration.

National Renewable Energy Laboratory 20 Innovation for Our Energy Future

Direct Cooled DBC

Entire 

Package

R”th,h-a: 2-phase, 

microporous

R”th,h-a: Sub. jet, 

skived
Future Work

i. Reliability:

– Investigate degradation of plain surface when subject to jet 

impingement including nozzle degradation over time,

– Investigate ability of enhanced surface to remain effective under 

long term use. 

ii. Synthesize/optimize additional coatings (e.g., using spray 

pyrolysis)

– Single-phase & two-phase applications (HFE7100, HFO-1234yf).

iii. Implement single-phase jet impingement with enhanced 

surfaces on a commercially available power electronics 

package (Semikron SKM). 

iv. Implement two-phase cooling with enhanced surface in a 

package.

v. Implement flow visualization/characterization to understand 

underlying physics/mechanisms behind surface 

enhancements

– PIV/micro-PIV, High speed video & Schlieren shadowgraphs.

National Renewable Energy Laboratory 21 Innovation for Our Energy Future
National Renewable Energy Laboratory 22 Innovation for Our Energy Future

Thermal interface materials 

for power electronics applications 
Thermal interface materials - project 

relevance

• Excessive temperature can degrade the performance, life, 

and reliability of power electronic components.

• Advanced thermal control technologies are critical to 

enabling higher power densities and lower system cost.

• Interfaces pose a major bottleneck to heat removal.

• Bonded interface materials (BIMs) based on solder are 

associated with thermomechanical reliability concerns 

under temperature cycling, as well as degradation at 

higher temperatures (>120°C).
The Problem

K. Stinson-Bagby, M.S. Thesis,

Virginia Tech, 2002.

DBA 

substrate

Crack in 

the solder 

layer

• Conventional TIMs do not 

meet thermal performance 

and reliability targets.

• Due to advantages from a 

packaging viewpoint, 

industry is trending toward 

bonded interfaces.

• Bonded interfaces such as 

solder degrade at higher 

temperatures, and are prone 

to thermomechanical failure 

under large temperature 

cycling.

BIM 2

Silicon die

Direct-bond-copper (DBC) or 

Direct-bond-aluminum (DBA)

Copper or Aluminum 

baseplate/coldplate

BIM 1
Sintered interfaces – based on silver nanoparticles

G.-Q. Lu, Virginia Tech

Synthesis of sintered interface

Sintered 

interface

Silver 

coating

Silver 

coating

Nickel 

coating

Nickel 

coating

Al 

or 

Cu

Al 

or 

Cu

• Sintered interfaces synthesized between silvered 

Cu-Cu and Al-Al disks (31.8 mm diameter) at 

Virginia Tech.

• A nickel coating (~2 μm) followed by silver coating 

(~ 2 μm) is applied on the copper and aluminum 

disks.

• For comparison, lead-free solder (SN100C) 

interface synthesized between Cu-Cu disks (31.8 

mm diameter).

• Different thicknesses fabricated (20 ~ 200 microns).

Fixture Sintering cycle
Sintered interfaces – preliminary experimental 

results

Samples Thickness

(μm)

Resistance

(mm2K/W)

Silvered

Cu-Cu

sintered

interface

20 5.8

27 8.0

64 5.4

Silvered

Al-Al

sintered

interface

28 14.9

103 25.2

144 5.0

Cu-Cu

soldered

interface

(SN100C)

80 1.0

150 4.8

200 3.7

• The thermal resistance tests 

were performed using the 

NREL ASTM TIM apparatus

– Average sample 

temperature ~ 65°C, 

pressure is 276 kPa (40 

psi).

• The silvered Cu-Cu sintered 

interface shows promising 

thermal performance.

• Results hint at some 

problems with the bonding 

of the silvered Al-Al 

interface.

• The lead-free solder 

(SN100C) interface initial 

thermal results are very 

promising.

ASTM test fixture
Thermoplastics with embedded carbon fibers 

Sample

• Thermoplastic films (provided by Btech) 

bonded between 31.8 mm diameter copper 

disks.

• Promising thermal results (8 mm2K/W for 

100 microns bondline thickness).

• Continuing work at NREL to further 

decrease contact resistance to approach 

target thermal performance, as well as 

characterize reliability.

Sequence of bonding steps 
Temperature Across a Package

- FEA analysis using Toyota package

Q heat 

100

110

120

130

140

150

160

0 2 4 6 8 10 12

Distance across the package (mm)

Temperature (

0C)

100 mm2K/W

20 mm2K/W

10 mm2K/W

5 mm2K/W

IGBT location Baseplate 

backside

Heat sink 

base

Glycolwater 

temperature

ΔTTIM

TJ,max

At 100 W/cm2 heat 

dissipation in the 

die, the maximum 

junction 

temperature 

(TJ,max) decreases 

by 16°C when TIM 

resistance 

decreases from 

100 to 8 mm2K/W.
Future Work

Remainder of FY10

• Work with Btech to develop and test (via 

ASTM steady-state approach) improved 

and reliable thermoplastics with embedded 

carbon fibers meeting target thermal 

performance

– Reduce contact resistance via ion- implantation and metal 

evaporation/sputtering techniques.

• In collaboration with Virginia Tech and 

Btech, synthesize and characterize various 

joints between DBA/DBC and 

aluminum/copper baseplate

– Synthesis of soldered, sintered, brazed and 

thermoplastic joints,

– Subject joints to thermal shock,

– Thermal resistance measurement after 

select cycles,

– CSAM after select cycles,

– High-potential test after select cycles,

– Modeling of the joint thermomechanical 

behavior (physics-of-failure) – end-of-life 

predictive model.

Tom Gennett, NREL
Future Work

FY11, FY12

• Detailed synthesis and characterization of thermal 

performance and reliability of joints based on the 

matrix given below

– Synthesis of bond/joint, 

– Subject joint/bond to thermal shock,

– Thermal resistance measurements after select cycles,

– Joint quality characterization (CSAM) after select cycles,

– High-potential test after select cycles,

– Modeling of thermo-mechanical behavior of the joints,

– Degradation/end-of-life model of the joints and the package. 
Summary

• Thermal management plays an important part in the cost 

of electric drives in terms of power electronics packaging.

• Very promising results from microporous coatings and 

skived surfaces in conjunction with single and two-phase 

flows.

• Sintered materials and thermoplastics with embedded 

fibers show significant promise as TIMs.

• Appropriate cooling technology depends on:

• Package application,

• Reliability.
Acknowledgments

32

• Susan Rogers, U.S. Department of Energy

• Kevin Bennion, Charles King, Mark Mihalic, Gilbert 

Moreno, Michael O’Keefe, Suraj Thiagarajan, Travis 

Venson, Tom Gennett, Kim Jones, Bobby To (NREL)

• Wei Wang and Ronggui Yang (CU Boulder)

• Jay Browne (Btech)

• Gary Eesley (Delphi)

• Greg Smith (GM)

• Sy-Jenq Loong (Wolverine Tube Inc.)

• Phil Tuma (3M)

• G.Q. Lu (Virginia Tech)