Power Electronics Thermal 

Management

P.I.: Gilbert Moreno

National Renewable Energy Laboratory

June 19, 2018

DOE Vehicle Technologies Office

2018 Annual Merit Review and Peer Evaluation Meeting

This presentation does not contain any proprietary, confidential, or otherwise restricted information.

NREL/PR-5400-71209

Project ID ELT078 
NREL | 2

Overview

• Project start date: 2017

• Project end date: 2019

• Percent complete: 60%

• Size and weight

• Cost

• Performance and lifetime

Timeline Barriers

• John Deere

• Elementum3D

• Dielectric fluid manufacturers

• Oak Ridge National Laboratory 

(ORNL)

• Project Lead– National Renewable 

Energy Laboratory

Partners

• Total project funding: $968K

– DOE share: $968K

• Funding for FY 2017: $493K

• Funding for FY 2018: $475K

Budget
NREL | 3

Relevance

• Thermal management is essential to increase reliability and 

power density

• Objective: Develop thermal management techniques to enable 

achieving the DOE power density target of 100 kW/L 

– Challenge is to create a thermal solution that allows for packaging high 

temperature (250°C) wide-bandgap (WBG) devices next to capacitors that 

typically cannot exceed 85°C

From 2017 EETT Roadmap

AIPM: advanced integrated power module
NREL | 4

Power Electronics Thermal Management

Thermal Management Technologies to 

Enable 100-kW/L Power Density Target

Approach

Two-Phase Cooling for High Packaging 

Density Planar Inverter (Cooperative 

Research And Development Agreement 

[CRADA])

Photo Credit: Gilbert Moreno (NREL)
NREL | 5

Approach

Define the 

thermal target 

required to 

achieve 100 kW/L

Compare potential 

cooling strategies

Design the cooling 

system via modeling

Component Volume used for 100 kW/L 

inverter estimate [L]

Source

Gate driver (includes current 

sensors) 

0.28 2015 BMWi3 (125 kW)

Control board 0.23 2012 Nissan LEAF (80 kW)

Capacitor 0.25 2015 BMWi3 (125 kW): Assumption: capacitor 

volume decreased by 50% to account for a decrease in 

capacitor requirements for WBG devices

Remaining volume for power 

module and cold plate

0.24

Toyota Engineer Speaks on Advantages, Disadvantages of Silicon Carbide (SiC) Power Devices

“...if switching frequency is improved by eight times by replacing a Si power device with a SiC power device, the 

volumes of capacitors and reactors can be reduced by 70-80%... ”

http://tech.nikkeibp.co.jp/dm/english/NEWS_EN/20120207/204483/
NREL | 6

Approach

Define the 

thermal target 

required to 

achieve 100 kW/L

Compare potential 

cooling strategies

Design the cooling 

system via modeling

Component Volume used for 100 kW/L 

inverter estimate [L]

Source

Gate driver (includes current 

sensors) 

0.28 2015 BMWi3 (125 kW)

Control board 0.23 2012 Nissan LEAF (80 kW)

Capacitor 0.25 2015 BMWi3 (125 kW): Assumption: capacitor 

volume decreased by 50% to account for a decrease in 

capacitor requirements for WBG devices

Remaining volume for power 

module and cold plate

0.24

• Heat dissipation requirements: 2,150 W (assuming 100 kW system, 98% WBG inverter 

efficiency, and 95% motor efficiency) 

• Assuming Tj, maximum = 250°C and T coolant = 65°C, volumetric thermal resistance target is 21 

cm3-K/W
NREL | 7

1

10

100

1,000

10,000

100 1,000 10,000 100,000

Specific thermal resistance R'' j-c (mm2·K/W)

Heat transfer coefficient (W/m2·K)

DBC cooled

baseplate cooled

device cooled

Approach

Define the thermal 

target required to 

achieve 100 kW/L

Compare potential 

cooling strategies

Design the cooling 

system via modeling

Device cooling 

• Provides the lowest thermal resistance 

• Enables cooling the electrical leads (decrease 

capacitors and board temperatures)

Image credit: Xuhui Feng (NREL)

Baseplate

Device 

DBC

Electrical leads in 

the 2015 BMWi3 

power module

DBC: direct bond copper
NREL | 8

Approach

Define the thermal 

target required to 

achieve 100 kW/L

Compare potential 

cooling strategies

Design the cooling 

system via 

modeling

• Propose a single-phase cooling approach

 Easier to seal (compared to two-phase 

system)

 Potential to use automatic transmission 

fluid (ATF) (decrease cost)

x Low heat transfer. Propose to use jet 

impingement to improve performance

• Cool the electrical interconnects

• Replace expensive ceramic dielectric material 

with cost-effective alternatives 

Dielectric cooling of planar package

Dielectric fluid

Inexpensive dielectric material

MOSFET

Electrical conductor

Electrical conductor

Electrical lead

MOSFET: metal–oxide–semiconductor field-effect transistor
NREL | 9

Approach/Milestones

Date Description of Milestone or Go/No-Go Decision

December 2017 

(complete)

Milestone: Conduct two-phase modeling of the cooling system 

using computational fluid dynamics multiphase modeling tools 

to support the research and analysis of an advanced compact 

power module

March 2018 

(complete)

Go/No-Go: Research thermal management strategies for a 

planar, compact inverter capable of high-temperature operation

September 2018

(in-progress) Milestone: Create a report to summarize the research results
NREL | 10

Dielectric Coolant Selection

• Selected synthetic hydrocarbons that are used in electronics cooling 

(single-phase) applications 

– ElectroCool EC-140: Engineered Fluids (used this fluid for the thermal analysis) 

– Alpha 6: DSI Ventures (other possible option)

• Ultimate goal is to develop a system that uses ATF as the dielectric 

to decrease cost, use fluid already qualified for automotive use, 

enable motor – inverter integration

Thermal conductivity 

[W/m-K]

Specific heat 

[J/kg-K]

Density 

[kg/m3]

Viscosity 

[Pa-s]

Flash point 

[°C]

Pour point 

[°C]

0.16 2,300 797 0.017 280 -52

ElectroCool EC-140 properties at 70°C temperature (used for thermal modeling)

Technical 

Accomplishments

Thermal conductivity [W/m-K] Specific heat [J/kg-K] Density [kg/m3] Viscosity [Pa-s]

0.42 3,494 1,038 0.00126

Water/ethylene glycol (50 /50) properties at 70°C (provided for comparison)
NREL | 11

w = slot width

Fluid

Copper metallization

Copper metallization

Filler material

SiC MOSFET (5 × 5 × 0.18 mm)

Slot jet inlet

5 mm

CFD Jet Impingement Model Description

Copper metallization

Copper metallization

Filler material

SiC MOSFET (5 × 5 × 0.18 mm)

Fluid

circular jet inlet (d = jet diameter)

Slot jet model (sectional view)

Circular jet model (sectional view)

outlet

s = spreader width

nozzle-to-surface distance: 

3 mm

nozzle-to-surface distance: 

3 mm

s =spreader width

• Evaluated effect of jet velocity ( 1 m/s maximum), heat spreader size, nozzle 

characteristic length (w, d) 

• Tinlet = 65°C, used laminar flow since Reynolds numbers < 300

CFD: computational fluid dynamics

Technical 

Accomplishments
NREL | 12

0

5

10

15

20

25

30

35

1.E-06 1.E-05 1.E-04 1.E-03 1.E-02

Thermal resistance, j-c [K/W]

Pumping power [W]

Circular Versus Slot Jet Performance Comparison

• Predict circular and slot jet 

to have similar performance 

for w, d = 3 mm

• Best performance yields a 

dismal thermal resistance of 

4.7 K/W. Would require 60 

devices to dissipate 2.2 kW

• Need to improve thermal 

performance. Evaluated 

using finned surface to 

improve performance.

spreader width (s) = 10 mm

○ circular jet

□ slot jet

white symbols: w, d = 1 mm

black symbols: w, d = 3 mm

Subscripts j-c: junction-to-coolant 

Technical 

Accomplishments
NREL | 13

Finned Heat Spreader Concept

side views top view

1-mm-tall 

fins

2.5-mm-tall 

fins

3 mm

3 mm

• Fin thickness, channel spacing = 0.2 mm

• Finned area extended 1 mm beyond 5 x 5 mm 

perimeter of the MOSFET area

• Fins only modeled for the slot jet case. Future work 

will model effect of fins on circular jet cases.

slot 

jet 7 

mm

7 mm

5 x 5 mm MOSFET footprint

finned area

Technical 

Accomplishments

Fins can be fabricated using a skiving process. Image fin dimensions: 

fin thickness = 0.09 mm, channel width = 0.18 mm, fin height = 1 mm

Image credit: Gilbert Moreno (NREL)
NREL | 14

0

5

10

15

20

0.0 0.2 0.4 0.6 0.8 1.0

Thermal resistance, j-c [K/W]

Jet velocity [m/s]

Effect of Fins: Slot Jet

• Reduced thermal resistance 

by ~80% using finned 

surfaces

– 50% lower thermal 

resistance (per device area) 

compare to the 2014 Accord 

• Finned surfaces increase 

pumping power 

requirements

spreader width (s) = 10 mm, slot width (w) = 1 mm

no fins

1-mm-tall fins

2.5-mm-tall fins At 0.5 m/s, 24 devices can 

dissipate 2,150 W of heat. 

Each device would dissipate 

~90 W at Tj < 250°C. 

Technical 

Accomplishments
NREL | 15

Initial Thermal Design

Initial results

• Maximum Tj = 234°C

• Each device dissipates ~90 W

• 24 devices can dissipate 2,150 W

• Heat flux = 358 W/cm2

Volume: 0.06 liters (1/4 of the 

volume available for the power 

module and cold plate)

Flow rate requirements: 3.6 LPM 

(at this flow rate, the outlet fluid 

temperature is predicted to be 

82.4°C) 

CFD temperature contours (sectional view)

Slot jet: 2.5-mm-tall fins, w = 1 mm, jet velocity = 0.5 m/s

10 mm

10 x 10 mm 40 mm

60 mm

assume 25.4 

mm tall

top view

Space for fluid manifold and case side view

Technical 

Accomplishments
NREL | 16

Responses to Previous Year Reviewers’ 

Comments

• Reviewer comment: “The reviewer commented the project is not specifically 

advancing the present state-of-the art: capacitor heating by power module, thermal 

interface material (TIM) degradation, capacitor active thermal management, 

eliminating the TIM layer, bus bar cooling, etc. are all known industry solutions.”

• Response: For 2018 and future years we are focusing our research efforts on the use 

of more novel power electronics cooling strategies (e.g., device cooling with 

dielectric fluids) that are typically not used or considered by industry.

• Reviewer comment: “The reviewer said that this work is directed towards capacitor 

cooling. Computed capacitor temperatures were given but they were not verified by 

experimental measurements.”

• Response: We plan to experimentally validate the concept through fabrication and 

testing after the model design work is completed. The validation work should be 

completed in 2018.
NREL | 17

Collaboration and Coordination with 

Other Institutions

• John Deere (industry): Two-phase cooling for high packaging 

density planar inverter (CRADA)

• Elementum3D (industry): Provide 3D-printed metal parts to 

evaluate new heat exchanger concepts

• ORNL (national laboratory): Interactions related to ORNL’s 

power electronics characterization research

• Interactions with other industry contacts

– Dielectric fluid manufacturers
NREL | 18

Remaining Challenges and Barriers

• Creating a reliable, leak free cooling system: main 

challenge is sealing the electrical leads that penetrate 

through the power module

• Decreasing cost: ultimate goal is to use transmission 

fluid as the dielectric coolant in an effort to decrease 

cost

• Fluid compatibility with power electronics materials: 

selected fluids should be compatible with electronics 

materials but tests should be conducted to verify 

compatibility

• Pumping power requirements at low temperatures 

due to higher fluid viscosity
NREL | 19

Proposed Future Research

• Optimize jet impingement cooling with dielectric coolants

• Conduct module-scale simulations to evaluate cooling multiple 

devices and the electrical interconnects

• Evaluate cooling solution for transient conditions (effect of 

switching frequency and short-circuit heating)

• Experimental validation

• Evaluate using ATF as the dielectric coolant

• Evaluate phase-change cooling to enable greater power density

“Any proposed future work is subject to change based on funding levels”
NREL | 20

Summary

Relevance

• Effective thermal management is essential to achieve the 2025 DOE 100-kW/L power density target

Approach/Strategy

• Define a thermal target required to achieve the 100 kW/L power density

• Design a dielectric fluid-based device cooling system to meet the thermal target 

Technical Accomplishments

• Developed thermal models to evaluate the performance of jet impingement cooling (single-phase) 

using dielectric fluids 

• Computed the effects of slot versus circular jets, varying the nozzle size, adding finned features, 

varying the jet velocity, and increasing the spreader/electrical conductor size on jet- impingement 

thermal performance

• Developed a cooling concept that can meet the volumetric thermal targets and thus enable achieving 

the 100 kW/L power density target

• Collaborating with John Deere to develop a two-phase cooling strategy for their inverter

Collaborations

• John Deere

• Elementum3D 

• Dielectric coolant manufacturers

• Oak Ridge National Laboratory
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.

www.nrel.gov

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.

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.

Thank You

Acknowledgments

Susan Rogers, U.S. Department of 

Energy

EDT Task Leader

Sreekant Narumanchi

Sreekant.Narumanchi@nrel.gov 

Phone: (303)-275-4062

Team Members

Kevin Bennion (NREL)

Emily Cousineau (NREL)

Xuhui Feng (NREL)

Bidzina Kekelia (NREL)

Jeff Tomerlin (NREL)

Tim Burress (ORNL)

For more information, contact

Principal Investigator

Gilbert Moreno

Gilbert.Moreno@nrel.gov

Phone: (303)-275-4450
Technical Back-Up Slides
NREL | 23

Other Candidate Dielectric Coolants

• ElectroCool EC-100: Engineered fluids

• Opticool-MIL Fluid: DSI Ventures 

• Automatic transmission fluid: Afton

• Xceltherm 600: Radco 

• Silicone oils: ClearCo 

• Fluorocarbons: 3M
NREL | 24

Thermal Resistance versus Pumping Power for All 

Cases Evaluated

1

10

1.E-06 1.E-05 1.E-04 1.E-03 1.E-02

Thermal resistance, j-c [K/W]

Pumping power [W]

Legend:

○ circular jet

□ slot jet

∆ slot jet with 1-mm-tall fins

◊ slot jet with 2.5-mm-tall fins

white symbols: w, d = 1 mm

gray symbols: w, d = 2 mm

black symbols: w, d = 3 mm