electric drive technologies for future electric...

Electric Drive Technologies for Future Electric Vehicles

Professor Daniel CostinettThe University of Tennessee

November 8th, 2017

With contributions from Burak Ozpineci, ORNL, and Dragan Maksimovic, CU

WHY ELECTRIFY VEHICLES?

Motivation

Transportation ElectrificationMotivation

• Improve efficiency: reduce energy consumption

• Displace petroleum as primary energy source

• Reduce impact on environment

• Reduce cost

EIA:

• Transportation accounts for 28% of total U.S. energy use

• Transportation accounts for 33% of CO2 emissions

• Petroleum comprises 93% of US transportation energy use

Vehicle Kinematics

0 100 200 300 400 500 6000

20

40

60

80

100v [

mph]

0 100 200 300 400 500 600-60

-40

-20

0

20

40

60

80

Pv [

kW

]

time [s]

Example: US06 driving cycle

0 100 200 300 400 500 6000

20

40

60

80

100

v [

mph]

0 100 200 300 400 500 600-60

-40

-20

0

20

40

60

80

Pv [

kW

]

time [s]

Example: Prius-sized vehicle

Ve

hic

le s

pee

d [

mp

h]

10-min8 miles

Pro

pu

lsio

n p

ow

er

[kW

]

Time [s]

Average power and energy

0 100 200 300 400 500 6000

20

40

60

80

100v [

mph]

0 100 200 300 400 500 600-60

-40

-20

0

20

40

60

80

Pv [

kW

]

time [s]

Prius-sized vehicle

Dissipative braking

Pvavg = 11.3 kW

235 Wh/mile

Regenerative braking

Pvavg = 7.0 kW

146 Wh/mile

0 100 200 300 400 500 6000

20

40

60

80

100

v [

mph]

0 100 200 300 400 500 600-60

-40

-20

0

20

40

60

80

Pv [

kW

]

time [s]

Vehicle speed [mph]

Propulsion power [kW]

Ve

hic

le s

pee

d [

mp

h]

Pro

pu

lsio

n p

ow

er

[kW

]

Time [s]

Gas Engine vs Electric Drive

Internal Combustion Engine (ICE) Electric Drive (ED)

• ηED,pk ≈ 95%; ηICE,pk ≈ 35%• ED offers full torque at zero speed

• No need for multi-gear transmission

Max Efficiency

≈ 34% Max Efficiency

> 95%

Conventional Vs. Electric Vehicle

Tank + Internal Combustion Engine

Electric Vehicle (EV) Battery + Inverter + AC machine

Regenerative braking

NO YES

Tank‐to‐wheel efficiency

20%

1.2 kWh/mile, 28 mpg

85%

0.17 kWh/mile, 200 mpg equiv.

Cost 12 ¢/mile [$3.50/gallon] 2 ¢/mile [$0.12/kWh]

CO2 emissions (tailpipe, total)

300, 350) g CO2/mile(0, 120) g CO2/mile

[current U.S. electricity mix]

Energy Costs (10-yr, 15k mi/yr)

$18,000 $3,000

(Commuter Sedan comparison)

Energy and Power Density of Storage12000

2016 Camaro 6.2L V8

Mazda RX-81.3L Wankel

Conventional Vs. Electric VehicleTank + Internal Combustion

Engine(Ford Focus ST)

Electric Vehicle (EV) Battery + Inverter + AC machine

(Ford Focus Electric)

Purchase Price $24,495 $39,995

Significant Maintenance

$5,000(Major Engine Repair)

$13,500(Battery Pack Replacement)

Range > 350 mi < 100 mi

Curb Weight 3,000 lb 3,700 lb

Energy storage Gasoline energy content

12.3 kWh/kg, 36.4 kWh/gallon

LiFePO4 battery

0.1 kWh/kg, 0.8 kWh/gallon

Refueling 5 gallons/minute

11 MW, 140 miles/minute

Level I (120Vac): 1.5 kW, <8 miles/hour

Level II (240Vac): 6 kW, <32 miles/hour

Level III (DC): 100 kW, <9 miles/minute

(Commuter Sedan comparison)

CO2 emissions and oil displacement studyWell-to-Wheel Analysis of Energy Use and Greenhouse Gas Emissions of PHEVs

(2010 report by Argonne National Lab)

The Impact of Policy

Peter Savagian, “Barriers to the Electrification of the Automobile,” Plenary session, ECCE 2014

EV Everywhere Grand Challenge

Today$12/kW

1.2 kW/kg3.5 kW/l

>93% efficiency

Power Electronics

• Electrification of transportation is an enabling technology for safe, reliable, and environmental future

• Issues:• Battery energy storage and charging times inadequate for consumer

expectations

• 33% of total cost of an HEV is due to power electronics

• Weight and volume miniaturization required; Thermal management and packaging increase weight and require overdesign

• Materials (semiconductor, inductor, capacitor) materials not cost-effective

• Intelligent and comprehensive design of power electronics is necessary to continue development of electric vehicles.

ROLE OF POWER ELECTRONICS

Background

Power Electronics in Electric Vehicles

Peter Savagian, “Barriers to the Electrification of the Automobile,” Plenary session, ECCE 2014

EV Drive Train

Up to 700 VDC, depending on driving

conditions

BatteryPHEV (~320VDC)HEV(~259VDC)

3-ф AC to motor

Low Voltage modules (4-60VDC)

Cooling Systems – Audi Q5 Example

• Engine coolant in gasoline vehicles typically 105°C• Silicon power electronics begin to break down at 125°C• Separate, dedicated cooling loop for electrical systems• SOA: 1kW to cool 4kW of power losses

Design of Power Electronics

Powertrain Components

InverterBi-directional Converter

Battery Charger

Torque

to Drive

Wheels

(200–450 V DC)

120 V AC/ 240V AC/ Fast Charger

Example EV Power Electronics Module

• Component buildup of an EV Inverter built at ORNL

Substrate

• A ceramic piece sandwiched between two metal layers.

• Typically Direct Bonded Copper (DBC), Direct Bonded Aluminum (DBA)

• Function: Electrical conduction (horizontal plane) and insulation (vertical plane).

Power Device Chips

• The power device chips are soldered on the DBC.

• Function of Die Attach: Electrical connection and mechanical joints.

Interconnect

• Wire or Ribbon Bonds

• Typically aluminum or gold.

• Function: Electrical connection.

Baseplate (Base Metal)

• Typically copper or metal matrix composite (e.g. AlSiC).

• Function: Mechanical support and heat spreading.

Encapsulate

• Silicone Gel

• Function: Electrical insulation, mechanical protection (damps mechanical shock), and heat spreading.

Housing and Terminals

• The module is covered with a plastic package.

• The interconnects connect the power device chips to the copper terminal which protrudes from the package.

• Function: Mechanical protection and electrical connection.

Liquid Cooled Heatsink

• Typically, an aluminum liquid-cooled heatsinkis used to cool inverters.

Power Modules

• The power modules are mounted on the heat sink with a thermal interface material (TIM) under the modules to increase the thermal conductivity.

• TIM is still responsible for around 40% of the thermal resistance from the junction to the ambient.

Gate Driver Boards

• The gate driver boards convert the computer signals into gate signals to switch the power devices on and off.

• The gate driver boards need to be as close to the power modules as possible to reduce the length of interconnections.

Sensors

• Current and voltage values are required as feedback for closed loop controllers to control torque (using current values) and speed (or flux using voltage values) of an electric machine.

Bus Bars

• Copper busbars are used for electrical connections between every power component of the inverter.

• For most inverters, bus bars must be designed to have minimum parasitic inductance.

Controller

• The controller boards are usually much more crowded than the one shown on the left and they include circuits for

– a microprocessor, typically a digital signal processor (DSP)

– analog and digital interfaces with the sensors and gate drivers

Capacitors• Dc link capacitors are

required in voltage source inverters (VSI). Capacitors generally occupy a third the volume of an inverter and weigh as much. They cost 15-20% of the inverter.

• In a VSI, they filter the dc link current and keep the dc link voltage constant.

• A current source inverter (CSI) would have an inductor instead of the dc link capacitor.

• The inverter shown on the left has significantly lower dc link capacitor requirements.

ORNL-built Segmented Inverter with Reduced Capacitance

• The advantage of this inverter is 60% reduction dc-link capacitor

HOW WE CAN IMPROVE POWER ELECTRONICS

Research

State of the Art (SOA):

No WBG inverters commercially available

Problems associated with power electronics for advanced vehicle applications include:

Low efficiency at

light load conditions for inverters and

converters

Low current density and

device scaling issues for

high power converters

Lack of reliable higher

junction temperature

devices

High cost of devices and

power modules

especially for WBG and advanced

silicon devices

Low power density for

the low voltage

electronics and cost of

inter-connects

Lack of adequate

protection for the devices

High cost for low loss

magnetics and high

temperature films for

capacitors

Power Electronics for Electric Drives

Reduce size and weight of the inverters to meet the 2022 targets of 13.4 kW/L and 14.1 kW/kg

• Increase the overall efficiency of the traction drive system

• Reduce the size of the passives with high frequency operation

• Reduced thermal requirements with high temperature operationWBG technology

• Reduce cost and volume of the passives

• Integrate more functionality and reduce cost through component count

Integrated topologies and passives

• Increase the safe operating area of the WBG devices using advanced gate driver circuits

• Increase the reliability of the system using protection circuits• Design algorithms to optimize efficiency for light load conditions

Control algorithms and novel circuits

• Minimize parasitics and increase heat transfer using advanced packaging

• Reduce cost through novel interconnects

• Reduce cost through process optimization

Novel system packaging and advanced manufacturing techniques

Research Areas

Functional Integration

InverterBi-directional Converter

Battery Charger

Torque

to Drive

Wheels

(200–450 V DC)

120 V AC/ 240V AC/ Fast Charger

BIDIRECTIONAL DC-DC CONVERTER

Example #1

Drivetrain DC-DC ConverterOak Ridge National Lab, “Benchmarking of Competitive Technologies”

• A DC-DC converter to boost battery voltage is included in many EVs

• Allows higher efficiency operation of ED• Wider operating range• Lower pack voltage

BatteryElectric Drive

η = 92%

η = 86%

Vdc = 650 V

Bidirectional DC-DC Converter





η = 92% 94%

η = 86% 88%

Vdc = 650 VVdc = 500 V







η = 92% 94%

η = 86% 88% 93%

Vdc = 650 VVdc = 500 VVdc = 225 V

Optimal Bus Voltage

• Bus voltage for maximum efficiency of electric drive varies with speed and torque

• Maximum ED efficiency only obtained when power electronics vary bus voltage dynamically

• Often, only high power operating points are considered in design, but…

• … under normal driving conditions, ED operates predominately at low power

0 100 200 300 400 500 6000

20

40

60

80

100

v [

mph]

0 100 200 300 400 500 600-60

-40

-20

0

20

40

60

80

Pv [

kW

]

time [s]

US06 driving cycle

Integrated DC/DC and Charger

• Designed combined drivetrain DC-DC and isolated battery charger

• Converter designed for high energy efficiency during normal drive cycles

• Highly efficient, low power isolation converter used as drivetrain converter at low power

Traditional 2-stage drivetrain topology

Combined isolated/non-isolated topology Prototype Measured Efficiency

Charger Mode

DC/DC Mode

Reducing Cooling

• Partnered with ORNL to integrate and test immersion cooled SiC power module

• SiC gate driver tested to 300°C

• 3D printed aluminum endcaps function as electrical, mechanical, and thermal components

Complete 50 kW phase leg assembly

BATTERY MANAGEMENT SYSTEM

Example #2

Battery Management System

Battery Management System

• EV battery consists of many (100’s) series battery cells (LFP, Li-ion, NiMH)• Cells share a charging and discharging current, but may have mismatches in series

resistance, capacity, operating temperature, health, or dynamics• Cells binned by manufacturer to limit mismatch at beginning-of-life

Cell Balancing

SOCmax

SOCmin

+ Vpack –

• Small differences in cell characteristics are exacerbated over multiple charge and discharge cycles due to their series connection

Cell Balancing

SOCmax

SOCmin

+ Vpack –

• Discharge is limited by the first cell to reach the minimum allowable State-of-Charge (SOC).

Cell Balancing

SOCmax

SOCmin

+ Vpack –

Inaccessible Capacity

• Discharge is limited by the first cell to reach the minimum allowable State-of-Charge (SOC).

• Effective pack capacity limited to the capacity of the lowest cell (in Amp-hours)

Cell Balancing

SOCmax

SOCmin

+ Vpack –

Uncharged Capacity

• When recharged, charging is stopped when the first cell reaches maximum capacity, leading to incomplete charging of some cells and lower total pack capacity

Passive (Dissipative) Balancing

SOCmax

SOCmin

+ Vpack –

• Option 1: Dissipate, then recharge• Repeat a number of cycles to balance all cells at full charge

Active Balancing

SOCmax

SOCmin

+ Vpack –

• Option 2: Redistribute• Requires more advanced circuits• Can be run continuously, during runtime

Incorporation of Functionality

• Combining the necessary HV-to-LV converter with balancing converters allows• Effective (differential) efficiency of 100%• Nearly zero effective cost

R. Zane, M. Evzelman, D. Costinett, D. Maksimovic, R. Anderson, K. Smith, M. S. Trimboli, and G.

Plett, “Battery control,” U.S. Patent 14/591,917, 2012.

WHERE POWER ELECTRONICS WILL TAKE US

Future Work

Vehicle-Grid Interactions

• EV batteries used as energy storage for grid

• Intermittency mitigation coupled to renewables

• Load shifting

• Power compensation

SAE Ground Vehicle Standards SmartGrid

Wireless Charging• Wireless charging opportunity:

• Provides convenience to the customer.

Plug-in and battery electric vehicles (PEVs) can be charged conveniently and safely without need for cable and plug.

With WPT the charging process can be fully autonomous.

• Reduce on-board ESS size using dynamic on-road charging.

Wireless Charging

Stationary Charging Opportunity/Quasi-Dynamic Charging

In-motion/Dynamic Charging

Wireless Charging

Stationary Charging In-motion/Dynamic ChargingChevy Equinox andToyota Prius Conversion Plugin

GEM Vehicle

World’s First 3D Printed Car Makes Debut

Layer by layer, inch my inch, the world's first 3-D printed vehicle seemingly emerged from thin air during the 2014 International Manufacturing Technology Show. In a matter of two days, history was made at Chicago's McCormick Place, as the world's first 3-D printed electric car --named Strati, Italian for "layers"-- took its first test drive.

September 2014

ORNL 3D-Printed Electric Shelby Cobra

3D Printed Liquid-Cooled 80 kW InverterNissan LEAF Inverter (80kW)80kW ORNL COMPACT Inverter

Conclusions

• Power Electronics play a critical role in the future adoption of electric vehicles

• The Power Electronics engineer must holistically consider in-system performance, not solely efficiency in isolation

• Future technologies including wireless power transfer and 3D printing can revolutionize the industry

QUESTIONS:

[email protected]

curent.utk.edu potenntial.eecs.utk.edu ornl.gov

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