2015-01-1201
Published 04/14/2015
Copyright © 2015 SAE International
doi:10.4271/2015-01-1201
saealtpow.saejournals.org
Power Dense and Robust Traction Power Inverter for the Second-Generation
Chevrolet Volt Extended-Range EV
Mohammad Anwar
General Motors Corporation
Monty Hayes
Delphi Electronics & Safety
Anthony Tata, Mehrdad Teimorzadeh, and Thomas Achatz
General Motors Corporation
ABSTRACT
The Chevrolet Volt is an electric vehicle with extended-range that is capable of operation on battery power alone, and on engine power
after depletion of the battery charge. First generation Chevrolet Volts were driven over half a billion miles in North America from
October 2013 through September 2014, 74% of which were all-electric [1, 12]. For 2016, GM has developed the second-generation of
the Volt vehicle and “Voltec” propulsion system. By significantly re-engineering the traction power inverter module (TPIM) for the
second-generation Chevrolet Volt extended-range electric vehicle (EREV), we were able to meet all performance targets while
maintaining extremely high reliability and environmental robustness. The power switch was re-designed to achieve efficiency targets
and meet thermal challenges. A novel cooling approach enables high power density while maintaining a very high overall conversion
efficiency.
CITATION: Anwar, M., Hayes, M., Tata, A., Teimorzadeh, M. et al., "Power Dense and Robust Traction Power Inverter for the SecondGeneration Chevrolet Volt Extended-Range EV
," SAE Int. J. Alt. Power. 4(1):2015, doi:10.4271/2015-01-1201.
INTRODUCTION
General Motors (GM) was the first in modern days to offer complete
electrification when it's Battery Electric Vehicle (BEV) EV1 was
introduced in 1996 [3]. Subsequently GM is enhancing level of
electrification [2] by introducing hybrid electric vehicle (HEV),
plug-in HEV (PHEV), Extended Range EV (EREV) 1st Generation
VOLT [5,8] and most recently 2nd Generation VOLT. Optimal design
of TPIM power electronics is one of the enablers to achieve
aggressive fuel-economy, performance, reliability, cost and mass
targets for these electrified vehicles. EREV, first introduced by GM
[5], has full functionality as an EV, better electrification than PHEV
and blended operation of gas and electricity as necessary. EREV
electric drive system contains two PM motors that are commuted by
two traction inverters, PIM-A (power inverter-A), PIM-B (power
inverter-B) and an oil pump motor run by a third inverter. All these
three inverters are contained in one package named TPIM situated
between electric motors and battery.
A comprehensive analysis, design and validation plan was tailored to
the 2nd Generation VOLT TPIM. Worst-case analyses were
performed and confirmed with a comprehensive early design
demonstration. Thermal measurements were made to understand the
effectiveness of cooling design and to map peak temperatures and
gradients predicted analytically. In addition, design failure modes and
effects analysis were used to refine the design. This paper focuses on
system architecture, power electronics component design
optimization, efficiency, size, cost, performance and safety of 2nd
Generation VOLT TPIM. Mechanical interfaces, assembly features
and environmental robustness of the TPIM are also presented in this
paper. Section-II describes propulsion, electric drive and HV system
requirements overview. Section-III presents power electronics design
optimization, power stage selection criteria, TPIM performance,
thermal robustness and efficiency metrics. Mechanical orientation,
stiffness/ vibration robustness features of TPIM and transmission
assembly features, coolant interface are discussed in Section-IV.
Section-V compares features between 1st Generation VOLT and 2nd
Generation VOLT TPIMs. System and TPIM level test results are
included in Section-V. Figure 1 shows 2nd Generation VOLT HV
components vehicle layout.
Anwar et al / SAE Int. J. Alt. Power. / Volume 4, Issue 1 (May 2015)
d.
Drive quality: Understanding worst-case drive profiles,
balancing between performance and tractive efforts.
e.
Safety integrity: Protection and sensor circuit design,
microprocessor selection and software design for fault
protection, diagnostic and calibration schemes.
Table 1. TPIM High Level Requirement Comparison.
Figure 1. 2nd Generation VOLT HV Power Electronics Components Vehicle
Layout.
SYSTEM AND REQUIREMENT OVERVIEW
2nd Generation VOLT electric drive system has very aggressive
targets for cost, all electric range, fuel economy and vehicle tractive
performance where TPIM power density, efficiency, thermal strength
and structural robustness, mounting on transmission manifold,
elimination of AC cables are some of the major contributors to meet
this target.
Table 2. TPIM Feature and Improvement Comparison.
Improvement Comparison
Table 1 shows TPIM high level requirements comparison between 1st
Generation VOLT and 2nd Generation VOLT. In 2nd Generation
VOLT architecture, realization of a better power flow between engine
and two electric motors reduces peak simultaneous AC power kVA by
19% and maximum phase current on PIM-B (power inverter-B) by
24% while increasing PIM-A current by 48%. Better understanding
of the electric drive system, interdisciplinary nature of power
electronics technology are among the design considerations to
achieve projected vehicle EV range increase of 30%, charge
sustaining (CS) label fuel economy increase of 10% and improved
vehicle performance both as an electric vehicle and in extended range
mode, compared to 1st Generation VOLT [12]. 2nd Generation VOLT
TPIM has improvements in efficiency, performance, power density,
mass and volume as shown in Table 2. Below is a list of main design
targets for 2nd Generation VOLT TPIM a.
Low system cost: Understand vehicle drive profiles and
mechanical integration of drive unit. Efficient power flow
between two motors, lower part counts, elimination of AC
cables and less assembly labors
b.
Improved Robustness: Improved interconnect and packaging
technique to ensure durability, robustness.
c.
Fuel economy and electric range: State of the art silicon
and module technology, electric drive system efficiency
optimization.
Propulsion and Transmission System
2nd Generation VOLT transaxle has gears, clutches, shafts and
controllers to execute multiple kinematic modes including electric
vehicle operation and charge sustaining hybrid operations. The 2nd
Generation VOLT transmission allows the efficient sharing of tractive
loads across both electric motors. This load sharing enabled the peak
torque requirements for the PIM-B electric motor to be reduced from
the level required on the 1st Generation VOLT. This reduction allows
a corresponding reduction in the peak current from the TPIM.
Lowering the peak current requirement for each inverter as well as
selecting power dense TPIM architecture allows a package size that
could be fit into a cavity on top of the transmission as shown in
Figure 2 (a).
The transmission internal location for the TPIM simplifies the
mechanical integration of the electric drive system into the vehicle
that lowers part counts and assembly labors. Figure 2 (b) shows
TPIM orientation that is chosen to minimize impact during vehicle
crash. Figure 2 (c-d) shows TPIM assembly on transmission and
coolant interface implementation.
Anwar et al / SAE Int. J. Alt. Power. / Volume 4, Issue 1 (May 2015)
TPIM arrives at the manufacturing plant as a single component that is
bolted into the transmission in a single operation. TPIM Interface to
3-phase AC motor done with ridged bus bars that pass through a cast
wall in the transmission case. Coolant interface is done through a
gasketed joint. Coolant is routed through the transmission case to an
external cooling pipe assembly. The coolant pipe assembly can be
unique for each vehicle application to keep the plant interface
common for all versions of the transmission.
Figure 3. 2nd Generation VOLT TPIM (a) Exploded View and (b) “Power
Board” (10).
ELECTRICAL DESIGN OPTIMIZATION
Figure 2. (a) Exploded view of Transmission showing TPIM Assembly (b-d)
TPIM Features and Interfaces.
TPIM and Electric Drive System
The TPIM interfaces with HV battery, motors, inverter cooling and
the rest of the vehicle electrical systems and controls. The main
building blocks of the TPIM are power board, DC bus capacitor, EMI
filters, control and gate drive boards, sensors and busbar. The motor
controls algorithms are programmed into the control boards. Figure 3
(a) shows the exploded view of the TPIM showing these components.
All 12 power switches (Silicon IGBT and Diode for PIM-A and
PIM-B) are connected to the heat sink assembly from both sides and
they are placed on a specially designed double sided printed board
(item 9, 10, 12) with other essential circuits. This board is called
“Power Board” and is shown in Figure 3 (b).
2nd Generation VOLT TPIM power stage has been designed to meet
mainly efficiency, performance and durability requirements. Silicon
technology, tradeoffs between switching and conduction loss
parameters, silicon size and thickness, thermal impedance, loop
inductance and PWM switching techniques are carefully crafted to
achieve an optimized power module design that meets the high level
design target described in System Overview section. To meet high
mile-per-gallon target, special attention was paid to achieve high
TPIM efficiency for the FTP drive cycles. Figure 4 shows TPIM
efficiency for a torque-speed zone that is heavily used for all FTP
cycles. As mentioned earlier, better power flow between Inverters,
better efficiency (Figure 5) and thermal robustness (will be discussed
in next section) enabled, average electric drive system FTP city
efficiency improvement of 6%, projected CS label fuel economy
increase of 10%, and both ‘Low End Torque’ and ‘High Speed
Power’ improvements. Table 3 (a) shows silicon level improvements
and some of the attributes of ‘power board’ as compared with 1st
Generation VOLT power module. Besides ‘power board’, bulkcapacitor is another expensive and bulky parts on the TPIM that
needs to fit inside the inverter package, meet ripple requirement for
the HV DC bus and sustain additional thermal stress for this
transmission mount TPIM. Table 3(b) shows bulk-capacitor and
connector improvements and features.
Anwar et al / SAE Int. J. Alt. Power. / Volume 4, Issue 1 (May 2015)
Power Device Design
Delphi's novel dual-side cooled Viper [9, 10], as shown in Figure 5, is
used as power device for this TPIM. It contains silicon IGBT and
diode in an electrically isolated package that is thermally conductive
on both sides and provides a low profile compact solution for
inverter. Unlike conventional power module, due to the better silicon
and dual-side cooled Viper, silicon footprint became minimal (refer to
Table 2, 3) and thus allowed layout flexibility and reduced cost (refer
to Figure 2, 3).
The Viper provides a more uniform current density than traditional
single-sided power modules and a CTE (coefficient of thermal
expansion) matched material stack. The current capability of the
device is improved by using near symmetrical drain and source
copper terminals that have similar cross sectional area. Source and
gate wirebonds, which are one of the capability limitations in
standard packages, have been eliminated by using topside solderable
interconnects technology. Both sides of the Viper are attached to
ceramic substrates that are CTE matched to the silicon. These low
stress interfaces significantly reduce the die attach solder cracking
problem commonly found in the TO (transistor outline) series
packaging. All these factors increased reliability and durability of 2nd
Generation VOLT TPIM.
Figure 4. Inverter efficiency for torque-speed zone that is heavily used for all
FTP cycles. (a) PIM-A and (b) PIM-B.
Table 3. Comparison on (a) Power Board (b) Bulk-Capacitor and Busbar (p.u
= per unit)
Figure 5. (a) Power Device Exploded View, (b) Power Device Sandwich, (c-d)
Thermal Analysis.
Heat Sink Design and Thermal Performance
For the inverter system chosen the design of the heatsink was critical.
In order to handle the large phase currents, a copper metal injected
molded heatsink was used (MIM). This unique MIM device shown in
Figure 6, provides a low pressure drop with a high heat convection to
handle the losses for each specific IGBT and Diode. This unique
heatsink and housing loop also passively cools the 3-phase AC busbar
and capacitor assembly.
Anwar et al / SAE Int. J. Alt. Power. / Volume 4, Issue 1 (May 2015)
From flow analysis the heat sink system provides a pressure drop of
26 kPa across the entire fluid domain for 105°C ambient, 75°C
coolant running at 10 Liter/min with a thermal split of 49.4% and
50.6% through the bottom and top heat sinks respectively. For the
same coolant conditions and peak phase currents as mentioned in
Table 1, thermal analysis of IGBT/ Diode are presented in Figure 5.
Due to a temperature rise along the coolant path, analysis ensures all
the devices of PIM-A and PIM- B experience beginning-of-life as
well as end-of-life junction temperature that is within specified limit
to ensure robustness and durability. For all the viper devices a unique
statistically proven end-of-line screening on electrical and thermal
parameters are in place during manufacturing process that ensures
efficiency and performance for the entire operating trajectory.
2nd Generation VOLT TPIM also includes a small oil transmission
pump module that is cooled through a thermal insulator material to
the housing coolant loop. Thermal analysis for this module, with
appropriate voltage and current, convection and gap pad assumptions
was also carried out to ensure design sufficiency.
Figure 6. Cooling System and MIM (Metal Injected Mold) Design Flow
Analysis Velocity Magnitude (m/s).
MECHANICAL DESIGN OPTIMIZATION
The 2nd Generation VOLT electric drive system was architected to
maintain the performance the vehicle customer came to expect from
the 1st Generation VOLT but to do it a substantially reduced cost.
The cost comes from the electrification components as well as
impacted by the integration of those components into the vehicle. The
TPIM team worked to reduce the cost on both of these fronts.
Optimize Integration Cost
The key enabler to reduce integration cost was to mount the TPIM
inside the transmission, as shown in Figure 3 and thus eliminate the 3
phase cables that would normally run between the TPIM and the
transmission. This saving is bigger than just the cable cost. The
cables occupy significant real estate where the under hood
environments of most modern non-electrified vehicles is packaged
tightly. When electric drive components are added to this
environment it drives many base vehicle systems to be redesigned to
accommodate more constrained packaging requirements. This cost
contributes to the overall cost of electrifying a vehicle. By integrating
the TPIM in to the transmission the total under hood volume taken up
by the TPIM, 3-phase conductors and transmission is significantly
reduced from the traditional approach of mounting the TPIM on the
vehicle body with a bracket and running 6 large cables from the
TPIM to the transmission.
In addition to piece cost savings the TPIM integration also saved on
total assembly labor between the transmission manufacturing plant
and the vehicle assembly plant. At the transmission manufacturing
plant the TPIM is mounted inside a cavity in the transmission that is
sealed from the outside environment. All the interfaces between the
TPIM and the other transmission mounted electrical components
(motors, sensors and actuators) is accomplished inside the boundaries
of the transmission housing. The TPIM is attached to the transmission
case by 11 bolts, 3 bolts attach the TPIM front and compress the
gasket that seals the IGBT cooling passages, 2 bolts secure the TPIM
rear. The last 6 bolts make the electrical connection between the
TPIM and the two motors. These connections go to bus bars that pass
from the dry TPIM cavity of the transmission into the wet ATF area
of the transmission where the electric motors are located. All 11 bolts
are the same part number and are driven in the same direction. These
simple bolted joints replace all the external high voltage AC wiring/
connectors and TPIM attachment brackets in a conventional
embodiment. After the TPIM is bolted to the transmission case 3
electrical connections are mated. Two connectors are for I/O between
the TPIM and sensors/actuators inside the transmission. The third
connection is to the high voltage pump drive motor also inside the
transmission. A cast cover is installed over the unsealed TPIM to
provide protection from the under hood environment. The high
voltage/current DC interface and a signal connector pass through this
cover casting. The only other external interface to the TPIM is the
liquid cooling port assembly that is bolted to the side of the
transmission case.
Improved Safety Orientation
The specific TPIM location on the top of the transmission was
selected to minimize contact with the condenser, fan, and radiator
module in the event of a frontal crash. This top location also
minimizes interaction with the vehicle body structure and suspension
components during a left hand side impact. The power inverter is also
oriented low in back and high in front. This affords the inverter
adequate packaging space while allowing it to travel under the brake
system master cylinder during a frontal crash. Some of these features,
orientation and assembly are shown before in Figure 2.
Anwar et al / SAE Int. J. Alt. Power. / Volume 4, Issue 1 (May 2015)
Friction Stir Welding (FSW)
High-power electronics for electrified vehicles need liquid-cooling
for heat dissipation. Typically, coolant passages and chambers with
internal fins need to be formed from multiple pieces and joints to
achieve low-cost fabrication. These joints need to be strong to handle
the pressures and vibrations in a vehicle, and to be leak-proof to
eliminate the risk of leakage inside a high-power electrical enclosure.
Conventional design uses pressure seals with rubber gaskets and
many screws, typically with a thick die-cast cover. The 2nd
Generation VOLT TPIM housing and liquid cooled chamber, shown
in Figure 7, uses FSW technology adapted by Delphi. This design
was validated to several thousand pressure cycles without failure.
Delphi's FSW method produces liquid-cooled heat exchangers
integral to product housing and offers most compact, reliable, lowest
cost, corrosion-free and leak-proof joint solution and avoids defects
caused by surface tension and wetting issues. This process firstly,
integrates a liquid heat exchanger to the body of the electronic
module with robust leak-free hermetic joints and secondly, FSW to
weld aluminum covers to cast aluminum cases to enclose the product
replacing glue-and-screw processes. This process outperforms the
industry-standard with 45% smaller footprint, fast change tooling,
3-D force monitoring, 50% shorter process time that results in
reduced carbon footprint and 40% power savings.
Figure 8. Three-phase AC Busbar Cooling and Thermal Analysis.
TESTING, DEVELOPMENT AND
VALIDATION
Besides extensive component level validation, more than 100 fully
functional and instrumented TPIMs were used to validate the product
and demonstrate a highly robust design in laboratory and vehicle set
up. These tests included high temperature durability (HTD), thermal
shock, powered temperature cycling (PTC), random vibration,
mechanical shock, humidity, salt exposure, AC/DC electrical stress,
and EMC. Highlights of the comprehensive validation plan the
includes (a). Full functional test operation cycles covering the entire
operational envelope at −40°C, 25°C, and 105°C
(b). Continuous monitoring of product integrity over its lifecycle,
(c). Field correlated thermal, vibration, and salt exposure,
(d). Emulated vehicle drive cycles using motor dynamometers
(e). Rapid failure mode precipitation test (RFMPT) for reliability
growth demonstration and robustness evaluation and
(f). Coupling with drive unit systems on abusive gas fired
dynamometer testing.
Figure 7. Friction Stir Welded Housing for 2nd Generation VOLT TPIM.
AC Interface Cooling
Among all the interfaces to the TPIM, DC and AC interconnects need
additional care as they handle high voltages and currents. For 2nd
Generation VOLT TPIM AC busbars are connected directly to the
3-phase motor windings by motor rod termination that can reach very
high temperatures. Mitigation scheme was chosen to provide cooling
from the interconnect to the power inverter housing. The cooing is
provided from a thermal insulated materials to the stamped aluminum
sheet that is FSW welded to the inverter housing. Thermal analysis,
as in Figure 8, ensures maximum allowable busbar assembly
temperature with appropriate operating and environmental conditions.
The RFMPT is designed to identify potential design weaknesses by
subjecting TPIM to multiple simultaneous environmental stresses
using field correlated test acceleration factors and exposure limits.
Vibration tests, in Figure 9, are derived from the Road load data
acquisition (RLDA), correlated to severe customer and tailored by
vehicle orientation with separate durability profiles for fore/aft,
lateral, and vertical.
Motor dynamometers are used for long-term high temperature
durability testing using actual application motors simulating severe
driving events requiring peak torques and re-gen occurrences. Finally,
before integrating into RLDA vehicles the electric drive system is
durability tested in a gas fired dynamometer cell, as in Figure 10 that
includes engine, transmission, electric motors, and TPIM. The test
simulates extremely severe customer driving conditions over multiple
design lives while enduring an aggressive vibration environment
arising from second order engine harmonics.
Anwar et al / SAE Int. J. Alt. Power. / Volume 4, Issue 1 (May 2015)
Figure 9. (a) TPIM Vibration Test Set Up, (b) Internal Accelerometer
Locations.
Figure 12. (a) PTC Chamber and HTD Test Stand, (b) Signal and Load Rack.
The sound pressure level of the TPIM shall be at or below some
indicated levels at four vertical sides and at the top surface, if
measured at locations 10 cm from the surface of the TPIM at the
geometric center of that surface. Test results shows (20 to 20,000 Hz),
in all cases and conditions the sound levels are far below the
maximum allowable limits. Figure 14.
Figure 10. (a) Gas Dynamometer and (b) Road Load Data Acquisition (RLDA
Vehicle Test Set up.
Test Results
Figure 11 shows some results during vibration test. They are tested
under currents pushing through IGBT and diode. During the test any
cracks are watched and if any lead fail the temperature image will be
reflected as ‘cool’.
Figure 12 (a) shows PTC chamber and HTD test stand, subjecting as
many as 12 TPIM's simultaneously to grueling durability cycles
simulating under hood conditions with inductive loads and rapid
temperature cycles between −40 and 105 °C. All hardware-inputoutput (HWIO) was monitored during testing to identify anomalies,
capture a snapshot of environmental conditions present when
anomalies occur, and track the amount of test exposure to correlate
with retail customer field time. Figure 13 show thermal results for 3
cycle HTD profile.
Figure11. (a) Vibration Test Results, (b) Lead Crack Inspection (Zoomed).
Figure 13. Thermal Results for 3 Cycle High Temperature Durability (HTD)
Profile.
SUMMARY/CONCLUSIONS
This paper describes 2nd Generation VOLT TPIM design to achieve
cost and power density targets and yet maintain extremely high
reliability and environmental robustness. System requirement,
architectural overview, electrical design of the power switch to
achieve efficiency target and thermal challenges are described with
illustrations. Mechanical design to provide stiff frame required for
powertrain environment, to optimize integration and assembly cost,
novel cooling and welding approach by Delphi made this TPIM
unique. All major TPIM test items are mentioned, where more than
100 fully functional and instrumented TPIMs were used to validate
the product and demonstrate a highly robust design in laboratory,
dynamometer and vehicle test set up.
Anwar et al / SAE Int. J. Alt. Power. / Volume 4, Issue 1 (May 2015)
Figure 14. (a) PTC Chamber and HTD Test Stand, (b) Thermal Results for 3
Cycle HTD Profile.
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CONTACT INFORMATION
Mohammad Anwar can be reached by email at
mohammad.anwar@gm.com.
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