2012-01-0339
Progress in the Development of a High-Performance Heat Sink for Hybrid Electric Vehicle Inverters Author, co-author list (Do NOT enter this information. It will be pulled from participant tab in MyTechZone) Affiliation (Do NOT enter this information. It will be pulled from participant tab in MyTechZone) Copyright © 2012 SAE International
ABSTRACT We have previously reported on the development of a novel active heat sink (AHS) for high-power electronic components offering unparalleled capacity in high-heat flux handling and temperature control. AHS employs convective heat transfer in a working fluid circulating in a miniature closed and sealed flow loop. High flow velocity, good flow attachment, and relatively high thermal conductivity of working fluid lead to ultra-low thermal resistance around 0.1 deg C/W. AHS appears very suitable for directly interfacing a hybrid electric vehicle (HEV) inverter to engine coolant loop. Alternatively, AHS may be used to interface inverter electronics to an air-cooled heat exchanger. As a result, the traditional dedicated liquid coolant loop for thermal management of the inverter can be eliminated, the inverter subsystem can be greatly simplified, and the power train electronics made much more compact. Our previous work focused on AHS with working fluids based on liquid metals. This paper reports on AHS development testing with water-based working fluid, which are more suitable for automotive applications. Also presented are an AHS concept updated for high-volume, low-cost series production, and a concept for AHS integration into an automotive inverter.
INTRODUCTION Hybrid electric vehicles (HEV) and future plug-in hybrid electric vehicles (PHEV) require further advances in the areas of power electronics and electric machines to effectively meet anticipated benefits of reduced energy consumption and lower greenhouse gas emissions. The HEV/PHEV electric power train uses a solid-state electronics inverter to convert a direct current (DC) from a battery to a 3-phase alternating current (AC) to drive a propulsion motor, Figure 1. Key semiconductor components in the inverter are insulated gate bipolar transistors (IGBT) and their associated shunt diodes. Silicon (Si)-based semiconductors are sensitive to temperature and must operate below 125°C (or very briefly up to 150°C) to maintain acceptable efficiency, reliability, and to avoid early degradation.
Figure 1- A 3-phase inverter feeding an AC motor from batteries During vehicle acceleration, the inverter must generate tens of kilowatts (kW) of AC power on demand. Although HEV/PHEV power train electronics is ~96% efficient, at the very high power throughput even a small residual power loss translates to a heat load on the order of several kW, which is deposited into the inverter solid-state electronics. This heat must be removed from the semiconductor chips in real-time to prevent their overheating and failure. At peak power, the projected thermal loads and fluxes to the semiconductor Page 1 of 10
chips may approach 500 W and 450 W/cm2, respectively, Figure 2. High heat fluxes in IGBT /diodes, and resulting high temperatures decrease component lifetimes by accelerating failure mechanisms in materials, especially in electrical and mechanical connections. The trades between technical choices and the associated economics are better understood considering that silicon chips represent about 50% of the inverter cost [1]. (For silicon carbide [SiC] chips, this fraction will likely be even higher.) The current trend is to reduce assembly cost and interconnection reliability by using as few IGBTs and diode chips as possible. With the increasing power per chip, the manufacturers are forced to use larger, more expensive dies to keep the resulting heat flux within limits. The resulting fewer but larger chip dies are also more susceptible to solder reliability due to a mismatch in coefficient of thermal expansion (CTE) with respect to the substrate. On the other hand, smaller dies are less costly, but they operate at higher heat flux, which requires heat sinks with very low thermal resistance.
Chip Heat Flux [W/cm2]
500
Future Full EV with SiC Chips*
400
Future Full EV with Si Chips
300 200 100
DOD FreedomCAR (2015)
0
400 200 300 Chip Heat Load [W]
100
500
Figure 2 - Heat loads to power semiconductor chips at maximum electric output (based on [2]) The current approach for cooling HEV electric power inverters uses a separate cooling loop (Figure 3) with water-ethylene glycol coolant at about 70°C [1]. Such a dedicated cooling loop undesirably adds to system complexity, weight, volume, and cost. 3Ø AC
DC
HEX
Dedicated Cooling Loop
El. Pump
Combustion Engine
Engine Cooling Loop
Eng. Driven Pump
Existing Hardware
Main Radiator
Waste Heat
Sub-Radiator
Inverter
Dedicated to the Hybrid
Figure 3 - Current approach for cooling HEV electric power inverters uses a dedicated cooling loop A preferred approach is to avoid the dedicated cooling loops and use the existing loop from the engine running through the radiator to cool the inverter, Figure 4. In this scenario, the coolant may reach temperatures as high as 105°C at the inverter inlet. Removing 200500 W of waste heat at heat fluxes around 250 W/cm2 from today’s chips and nearly 450 W/cm2 from future chips with only 20°C temperature gradient is extremely challenging because it requires heat sink with thermal resistance on the order of 0.1°C/W. Using air for cooling inverter electronics is an attractive option because it eliminates the need for plumbing to the engine cooling loop and provides the designer with more freedom in locating inverter hardware. However, cooling by air is considered even more technically challenging than liquid cooling by engine loop. Page 2 of 10
While future trench IGBTs and possible silicon-carbide (SiC) or gallium nitride (GaN) devices have more relaxed temperature limits, the ultimate temperature limit may be set by the thermomechanical stresses and fatigue failure in the chip dies as well as the in electrical and mechanical interfaces. Already today, the deep temperature cycling of inverter chips has been responsible for their limited reliability and shortened lifetime. Considering the very high cost of the inverter, the inverter manufacturers today design the hardware to specifically allow for inverter chip replacement [3], which complicates the design. According to some estimates, a present day inverter represents about a quarter of the electric power train cost, not counting its dedicated cooling loop [4]. Packaging the required inverter electronics in a compact, lightweight, and inexpensive unit has been a very challenging technical problem. There is a strong need for technologies offering to make HEV/PHEV more cost effective to consumers. In particular, eliminating the dedicated cooling loop requires an ultra-low thermal resistance means for interfacing the high-wattage, high-heat flux load of the inverter chips to engine coolant or ambient air. 3Ø AC
DC
Inverter Waste Heat
Dedicated Cooling Loop
Combustion Engine Eng. Driven Pump
Existing Hardware
Main Radiator
AHS
Dedicated to the Hybrid
Figure 4 – Inverter cooled by engine coolant loop
LIMITATIONS OF CURRENT INVERTER COOLING TECHNOLOGIES Three things affect the junction temperature of inverter semiconductors: motor drive current, thermal path, and coolant temperature. The first one is the cause of the heat load. The second one accounts for everything between the semiconductor material and the coolant. Figure 5 shows the different layers constituting a conventional IGBT package in an inverter. Each layer increases the thermal resistance from junctions to coolant. In the 2010 Toyota Prius, the stack-up was reduced to 3.8 mm. While this allowed Prius to increase the electrical output, it did not eliminate the dedicated cooling loop. A variety of techniques has been considered to reduce the junction-to-coolant thermal resistance, thus reducing the chip temperature, increasing the chip output, or both:
Figure 5: Thermal resistance stack-up for the IGBT and its associated diode in Toyota Prius [5] Passive Heat Spreaders use materials with very high thermal conductivity to “spread” high-flux heat load over a larger area, thereby reducing the heat flux to downstream components. Traditional heat spreading materials have insufficient performance. A variety of novel materials with non-isotropic thermal conductivity approaching 1,000 W/m°K are now being developed for military electronics, including diamond film, carbon fibers, and metal-carbon composites. Novel carbon-based materials with thermal conductivity in excess of 10,000 W/m°K are also being forecast. Even when mature, the high price of these materials and the lack of joining methods may keep them out of reach for the automotive market. Page 3 of 10
Two-Sided Active Cooling applied to an inverter chip provides a second thermal path, thus theoretically reducing chip temperature rise over the heat sink by as much as a factor of four. One of the most advanced active two-sided cooling is now employed in the Lexus 300 HEV, Figure 6. The IGBTs and diodes are placed on vertical cards, which are contacted via thermal grease to flat cooling tubes running between supply manifolds. While this approach delivers 55 kW of electric power to Lexus traction motor, it does not eliminate the need for the dedicated cooling loop. The drawbacks of double-sided cooling include complexity, increased risk of coolant leaks (more connections), and impeded access for servicing to replace failed chips. In particular, to facilitate chip replacement, the cold shoes used to draw heat from the chips are not soldered but contacted via thermal grease, resulting in a significant thermal resistance.
a) Concept
b) Implementation in 2010 Lexus 300 Figure 6 - Two-sided active cooling approach [5] Impinging Jets, Evaporative Sprays, and Microchannels are well-known means for enhancing the coefficient of heat transfer from a component to a coolant. To be effective, these techniques generally require a low viscosity, pure, and highly filtered coolant, or operating in a dedicated cooling loop including pumps, compressors, and evaporators (for evaporative sprays [6]). While capable of removing heat at high flux, these approaches are incompatible with the goals of eliminating separate cooling loops. Direct Back-Side Cooling (DBSC) technique that received attention in recent years [7] provides jet impingement cooling directly to the chip substrate and, thus, beneficially eliminates the heat sink and TIM. However, direct integration of DBSC with the engine cooling loop is very challenging because of 1) high viscosity of polyethylene glycol coolant, 2) the need for a booster pump to overcome the nozzle pressure loss, and 3) susceptibility of the chips to flow induced mechanical vibrations and consequential mechanical failure. In summary, many inverter cooling technologies have been proposed and examined in detail; however, to date no commercially acceptable approach for eliminating the dedicated cooling loop has been found.
ACTIVE HEAT SINK (AHS) We have previously reported on the Aqwest’s novel high-performance AHS concept for thermal management of HEV inverter electronics [4]. AHS offers interfacing the IGBT and diode chips directly to engine coolant or ambient air, thereby eliminating the dedicated cooling loop. The AHS employs a convective heat transfer by a working fluid flowing at high speed inside a miniature sealed flow loop, Figure 7. The working fluid receives waste heat at a high flux, transports it by forced convection, and rejects it at a much lower heat flux to either engine coolant (Figure 7a) or ambient air (Figure 7b). Much of our prior work [8, 9, 10] focused on Page 4 of 10
working fluids based on liquid gallium alloys having a low melting point as low as -35°C [11] and thermal conductivity around 30 times higher than water. In the past, we have successfully tested AHS with liquid metal working fluid with a heat load of 260 W and heat flux of 270 W/cm2 (inferred thermal resistance 0.1°C/W), and with a heat load of 166 W and heat flux of 369 W/cm2 (inferred thermal resistance 0.09°C/W) [12], Figure 8. Liquid metals have a good electrical conductivity, which makes it possible to flow them around the loop by using electromagnetic induction and without any moving parts. The high thermal conductivity of liquid metal makes it possible to remove high-flux heat at relatively modest flow velocities and laminar flow.
Active Heat Sink (AHS)
A
IGBT / Diode
Diode
Nearly Isothermal Liquid Flow Loops
dQ/dt
IGBT
dQ/dt
RL Macro-Channel HEX for Engine Coolant
A
VIEW A-A
a) Rejecting heat to engine coolant Active Heat Sink (AHS)
Nearly Isothermal Liquid Flow Loops
A
IGBT / Diode
Diode
dQ/dt
Air Flow
dQ/dt
RL
Finned Air-Cooled HEX
IGBT
A
VIEW A-A ~100 mm
b) Rejecting heat to ambient air Figure 7 - Schematic diagram of the innovative AHS concept for cooling of IGBT/diodes
Chip Heat Flux [W/cm2]
500 400
Future Full EV with SiC Chips 2011
Future Full EV with Si Chips
300 2010 200 100
0
2009 Test Data
100
DOD FreedomCAR (2015)
400 200 300 Chip Heat Load [W]
500
Figure 8 - Test data plotted onto chart from Figure 2 [12] Page 5 of 10
Alternative working fluids include various aqueous solutions having an acceptably low melting point. Compared to liquid metal working fluids, water-based fluids offer lower cost and simpler handling during AHS fabrication. To attain targeted heat transfer rates in water-based working fluids, the flow velocity is increased to turbulent flow regime. Preferred method for flowing such working fluids uses a mechanical impeller. Figure 9 shows an AHS comprising a copper spool with a cooling jacket having high-density channels for flowing engine coolant. An impeller drum on a drive shaft suspended between two flanges is installed inside the spool and magnetically coupled to a motor. The space between the impeller drum and the spool bore forms a flow channel, which is hermetically sealed and filled with a suitable working fluid. IGBT and diode chips are installed via electrically insulating pads on the exterior surface of the spool. In operation, rotation of the impeller induces the working fluid to flow a Couette (shear flow) regime. The working fluid flows at high flow velocity of several meters per second to receive high-flux heat from the chips with very low thermal resistance. Acquired heat is carried by the working fluid to the engine coolant channels. These channels have a surface area nearly two orders of magnitude larger than the area of the chips, which makes it possible to transfer heat with very low thermal resistance to engine coolant flowing at modest velocities. Preferably, AHS would be engaged only during vehicle acceleration. To enable fast on-demand response, the impeller drum is made from lightweight materials so that it can reach full speed in less than 0.3 second. The motor may be formed as a 3-phase winding powered by the inverter output and magnetically engaging a permanent magnet attached to the drive shaft. IGBT/diode Chip, 6 PL
A
Insulator
IGBT/diode Chip, 6 PL
Insulator
Jacket
Motor/ Drive Magnetic Shaft Drive
Flange w/ Bearings
Impeller Drum Spool Engine Coolant Channels
A
Working Fluid Flow Channel
Impeller Drum
Spool Jacket
VIEW A-A
Figure 9 - AHS for use with alternative working fluids An engineering concept design of AHS suitable for high-volume production is shown in Figure 10. This AHS design carries up to 6 chips each; 3 IBGTs and their associated shunt diodes. AHS perimeter is generally divided into six segments, three for mounting the chips (2 chips per segment) and three for rejecting heat to engine coolant. AHS comprises a spool, cooling jacket, impeller, flanges, motor, and working fluid fill. The spool transfers heat from the chips to the working fluid and from there to engine coolant. The spool is made of copper and it has three flat segments for mounting of the chips and three segments with channels for flowing engine coolant. The jacket is formed as a cylindrical shell hard soldered over the copper spool. Three cutouts in the jacket align with the flats on the spool and allow for installation of the IGBT and diode chips on an electrically insulating leaf of aluminum nitride. The jacket has internal passages for directing engine coolant to and from the cooling channels in the spool. The working fluid can be aqueous solution with a suitably low freezing point. The channel containing the working fluids is hermetically sealed and capable of holding internal pressure. Impeller and Working Fluid Channel
Motor
Jacket Flange w/Seal Engine Coolant
Spool w/ Engine Coolant Channels IGBT/Diode Chips
Figure 10 – Engineering concept design of AHS high-volume production
Page 6 of 10
TESTING WITH WATER-BASED WORKING FLUID An AHS test article for operating in Couette regime was developed to generally follow the principles shown in Figures 8 and 9. This unit (Figure 11) has a water-cooled jacket with coolant flowing in azimuthal direction opposite to the rotation of the impeller. Tubes attached to the jacket are connected to a water supply and return. The flat portion of the spool allows for installation of up to two heat loads representing an IGBT-diode pair. The heat load was a 35-W resistor in TO-220 package modified to remove a portion of the copper screw tabs, thus, reducing the available thermal interface area to the AHS to about 0.45 cm2 each. Because the AHS provided excellent removal of heat, the resistors could be safely overloaded by more than a factor of 3. The test article was mechanically coupled to an electric motor equipped with a variable speed drive and a tachometer. The unit was operated with a water-based working fluid at several rotational speeds and heat loads up to about 100 W and heat flux of about 220 W/cm2. Figure 12 shows the inferred AHS thermal resistance (heat load interface to water coolant) as a function of impeller speed. Evidently, at high rotational speeds, the thermal resistance of this un-optimized test article approaches the stated goal of 0.1°C/W. Engine Coolant Port (2 PL)
Heat Load
Impeller Drum Assembly
Spool-Jacket Brazed Assembly
Figure 11 – Partially assembled AHS test article
Thermal Resistance [degC/W]
0.3 0.25 0.2 0.15 0.1 0.05 0 0
1000
2000
3000
4000
Impeller Speed [rpm]
Figure 12 – Inferred AHS thermal resistance
INVERTER INTEGRATION CONCEPT According to current practice in inverter integration, chips are placed on a printed circuit board (PCB) and cooling is routed to them. Our approach to design the inverter around the AHS cooling concept. In particular, the AHS and an engine coolant manifold form a “thermal ground plane” as shown in Figure 13. The manifold is mounted under the PCB and the AHS is installed onto the manifold through the PCB cutouts. Engine coolant flows into and out of the AHS. Because of the strong heat spreading effect of the working fluid flowing at high velocity, the AHS is maintained nearly isothermal at about 105°C. Electrical connections are made from the IGBT /diode via PCB buses to high-current terminals, Figure 14. IGBT/diodes can be easily replaced by removing and replacing the entire AHS. Page 7 of 10
AHS Cutout
Inverter PCB Card Gasket
Coolant Manifold Engine Coolant Lines Figure 13 - Innovative inverter concept using AHS and manifold to form a thermal ground plane (exploded view)
3Ø Terminals
AHS IGBT/Diode Terminals
IGBT/Diode Chips
Inverter Card Battery Terminals Engine Coolant
Coolant Manifold (under card)
Figure 14 - Isometric view and cross-section of assembled inverter
SUMMARY/CONCLUSIONS An update on the development of AHS for thermal management of automotive inverters was presented. An AHS for operating with Couette flow was presented. Test data indicate that AHS has a strong potential for cooling of high-current electronics in HEV and PHEV. In particular, AHS appears very suitable for directly interfacing inverter electronics to engine coolant loop or to an air-cooled HEX. This approach to the thermal management of HEV/PHEV inverters would eliminate the presently used dedicated liquid coolant loop, thereby reducing the volume, weight, and complexity of the electric power train subsystem. According to our preliminary analysis, AHS-based thermal management offers (depending on specific application) reducing the system weight by about 4-7 kilograms (including the weight of the conventional system coolant), the volume by about 4-5 liters, and operating power by about 80 W while presenting significant savings in hardware and assembly costs. In addition, AHS ultra-low thermal resistance enables Page 8 of 10
smaller die size (or fewer dies) for target die temperature rise above coolant. Besides the cost savings in silicon (projected to be $100200), smaller dies have a smaller CTE mismatch to substrate and are, thus, conducive to more reliable packaging. Our future work includes AHS operation with the engine coolant used directly as the AHS working fluid. This approach promises reduced thermal resistance and simpler hardware. A detailed comparative analysis between conventional and AHS-based thermal management is also planned. In summary, the AHS family of high-performance heat sinks offers non-traditional solution for inverter cooling that is simple, compares well with alternative approaches (Table 1), and is adaptable to many existing and future HEV/PHEV power trains. Table 1 – AHS compares favorably to alternative solutions for inverter cooling Known Cooling Concept
High Heat Flux Handling
AHS Conventional with Large HEX Jet Impingement & Microchannels Spray Coolers (2-Phase) Direct Back Side Cooling Microchannel Air Cooling
Yes No Yes Yes Yes No
Can Reject Heat to Engine Coolant Yes No Not directly Not directly Yes No
Can Reject Heat to Air Yes Not directly Not directly Not directly No Yes
Does Not Require Micro-Filtered Coolant Loop Yes Yes No No No No
Compact Yes No No No Yes Yes
REFERENCES 1.
“Plug-In Hybrid Electric Vehicle R&D Plan,” Working Draft by US DOE, June 2007 (available at www.usdoe.gov )
2.
US Department of Energy, Sep 21, 2010, “FreedomCAR and Fuel Partnership Teams”, http://www1.eere.energy.gov/vehiclesandfuels/about/partnerships/freedomcar/fc_teams.html
3.
G. Eesley, Delphi Electronics & Safety, Communications, December 2010
4.
B. Moor, “Power Electronics for Electric Drive,”Electric Drive Transportation Association, ’08 Conference & Exposition, Washington, DC, December 2-4, 2008
5.
Burress, “The Progression of Commercially Available EV/HEV Technologies and Ongoing Research,” International Energy Conversion Engineering Conference, Nashville, TN, July 26, 2010
6.
J. S. Hsu and L. D. Marlino, “Emerging Two-Phase Cooling Tecchiques for Power Electronic Inverters, ORNL/TM2005/156, July 2005
7.
T.P. Abraham, “Characterization and Development of Advanced Heat Transfer Technologies,” NREL/PR-540-42344, also in J.K. Kelly et al., “Assesement of Thermal Control Technologies for Cooling Electric Velicle Power Electronics,” NREL-CP540-42267, January 2008
8.
J. Vetrovec, “Active Heat Sink for Automotive Electronics,” SAE Paper No. 2009-01-0965, 2009.
9.
J. Vetrovec, “High-Performance Heat Sink for Hybrid Electric Vehicle Inverters,” ASME Paper DETC2010-28776 (August 2010)
10. J. Vetrovec, “High-Performance Heat Sink for Interfacing Hybrid Electric Vehicles Inverters to Engine Coolant Loop,” SAE paper No. 2011-01-0349, April 2011 11. S. D. Brandeburg et al. in the US 7,726,972, June 1, 2010 12. J. Vetrovec, “High-Performance Heat Sink for Interfacing Hybrid Electric Vehicles Inverters to Engine Coolant Loop,” SAE paper No. 2011-01-0349, view charts presented in session T11PFL at the SAE Congress, April 2011
CONTACT INFORMATION Jan (John) Vetrovec is the founder and president of Aqwest LLC, a science and technology innovation company in Larkspur, CO, USA; www.aqwest.com. John has 28 years experience in the aerospace industry at TRW Defense & Space Technologies (now Northrop Grumman Space Technologies) in Redondo Beach, CA and The Boeing Company in Canoga Park, CA. During his aerospace career, he led R&D projects in rocket engines, spacecraft, plasma physics, vacuum systems, thermal management, highenergy lasers, electro-optics, and missile interceptors. In 2006, he retired as a Technical Fellow from Boeing and started Aqwest. His Page 9 of 10
professional interests include adapting selected aerospace technologies to commercial uses. John holds a BA and an MA in mathematics, and MSEE in electro-optics, all from UCLA. He is a member of SAE, ASME, IEEE, AIAA, SPIE, and DEPS. John is an author of over 70 technical publications and has over 50 patents issued or pending. John may be reached at
[email protected].
ACKNOWLEDGMENTS This work was in-part funded by the National Science Foundation grant no. 0944663.
DEFINITIONS/ABBREVIATIONS AC AHS CTE DBSC DOE DC GaN HEV HEX IGBT PCB PHEV Si SiC 2D
- Alternating current - Active heat sink - Coefficient of thermal expansion - Direct backside cooling - Department of Energy - Direct current - Gallium nitride - Hybrid electric vehicle - Heat exchanger - Insulated gate bipolar transistor - Printed circuit board - Plug-in hybrid electric vehicle - Silicon - Silicon carbide - Two-dimensional
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