motorola's modular power systems in phoenix, arizona uses maya's electronic system cooling software to develop the next generation of liquid-cooled power inverters for electric vehicles.
unplug the car and let's go
since the first half of the century, science and technology publications have been promising an environmental revolution that would fill our roads and highways with eco-friendly cars and trucks. it's been a long time coming, but it looks like the revolution is finally here. no longer the preserve of futurists and science writers, the electric vehicle (ev) is finally gliding off the drawing board and into our lives.
of the variety of zero emission vehicle technologies currently available, the most promising appears to be the electric car. the automotive industry has spent the last decade working overtime to produce a commercially viable line of safe, fun and affordable evs. many automotive manufacturers, such as general motors, ford motor company and toyota have already unveiled electric vehicles.
designed to meet our growing concerns over the environment, and the future availability of fossil fuels, today's electric vehicles have been re-designed from the ground up. lighter composite materials and more efficient electronic and propulsion systems have resulted in vehicles that are cheaper to run, nimbler on their feet, and environmentally friendly too.
the power inverter
deep at the heart of any electric vehicle lies a crucial component - the power inverter. this is a multi-purpose electrical power module that not only converts dc supplied by the batteries into ac to drive the motor, but also does the reverse job, converting ac generated by the motor during braking to dc to recharge the batteries.
the design of this crucial component is a challenge for electrical and packaging engineers. a typical inverter switch is composed of an array of insulated gate bipolar transistors (igbt), each of them rated at several hundred amps. unless the inverter is adequately and uniformly cooled it will suffer material degradation and premature failure. the search for a thermally viable inverter design is one of many challenges facing the electric vehicle industry today.
tom hatfield, operations manager and pablo rodriguez, project engineer, of the modular power systems group at motorola's semiconductor products sector are very familiar with the problem. and with the help of maya's electronic system cooling (esc) package, they just might have solved it, giving the ev industry a green light in the process.
the power inverter assembly mounts directly to the fluid filled heat sink.
better design tools for better designs
"what we wanted to do," explains hatfield, "was perform a feasibility study as to how well we could integrate liquid cooling with this power module." to do that, they needed a powerful cfd tool that would allow them to design a model that was true to their manufacturing and packaging needs, and not limited by primitive geometric constraints built into the modeling software.
motorola was already taking advantage of the superior 3d modeling and meshing properties of sdrc's i-deas master series suite, and was delighted to find that maya's thermal modeling packages fit seamlessly with i-deas. the wide range of supported element shapes also impressed them. "esc supports many element types; lump masses, beams, triangular and quad shells, bricks, wedges, and most important, tetrahedral elements which are needed for the really complicated geometry - our geometry just isn't rectangular", states hatfield.
armed with the right software, motorola was able to model a new approach to inverter construction, using their patented metal matrix composite (mmc) technology. this meant they could avoid the problems introduced by traditional construction materials. typical methods have involved building a multi-layered thermal stack, on a copper base-plate, topped with an out-sourced heat sink.
studies revealed, however, that this technique suffers from uneven cooling, and the inevitable discrepancies between the rates of thermal expansion of the components and the copper base-plate in turn lead to degeneration in interface integrity, and the eventual break down of the component.
maintaining a uniform fluid flow across the heat exchanger was critical for uniform cooling
of the inverter switch array. adequate flow characteristics were achieved with a single fluid inlet
and outlet. the software allowed the designers to accurately represent the geometry and
optimize pin size, shape and spacing.
the motorola team designed a new base-plate, using an aluminum silicon carbide mmc. this offered a better thermal expansion match with the other components, and in turn reduced stresses, and improved reliability. since the mmc could be molded instead of machined, they realized that they could take a fresh look at the module's design. the traditional and troublesome finned heat sinks were replaced with a new form-fitting fluid-filled design that combined the functions of both base-plate and heat sink, enabling them to combine the heat-sink with the inverter itself.
two important objectives were that the heat sink had to provide for uniform operating temperatures across the multiple igbts that compose a single device, and that the footprint was to be as small as possible. a low-profile fluid filled design was developed, whereby a coolant is circulated through a simple heat exchanger. the heat exchanger is designed to provide uniform fluid flow across the width of the device. within the heat exchanger, the fluid circulates around a series of diamond shaped pins arranged to draw heat away from the transistors.
smaller meshes, faster solves
the part design was developed in i-deas then simplified for meshing. since the design was symmetrical along the fluid inlet/outlet axis the size of the thermal model was halved. also, since esc provides a thermal assembly tool, called thermal couplings, for joining dissimilar meshes, it was easy to use different mesh densities for the various major sub-components. this unique coupling capability further reduced the number of elements required to model the inverter assembly and simplified the thermal modeling task.
the thermal model simulated the steady state heat transfer and fluid flow and a relatively coarse mesh was used to model the power substrate. the fluid mesh was further subdivided. in regions of particular interest, such as around the heat-exchange fins, a fine mesh was used, while a coarse one was used in less detailed regions. the flexibility of free meshing was critical for effectively modeling the geometry and maintaining a reasonable mesh size.
maya's thermal assembly tools were key to simplifying the meshing process and
reducing model size. the components, substrates, and heat exchanger could all be
"meshed independently. the fluid mesh does not even need to match the mesh
on the solid parts. thermal couplings are used to create a path for heat
to flow between parts in an assembly.
through the use of an integrated heat exchanger, motorola was able to dissipate 3kw of heat
and maintain an average die temperature of 112c. this was achieved with relatively
low flow rates. in addition, equalization of die junction temperatures for each
inverter switch was readily achieved.
the mesh on the sub-components was then combined, and the reduced number of elements led to significantly shorter computational times than would otherwise have been needed with older cfd technology. the advantage of smaller meshes and shorter solve times meant that they were able to see the results of changes in hours instead of days.
in fact, the combination of faster solve times, and ease of use enabled rodriguez and hatfield to complete the feasibility study in only two months, an undertaking that would have been difficult with other software packages.
accelerating into the future
the results from this project indicated that the use of an internally cooled power module was a feasible and effective approach to manufacturing an affordable, reliable power train. "basically, by integrating the heat exchanger, we've minimized piece part count, we've improved thermal performance, and we've reduced overall system cost for the customer" states hatfield. "by using maya's software to accomplish this, we brought new technology to market quicker than anyone else."
in fact the results of this effort were so promising that motorola was able to promote the viability of this new integrated cooling technology and win additional development contracts. motorola is now actively pursuing product definition with potential customers and expect to release a product in early 1999.
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