EVrelated(UCLA water Cooled Hot EV Chips)

[Bruce EVangel Parmenter <brucedp@yahoo.com>]

EVrelated(UCLA water Cooled Hot EV Chips)
[The Internet Electric Vehicle List News. For Public EV
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 --- {EVangel}
http://www.sciencedaily.com/releases/2002/07/020729073605.htm
Source: University Of California - Los Angeles
(http://www.ucla.edu/) Date: Posted 7/29/2002

UCLA Researchers Cool Hot Silicon Chips By Spraying Them
With Water

Borrowing from a method often used to cool down on a hot
summer day, researchers at the UCLA engineering school are
coaxing more efficiency out of hot silicon chips by spraying
them with water.

The technology has numerous applications including improving
the efficiency of the communications platform aboard
unmanned aircraft and the performance of electric car and
train motors.

Maintaining lower operating temperatures allows transistors
to be driven harder, causing them to produce more power. It
also allows chips to survive in harsh temperature
environments that would otherwise cause them to fail.

Under the leadership of Elliott Brown, professor of
electrical engineering and Vijay K. Dhir, interim dean of
the Henry Samueli School of Engineering and Applied Science,
researchers found that liquid spray-cooling could improve
the performance of transistors as much as 34 percent. Their
findings were presented in June in San Diego at the Eighth
Intersociety Conference on Thermal and Thermomechanical
Phenomena in Electronic Systems.

Researchers were able to achieve "significantly greater
power than can be achieved with the same chips using
conventional cooling," Brown said. He called it "a promising
approach to improving thermal management at higher heat
density or elevated temperature."

He and Dhir discovered that compared to other methods, such
as liquid immersion or forced-convective (fan) cooling,
spraying improves the transfer of heat away from the chip by
combining the effects of convection with vaporization.

Although applying the concept to electronics isn't
completely new — there are commercially available products
that spray-cool the entire package of components, including
the circuit boards — the UCLA team is the first to employ
micro-spraying, which isolates the spray to each individual
chip. For their research, the spray-cooling equipment was
scaled down, with the size of the nozzles tailored to match
the size of the chip.

Electrons speeding through transistors create heat. In
devices such as cell phones, the heat is usually negligible.
But when the circuitry must generate large amounts of power
— to drive motors or operate radar equipment — their
temperatures can exceed the boiling point of water (100
degrees Celsius). Above temperatures of 150 degrees Celsius,
chips break down faster and the results they produce become
unreliable. At 200 degrees Celsius, they cease to function.

In addition to increasing their power, creating a method of
keeping chips below 150 degrees Celsius also allows
power-amplifier chips to operate in harsh temperature
environments such as those aboard unmanned aerial vehicles
(UAVs).

In the cramped confines of a drone, where space and weight
come at a high premium, cooling systems must be small and
light. In addition to the operating temperatures of the
chips, ambient temperatures aboard a drone flying over the
desert vary by as much as 100 degrees Celsius between day
and night, creating "conditions that lead to very high
temperatures — too high for silicon to survive," Brown said.
The cooling system Brown and Dhir have designed is small,
lightweight and consumes only a small amount of power.

Two types of chips were tested: Insulated Gate Bipolar
Transistors (IGBTs), used to drive electric motors in
trains, electric cars and elevators, and LD- MOSFET
transistors used in 500-MHz radio frequency power
amplifiers. Such chips traditionally power radar base
stations.

Results for the IGBTs were impressive, boosting performance
by as much as 34 percent. Using the same technique on the
LD-MOSFETs was an order of magnitude more effective at
removing heat.

For example, Brown said, in a 60-watt radio frequency power
amplifier, spray-cooling disburses about 20 watts of heat.

Power amplifier chips, which operate in the radio frequency
range, produce very high temperatures. Inside the fuselage
of a drone circling above the desert, operating temperatures
of these chips are even further challenged.

Calculating the proper flow rates required Dhir's expertise
in phase change heat transfer.

UCLA researchers found that in this temperature range, water
is the best high- density liquid flux. Heat is disbursed by
both thermal convection and evaporation. Heat dissipation by
convection and evaporation were found to be about equal.

"Like spraying your face with an atomizer on a hot day,
atomizing the water increases the surface area, disbursing
every cubic centimeter into a zillion droplets. And each of
those droplets removes heat as it evaporates." Brown said.

The performance improvements were achieved by spray-cooling
directly on the top of the transistor die. The top surface
of silicon die was coated with Parylene-C, a conformal
polymer with excellent dielectric properties.

Measuring 4.86 mm x 1.53 mm, the nozzle matrix used for the
LD-MOSFETs consisted of 28 holes horizontally and 18 holes
vertically. Brown pointed out that the size and matrix of
the nozzle array was constructed "to exactly match the
layout of the active cells." He said they "tailored the
design of the nozzle to the heat source distribution of the
transistor."

Because silicon is so chemically robust with respect to
acids and other harsh chemicals and inexpensive to
mass-produce, it is an ideal material for the nozzle array,
Brown said. Reactive-ion etching, the same process used to
create the transistors themselves, was used to create the
nozzles 35 microns in diameter. The process produces very
smooth sidewalls compared to any known mechanical machining,
so there is less of a tendency to trap contaminants and
become clogged.

Researchers also found that at higher temperatures, an even
larger amount of heat can be dissipated by spray-cooling.

Looking ahead, Brown plans to experiment with the use of
spray-cooling on wide band gap semiconductors, which run
even hotter than LD-MOSFETs.

Editor's Note: The original news release can be found at
http:// www.newsroom.ucla.edu/page.asp?id=3367

Note: This story has been adapted from a news release issued
by University Of California - Los Angeles for journalists
and other members of the public. If you wish to quote from
any part of this story, please credit University Of
California - Los Angeles as the original source. You may
also wish to include the following link in any citation:
-




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