Vaporizable Dielectric Fluid Cooling of IGBT Power Semiconductors ...

ehavior at temperature extremes, or another variant filtered
and controlled to meet certain requirements. Requirements for
purity, corrosively, and electrical resistivity for water and
electrically-conductive liquids may be highly specific in certain
electronic cooling systems. [1, 2]
In avionics and other airborne applications, a common
liquid coolant is polyalphaolefin (PAO). In semiconductor
manufacturing equipment, test equipment, certain types of
laboratory instruments, and immersion cooling applications in
locomotive traction and other specialized markets, use of
dielectric electronic liquids such as 3M Company’s
Fluoroketone (FK) liquids has been documented in the
literature. [3]
Use of each of these liquids represents an engineering
selection process by the system thermal/mechanical
engineering team to meet performance and cost requirements
and program objectives (availability and logistics for certain
liquids, electrical requirements, availability for field
replacement, among others). These requirements are specific
to each application type; the liquid selection is one component
of the liquid cooling design process.
A major system decision point is the selection of a single-
versus two-phase liquid system design, primarily determined
by source heat fluxes and design targets for maximum thermal
performance. Use of single-phase liquid cooling systems with
a water coolant [either deionized water or water/glycol
mixtures such as propylene glycol (PEG), or ethylene glycol
(WEG)] is the predominant liquid system solution globally for
electronics thermal management. Deep engineering knowledge
exists at many OEM companies, universities, and research
institutions. There is a large and well-established component
vendor base worldwide. Principal system components include
a pump, heat exchanger, fans, one or multiple liquid cold
plates, filters, expansion tank, flow meters and other control
components, and a deionization unit (with field-replaceable
deionization cartridge, for deionized water systems).
Single-phase water/ethylene glycol cooling is also the
predominant liquid cooling solution for the automotive market
globally, for internal combustion engine (ICE) cooling. The
same is also true for the traction market globally, for dieselelectric
and electric-drive passenger and freight locomotives,
high-speed trainsets, and electric multiple unit and urban transit
railcars in many forms. The use of single-phase water cooling
is obviously well-established and field proven for simplicity,
even in the simplest implementation for engine block heat
removal – where there traditionally have been no concerns for
relatively temperature-sensitive power semiconductors and
microprocessors and other electronic devices.
Use of pumped vaporizable dielectric liquid systems where
boiling is induced is less common in electronic system thermal
management. Historically, much research work has been
conducted in two-phase cooling at universities and industry,
including large heat exchange and cooling systems for power
plants and other non-electronics applications. In computing
and electronics, development work has included such concepts
as IBM Corporation’s Liquid Encapsulated Module (LEM),
from the early 1970s, and more recent research and production
implementation of both water cooling and refrigeration for
large enterprise servers. [4]
Significant current research and development work
continues today with liquid boiling and liquid immersion,
conducted by universities, research institutions, and at coolant
liquid suppliers such as 3M Company. [5]
C. Why Pumped Vaporizable Dielectric Fluid Solutions?
Very strong market needs for improved thermal
management solutions, as source heat fluxes increase rapidly
in many types of systems, has led to renewed investigation of
two-phase systems which offer a practical and cost-effective
next step beyond single-phase liquid cooling. The
development and use of two-phase systems which utilize a
pump but not a compressor (as is used for vapor cycle
refrigeration) yields an additional tool in the thermal engineer’s
toolbox that does not require the further step to full vapor cycle
compression refrigeration. A two-phase thermal management
system may be appropriate for power inverter designs for HEV
applications where sub-ambient cooling is not required.
The primary objective in system design for considering a
two-phase liquid system, as compared to single-phase liquid, is
to harness the substantial increase in overall thermal
performance obtained with boiling phenomenon. A typical rule
of thumb for two-phase systems is that the heat of vaporization
principle will yield a two- to four-times increase in total heat
transport capability.
Heat of vaporization is utilized in simple and effective heat
pipe and vapor chamber designs used widely in electronic
systems. Both heat pipes and vapor chambers are well
understood and represent a highly useful component that can be
easily applied to solving thermal problems without the
encumbrance of pumps, compressors, and other components of
a loop liquid cooling system. However, the total capacity of
heat pipe and vapor chamber designs is significantly lower than
that of a pumped two-phase system.
In operation, pumped VDF thermal management systems
are self-optimizing. System operation is controlled by the heat
load: as the power load increases as switching increases and
heat dissipation rises, the amount of boiling increases to
withdraw the incremental heat load. As the power load
decreases as work slows, the boiling rate decreases. These
near-instantaneous changes provide a self-regulating feature for
two-phase systems.
A pumped VDF system provides an excellent intermediate
step after single-phase water and heat pipes and vapor
chambers have been considered. A pumped two-phase system
employs heat of vaporization principles and adds the benefit of
a pump to multiply the potential heat removal capability.
Why might traditional vapor cycle compression not be the
next obvious step, to move directly to the use of refrigeration
and the obvious advantages of obtaining sub-ambient cooling?
Refrigeration provides an excellent, compact solution for high
heat flux thermal management. Vapor cycle compression is
required where maximum operating temperatures must be
observed with a relatively low rise over ambient temperature;