Vaporizable Dielectric Fluid Cooling of IGBT Power Semiconductors ...

The system design engineer may set the dielectric liquid
coolant saturation temperature by adjusting the system
operating pressure. This has a principal benefit for thermal
management and system component design and cost:
• System fluid saturation temperature can be increased
by setting a higher system operating pressure;
• A higher fluid saturation temperature will enable a
higher junction temperature (within the limits of the device to
be cooled);
• Setting a higher operating temperature (where feasible
for devices) adds another attribute for system design: either a
smaller condenser can be used, further reducing system size, or
a lower airflow can be set, reducing acoustic noise and power
consumption or the ability to dissipate in high ambient
conditions.
Other key points of the VDF cooling loop concept that
affect system design are:
• System is gravity fed;
• Pump must be located below liquid cold plates in the
same cooling loop;
• Heat exchanger must be located above cold plates;
• Heat exchanger may be fluid-to-air (traditional tubeand-fin
style) or fluid-to-water (shell-and-tube for external
chilled water or cooling tower).
• Liquid cooling is employed for the critical high heat
sources in typical system design with air cooling for parasitic
heating in other areas, such as capacitors; these decisions are
design-specific.
D. Maximum Coolant Operating Temperatures
While a two-phase pumped system is a pressurized system,
it is important to note that there is no use of a compressor;
therefore, there is also no opportunity to operate at sub-ambient
temperatures. The minimum fluid temperature in pumped twophase
system will be the heat exchanger medium temperature.
This will reflect whatever system operating pressure has been
selected for an expected maximum system ambient temperature
design point.
This is particularly important in the discussion of vehicle
systems where a battery pack or ultracapacitor bank must also
be cooled. For lithium-ion battery packs which require a
maximum operating temperature of 40°C, for example, an
assumption that a heavy truck or construction vehicle must
operate in ambient outdoor temperatures as high as 32-35°C
would suggest that only vapor compression cycle refrigeration
systems will provide reasonable cooling for the battery pack.
Ideally, a single liquid cooling system to cool both the
inverter(s) for a hybrid vehicle powertrain system and the
battery pack would reduce system complexity and cost.
However, the limited thermal design headroom available for
battery pack cooling, in an assumed 32-35°C outdoor ambient
temperature, will require that other design considerations will
be needed to maintain a 40°C maximum battery pack enclosure
temperature. For ultracapacitor banks and other types of
battery technologies where maximum operating temperature is
not so tightly limited, a two-phase pumped liquid cooling
system may be an appropriate selection.
These VDF cooling systems are considered to be
refrigerant-agnostic; a number of other vaporizable dielectric
fluids can be used or will become available, such as HFO-
1234yf. One example is where very high system operating
temperatures must be anticipated; the solution may be selection
of a refrigerant with a chemistry developed for very high
temperature regimes.
Certain types of power electronic systems must be designed
for unusually high temperature operating conditions, either for
pulsed operation or for other requirements. When water is used
in such systems (if possible), concern for algae and other
biological growth requires use of algaecides and biocides; a
pulsed laser system is one example. Typically, no biological
control agent or other control additive is needed for two-phase
dielectric liquid systems which utilize a coolant such as R-134a
or HFO-1234yf (intended as a future drop-in replacement).
E. Minimum Coolant Operating Temperatures
A refrigerant or other vaporizable dielectric fluid used in
place of water as a coolant for electronic systems does not
require addition of ethylene glycol for temperature protection,
even for subzero temperatures. Therefore, no thermal derating
is required. Derating can be substantial for glycol/water
systems, rising to derating values of 35% or greater for the
most challenging railroad traction values (such as may be
quoted per railroad cold-start requirements of -55°C for
railroad locomotives supplied to Russia).
III. PROTOTYPE 750KW/1000HP DRIVE SYSTEM DESIGN
A. System Design
A complete three-phase electrical drive system was
developed for industrial and commercial applications,
incorporating different thermal solutions for the primary heat
sources, the individual IGBT semiconductor devices. While
this system design is intended for a large stationary industrial
drive system, the thermal management system design and
performance can be described and results compared to water
cooling systems. These comparisons are useful for analysis of
performance, prior to system compaction and fitting to vehicle
system requirements for HEV inverters and related subsystems.
Each cooling system configuration was tested utilizing the
same IGBT module type, a dual 1700V 450A module packaged
in an industry-standard package design referred to as the
EconoDUAL design. [10] A similar package design is
common to several semiconductor manufacturers. The module
footprint measures approximately 122m in length and 62mm in
width; these modules have a large, flat nickel-plated copper
base plate of standard thickness as the primary heat-spreading
component for the module itself.
Typically, six large IGBT die and six smaller diode die are
arrayed in format consisting of two IGBT die and two diode die
each, soldered onto a direct bond copper (DBC) substrate
containing a ceramic dielectric layer and an additional layer