Challenges of heat dissipation in power electronics, microprocessors and lasers pose
serious hurdles in the ongoing efforts to improve performance. Changes in component
specifications and complexity have required transitions in cooling methods: from air to
liquid, then liquid to multi-phase, and then back to air.
During the last decade, along with usual and anticipated rise in the dissipated heat, a new
challenge has arisen: high heat density. Today this is the most serious challenge and the
biggest obstacle in the development of new electronic devices.
Fig. 1a and 1 b present comparative analysis of changes in overall dissipated heat and its
density using Intel XEON µP-Family processor as an example.
As one can see, maximum dissipated heat remains practically constant within the 90
to120 W range . Moreover, at some point during the transition to multi-core processors,
one can see a slight decrease. Heat density, on the other hand rises continuously.
Power Dissipation in W
160
1 core, 90nm
2 core, 90nm
2 core, 65nm
4 core, 45nm
6 core, 45nm
8core,
10core, 32nm
2005: Cranford
2005: Paxville
2006: Tulsa
2008: Dunnington
2008: Dunnington
2009: Beckton
2010: Westmere
1
2
3
4
5
6
7
8
9
60
40
20
0
0
Years from 2000 to 2010
45nm
1 core, 0,18µm
80
2001: Foster MP
100
1 core, 0,25µm
120
1998: Drake
140
10
Heat Flux of µPs in W/m2
3.00E+06
5.00E+05
5
6
Westmere
Beckton
Dunnington
Drake
1.00E+06
Foster
1.50E+06
Cranford
2.00E+06
Tulsa
Paxville
Dunnington
2.50E+06
0.00E+00
0
1
2
3
4
7
8
9
10
Anticipated increase of number of cores in processors will only cause this trend to
continue, if not accelerate. Developments in power electronics exhibit the same trend:
miniaturization of active elements of diodes and IGBT's causes an increase in heat
density.
Cool Technology Solutions, Inc. has developed Swirling Streams nozzle designs which
do not require additional mechanical devices, and has perfected utilization of such
streams within liquid heat exchangers.
Swirling of liquid streams is more complicated and less studied phenomenon. Swirling
Jet Stream Technology is not a classical method based on swirling of jets streams with
intermixing and turbulization of liquid streams with the help of injected gas. This
technology is based on quasi-swirling, and utilizes rectangular or oval nozzles and
uneven profile gaps to create dynamic jet streams (Fig. 3)
Fig.3 Hydrodynamic jet-streams flow
These figures show forming of turbulized flow within 1 to 5 nozzle calibers, It is seen
clearly that jet are swirled at the significant distance from the nozzle (located to the left)
and are not jet separation effects;; instead they prove that turbulization inside the heat
exchanger indeed takes place.
Capabilities to localize heat exchange are determined by ejection effects created by
secondary nozzles aimed onto areas with maximum heat fluxes. This allows the creation
of zones of high turbulization and destruction of the wall boundary layer. Stream
formation is highly dependent on the shape and geometry of the nozzle, therefore
formation zone can vary significantly, from 2.5 to 8 nozzle's calibers.
Interaction of cold plate microstructure with laminar rectangular flow creates local
swirling streams along heat transfer surface; this phenomenon is especially effective for
handling of local hot spots, since such micro turbulizations not only allow to significantly
increase average heat exchange across the whole surface, but prevent local overheating as
well. Ability to effectively control such local overheatings is the main task of localized
heat dissipation.
Such artificial turbulization at Reynolds numbers up to 350 allows to drastically increase
the efficiency of cold plates and heat exchangers.
Fig. 4 and 5 present comparison of various methods utilized in design of modern of cold
plates.
Swirling
TM
Jets-Stream in
wide range of Re
number > 350
Micro channels for
low Re numbers
Impingement Jets
Alternate horizontal jets
Standard Pin-Fin structure
Improving Performance
Best
Worse
Fig. 4 Thermal Performances (based on Thermal resistance)
Improving Performance
Best
Swirling JetsTM
Stream in wide range
of Re number > 350
Impingement Jets
for
low Re numbers
Micro channels
Alternate horizontal jets
Standard Pin-Fin structure
Worse
Fig. 5 Hydraulic Performance (based on pressure drop)
The most promising benefit of the implementation of this method is hot spot
management. Drastic improvement of the temperature splash in the hot spots, results in
significant increase in overall system reliability. Fig. 6 shows comparison of
aforementioned methods taking into consideration not just the average overall thermal
resistivity, but local resistivity in the hottest spots. Swirling Jets-Stream is very effective
way of dealing with local hot spots.
Clearly local heat dissipation at the source, i.e. within the 3D structure of the chip is most
preferred. However, so far such methods are in a preliminary research stage, and not
likely to be available in production for some years.
It is important to mention that thermal resistance is always defined relatively to average
overheating of the whole heat dissipation surface. When hot spots are present such a
parameter is not always the best in determining heat exchange characteristics.. Therefore
it is important to determine thermal resistance for spots with a maximum T junction for
heat dissipation. Thermal resistance calculated for Tj for areas with maximum hot spots
will really characterize and define the most capable method of localized heat dissipation.
Comparison of various methods of preventing local overheating is presented in Fig. 6
Technology’s Ability to dissipate heat from a local hot spot
Best
TM
Impingement Jets
Micro channels
Alternate horizontal jets
Improving Performance
Swirling Jets-Stream
Standard Pin-Fin structure
Worse
Fig.. 6 Max Thermal Performance for Max Heat Flux based on Local Thermal
Resistance
Fig. 7 and 8 present temperature distribution fields at the power board carrying 6 IGBT's
50 W each using standard pin-fin structure cold plate and plate utilizing Swirling Jet
Streams. Comparison shows that even at the modest liquid flow of 1.8 GpM gain is very
significant.
Pictures below exhibit efficiency of an approach. Thermal distribution modeling for
IGBT modules uncovers decrease of almost 30% in hot spots temperature influx
compared with best market cold plates with micro channels.
Pic.7 Temperature field simulation with micro channel Structure for cooling IGBT
module
Results: •Max. Temp of IGBT = +105°C  ∆T = +20°C Rth=0.4
Pic.8 Temperature field simulation Swirling Jet-Streams Structure for cooling the same
IGBT module Results: •Max. Temp of IGBT = +99°C  ∆T = +14°C Rth= 0.28
Fig. 7 and 8 show that maximum temperature Tj onto IGBT is 105ºС and at coolant's
temperature of 85ºС is 20ºC higher coolant's temperature at the dissipated power of 50
W. Utilization of Swirling Jets Technology allows to keep maximum temperature Tj onto
IGBT at only 97ºC, bringing temperature differential between IGBT and coolant to 14ºС
or almost 35% better.
Here are some different examples of the cold plates for microprocessors and power
electronics.
a)
b)
Fig. 9 a) Cold plate CTS-V-series for cooling microprocessors
b) Cold Plate CTS – W- series for cooling IGBT modules
Conclusions:
1) The main tendency in the market of microprocessors and power electronics increase in the density of heat flows.
2) Increase in the density of heat flows requires new cooling methods, capable not
only of managing significant amounts of heat integrally but at the same time be
effective in handling local hot spots on the surface of these elements.
3) One of the most promising methods is development of cooling devices with
artificial turbulization of flow with small hydraulic resistance and losses
4) Described here method of artificial turbulization Swirling Jet-Streams, developed
and offered by the company Cool Technology Solutions, Inc. offers optimistic
hope to be capable of handling effectively not just integral heat dissipation, but
also local heat dissipation from the local hot spots of electronic components and
devices.