Note: Descriptions are shown in the official language in which they were submitted.
CA 02522452 2005-10-06
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Power Supply Unit With Perforated Housing
Description
This invention relates to a power supply unit.
It is well known to provide computing devices such as personal computers
(PCs),
servers, media centers and the like with power supply units (PSUs). A PSU
receives
mains electricity, and provides regulated direct current (DC) power at one or
more
outputs. Since different components of computing devices have different power
1o supply requirements, it is usual to provide at different outputs power at
different
voltages and with different maximum power ratings.
A PSU typically comprises a cuboid housing made of a metallic sheet material.
The
metal prevents electromagnetic interference outside of the housing, which
interference may negatively affect the operation of other components in the
computing device. The housing houses components including one or more
transformers and one or more rectifiers which convert mains power, received
via a
mains power connector mounted in one side wall of the housing, into the
required
power supplies.
The side wall of the housing with the mains power connector is often also
provided
with an on/off switch. This wall is then supported in or against an aperture
of a
casing of the computing device when installed, so as to allow a mains power
cable
to be connected to the PSU. This makes it convenient for the wall also to be
used
to allow air to be expelled from the interior of the PSU housing.
The total power supplied by a PSU typically is of the order of hundreds of
Watts.
The components required to generate such power necessarily generate waste heat
within the housing which needs to be removed. Three different types of
arrangement can be used to achieve this. Each uses one or two fans.
A fan includes a motor and a fan shroud comprising blades mounted on a support
and rotatable by the motor. The fan shroud in generally circular, and rotation
of it
CA 02522452 2005-10-06
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by the motor causes air to be forced in a direction perpendicular to the plane
of the
fan component. A fan is supported next to an aperture in a wall of the
housing.
The aperture is about the same size and shape as the fan component. To prevent
a
user's fingers or similar being contactable with the blades of the fan, a
grill is
normally secured to the housing so as to cover the aperture without
restricting
significantly the flow of air through the fan aperture. This grill also
provides
grounding to prevent electromagnetic interference (EMI).
In one type of arrangement, two fans are provided, a first fan being supported
with
the upper wall and a second fan being supported with the side wall in which
the
mains connector is provided. In this type of arrangement, the first fan blows
inwards of the PSU and the second fan blows outwards. As a result, air flows
into
the PSU through the upper wall, is heated by the internal components of the
PSU
and the heated air then is expelled by the second fan to the exterior of the
housing
of the computing device. To minimise the influx of air heated by the
components
of the PSU into the housing of the computing device containing the PSU, the
PSU
housing in such an arrangement is not provided with apertures, although a
small
amount of heated air will sometimes be able to escape the PSU around
connectors,
wire harness exit apertures and the like. Heat also is conducted through the
walls of
the housing.
In another type of arrangement, a 1'SU includes a fan in the side wall that
includes
the mains power connector, and no other fans. The fan is arranged to blow
outwards of the PSU. To allow air to enter the PSU, the side wall opposite the
side
wall including the mains power connector is provided with apertures. Thus, the
fan
produces a reduction in air pressure within the PSLT housing. Air then flows
through the apertures by virtue of higher pressure air in the computing device
housing, is heated by the PSU components and is expelled to atmosphere by the
fan. To produce maximum cooling for a given fan throughput, the other side
walls
and the upper wall of the PSU housing are not usually provided with apertures.
This maximises the amount of air expelled by the fan being air which has
passed
over a heat-generating component of the PSU. The SilentX 300W PSU produced
by Ahanix is one example of this type of arrangement. This PSU includes a
series
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of parallel elongate slits some tens of mm long and a few mm wide in the side
wall
opposite the side wall which includes the mains power connector.
In a third type of arrangement, a single fan is included on an upper wall of
the PSU
and is arranged to blow inwards of the PSU. In this case, an exit for air is
provided
on the side wall which includes the mains power connector. This exit can take
any
of a number of different forms, fox example a grill. The other side walls are
not
provided with apertures, which enhances the channelling of air over the heat-
generating components of the PSU. One example is the FSP300-60GLS produced
7o by the FSP group of Taiwan. This PSU has a mesh on the side wall on which
the
mains connector is formed. The mesh allows air forced into the PSU by the fan
to
be expelled to the exterior of the housing of the computing device in which it
is
included.
In a fourth type of arrangement, an inwardly blowing fan is provided on the
rear
faces, and an outwardly blowing fan is provided on the front face. This makes
the
housing a tunnel through which air is blown.
In a fifth type of arrangement, inwardly blowing fans are provided on both
upper
2o and rear faces, and an outwardly blowing fan is provided on the front face.
This
arrangement tends to be effective, but requires three fans so is more
expensive.
It is known also to use two fans mounted adjacent one another in one wall of
the
PSU housing. These fans then operate in parallel, and can be considered to be
equivalent to a single fan of greater capacity mounted in that wall of the PSU
housing.
It is known in some multiple fan PSUs to include a small number of elongate
slits
over an area o~ a side wall. Such slits serve to compensate for any
differences in the
rate of air entering and leaving the housing by way of the fans. Thus, if
inwardly
blowing fans have a greater flow rate than outwardly blowing fans, then the
slits
allow air to leave the housing of the PSU. Without the slits, the inwardly
blowing
fans would be more stressed. Similarly, if outwardly blowing fans have a
greater
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flow rate than inwardly blowing fans, then the slits allow air to enter the
housing of
the PSU. Without the slits, the outwardly blowing fans would be more stressed.
Fanless PSUs are known, but suffer from certain disadvantages.
Although in a conventional fan PSU one or more fans are used to circulate air
through the PSU, the presence of internal components means that it is not
always
possible to ensure that there is sufficient air movement in all of the volume
formed
by the housing. As such, it is possible for air in some locations to be heated
to a
temperature significantly above the average air temperature within the
housing.
Such heated air is termed a hotspot. Since hot aix is less dense than cooler
air,
hotspots move once formed. When a hotspot moves to a location where there is a
temperature sensitive material or component, such as the plastic insulation of
a
wire, the plastic of a connector or a packaged semiconductor device, that
material or
75 component can become damaged. Since hotspots usually rise within the PSU,
damage is most likely to occur to components located near the upper wall of
the
housing.
The formation of hotspots can cause a fan PSU to cease working correctly, and
in
some cases cease functioning altogether. Fox this reason, fan PSU designers
choose
fans having flow rates sufficiently high to reduce the probability of hotspots
forming and causing damage to a minimum. This normally means that the flow
rates of the fans are higher than would be the case if hotspots could not
cause
problems. However, higher flow rate fans are generally more expensive and/or
noisier than lower flow rate fans. Thus, the need to avoid hotspots increases
to the
cost of fan PSUs and/or the noise levels that are generated by them.
According to a first aspect of the invention, there is provided a power supply
unit
for a computing device, the power supply unit comprising a housing, the
housing
having a generally cuboid shape with upper, lower and first to fourth side
walls,
wherein the upper wall of the housing is provided with holes over at least 50%
of
the area of the upper wall, disregarding any area of the upper wall
constituting a fan
aperture, wherein each hole has a maximum dimension through a centre point of
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the hole of 5mm, and wherein a distance between the centres of adjacent holes
is no
greater than l0mm.
According to a second aspect of the invention, there is provided a power
supply
unit for a computing device, the power supply unit comprising:
a mains power supply input including a mains connector;
at least one direct current power supply output each including a respective
connector;
a transformer, the transformer being arranged to transform a mains
70 electricity supply received at the mains electricity supply input and
having a mains
electricity supply voltage into at least one transformed electricity supply,
the at least
one transformed electricity supply having a voltage lower than the mains
electricity
supply voltage;
at least one rectifier, the at least one rectifier being arranged to convert
the
75 at least one transformed electricity supply into a rectified electricity
supply for
supply to the at one direct current power supply output;
a housing, the housing having a generally cuboid shape with upper, lower
and first to fourth side walls,
in which at least one of the upper and first to fourth side walls includes a
fan
20 aperture formed therein and the power supply unit includes at least one
fan,
wherein each fan:
has a fan shroud residing generally in a fan shroud plane and being
operable to force air in a direction generally perpendicular to the fan shroud
plane;
is associated with a respective fan aperture; and
25 is supported substantially in register with its respective fan aperture,
wherein the upper wall of the housing is provided with holes over at least 50%
of
the area of the upper wall, disregarding any area of the upper wall
constituting a fan
aperture, wherein each hole has a maximum dimension through a centre point of
the hole of 5mm, and wherein a distance between the centres of adjacent holes
is no
30 greater than l0mm.
CA 02522452 2005-10-06
The holes in the walls of the housing allow air to pass from the interior of
the PSU
housing to the exterior. This is disadvantageous in so far as it reduces the
effectiveness of the channelling of air over heat-generating internal
components of
the PSU to atmosphere. However, the inventors consider this to be an
acceptable
disadvantage in light of the function of the holes to remove hotspots within
the
PSU housing.
Since the holes reduce the formation of hotspots, the use of holes allows the
use of
reduced flow rate fans. This in turn reduces the cost of the PSU and/or the
noise
70 levels generated by the PSU. Preferably the holes are perforations.
Preferably at least three of the upper and first to fourth walls of the
housing is
provided with holes over at least 50% of the area of that wall
Optionally, at least three walls include the upper wall and first and second
side walls
which are adjacent the side wall which includes the mains power connector.
Conveniently, the holes can be generally circular.
The distance between a centre point of a first hole and a centre point of an
adjacent
hole may be between 1.1 and 2 times the maximum width of the first hole.
Preferably, the distance between a centre point of a first hole and a centre
point of
an adjacent hole is approximately 1.5 times the maximum width of the first
hole.
Having closely packed holes allows relatively free movement of air and thus
helps
prevent hotspots.
The holes may be in the range of 1 mm to 3mm in diameter.
The holes may be formed in regular parallel lines.
The holes on at least one of the walls preferably are provided over at least
80% of
the area of the wall.
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Conveniently, the holes may be formed by perforating a metal sheet.
An embodiment of the invention will now be described, by way of example only,
with reference to the accompanying drawings. In the drawings:
Figure 1 is a perspective view of a power supply unit according to the
invention;
Figure 2 is a plan view of the Figure 1 power supply unit;
Figure 3 is a right side view of the Figure 1 power supply unit;
Figure 4 is a left side view of the Figure 1 power supply unit with a cutaway
portion
revealing internal components thereof; and
Figure 5 is a front view of the Figure 1 power supply unit.
In Figures 1 to 5, like reference numerals refer to like elements.
~5 Referring to Figure 1, a power supply unit (PSU) 10 according to the
invention is
generally cuboid in shape and has a top or upper wall 11, a right side wall 12
(not
visible in Figure 1), a front wall 13 (not visible in Figure 1), a left side
wall 14, a rear
wall 15 and a bottom wall (not shown). The rear and front walls 13, 15 can be
referred to as side walls since they are at the side of the PSU 10. The walls
11-15
2o define an internal volume of the PSU 10.
The upper wall 11 has formed therein a circular aperture 16, which constitutes
a fan
aperture. Supported on the upper wall 11 of the PSU 10 is a fan aperture cover
17.
This is the same shape and size as the fan aperture 1 G, and is screwed to the
upper
25 wall 11. The fan aperture cover 17 comprises a number of circular metal
components connected together by radial metal spokes. The fan aperture cover
17
prevents the ingress of a user's fingers through the fan aperture 16. A first
fan 18 is
supported just behind the fan aperture cover 17. The first fan 18 includes a
number
of blades (visible more clearly in Figure 2) supported in a fixed relation to
one
30 another. The diameter of the first fan 18 is approximately the same as the
diameter
of the fan aperture 16.
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The front, back and bottom walls 13, 15 are formed from a first bent metal
sheet.
Extending either side of the bottom wall is a tab, one of which is shown at
19. The
tabs axe bent perpendicularly to the bottom wall so as to lie in the plane of
their
respective side wall 14. The tabs extends for the whole of the length of their
respective side wall 14. These tabs are between 5mm and l0mm wide.
'I'he left and right side walls 12, 14 and the upper wall 11 are formed from a
second
metal sheet bent into shape. A first edge 21 separates the right side wall 14
from
the upper wall 11. A second edge 22 separates the left side wall 12 from the
upper
l0 Wall 11.
The first metal sheet also includes tabs (not shown) at the uppermost ends of
the
front and back walls 13, 15. These tabs are between 5mm and l0mm wide. The
tabs are bent into the plane of the upper wall 11. Each of these tabs is
included
with two threaded holes. Screws 20 are inserted through corresponding holes in
the
upper wall 11 and screwed into the threaded holes, so as to secure the second
metal
sheet to the first metal sheet. t'1 lower end of the right side wall 14 is
located
inwards of the tab 19. This eliminates the need for separate fixings at the
lower end
of the right side wall. The lower end of the left side wall is similarly
secured behind
a corresponding tab (not shown).
Mounted in the rear wall 15 are first to sixth power connectors 23 to 28.
Electrical
power at low DC Voltages is provided at these connectors. Power is also
provided
through a wire harness (not shown) extending through a harness support 29.
The second metal sheet, forming the left and right side walls 12, 14 and the
upper
wall 11, and the first metal sheet, forming the front, back and bottom walls
13, 15,
together comprise a housing of the PSU 10. Through its construction, the
housing
is rigid.
In accordance with the invention, the second metal sheet is provided with
plural
perforations on each of the left and right side walls 12, 14 and the upper
wall 11.
The perforations axe circular in shape. The perforations are formed in a
regular grid
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pattern. The diameter of the perforations in this embodiment is 2mm. The
distance between the centre points of adjacent perforations is 3mm. The
perforations are formed by punching the second metal sheet before the sheet is
bent
into shape.
The perforations do not affect the ability of the housing to prevent the
escape of
EMI.
Referring to Figure 2, the upper wall 11 is shown. Blades of the first fan 18
are
clearly visible through the fan aperture cover 17. The arrangement of the fan
aperture cover 17 also is clearly visible. The centre part of the fan aperture
cover
17 does not need to have only small apertures since this is where a motor of
the fan
is present. Also visible in Figure 2 is a rocker power switch 30 and a mains
power
connector 32, both of which are mounted in the front wall 13. Connected to the
front wall 13 is a second fan cover 31.
As shown in Figure 2, the perforations in the second metal sheet are provided
over
almost all of the surface of the upper wall 11. Except for the fan aperture
16, where
there is no metal sheet and thus no perforations could be included, and narrow
margins where the tabs included as part of the first metal sheet overlap with
the
second metal sheet, the perforations cover the entire area of the upper wall
11. It
can be said that the perforations cover the entire useful area of the upper
wall 11.
Referring now to Figure 3, the right side wall 12 is shown. A tab 35 is formed
with
the first metal sheet. The bottommost part of the second metal sheet is
secured
behind the tab 35.
As shown in Figure 3, the perforations in the second metal sheet are provided
over
almost all of the surface of the right side wall 12. Except for narrow margins
at the
edges of the right side wall 12 adjacent the front and back walls 13, 15, the
perforations cover the entire area of the right side wall 12. It can be said
that the
perforations cover over 90% of the entire useful area of the right side wall
12. The
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margins at the edges of the right side wall 12 adjacent the front and back
walls 13,
15 are the same width as the margins on the upper wall 11 of the PSU 10.
Figure 4 is a view of the left side wall 14. A portion of the left side wall
14 is
cutaway to reveal components housed within the PSU 10. A tab which is formed
with the first metal sheet is shown at 36. The bottommost part of the second
metal
sheet is secured behind the tab 36.
As shown in Figure 4, the perforations in the second metal sheet are provided
over
70 almost all of the surface of the left side wall 14. Except for narrow
margins at the
edges of the right side wall 14 adjacent the front and back walls 13, 15, the
perforations cover the entire area of the left side wall 14. It can be said
that the
perforations cover over 90% of the entire useful area of the left side wall
14. The
margins at the edges of the left side wall 14 adjacent the front and back
walls 13, 15
are the same width as the margins on the upper wall 11 of the PSU 10.
Through the cutaway part of the left side wall 14, a transformer 40, a
rectifier 41
and other components 42 are shown mounted on a circuit board 43. These
components server to convert mains electricity received via the mains power
connector 32 into DC power at lower Voltage, and supply the result to the
first to
sixth power connectors 23 to 28, as well as to wires of the harness extending
through the harness support 29. The operation of the PSU 1U may be controlled
by
operation of the rocker switch 30 by a user. When switched off, the PSU 10
powers-down and ceases providing power. A relay and capacitor arrangement (not
shown) provide power to the fan for a while after the PSU 10 is switched-off,
so as
to provide further cooling. The components within the PSU 10 may be
conventional.
Figure 5 shows the front wall 13. The mains power connector 32 can be seen to
be
of the standard type used for PSUs, having three pins arranged in a triangular
arrangement, although any other type of mains connector may be used instead.
The
second fan cover 31 can be seen to be generally in register with a second fan
aperture 37. In turn, the second fan aperture 37 is generally in register with
a
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second fan 38, which is supported within the housing of the PSU 10. The front
wall
is not provided with perforations.
The arrangement of the first and second fans 18 and 38 will now be discussed.
The
first fan 18 is arranged to blow inwards of the PSU 10. The first fan 18 thus
blows
air onto the transformer 40, the rectifier 41 and the other components 42
mounted
on the circuit board 43. The second fan is arranged to blow outwards of the
housing. The second fan 38 thus blows air from the inside of the housing to
the
outside thereof.
ro
When installed within a casing of a computing device such as a PC or a server,
the
upper wall 11 of the PSU 10 is placed horizontally facing upwards. The upper
wall
11 typically is exposed to the internal volume of the computing device.
Accordingly, air blown by the first fan 18 into the PSU 10 typically is air
which has
r5 been warmed slightly by other components of the computing device, but still
is air
which is relatively cool compared to the air within the housing of the PSU 10.
The
front wall 13 of the housing of the PSU 10 typically is mounted adjacent a
corresponding aperture in the housing of the computing device. This ensures
that
air blown by the second fan 38 is blown to atmosphere. This thereby ensures
that
20 air heated by the internal components of the PSU 10 is blown to atmosphere,
not to
the internal volume of the computing device.
The flow rates of the first and second fans can be approximately equal. This
helps
direct air smoothly through the PSU 10, and does not encourage heated air to
be
25 vented to the housing of the computing device.
Preferably, though, the first fan 18 is controllable to adopt one of a number
of
speed settings. The speed setting of the first fan 18 is automatically
adjusted on the
basis of a signal provided by a thermal sensor (not shown) mounted on the
circuit
3o board 43.
When the PSU 10 is not running hot, as indicated by the thermal sensor, the
first
fan is controlled to adopt a low flow setting. In this low setting, the flow
rate of the
CA 02522452 2005-10-06
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first fan is less than the flow rate of the second fan 38. As a result, the
air pressure
within the housing of the PSU 10 is lower than the pressure outside. This
causes
some air to be entered into the housing through the perforations, following
which it
is vented to atmosphere by the second fan 38. This has the advantage that the
PSU
then cools the interior of the housing of the computing device in which it is
included.
When the PSU 10 is running hot, as indicated by the thermal sensor, the first
fan is
controlled to adopt a high flow setting. In this high setting, the flow rate
of the
70 first fan is greater than the flow rate of the second fan 38. As a result,
the air
pressure within the housing of the PSU 10 is greater than the pressure
outside. This
causes some air to exit the housing of the PSU through the perforations to the
interior of the housing of the computing device in which it is included.
However,
the inventor has found that this results in an increase in the pressure within
the
computing device housing, which in turn increases the air pressure within the
PSU
housing and thus increases the difference in pressure across the second fan
38. This
results in the flow rate of the second fan 38 being increased, and thus the
rate of
expulsion of heated air from the interior of the housing of the PSU being
increased,
providing improved cooling.
The perforations in the housing of the PSU 10 allow hotspots to be carried
through
the walls of the housing of the PSU, following which the hotspots are unable
to
damage internal components of the PSLT. Whereas in conventional PSUs the walls
of the housing might prevent heated air from moving further, thereby
encouraging
further the build-up of heated air, the movement of heated air is not impeded
by the
perforated housing of the invention. Since hotspots typically travel generally
upwards, a hotspot formed within the PSU typically will pass through the upper
wall
11 of the PSU. Thus, the perforations are of most benefit in the upper wall
11.
However, air currents caused by the fans 18, 38 and deflection by components
of
the PSU 10 can result in over-heated air being directed into contact with the
left and
right side walls. Whereas with conventional PSUs such heated air would be
contained by the housing, the perforations of the invention allow the heated
air to
CA 02522452 2005-10-06
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escape the PSU and thereby prevent the possibility of the heated air causing
damage
within the PSU.
The effect of the perforations in preventing hotspots is enhanced when the
flow
rates of the inward and outward blowing fans differs significantly. In a
conventional PSU with slits formed therein, heated air is able to leave the
housing
only through a fan aperture or a slit, and air is able to move only along but
remain
within the other parts of the walls of the housing. Thus, hotspots can be
allowed to
develop or be trapped near the walls of the housing. With the perforated
housing
of the invention, however, hotspots cannot be constrained by the walls of the
housing where the perforations are present and thus are free to exit the PSU
10.
As stated above, the flow rate of one or more fans in a conventional PSU is
selected
to be higher than would be the case if hotspots could not form. Since the
perforations reduce the formation of hotspots, the use of perforations allows
the
use of reduced flow rate fans. This in turn reduces the cost of the PSU and/or
the
noise levels generated by the PSU.
The existence of the perforations has a slightly negative effect on the
movement of
2o air within the housing of the PSU 10. In particular, the walls of the
housing not
being solid means that they axe less able to deflect air currents. In turn,
this makes
it possible for air blown by the first fan 18 into the PSU 10 to escape
through
perforations in the left and right side walls 12, 14 and the upper wall 11
instead of
being blown by the second fan 38 to atmosphere. This in turn can result in an
increase in the temperature within the housing of the computing device in
which the
PSU 10 is located, which is undesirable. However, the inventor considers that
this
disadvantage is outweighed by the hotspot amelioration benefits. Also, the
amount
of air escaping through the perforations can be minimised by ensuring that the
flow
rates of the first and second fans axe approximately equal.
As well as the above advantages, the perforated housing is considerably more
attractive to the eye than a PSU provided with slits.
CA 02522452 2005-10-06
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In other embodiments (not shown), a very similar effect is achieved through
the use
of a mesh material, instead of a perforated sheet. Here, the mesh material can
comprise for instance a first layer of plural metal bars or strips arranged in
parallel
and slightly separated from one another mechanically and electrically
connected to a
second layer of metal bars or strips similar to the first and laid
perpendicularly
thereon. Alternatively, the mesh material may comprise a woven metal
arrangement, such as a metal gauze. In either case, the mesh serves to shield
against
EMI and provide structural strength whilst including many small holes through
which hotspots can be allowed to pass.
Although the invention has been described with reference to a number of
particular
embodiments, the scope of the invention is not limited to these embodiments.
For example, the perforations need not be circular, but may alternatively be
oval,
square, triangular, irregular or any other suitable shape. Holes formed by a
mesh
material will normally be generally square, but may instead be any other
suitable
shape. Preferably the perforations or holes are not elongate, for instance by
having
a maximum width through a centre point of the perforation less than or equal
to
twice the minimum width through the centre point of the perforation. The
20 perforations or holes may be of different sizes and shapes, even on a given
wall of
the housing of the PSU.
The separation of the holes may be varied. Decreasing the distance between
holes
allows the freer movement of air through the housing but at the expense of
reduced
25 strength of the PSU housing. Decreasing the distance between adjacent holes
results in increased strength of the housing but reduced movement of air
through
the housing. The greater the distance between holes, the greater the chance is
that
hot air constituting a hotspot will not be dissipated quickly by the
perforations.
Preferably the distance between a centre point of a first hole and a centre
point of
3o an adjacent hole is between 1.1 and 2 times the maximum width of the first
hole.
Preferably the distance between a centre point of a first hole and a centre
point of
an adjacent hole is approximately 1.5 times the maximum width of the first
CA 02522452 2005-10-06
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perforation. If the holes, e.g. perforations, are circular, then the maximum
width is
the diameter of the hole.
The holes do not need to be 2mm across. Preferably though the holes are in the
range of 1mm to 3mm in diameter. Smaller holes are not so effective at
preventing
hotspots, whereas larger holes more easily allow articles to enter the PSU,
which is
undesirable.
The holes preferably are formed in regular parallel lines. However, these
lines need
70 not form a square grid pattern. For instance, the holes may instead be
formed in
regular parallel lines in three different directions, each direction being
separated
from each other direction by approximately 60 degrees.
Holes are formed in the upper wall 11, and preferably are formed in one or
both of
75 the left and right side walls 12, 14 aswell although this may not be
essential.
Providing holes in the rear wall 15 would normally be avoided since this could
reduce the efficiency of removal of air from within the housing by the second
fan
38. Although not shown, holes may be provided in the front wall 13 in the
parts of
that wall around the connectors 23 to 28 and the harness connector 29.
To maximum advantage, the holes are formed over as much of the walls as is
practicable, although much of the advantages obtained through the holes can be
achieved with less hole coverage. Preferably the holes on one or more of the
walls
are provided over at least 80% of the area of the wall, although the coverage
may be
as little as 50%.
Although fans axe provided in the upper wall 11 and the rear wall 15, fans may
instead be provided in any other suitable arrangement. For instance, a first
fan may
be provided in the rear wall 15 and a second fan may be provided in the front
wall
13. In this case, the output connectors 23-28 may be provided instead in the
bottom wall 16.
CA 02522452 2005-10-06
-16-
The scope is to be understood to be limited only by the scope and spirit of
the
invention as defined in the appended claims.
