AFL28XXS Series
When operating in the shared mode, it is important that
symmetry of connection be maintained as an assurance of
optimum load sharing performance. Thus, converter outputs
should be connected to the load with equal lengths of wire of
the same gauge and should be connected to a common
physical point, preferably at the load along with the converter
output and return leads. All converters in a paralleled set
must have their share pins connected together. This
arrangement is diagrammatically illustrated in Figure III
showing the output and return pins connected at a star
point which is located close as possible to the load.
As a consequence of the topology utilized in the current
sharing circuit, the share pin may be used for other functions.
In applications requiring only a single converter, the voltage
appearing on the share pin may be used as a “current
monitor”. The share pin open circuit voltage is nominally
+1.00V at no load and increases linearly with increasing
total output current to +2.20V at full load.
A conservative aid to estimating the total heat sink surface
area (A
HEAT SINK
) required to set the maximum case
temperature rise (∆T) above ambient temperature is given
by the following expression:
A
HEAT SINK
where
.
⎧ ∆T ⎫
−
143
⎬
−
3.0
≈⎨
⎩
80
P
0.85
⎭
∆T =
Case temperature rise above ambient
⎧
1
⎫
−
1
⎬
P
=
Device dissipation in Watts
=
P
OUT
⎨
⎩
Eff
⎭
As an example, it is desired to maintain the case temperature
of an AFL2815S at
≤
+85°C while operating in an open area
whose ambient temperature is held at a constant +25°C;
then
Thermal Considerations
Because of the incorporation of many innovative
technological concepts, the AFL series of converters is
capable of providing very high output power from a package
of very small volume. These magnitudes of power density
can only be obtained by combining high circuit efficiency
with effective methods of heat removal from the die junctions.
This requirement has been effectively addressed inside the
device; but when operating at maximum loads, a significant
amount of heat will be generated and this heat must be
conducted away from the case. To maintain the case
temperature at or below the specified maximum of 125°C,
this heat must be transferred by conduction to an
appropriate heat dissipater held in intimate contact with the
converter base-plate.
Because the effectiveness of this heat transfer is dependent
on the intimacy of the baseplate/heatsink interface, it is
strongly recommended that a high thermal conductivity heat
transferring medium is inserted between the baseplate and
heatsink. The material most frequently utilized at the factory
during all testing and burn-in processes is sold under the
trade name of Sil-Pad® 4001 . This particular product is an
insulator but electrically conductive versions are also
available. Use of these materials assures maximum surface
contact with the heat dissipater thereby compensating for
any minor surface variations. While other available types of
heat conductive materials and thermal compounds provide
similar effectiveness, these alternatives are often less
convenient and can be somewhat messy to use.
1Sil-Pad is a registered Trade Mark of Bergquist, Minneapolis, MN
∆
T = 85 - 25 = 60°C
From the Specification Table, the worst case full load
efficiency for this device is 83%; therefore the power
dissipation at full load is given by
⎧
1
⎫
P
=
120
• ⎨
−
1
⎬ =
120
•
(
0.205
)
=
24.6W
⎩
.83
⎭
and the required heat sink area is
.
⎧
⎫
−143
60
⎬
−
3.0
=
71 in
2
A
HEAT SINK
=
⎨
0.85
⎭
⎩
80
•
24.6
Thus, a total heat sink surface area (including fins, if any) of
71 in2 in this example, would limit case rise to 60°C above
ambient. A flat aluminum plate, 0.25" thick and of
approximate dimension 4" by 9" (36 in 2 per side) would
suffice for this application in a still air environment. Note
that to meet the criteria in this example, both sides of the
plate require unrestricted exposure to the ambient air.
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