AFL50XXS Series
A conservative aid to estimating the total heat sink surface
When operating in the shared mode, it is important that
symmetry of connection be maintained as an assurance of area (AHEAT SINK) required to set the maximum case temp-
optimum load sharing performance. Thus, converter outputs erature rise (∆T) above ambient temperature is given by
the following expression:
should be connected to the load with equal lengths of wire of
the same gauge and sense leads from each converter 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 outputs and return pins
connected at a star point which is located close as possible to
the load.
−1.43
⎧
⎫
⎬
⎭
∆T
0.85
⎨
⎩
A
HEAT SINK
≈
− 3.0
80P
where
∆T = Case temperature rise above ambient
⎧
⎨
⎩
⎫
⎭
1
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
output current to +2.20V at full load.
⎬
−1
P = Device dissipation in Watts = POUT
Eff
As an example, it is desired to maintain the case temperature
of an AFL5015S at ≤ +85°C while operating in an open area
whose ambient temperature is held at a constant +25°C;
then
Thermal Considerations
∆T = 85 - 25 = 60°C
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.
If the worst case full load efficiency for this device is 83%;
then the power dissipation at full load is given by
⎧
⎨
⎫
⎭
1
⎬ ( )
−1 = 120• 0.205 = 24.6W
P = 120•
⎩.83
and the required heat sink area is
−1.43
⎧
⎨
⎩
⎫
⎬
⎭
60
A
HEAT SINK
=
− 3.0 = 71in2
0.85
80 • 24.6
Since 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
Thus, a total heat sink surface area (including fins, if any) of
2
71 in in this example, would limit case rise to 60°C above
ambient. A flat aluminum plate, 0.25" thick and of
2
approximate dimension 4" by 9" (36 in 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.
1
trade name of Sil-Pad® 400 . 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.
1
Sil-Pad is a registered Trade Mark of Bergquist, Minneapolis, MN
www.irf.com
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