AFL120XXS Series
When operating in the shared mode, it is important that A conservative aid to estimating the total heat sink surface
symmetry of connection be maintained as an assurance of area (AHEAT SINK) required to set the maximum case
optimum load sharing performance. Thus, converter outputs temperature rise (∆T) above ambient temperature is given
should be connected to the load with equal lengths of wire of by the following expression:
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 sense pins
connected at a star point which is located close as possible
to the load.
−1.43
⎧
⎨
⎩
⎫
⎬
⎭
∆T
A
HEAT SINK
≈
− 3.0
0.85
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 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. The share pin voltage
is referenced to the output return pin.
⎬
−1
P = Device dissipation in Watts = POUT
Eff
As an example, it is desired to maintain the case temperature
of this device at £ +85°C in an area where the ambient
temperature is held at a constant +25°C; then
∆T = 85 - 25 = 60°C
Thermal Considerations
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
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.
⎧
⎨
⎫
⎭
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 = 71 in2
0.85
80 • 24.6
Thus, a total heat sink surface area (including fins, if any) of
71 in in this example, would limit case rise to 60°C above
Because 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
transferance 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
2
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
the 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 dissipator thereby compensating for
minor variations of either surface. While other available
types of heat conductive materials and compounds may
provide similar performance, these alternatives are often
less convenient and are frequently messy to use.
1
Sil-Pad is a registered Trade Mark of Bergquist, Minneapolis, MN
www.irf.com
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