AFL120XXD 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 temp-
optimum load sharing performance. Thus, converter out- erature rise (∆T) above ambient temperature is given by
puts should be connected to the load with equal lengths of the following expression:
wire of the same gauge and should be connected to a com-
mon physical point, preferably at the load along with the
−1.43
∆T
converter output and return leads. All converters in a par-
alleled 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.
A
HEAT SINK
≈
− 3.0
0.85
80P
where
∆T = Case temperature rise above ambient
As a consequence of the topology utilized in the current
sharing circuit, the share pin may be used for other func-
tions. In applications requiring only a single converter, the
voltage appearing on the share pin may be used as a “totall
current monitor”. The share pin open circuit voltage is nomi-
nally +1.00v at no load and increases linearly with increas-
ing total output current to +2.20v at full load. Note that the
current we refer to here is the total output current, that is,
the sum of the positive and negative outout currents.
1
P = Device dissipation in Watts = POUT
Eff
−1
As an example, assume that it is desired to operate an
AFL12015D while holding the case temperature at TC
+85°C in an area where the ambient temperature is held to
≤
a constant +25°C; then
∆T = 85 - 25 = 60°C
Thermal Considerations
Because of the incorporation of many innovative techno-
logical 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 effec-
tive methods of heat removal from the die junctions. This
requirement has been effectively addressed inside the de-
vice; 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 tem-
perature 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.
From the Specification Table, the worst case full load effi-
ciency for this device is 83% @ 100 watts: thus, power
dissipation at full load is given by
1
P = 100•
−1 = 100• 0.205 = 20.5W
(
)
.83
and the required heat sink area is
1.43
−
60
A
HEAT SINK
=
− 3.0 = 56.3 in2
80• 20.50.85
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 fac-
tory during all testing and burn-in processes is sold under
Thus, a total heat sink surface area (including fins, if any) of
2
56 in in this example, would limit case rise to 60°C above
ambient. A flat aluminum plate, 0.25" thick and of approxi-
2
mate dimension 4" by 7" (28 in per side) would suffice for
1
the trade name of Sil-Pad 400 . This particular product is
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 +25°C ambient air.
an insulator but electrically conductive versions are also
available. Use of these materials assures maximum sur-
face contact with the heat dissipater thereby compensating
for any minor surface variations. While other available types
of heat conductive materials and thermal compounds pro-
vide 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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