AFL50XXS 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 sense leads from each con-
verter should be connected to a common physical point,
−1.43
∆T
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 out-
puts 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
1
P = Device dissipation in Watts = POUT
Eff
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 “cur-
rent monitor”. The share pin open circuit voltage is nomi-
nally +1.00v at no load and increases linearly with increas-
ing output current to +2.20v at full load.
−1
As an example, it is desired to maintain the case tempera-
ture 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 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.
If the worst case full load efficiency for this device is 83%;
then 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
−1.43
60
2
A
HEAT SINK
=
− 3.0 = 71in
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 fac-
tory during all testing and burn-in processes is sold under
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
2
ambient. A flat aluminum plate, 0.25" thick and of approxi-
2
mate 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 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
7
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