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 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,
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 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
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 a single converter, the volt-
age 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
As an example, it is desired to maintain the case tempera-
ture of an AFL27015S 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 effi-
ciency for this device is 83%; therefore the power dissipa-
tion at full load is given by
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.
1
(
)
P
120
1
120 0.205 24.6W
=
•
−
=
•
=
.83
and the required heat sink area is
−1.43
60
A
HEAT SINK
=
− 3.0 = 71 in2
80• 24.60.85
Because effectiveness of this heat transfer is dependent
on the intimacy of the baseplate/heatsink interface, it is st-
rongly recommended that a high thermal conductivity heat
transferance medium is inserted between the baseplate a-
nd heatsink. The material most frequently utilized at the fa-
ctory 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 pro duct
is an insulator but electrically conductive versions are also
available. Use of these materials assures maximum surfa-
ce 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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