9
temperature will also be shared. Thus if one
member of a paralleled set is operating at a higher
case temperature, the current it provides to the load
will be reduced as compensation for the
temperature induced stress on that device.
When operating in the shared mode, it is important
that symmetry of connection be maintained as an
assurance of optimum load sharing performance.
Thus, converter outputs 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 sense pins
connected at a star point which is located close as
possible to the load.
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.
Thermal Considerations
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.
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 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 trade name of Sil-Pad 4001. 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 are
frequently messy to use.
A conservative aid to estimating the total heat sink
surface area (AHEAT SINK) required to set the maximum
case temperature rise (∆T) above ambient
temperature is given by the following expression:
A HEAT SINK ≈
−
∆T
P80 30
085
143
.
..
where
T
P P Eff
OUT
= = −
Case temperature rise above ambient
Device dissipation in Watts 11
As an example, it is desired to maintain the case
temperature of an AFL12015S at ≤ +85°C while
operating in an open area whose ambient
temperature is held at a constant +25°C; then
∆T = 85 - 25 = 60°C.
If the worst case full load efficiency for this device is
83%; then the power dissipation at full load is given
by
( )
P=•−
=•=120 1
83 1120 0205 246
.. . W
and the required heat sink area is
A =60
80 24.6 inHEAT SINK 0.85
•
− =
2
30 71
.
Thus, a total heat sink surface area (including fins, if
any) of 71 in2 in this example, would limit case rise
to 60°C above ambient. A flat aluminum plate,
0.25" thick and of approximate dimension 4" by 9"
(36 in2 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.
1Sil-Pad is a registered Trade Mark of Bergquist, Minneapolis, MN