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7

AFL50XXS Series

A conservative aid to estimating the total heat sink surface

erature rise (

∆T) above ambient temperature is given by

the following expression:

≈

⎧

⎨

⎩

⎫

⎬

⎭

−

−

∆T

P

80

30

085

143

.

.

.

where

∆T

PP

Eff

OUT

=

==

−

⎧

⎨

⎩

⎫

⎬

⎭

Case temperature rise above ambient

Device dissipation in Watts

1

1

and the required heat sink area is

If the worst case full load efficiency for this device is 83%;

then 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.

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 return 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.

1Sil-Pad is a registered Trade Mark of Bergquist, Minneapolis, MN

Thermal Considerations

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 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 can be somewhat messy to use.

As an example, it is desired to maintain the case temperature

of an AFL5015S at

≤ +85°C while operating in an open area

whose ambient temperature is held at a constant +25°C;

then

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.

()

P

=•

−

⎧

⎨

⎩

⎫

⎬

⎭

=•

=

120

1

83

1

120 0 205

24 6

.

.. W

A

=

60

80 24.6

in

HEAT SINK

0.85

•

⎧

⎨

⎩

⎫

⎬

⎭

−=

−143

2

30 71

.

.