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7

AFL270XXS Series

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 convinient and are frequently messy to use.

A conservative aid to estimating the total heat sink surface

erature rise (

∆T) above ambient temperature is given by

the following expression:

≈













−

−

P

80

30

085

143

.

.

.

where

∆T

PP

Eff

OUT

=

==

−













Case temperature rise above ambient

Device dissipation in Watts

1

1

()

P

=•

−













=•

=

120

1

83

1

120

0 205

24 6

.

.. W

and the required heat sink area is

A

=

60

80 24.6

in

HEAT SINK

0.85

•













−=

−143

2

30

71

.

.

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.

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

the trade name of Sil-Pad

 4001 . 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

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 out-

puts should be connected to the load with equal lengths of

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.

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.

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 approxi-

mate 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

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

Thermal Considerations