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5

AFL270XXS Series

ERROR

AMP

& REF

OUTPUT

FILTER

INPUT

FILTER

OUTPUT RETURN

DC INPUT

INPUT RETURN

.*

CONTROL

1

2

4

3

5

6

SYNC INPUT

CURRENT

SENSE

+ SENSE

- SENSE

SENSE

AMPLIFIER

ENABLE 2

SHARE

SHARE

AMPLIFIER

7

11

10

9

12

8

+ OUTPUT

SYNC OUTPUT

ENABLE 1

CASE

PRIMARY

BIAS SUPPLY

AFL270XXS Circuit Description

Figure I. AFL Single Output Block Diagram

Connection of the + and - sense leads at a remotely locat-

led load permits compensation for resistive voltage drop

between the converter output and the load when they are

physically separated by a significant distance. This con-

nection allows regulation to the placard voltage at the point

of application. When the remote sensing features is not

Figure II. Enable Input Equivalent Circuit

Pin 4 or

Pin 12

1N4148

100K

290K

150K

2N3904

+5.6V

Disable

Pin 2 or

Pin 8

used, the sense leads should be connected to their respec-

tive output terminals at the converter. Figure III. illustrates a

typical application.

Circuit Operation and Application Information

The AFL series of converters employ a forward switched

mode converter topology. (refer to Figure I.) Operation of

the device is initiated when a DC voltage whose magnitude

is within the specified input limits is applied between pins 1

and 2. If pin 4 is enabled (at a logical 1 or open) the primary

bias supply will begin generating a regulated housekeeping

voltage bringing the circuitry on the primary side of the con-

verter to life. Two power MOSFETs used to chop the DC

input voltage into a high frequency square wave, apply this

chopped voltage to the power transformer. As this switch-

ing is initiated, a voltage is impressed on a second winding

of the power transformer which is then rectified and applied

to the primary bias supply. When this occurs, the input

voltage is shut out and the primary bias voltage becomes

exclusively internally generated.

The switched voltage impressed on the secondary output

transformer winding is rectified and filtered to provide the

converter output voltage. An error amplifier on the second-

ary side compares the output voltage to a precision refer-

ence and generates an error signal proportional to the dif-

ference. This error signal is magnetically coupled through

the feedback transformer into the controller section of the

converter varying the pulse width of the square wave sig-

nal driving the MOSFETs, narrowing the width if the output

voltage is too high and widening it if it is too low.

Remote Sensing

Inhibiting Converter Output

As an alternative to application and removal of the DC

voltage to the input, the user can control the converter

output by providing TTL compatible, positive logic signals

to either of two enable pins (pin 4 or 12). The distinction

between these two signal ports is that enable 1 (pin 4) is

referenced to the input return (pin 2) while enable 2 (pin 12)

is referenced to the output return (pin 8). Thus, the user

has access to an inhibit function on either side of the isola-

tion barrier. Each port is internally pulled “high” so that

when not used, an open connection on both enable pins

permits normal converter operation. When their use is

desired, a logical “low” on either port will shut the con-

verter down.