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5

AFL120XXS Series

AFL120XXS Circuit Description

Figure I. AFL Single Output Block Diagram

Figure II. Enable Input Equivalent Circuit

Pin 4 or

Pin 12

1N4148

100K

290K

150K

2N3904

+5.6V

Disable

Pin 2 or

Pin 8

Circuit Operation and Application Information

Remote Sensing

Inhibiting Converter Output

1

DC Input

Enable 1

4

Sync Output

5

6

Sync Input

Case

3

2

Input Return

Input

Filter

Primary

Bias Supply

Control

FB

Output

Filter

Current

Sense

Error

Amp

& Ref

Share

Amplifier

Sense

Amplifier

7

+Output

10

+Sense

11

Share

12

Enable 2

9

-Sense

8

Output Return

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

converter to life. A power MOSFET is used to chop the DC

input voltage into a high frequency square wave, applying

this chopped voltage to the power transformer at the nomi-

nal converter switching frequency. Maintaining a DC volt-

age within the specified operating range at the input as-

sures continuous generation of the primary bias voltage.

The switched voltage impressed on the secondary output

transformer winding is rectified and filtered to generate the

converter DC output voltage. An error amplifier on the sec-

ondary side compares the output voltage to a precision

reference and generates an error signal proportional to the

difference. 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 signal

driving the MOSFET, narrowing the width if the output volt-

age is too high and widening it if it is too low, thereby regulat-

ing the output voltage.

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

load permits compensation for excessive resistance be-

tween the converter output and the load when their physical

separation could cause undesirable voltage drop. This con-

nection allows regulation to the placard voltage at the point

of application. When the remote sensing feature is not used,

the sense should be connected to their respective output

terminals at the converter. Figure III. illustrates a typical

remotely sensed application.

As an alternative to application and removal of the DC volt-

age 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 refer-

enced to the output return (pin 8). Thus, the user has

access to an inhibit function on either side of the isolation

barrier. Each port is internally pulled “high” so that when not

used, an open connection on both enable pins permits nor-

mal converter operation. When their use is desired, a logi-

cal “low” on either port will shut the converter down.