www.irf.com

5

AFL270XXS Series

Block Diagram

Figure 1. AFL Single Output

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

load permits compensation for resistive voltage drop

between the converter output and the load when they are

physically separated by a significant distance. This

connection allows regulation to the placard voltage at the

point of application. When the remote sensing features is

Figure 2. Enable Input Equivalent Circuit

Pin 4 or

Pin 12

1N4148

100K

290K

150K

2N3904

+5.6V

Disable

Pin 2 or

Pin 8

not used, the sense leads should be connected to their

respective output terminals at the converter. Figure 3.

illustrates a typical application.

Circuit Operation and Application Information

The AFL series of converters employ a forward switched

mode converter topology. (refer to Figure 1.) 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. 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

switching 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.

Remote Sensing

Inhibiting Converter Output (Enable)

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 isolation

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 converter down.

ERROR

AMP

& REF

OUTPUT

FILTER

INPUT

FILTER

OUTPUT RETURN

+ INPUT

INPUT RETURN

CONTROL

1

2

4

3

5

6

SYNC INPUT

CURRENT

SENSE

+ SENSE

RETURN SENSE

SENSE

AMPLIFIER

ENABLE 2

SHARE

SHARE

AMPLIFIER

7

11

10

9

12

8

+ OUTPUT

SYNC OUTPUT

ENABLE 1

CASE

PRIMARY

BIAS SUPPLY

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 secondary

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 MOSFETs, narrowing the width if the output

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