AFL28XXS Series
Block Diagram
Figure I. Single Output
+ Input 1
Input
Filter
Primary
Bias Supply
Current
Sense
Sync Output
5
Control
FB
Enable 1 4
Output
Filter
7
10
+Output
+Sense
Sync Input 6
Case
3
Error
Amp
& Ref
Share
Amplifier
Sense
Amplifier
11 Share
12 Enable 2
9
8
Sense Return
Output Return
Input Return 2
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
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 nominal
converter switching frequency. Maintaining a DC voltage
within the specified operating range at the input assures
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
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 MOSFET, narrowing the width if the output voltage
is too high and widening it if it is too low, thereby regulating
the output voltage.
not used, the sense leads should be connected to their
respective output terminals at the converter. Figure III.
llustrates a typical remotely sensed application.
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.
Figure II. Enable Input Equivalent Circuit
+5.6V
100K
Pin 4 or
Pin 12
1N4148
Remote Sensing
Connection of the
+
and
-
sense leads at a remotely located
load permits compensation for excessive resistance
between the converter output and the load when their
physical separation could cause undesirable voltage drop.
This connection allows regulation to the placard voltage at
the point of application. When the remote sensing feature is
290K
2N3904
150K
Pin 2 or
Pin 8
Disable
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