AFL120XXS Series
Block Diagram
Figure I. AFL Single Output
Input
Filter
1
4
5
+ Input
Output
Filter
+Output
+Sense
7
Primary
Bias Supply
Enable 1
10
Current
Sense
Sync Output
Share
Amplifier
Control
11 Share
Error
Amp
& Ref
Sync Input
Case
6
3
2
Enable 2
FB
12
Sense
Amplifier
9
8
Return Sense
Output Return
Input Return
Circuit Operation and Application Information
output terminals at the converter. Figure III. illustrates a
typical remotely sensed application.
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.
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.
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.
Figure II. Enable Input Equivalent Circuit
+5.6V
100K
1N4148
Pin 4 or
Pin 12
Disable
290K
Remote Sensing
2N3904
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
not used, the sense should be connected to their respective
150K
Pin 2 or
Pin 8
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