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5
AHP270XXS Series
Circuit Description
Figure I. Single Output Block Diagram
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 feature is not used, the sense
Figure II. Enable Input Equivalent Circuit
Pin 4 or
Pin 12
1N4148
100K
290K
150K
2N3904
+5.6V
Disable
Pin 2 or
Pin 8
leads should be connected to their respective output
terminals at the converter. Figure III. illustrates a typical
application.
Circuit Operation and Application Information
The AHP 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. 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.
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.
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 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 “l(fā)ow” 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
SENSE RETURN
SENSE
AMPLIFIER
ENABLE 2
SHARE
AMPLIFIER
7
11
10
9
12
8
+ OUTPUT
SYNC OUTPUT
ENABLE 1
CASE
PRIMARY
BIAS SUPPLY