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5
AFL270XXD Series
AFL270XXD Circuit Description
Figure I. AFL Dual 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
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 pins 4 and 12 are 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 excluded from the bias voltage
generator and the primary bias voltage becomes internally
generated.
DC Input
Enable 1
Input
Filter
Sync Input
Primary
Bias Supply
Control
Sync Output
Input Return
Case
4
1
5
6
3
2
Error
Amp
& Ref
Output
Filter
Current
Sense
Output
Filter
Share
Amplifier
+ Output
Output Return
-Output
Share
Enable 2
Trim
7
8
9
11
12
10
The switched voltage impressed on the secondary output
transformer windings is rectified and filtered to provide the
positive and negative converter output voltages. An error
amplifier on the secondary side compares the positive out-
put 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 control section of the converter varying the pulse width
of the square wave signal driving the MOSFETs, narrowing
the pulse width if the output voltage is too high and widening
it if it is too low. These pulse width variations provide the
necessary corrections to regulate the magnitude of output
voltage within its’ specified limits.
Because the primary portion of the circuit is coupled to the
secondary side with magnetic elements, full isolation from
input to output is maintained.
Although incorporating several sophisticated and useful
ancilliary features, basic operation of the AFL270XXDseries
can be initiated by simply applying an input voltage to pins 1
and 2 and connecting the appropriate loads between pins 7,
8, and 9. Of course, operation of any converter with high
power density should not be attempted before secure at-
tachment to an appropriate heat dissipator. (See
Thermal
Considerations,
page 7)
Inhibiting Converter Output
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 “l(fā)ow” on either port will shut the converter down.