AHP2800D_15 Datasheet, PDF (5/11 Pages) International Rectifier – HIGH RELIABILITY HYBRID DC/DC CONVERTERS

AHP28XXD Circuit Description
Figure I. AFL Dual Output Block Diagram
AHP28XXD Series
DC Input 1
Enable 1 4
Input
Filter
Primary
Bias Supply
Sync Output ₅
Sync Input ₆
Case 3
Control
Error
Amp
& Ref
Output
Filter
Current
Sense
Output
Filter
Share
Amplifier
7 + Output
8 ^{Output Return}
9 -Output
11 Share
12 Enable 2
10 Trim
Input Return 2
Circuit Operation and Application Information
The AHP series of converters employ a forward switched
mode converter topology. (refer to the block diagram in
Figure I.) Operation of the device is initiated when a DC
voltage whose magnitude is within the specified input voltage
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. By maintaining a DC voltage within specified
operating range at the input, continuous generation of the
bias voltage is assured.
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 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 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 AHP28XXD
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series 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. As is the case with any high power density
converter, operation should not be initiated before secure
attachment to an appropriate heat dissipator. (See Thermal
Considerations, page 7) Additional application information
is provided in the paragraphs following.
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
“low” on either port will shut the converter down.
Figure II. Enable Input Equivalent Circuit
+5.6V
Pin 4 or
Pin 12
1N4148
100K
270K
Disable
2N3904
Pin 2 or
Pin 8
200K
5