DescriptionSimple Switcher Step-up Voltage Regulator
ManufacturerNational Semiconductor Corporation

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Application Hints
(Continued)
FLYBACK REGULATOR
A Flyback regulator can produce single or multiple output
voltages that are lower or greater than the input supply
voltage. Figure 18 shows the LM1577/LM2577 used as a
flyback regulator with positive and negative regulated out-
puts. Its operation is similar to a step-up regulator, except the
output switch contols the primary current of a flyback trans-
former. Note that the primary and secondary windings are
out of phase, so no current flows through secondary when
current flows through the primary. This allows the primary to
charge up the transformer core when the switch is on. When
the switch turns off, the core discharges by sending current
through the secondary, and this produces voltage at the
outputs. The output voltages are controlled by adjusting the
peak primary current, as described in the step-up regulator
section.
Voltage and current waveforms for this circuit are shown in
Figure 17, and formulas for calculating them are given in
Figure 19.
FLYBACK REGULATOR DESIGN PROCEDURE
1. Transformer Selection
A family of standardized flyback transformers is available for
creating flyback regulators that produce dual output volt-
±
±
ages, from
10V to
15V, as shown in Figure 18. Figure
20lists these transformers with the input voltage, output
voltages and maximum load current they are designed for.
2. Compensation Network (C
, R
) and
C
C
Output Capacitor (C
) Selection
OUT
As explained in the Step-Up Regulator Design Procedure,
C
, R
and C
must be selected as a group. The following
C
C
OUT
procedure is for a dual output flyback regulator with equal
turns ratios for each secondary (i.e., both output voltages
have the same magnitude). The equations can be used for a
single output regulator by changing
I
in the following equations.
A. First, calculate the maximum value for R
Where
I
tude) required from both outputs. Select a resistor less than
or equal to this value, and no greater than 3 kΩ.
B. Calculate the minimum value for
at both outputs) using the following two equations.
The larger of these two values must be used to ensure
regulator stability.
FIGURE 17. Flyback Regulator Waveforms
to I