Flyback Regulator:
The
Flyback is the most versatile of all the topologies, allowing the designer to
create one or more output voltages, some of which may be opposite in polarity.
Flyback converters have gained popularity in battery-powered systems, where a
single voltage must be converted into the required system voltages (for
example, +5V, +12V and -12V) with very high power conversion efficiency. The
basic single-output flyback converter is shown in Figure.
The most
important feature of the Flyback regulator is the transformer phasing, as shown
by the dots on the primary and secondary windings. When the switch is on, the
input voltage is forced across the transformer primary which causes an
increasing flow of current through it.
Note that
the polarity of the voltage on the primary is dot-negative (more negative at
the dotted end), causing a voltage with the same polarity to appear at the
transformer secondary (the magnitude of the secondary voltage is set by the
transformer seconday-to-primary turns ratio).
The
dot-negative voltage appearing across the secondary winding turns off the
diode, reventing current flow in the secondary winding during the switch on
time. During this time, the load current must be supplied by the output
capacitor alone. When the switch turns off, the decreasing current flow in the
primary causes the voltage at the dot end to swing positive. At the same time,
the primary voltage is reflected to the secondary with the same polarity. The
dot-positive voltage occurring across the secondary winding turns on the diode,
allowing current to flow into both the load and the output capacitor. The
output capacitor charge lost to the load during the switch on time is
replenished during the switch off time. Flyback converters operate in either
continuous mode (where the secondarycurrent is always >0) or discontinuous
mode (where the secondary current falls to zero on each cycle).
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