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Flyback transformers, popularly known
as the Line Output Transformers, is a special mechanism of converting the
energy supply, both voltage and current, into electronic circuits. Although it
is termed as a transformer, it works against the typical functions of a
conventional transformer, and is exploited more as energy storage equipment.
When the primary switch is on, the energy is stored on ferrite core that has
the air gap in it. However, when the primary switch is off, the energy is not
stored but transferred to the outputs. The current will flow either in the
primary winding, or the secondary one, but not both at the same time. Thus, a
flyback transformer is often misguided to be an inductor having the secondary
windings.
The primary winding of the flyback
transformer is
wound first around a ferrite rod, and then the secondary is wound around the
primary. This arrangement minimizes the leakage inductance of the
primary. A ferrite frame is wrapped around the primary/secondary assembly,
closing the magnetic field lines. Between the rod and the frame is an air gap,
which increases the reluctance. The secondary is wound layer by layer with enameled wire.
The primary winding of the
flyback transformer is driven by a switch from a DC supply (usually a
transistor). During the switch on time, there wont be
any power conversion from primary to secondary side, since secondary side diode
will be reverse biased, hence energy is stored in the inductor itself. In order
to store the energy in magnetic field we use airgap in the inductor. but if it
is too large, leakage inductance problem will occur. So provide airgap with
minmum as per calculation obtained.
When switch is off, the stored energy in the inductor(primary) will be transfered to the secondary. Through diode the capacitor will get charged. and this stored energy in the capacitor will be discharged when switch is in ON. The cycle then can be repeated. If the secondary current is allowed to discharge completely to zero (no energy stored in the core) then it is said that the transformer works in discontinuous mode. When some energy is always stored in the core then this is continuous mode.
When switch is off, the stored energy in the inductor(primary) will be transfered to the secondary. Through diode the capacitor will get charged. and this stored energy in the capacitor will be discharged when switch is in ON. The cycle then can be repeated. If the secondary current is allowed to discharge completely to zero (no energy stored in the core) then it is said that the transformer works in discontinuous mode. When some energy is always stored in the core then this is continuous mode.
Once the voltage reaches such level as to
allow the secondary current to flow, then the current in the secondary winding
begins to flow in a form of a descending ramp signal.
The current does not flow simultaneously in primary and
secondary (output) windings. Because of this the flyback transformer is really
a loosely coupled inductor rather than classical transformer, in which currents do
flow simultaneously in all magnetically coupled windings.