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Flyback Converters


All the dc-dc converter mentioned previously do not has electrical isolation between the input stage & the output stage. If the input supply is grounded, the output stage will share the same ground point. A dc-dc converter that provides isolation between the input and output is the flyback converter. We begin the evolution of a flyback converter from a basic Buck-Boost converter as shown below:
A) Start with a basic Buck-Boost converter:
 

Buck-Boost converterBuck-Boost converter
B) Inductor L may be produced by 2 windings in parallel on same core:Inductor L may be produced by 2 windings in parallel on same core
C) As there is tight magnetic coupling between both coils of the inductor, it is no longer necessary to have them linked electrically, also not really necessary to have same number of turns on each winding. Re-arranging winding polarity.As there is tight magnetic coupling between both coils of the inductor, it is no longer necessary to have them linked electrically, also not really necessary to have same number of turns on each winding. Re-arranging winding polarity
D) Switch S could be put in the return side of the supply E in the primary windings to give the basic Flyback converter circuit.Switch S could be put in the return side of the supply E in the primary windings to give the basic Flyback converter circuit
E) The input & output relationship is given by
9
 A basic flyback converter with its equivalent ideal transformer model incorporating the equivalentFig : A basic flyback converter with its equivalent ideal transformer model incorporating the equivalent
magnetizing inductance Lm
The operation of the flyback converter is similar to that of the buck-boost converter. The output voltage of the converter can also be smaller or larger than the input voltage depending on the duty ratio of the switch as well as on the turn ratio of its transformer. When the switch is closed or on, the diode is reverse-biased & thus isolating the output stage from the input source and the transformer. The energy is first stored in the magnetizing inductance Lm (or the transformer core) while the capacitor which is previously charged up will provide energy to the output load. When the switch is opened or off, the diode is forward biased & the output stage receives energy from the inductance Lm via the transformer. The energy stored in the inductance is charging up the capacitor & transferred to the load at the same time. Most popular choice of converter for applications required less than 100W of output power at audio range frequency (a few tens of kHz). By adding additional secondary windings to the transformer core, multiple output terminals are also possible. Thus, MOSFET is normally a preferred choice of switch to BJT and IGBT.
previous DC-DC converters with electrical isolation
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