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Duty Cycle
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Turns Ratio (Np:Ns1)
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Primary Inductance
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Primary Peak Current
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Primary Turns
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Secondary Turns (Ns1)
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A flyback converter is a buck-boost DC-DC converter that provides galvanic isolation between input and output circuits through a transformer. It can step voltage up or down while providing electrical isolation, making it popular for isolated power supplies up to 120W.
When the switching transistor turns on, the primary winding is energized and stores energy in the magnetic field. When the transistor turns off, this stored energy is transferred to the secondary windings and delivered to the load. The dots on primary and secondary windings are reversed compared to forward converters.
Flyback converters offer excellent isolation, can provide multiple output voltages (positive or negative), have fewer components than other isolated topologies, and are cost-effective for low to medium power applications. They're also relatively simple to design and control.
In flyback converters, energy is stored during the on-time and transferred during off-time, with reversed winding dots. Forward converters transfer energy continuously when the switch is on, with aligned winding dots. Flyback is better for lower power, while forward is preferred for higher power applications.
The turns ratio is calculated based on input voltage, output voltage, duty cycle, and diode voltage drop. The formula considers the voltage reflection across the transformer during the energy transfer phase to ensure proper voltage conversion.
AL value represents the inductance per turn squared of the transformer core, typically expressed in nH/turn². It's a core-specific parameter that determines how much inductance you get for a given number of turns, essential for calculating the required primary turns.
Flyback converters have higher voltage stress on the switching transistor, poor transformer utilization, and are generally limited to lower power applications. They also have higher output ripple compared to continuous-mode converters and require careful design to prevent core saturation.
Primary inductance is calculated based on the input power, switching frequency, duty cycle, and desired peak current. It must be sized to store enough energy during the on-time to supply the load during the off-time, while avoiding core saturation.