Flyback converter question?

At 1:55 she didn't explain when transistor is off, why current through primary reverses direction. We know if battery is cut off and load remains connected, inductor will always keep the current going in the SAME direction. When transistor breaks the circuit and inductor has no other way to discharge i assume current now goes in reverse (to charge the cap).

Update 2:

"The windings in the circuit shown are so wound that if primary current is top to bottom, then secondary is bottom to top." i dont understand how is that supposed to be, i mean, current induced in the secondary will always be in opposite direction to the primary, no matter if it's wound CW or CCW, Lenz's law.

And thus your other point about the core delivering energy to the secondary, forward biasing the diode is also not clear, when mosfet is off, flux in a core is still in the wrong direction

Update 3:

Little correction there, magnetic field will always be such to oppose the primary, but current direction will depend on whether its CW or CCW wound.

Update 4:

All clear now.

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1 Answer

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  • Ash
    Lv 7
    1 month ago
    Favourite answer

    I agree with you, the reverse polarity at primary is misleading. The transformer used in flyback convertor is a coupled transformer. The windings in the circuit shown are so wound that if primary current is top to bottom, then secondary is bottom to top. It is usually shown in the circuit by dots.

    When MOSFET is activated, the core stores the energy, while the secondary tries to flow current into the cathode but is stopped by reverse polarity diode.

    When MOSFET is off, the core delivers energy to the secondary. Now, similar to primary, the current tries to flow from the dot end into the inductor. As the dot is at the bottom, in the secondary, the current flows into anode of diode. 

    Hope this clarifies...   

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