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Flyback converter circuit

If you look at the flyback converter circuit above, the current only flows in one of the coils at any instant of time. What stops the magnetic field generated by the coil in series with the MOSFET to induced a current in the coil in series with the diode resulting in a simultaneous flow of current through the diode and the MOSFET at the same time?

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    \$\begingroup\$ Hint : diodes only conduct one way round. \$\endgroup\$ – Brian Drummond Sep 25 '14 at 8:25
  • \$\begingroup\$ Yeah, I know that. Why can't the induced current create the positive bias? ( like it does when the MOSFET is switched OFF) \$\endgroup\$ – am3 Sep 25 '14 at 9:01
  • \$\begingroup\$ Because of the direction of the windings. When the FET switches off the current was from the point-side to the transistor, so the induced current in the secondary is also from the point-side, and through the diode. \$\endgroup\$ – Wouter van Ooijen Sep 25 '14 at 9:04
  • \$\begingroup\$ The direction of the windings is indicated by the dots in the schematic. \$\endgroup\$ – Bommelchen Sep 25 '14 at 9:34
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What stops the magnetic field generated by the coil in series with the MOSFET to induced a current in the coil in series with the diode resulting in a simultaneous flow of current through the diode and the MOSFET at the same time?

The dots on the windings on the transformer tell you that the voltage on the top of the primary is in phase with the voltage on the bottom of the secondary. This means that when the MOSFET conducts, there can be no current in the secondary because the secondary voltage produced will not be able force current backwards thru the diode: -

enter image description here

This is what happens in flyback designs during the energy charge part of the switching cycle. When charging is complete, the secondary flips polarity and drives current into the load and capacitor. The polarity flip is the flyback effect.

In reality there will be a little leakage current through the diode during the charge part of the cycle but nothing to alter the basic premise of this answer.

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  • \$\begingroup\$ @am3 if you are happy with this answer please consider formally accepting it or add a comment explaining what you don't understand. \$\endgroup\$ – Andy aka Apr 6 '18 at 11:48

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