Switch Mode Power Supply Transformer Design for Beginners

Comments · 31

• D6

  @D614G_aka_Doug · 3 years ago (edited) · pinned

  With a gapped core almost 100% of the energy is stored in the air gap.<br><br>When a transformer is used as a true transformer, the flux in the core does not change appreciably with load current, since the primary ampere•turns are cancelled by the secondary ampere•turns. As power increases, for a given frequency, the core size must increase to allow enough window area to fit windings heavy enough to handle the currents.<br><br>Extra windings are not used to discharge leakage inductance. Leakage inductance is there because of imperfect magnetic coupling between the windings that are already there. You might split a winding for better coupling to reduce leakage inductance. A reset winding is used in a forward converter where there is no means of discharging the energy stored in the magnetizing inductance.<br><br>Normally you don&apos;t fill a bobbin fully end-to-end because of the need for safety margins for &quot;creepage and clearance.&quot; Triple insulated wire can be a boon, though an expensive one, because the insulation removes the need for the c &amp; c allowance, letting you use the full bobbin. Typically it is used either for the primary or the secondary(s), but not often both.<br><br>A copper foil external shorting band is normally used only on a gapped transformer, which means only (in general) for a flyback converter.<br><br>I&apos;ve designed 1 kW switchers. I&apos;d hang my head in shame if they were as horrible as only 80% efficient. I&apos;ve achieved much better than that overall in designs with active power factor correction (so the overall circuit is essentially a specialized boost converter followed by a half-bridge). It <b>is</b> difficult to get really high efficiency with very low output voltage due in large part to diode losses. Synchronous rectifiers can be a big help there.<br><br>Ignoring the skin effect issues, wire size is based on RMS current, not peak or average, though of course they all go together, but the ratios depend on the waveforms.<br><br>I don&apos;t think the wire shown as litz wire actually is, but it&apos;s hard to tell.. It looks like it is merely a large bundle of fine strands each of which is individually insulated.<br>In true litz wire the design is much more complex and the strands are arranged to that all of them &quot;spend the same time&quot; at the available positions in the bundle. That is, a conductor will move from the centre of the bundle out to the periphery and then back toward the center. This isn&apos;t necessarily done for each and every individual conductor, though that is often the case for small litz wires, but for small bundles of conductors.<br><br>Litz wire is expensive, nice to wind because it is very flexible, and hard to terminate because each strand is insulated.<br><br><br>Copper foil for windings has some significant benefits, but like everything else in a switcher there are compromises that must be evaluated. I&apos;ve used it in several designs. If you go to a contract manufacturer for transformers they may prefer using parallel strands of magnet wire because their equipment is set up to handle it well.<br><br>A copper foil external shorting band is normally used only on a gapped transformer, which means only (in general) for a flyback converter.

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• SA

  @SanelKeys · 7 months ago

  When you want again and again to give the video thumbs up, you know the video is great!

• RE

  @republicoftutorials6068 · 2 years ago

  Great explanation thank you

• ST

  @stevenbliss989 · 2 years ago

  Great BASICS! :)

• WP

  @WPF465B · 2 years ago

  well done, thank you. Great detailed explanation without all the math, very practical.

• MA

  @maxwang2537 · 3 years ago

  Interesting video to someone like me relatively new to, or not that deep in this subject. Thanks!

• JO

  @JosephOto-s5w · 9 months ago

  Am just a beginner<br>Thank you

• AL

  @AllTheFasteners · 2 years ago

  Very interesting video. Does the copper tape around the edge to prevent EMI actually need to be connected to earth?

• RE

  @redesign3dp · 3 years ago

  Why don&apos;t all these experts in the comments make their own videos? You did just fine.

• AN

  @anlpereira · 3 years ago

  Very nice video. Can you show in future videos the topology of transformer and how to winding them? Thanks

• GR

  @gregorymirsky8707 · 3 years ago

  What is described here is not a transformer at all, it is a multi-winding inductor. Inductors store magnetic energy. For transformers storing magnetic energy is calamity.

• XE

  @xenoxaos1 · 3 years ago

  <a href="https://www.youtube.com/watch?v=aHGeJMkUgto&amp;t=628">10:28</a> this allows for better magnetic coupling.