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Today I dismantled (I hope this is the correct verb) a flyback transformer. The thing that I've never seen is that one end of one of the windings is "glued" to the ferrite core (Before you ask; yes, the glued end is coated with solder).

Here's the schematic of the transformer (neglecting the dots):

schematic

simulate this circuit – Schematic created using CircuitLab

EDIT:

Some details:

  • Core: EE16 with center air gap

  • Bobbin: EE16 horizontal 2x5pin

Winding structure (from inner to outer):

1) 56 Turns - half of primary (Np1)

2) 35 Turns - Ng

3) 11 Turns - Secondary (Ns)

4) 13 Turns - Auxilliary (Na)

5) 50 Turns - Remaining half of primary (Np2)

One end of the Ng winding is GND and the other end is glued directly to the ferrite core. Ng is in phase with Na wrt GND.

So, I have a few questions:

1- What is the purpose of Ng winding? For better EMI performance?

2- Related to Q-1, is there any difference between direct ground connection and connection through a winding? Also, what if there was an external inductor (e.g. I-Core) instead of the Ng winding?

I know that iron-core power transformers are grounded for safety. But I think that this is different.

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  • \$\begingroup\$ I've never seen that done. I'd GUESS that Ng = noise ground. Guess. How many turns on Na and Ng and what voltage is Vdd. When included on the core Ng ENSURES that the core is driven at AC voltages not at ground - which seems a very strange choice. It MAY be that induced noise or coupling from other sources has been established (possibly experimentally) and that the core is being driven ~~~~= antiphase to these. Perhaps :-). Merry Christmas. \$\endgroup\$
    – Russell McMahon
    Dec 25, 2018 at 8:29
  • \$\begingroup\$ Maybe you should also post a picture? How did the transformer windings acquire the names Ng, Na etc.? \$\endgroup\$
    – Andy aka
    Dec 25, 2018 at 11:37
  • \$\begingroup\$ @Andyaka I cannot put a picture because I broke it apart. But I can put a simplified winding structure showing number of turns. I put the names as used in flyback converters (Np: Primary, Na: Auxilliary etc). Ng is just a name to be used in the question. \$\endgroup\$ Dec 25, 2018 at 12:41
  • \$\begingroup\$ My best bet is they had those transformers in stock. \$\endgroup\$
    – Janka
    Dec 25, 2018 at 13:40
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    \$\begingroup\$ I have seen this done in an RF application to reduce capacitance to the ferrite core... It was used around 400kHz. \$\endgroup\$
    – MadHatter
    Dec 25, 2018 at 18:26

1 Answer 1

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I recently watched the Würth Webinar Transformer Design for EMC which explains what this winding arrangement is intended for.

It is a shielding layer between primary and secondary to decrease the interwinding capacitance and therefore improve EMI performance. It is connected to a quiet circuit node (PRI-DC GND) and the other end is left floating to prevent creating a shorted turn/transformer: enter image description here

The connection to the core is a second measure to reduce EMI by grounding the ferrite core. This is also mentioned in the webinar: enter image description here

It looks like these two design options were combined in this particular flyback transformer.

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