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):


simulate this circuit – Schematic created using CircuitLab


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.

  • \$\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 '18 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 '18 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\$ – Rohat Kılıç Dec 25 '18 at 12:41
  • \$\begingroup\$ My best bet is they had those transformers in stock. \$\endgroup\$ – Janka Dec 25 '18 at 13:40
  • 1
    \$\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 '18 at 18:26

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