Flyback Transformer - Grounding the Ferrite Core

Question

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}

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.

Answer 1

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:

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

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