USB-C PD wall adapter flyback transformer confusion

Power Power Supplies (board mount)
1

I want to sanity-check the architecture.

We were operating under the assumption that I was looking for an AC/DC flyback transformer for a 65 W USB-C PD wall adapter, based around the following target hardware:

  • Controller basis: INN3879C USB-PD flyback design

  • Core: ETD29/16/10-3C97

  • Bobbin: CPH-ETD29-1S-13P

  • Clip: CLI-ETD29

My design target is a universal-input offline flyback supply for USB-C PD, with:

  • Input: 85–265 VAC, 50/60 Hz

  • Power: 65 W

  • Output profiles: 5 V / 9 V / 12 V / 15 V / 20 V

  • Target primary inductance: about 337.7 µH

  • Example winding sets under consideration:

    • Np = 40, Ns = 5, Naux = 10

    • Np = 48, Ns = 6, Naux = 12

  • Primary:secondary ratio: about 8:1

  • Primary:auxiliary ratio: 4:1

  • Auxiliary winding use: controller / primary-side bias supply

What I want reassurance on is this:

For an offline AC-DC flyback, my understanding is that the wall AC first goes through the bridge rectifier and then through the bulk/filter capacitor, which creates a high-voltage DC bus, not AC. So the flyback transformer is not being driven by sinusoidal mains AC directly. Instead, the controller switches the primary with high-frequency pulses from that rectified DC bus.

So my question is: is that correct?
In other words, after the bridge rectifier and bulk cap, the input to the flyback stage is effectively DC, and the transformer is really a high-frequency switched flyback transformer, not a 50/60 Hz AC mains transformer.

I also want to make sure I am not misunderstanding the role of the auxiliary winding. My current understanding is:

  • the auxiliary winding is mainly used for primary-side bias / controller supply

  • output voltage selection for USB-C PD (5 V, 9 V, 12 V, 15 V, 20 V) is not done by the aux winding itself

  • instead, the USB-PD controller / feedback loop adjusts the converter operation so the secondary output regulates to the negotiated voltage

If that understanding is wrong, I would appreciate correction.

Second question: if I want an off-the-shelf transformer or module close enough to this design, how would you recommend searching for it efficiently?

Would the best approach be to look for parts matching:

  • universal AC input flyback use

  • roughly 65 W power level

  • similar primary inductance

  • similar turns ratio

  • auxiliary winding included

  • compatibility with an offline flyback controller like the INN3879C

Or is the more realistic path to look for a reference-design transformer tied to a specific controller family, rather than trying to find a generic off-the-shelf flyback transformer by core and turns ratio alone?

I would appreciate any guidance on:

  1. whether my understanding of the rectified input bus is correct

  2. whether the auxiliary winding role above is correct

  3. how to search for a close off-the-shelf part without wasting time

  4. whether a 65 W USB-C PD wall adapter is already in the range where a custom transformer is usually the correct approach

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2

I can provide input in a couple of your questions, first, the purpose of the auxiliary winding is quoting the Reference Design Example Report:

In case of overvoltage at output of the converter, the auxiliary winding voltage increases and causes breakdown of VR2 which then causes excess current to flow into the BPP pin of InnoSwitch3-PD IC. If the current flowing into the BPP pin increases above the ISD threshold, the InnoSwitch3-PD controller will latch off and prevent any further increase in output voltage. Resistor R8 limits the current injected to BPP pin when the output overvoltage protection is triggered

Secondly, on how to source an off the shelf transformer for this, the Design Example Report goes into such detail on how to construct the transformer that assuredly no off the shelf transformer for this exists, they provide the blueprints to have one made.

If I had to give my advice in that respect some of the larger semiconductor manufacturers in the USB-C PD charger space at times partner with transformer manufacturers so their reference designs have more readily available bill of materials, although there is still often manufacturer minimum quantites required for the transformers.

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thak you sir I will apply this knowlegde