[SOLVED] Flyback snubber not working!

[coates]

coates claimed 3.5 µH leakage throughout this thread, unfortunately the number doesn't fit the observations of burned snubber resistor. But we didn't yet see an actual snubber circuit with component values. Of course it's possible to get much more than leakage inductance energy by choosing unsuitable values. Also, can we be sure about correct secondary winding polarity?

I've considered that the secondary winding polarity must be correct in order to operate as a flyback. I'm pretty sure it's correct.

The RCD snubber components are UF4007, 18K enamelled wirewound 11W resistor, and 10n 1600V polyprop capacitor.

I went back to basics and realized that I may have something fundamentally wrong. I wound the primary coil as a single layer, closest to the former. Then I wound 3 secondary coils on top of the primary coil+insulation, individually and side-by-side. (Not the best construction, I now realize). The layer stack-up is shown below.

----insulation----
--S1---S2---S3--
----insulation----
---------P---------
-----former------
---centre limb---
-----former------
---------P---------
----insulation----
--S1---S2---S3--
----insulation----

When I was measuring the primary leakage inductance I shorted ALL the secondary windings and got the 3.5uH. However, when I was testing the circuit I was only using ONE winding because I wanted to test the low current. I think this was wrong. The real the leakage inductance would've been much higher with only a single winding particularly because it wouldn't been wide enough to span the primary winding.

I've now rewound the transformer with a few more turns, interleaved secondary windings and the following stack-up:

---------insulation------------
--S1/S2/S3 S1/S2/S3 ...--
---------insulation------------
--------------P----------------
----------former--------------
--------centre limb-----------
----------former--------------
--------------P----------------
---------insulation------------
--S1/S2/S3 S1/S2/S3 ...--
---------insulation------------

The secondary windings completely cover the primary winding.

Primary inductance has now changed to 150uH. Measured the leakage as approximately 8uH (5% of primary) which will now explain the high snubber dissipation. When I'm doing future tests I will use ALL the secondary windings!

---

Can someone please confirm that the leakage inductance with those 2 extra secondaries left open is what would have caused the problem?

Also, other than getting a professional to do it, is there an easy way for me to reduce the leakage inductance further? Would "sandwiching" the primary reduce the leakage?


Many thanks.

 

[treez]

AN18 by power integrations tells you want you need to know for flyback transformer.

What would be best, is to split the primary into two in series, then sandwich all the secondaries between them.
And yes, every individual winding should cover the whole bobbin.

Have the main secondary winding next to one of the primaries, so to speak.

The Power integrations PI Expert software might even design the whole thing for you, transformer and all..try it and see

- - - Updated - - -

If you have a main secondary winding, and the other two secondarys are low power, then just have one main secondary winding, and get your other two secondary rails from secondary side smps’s. Then you can concentrate to get good coupling on the one main secondary….

 

[c_mitra]

Transformer making is a black art.

 

[treez]

transformer for flyback is here
https://ac-dc.power.com/design-supp...on-notes/an-18-topswitch-flyback-transformer/

-try their "pi expert" software, it does the trafo for you sometimes