Question about power supply output parasitics (scope shots attached)

[jim hardy]

what I did was take out one of the two transformers , disconnected it electrically , the one I wasn't sure about whether it dosn't have an airgap in the middle.
Now the noise is much much lower.Before it was about from peak to peak about 2 volts in amplitude now the shape has changed and i's amplitude is 0.2v or about 150-200 mV.when I connected the speakers I couldn't hear any noise anymore just the typical " SHHSHSHSH" hissing type noise coming through an amp if it's input (pc soundcard this time) is turned all the way up with no signal.

Aha there's something about that transformer ?
Could be
core - high reluctance draws more magnetizing current ? Does it rum warmer than other one?
windings - leakage inductance, interwinding capacitance
leakage flux, especially if core has air gaps.

or its physical location on the board...
Again - how does its primary current flow?

 

[Salvador]

i don't know exactly but I think the core differs in material from the other one.
I took the core apart and it seems like it's not ferrite but that other material they used back in day for TV smps which was running on lower power and most importantly frequency.
Metglass or something similar.i broke the core apart and it has that weird electronics smell I think ferrite didn't had that smell.

I basically hope to simply buy a matching or one that is up to spec with my needs ferrite core and put into the already wound bobbin from which i took the cube shaped core out.

what do you mean by?

Again - how does its primary current flow?

 

[jim hardy]

what do you mean by?

Trace the current path on the board

like in post 8
(though i have no idea how current flows on your board, that's just a 'how to' example)

apply right hand rule

there's just one turn on the circuit board of course,

does the magnetic flux B created by the current loop couple your output wires?

 

[the_emi_guy]

well as for the EMI test yes Jim I connected simply a straight piece of wire between the scope gnd crocodile clip and ...

Get rid of the crocodile clip that came with the scope probe and unscrew the plastic sleeve. Use a dental pick or equivalent to make low inductance ground connection between PCB ground and ground sleeve on probe tip. Next, determine how much of the noise is instrumentation by touching probe tip to the exact same place as ground connection.

 

[Salvador]

nop it's not the measuring wire fault I've tried many times attaching wires leaving the tips hanging in mid air etc etc and nothing produces such waveforms as can be seen attaching the leads o the smps output, he parasitic is definitely there.
ok here's the brightside I've attached 10k resistors across gate to source to tie down the gates as noted in some application notes by semiconductor makers.To my surprise when I plugged the thing it the IGBT's went bang and short.So now that's I'm changing the igbt's with my spare ones I also ordered a new etd59 transformer , previously I used some off the shelf ferrite cores let's see how a good solid ferrite core maybe changes this problem because it has atleast partially to do with the transformer as I already found out earlier , why the IGBT's blw with the gate resistors not sure , maybe the gates had dropped the driving signal too much and left the gates either floating or half opened at full current.

either way the transformer will be here today so I will see how this goes for me.
the thing I hate once again about semiconductors is that they are so demanding I can't even get to see what the scope shows that the gosh darn thing's already let it's magic smoke out.

(editted by mentor to change some words to protect sensitive eyes and ears)

 

[the_emi_guy]

Firstly, what is the switching frequency and what is the frequency of that ringing?

Salvador,
The first responder to your query asked about the frequency of the ringing. I don't think you provided this yet and its quite important info for debugging this.
There is no way to tell from your scope shots if we are dealing with KHz noise of MHz noise.

We can often use the ringing frequency as a way to figure out what is resonating. For example, if you suspect that the capacitance of the resonating tank circuit is a reversed biased diode, then you can put a small capacitor in parallel with the diode and see if it shifts the ringing frequency. This is a common method used to design snubbers.

 

[Salvador]

ok sorry for not telling that fully , it's just that I'm mostly doing everything myself including the thinking because I lost faith in this or any other internet forum for that mater long ago, also because nobodies responding etc.
the frequency fs the output ripple like pulses matches the turn on and off times of each IGBT, I saw this while measuring both the output and then the gate drive signal and it had this ringing , bigger at turn on less at turn off but it's always the moment when the square wave just begins to rise or is about to cut off.

I have a big ETD59 transformer which I am about to wind right now as i finish this and well let;'s attach that and see how this goes.

 

[jim hardy]

I lost faith in this or any other internet forum for that mater long ago, also because nobodies responding etc.

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