smps problem (strange waveforms)

[PM650]

I have an smps with a bit of a problem. It works, but the gate drive and primary waveforms look quite nasty. Also, the transformer makes a noise that is unaffected by any amount of physical manipulation. The supply is tl494 based with regulation, and does provide the correct dc outputs.

Gate drive:
http://www.mustangmods.com/ims/u/1363/4389/207366.jpg
http://www.mustangmods.com/ims/u/1363/4389/207367.jpg
Primary:
http://www.mustangmods.com/ims/u/1363/4389/207368.jpg

Any ideas?

[gootee]

Well, you could use a faster shutter speed, and turn down the scope's intensity. :-)

From what I can see, there is some evidence that you might need a snubber or two.

There is a good paper about snubber design on the Cornell Dubilier website, at http://www.cde.com , in the technical papers section:

http://www.cornell-dubilier.com/design.pdf

There are some other good ones on line, too:

http://www.hagtech.com/pdf/snubber.pdf

http://www.ridleyengineering.com/snubber.htm

http://www.maxim-ic.com/appnotes.cfm...te_number/3835

http://archive.chipcenter.com/circui.../c1100rp58.htm

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

[poobah]

Looks like you need a better trigger source to look at your waveform. If you're using delayed or expanded sweep... that could be a problem.

What you are seeing is "jitter"...

Your signals really don't look that bad... they are just not getting on the screen very well.

Getting a good look at gate drives is usually difficult on good day!

Also, what topolgy of SMPS are we talking about here?

A schematic would help a bit.

[PM650]

The supply already has snubbers on the primary and gate drive as far as I know - I won't be able to look at it until i get home tomorrow. I guess those will be the first thing I check.

The supply is a standard half brigde with regulation; both +/- outputs on common mode inductor. I suppose it's worth noting that this is from a fairly reputable commercial manufacturer, not a diy. I can't remmember off-hand what settings I used on my scope, but I did use it earlier to test a smps and it provided no strange waveforms.

[poobah]

PM,

It is not about your scope... it's more about how you are triggering the waveform...

How are you doing that?

[PM650]

Originally posted by poobah
PM,

It is not about your scope... it's more about how you are triggering the waveform...

How are you doing that?

Yes I understood your point from the first post. The point I attempted to make was that I used the scope with the same trigger setting on another smps and everything seemed fine. I believe it's set to auto/int, whichever this scope has (I don't have it in front of me).

I'm afraid I don't have a schematic available to me; it would probably take a few hours to draw one up and I really don't want to take this thing appart again if I can avoid it.

[poobah]

I got ya...

The trick with gat drive waveforms, because of the constantly changing duty cycle, is to trigger off the same edge you wish to view. Try taking "auto off" and setting the trig level manually.

From what I could see your gate drive looked pretty good... any time you can get a good clear view of the MILLER step... you're doing prtty good.

[PM650]

Ok, the settings I had before were not auto/int but now they are; this allowed manual adjustment and I got more or less the same gate drive waveform. The transformer primary waveform came out to be:


I seriously don't think everything is right with this thing. The transformer didn't make noise before and I'm not quite sure what to think.

[luka]

Hi

I think if you will load trafo a bit, that sinus will go away... try that

[Eva]

The sinus is normal behaviour at low duty cycles, it's produced by transformer magnetizing inductance resonating with the capacitance or whatever stuff is connected to the windings.