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Old 9th February 2004, 01:24 PM   #1
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Default Switching Mosfet overshoot

Well, I've done a quick search on the site, and haven'tquite been able to find what I am looking for..

Basically, I am about half way finished with my first SMPS, I've mounted everything but the rectification and reservior capacitors. While testing the waveform though, I get a good square out of the controller chip, but I get a fairly large spike on the rising edges of the primary winding wave. Since this is my first shot, I'm not really sure what I am looking for, or how to correct the problem. I've tried using 100nF caps to 'snub'. I've also considered that this may diminish once I add the reservoir caps and diodes. Any suggestions?
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Old 9th February 2004, 04:14 PM   #2
maylar is offline maylar  United States
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Put a load on the secondary and look again.
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Old 9th February 2004, 04:28 PM   #3
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Quote:
I've tried using 100nF caps to 'snub'.
Where and how did you put them ?

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Charles
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Old 9th February 2004, 05:01 PM   #4
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I placed one from the center tap on the primary to each primary winding, with a resistor. After a little more experimentation, I've found that hooking one of these networks from the center tap to one of the primaries does nothing, while without hooking one up to the other primary, my spike becomes a ringing, so instead of a plateau, it lust looks like I have a couple of spikes in descending amplitude. I since I have a fairly clean square wave from the controller, do you think it's one of the mosfets?
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Old 9th February 2004, 09:59 PM   #5
subwo1 is offline subwo1  United States
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In a push pull SMPSs, you can expect to see such overshoot because the winding is not directly but inductively coupled to the free wheeling (body) diode of the mosfet on the other winding. Continue construction of the secondary part of the circuit and place a small load on it and see what happens to help determine what kind of snubbers you may then want on the primary side.
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Old 9th February 2004, 10:25 PM   #6
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That's just what I needed to hear. I had been thinking that it was due to the magnetic field collapsing back into the core on the falling edge or something, but I'll finish it and see if I still have problems.
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Old 10th February 2004, 07:27 AM   #7
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Quote:
After a little more experimentation, I've found that hooking one of these networks from the center tap to one of the primaries does nothing, while without hooking one up to the other primary, my spike becomes a ringing, so instead of a plateau, it lust looks like I have a couple of spikes in descending amplitude.
This is showing that your snubbers actually work as intended. They do in fact nothing but emulating an added lossy capacitor which 1.) lowers the parasitic resonant frequency and 2.) lowers its Q.

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Charles
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Old 10th February 2004, 02:20 PM   #8
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The big problem was my transformer core. I was told that Iron Powder would work, but I don't think the permeability is great enough at the frequency I am running, and it lets too much current through. I should actually do the math BEFORE I start, what a novel idea. Anyway, I found that I had blown one of the mosfets (probably due to this). Replacing it put the ringing in its place! There is still a tiny spike on the rising edge, but no huge ocillation as before.


Ok, now I've put it all together and everything works fine....except that I have +/-60V rails now instead of the intended 30V. Looking at the primary, I have 23V pek square waves on each primary. Not sure what went wrong there. On the primary I did 4 turns, center tap, 4 turns, the secondary was 10 turns, center tap 10 turns, and I only have a 12V supply. I'm wondering if I made some minor mistake and it's acting as an auto-transformer. Back to winding again...
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Old 10th February 2004, 03:30 PM   #9
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Originally posted by tennisballg
The big problem...............................

Do you know anything about your core??
Al for instance?
Or could you at least tell the physical dimensions, eg. Od, Id, height?
What's the switching frequency?
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Old 10th February 2004, 04:13 PM   #10
subwo1 is offline subwo1  United States
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Quote:
Ok, now I've put it all together and everything works fine....except that I have +/-60V rails now instead of the intended 30V. Looking at the primary, I have 23V pek square waves on each primary. Not sure what went wrong there. On the primary I did 4 turns, center tap, 4 turns, the secondary was 10 turns, center tap 10 turns, and I only have a 12V supply. I'm wondering if I made some minor mistake and it's acting as an auto-transformer. Back to winding again...
In push-pull circuits it is normal for the voltage output to be the same as a half-bridge operating on twice the voltage. Instead of the two power supplies being stacked on top of each other, they are side-by-side. The mosfets still have to handle as much voltage as with a totem pole because when the opposing mosfet goes low, it forces the drain of the "off" one to go high.
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