Antec SL-400 5vsb Mod with NCP1200

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

I finally got around to hooking up my ring tester to the 5vsb transformer. It only went up to three red LEDs so I think the transformer must be damaged. Well this is not surprising considering what I was trying to do.

A couple of weeks ago I tore apart a 5vsb transformer from a scrap supply just to see what it would take to rewind one. Well lets just say I have better things to do with my time and I will wait until I can find a another Antec supply from which I can scavenge the 5vsb transformer.

[ben7]

Re: Antec SL-400 5vsb Mod with NCP1200

I probed around the supply tonight and I did not find any points of failure I could identify so I decided to power it on an see what would happen. I just had my standard ATX tester connected to it along with my DMM for voltage reading. Much to my surprise it worked, with a reading of just over 5.1V, (I do not remember what the last two digits were.)

It does appear that the waveform on the secondary diode has significantly changed. So the theory about the transformer being damaged my be correct, it just may have been pushed too far. If this is the case I am not really surprised, the only thing that surprises me is that it did not happen sooner.

It looks like it could be time for someone to learn how to wind a transformer.

If your going to wind your own transformer, you might as well make your own smps, with pcb... xD

There are plenty of things online that are about designing smps transformers

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

I probed around the supply tonight and I did not find any points of failure I could identify so I decided to power it on an see what would happen. I just had my standard ATX tester connected to it along with my DMM for voltage reading. Much to my surprise it worked, with a reading of just over 5.1V, (I do not remember what the last two digits were.)

It does appear that the waveform on the secondary diode has significantly changed. So the theory about the transformer being damaged my be correct, it just may have been pushed too far. If this is the case I am not really surprised, the only thing that surprises me is that it did not happen sooner.

It looks like it could be time for someone to learn how to wind a transformer.

[kaboom]

Re: Antec SL-400 5vsb Mod with NCP1200

If it's powered up again, it'll probably be unstable now for the same load it was stable at before it overheated. If the transformer is unwound, the wire varnish will at least be darkened, possibly cracked.

Also, if there's no, or a too-low value, resistor feeding the aux supply limiter zener (tertiary/line side), said zener could be bad.

[everell]

Re: Antec SL-400 5vsb Mod with NCP1200

^Agreed +3. I am having a similiar problem with my Bestec

[ben7]

Re: Antec SL-400 5vsb Mod with NCP1200

Sounds like (punny- ) your transformer developed some shorted turns. The ratio has changed, throwing off the compensation.

Did you mean you still had 5VSB output when you shut it off? Even with the smell? If so, I'd think you really overheated that transformer...

The smell matches closest to the smell of an overheated transformer, I think something has happened with it.

I agree with the shorted turns theory too.

[kaboom]

Re: Antec SL-400 5vsb Mod with NCP1200

At thirty minutes into the test I look over and notice the output on the scope was not longer stable. As I go to kill power to the supply I hear some bad noises coming from the supply and then I smell burnt electronics.

Sounds like (punny- ) your transformer developed some shorted turns. The ratio has changed, throwing off the compensation.

Did you mean you still had 5VSB output when you shut it off? Even with the smell? If so, I'd think you really overheated that transformer...

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

What was the smell like? Like what kind of component burning? Resistor? Capacitor? Semiconductor? Etc...

I wish I could tell you the answer to this as it would make my troubleshooting much easier. I pretty much know what an electrolytic capacitor that has popped smells like but beyond that burning electronics all pretty much smell the same to me.

Also, you said you took it off the psu to do this testing, is the main MOSFET still on a decent heatsink?!

I am not sure what you mean by this. The supply is just not in its' metal housing but the rest of it is the way it is supposed to be, original heat sinks and all. The attached picture is of one of my previous tests so you can see how I set it up. (This picture is not from any recent test, just one I had on my drive.)

[ben7]

Re: Antec SL-400 5vsb Mod with NCP1200

Although I have one of those thermometers I did not bother with it at the time as I am not sure where it was stashed. You may be correct that the transformer is the problem but I will have to find out what went bad to know for sure. Currently, the transformer windings are still showing the same resistance as it had before it died so I do not think it was damaged.

What was the smell like? Like what kind of component burning? Resistor? Capacitor? Semiconductor? Etc...

Also, you said you took it off the psu to do this testing, is the main MOSFET still on a decent heatsink?!

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

My guess is the transformer is overloaded. It's simply not designed for 25 watts!... Did you check to see what parts were hot? It's handy to have an infra-red thermometer around...

Although I have one of those thermometers I did not bother with it at the time as I am not sure where it was stashed. You may be correct that the transformer is the problem but I will have to find out what went bad to know for sure. Currently, the transformer windings are still showing the same resistance as it had before it died so I do not think it was damaged.

[ben7]

Re: Antec SL-400 5vsb Mod with NCP1200

My guess is the transformer is overloaded. It's simply not designed for 25 watts!... Did you check to see what parts were hot? It's handy to have an infra-red thermometer around...

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

While I was working on another supply today I decided it was a good opportunity test this supply off to the side. My goal was to get 5A out of the 5vsb for a minimum of 1 hour.

Everything started off good. I had a load setup of 1.0 ohms. I was getting 4.935V out of the supply. Fifteen minutes later I check on it and I am getting 4.945V, everthing is looking good. I had my scope connected to the output diode the same as when I was testing the snubber just as another way to monitor the operation. At thirty minutes into the test I look over and notice the output on the scope was not longer stable. As I go to kill power to the supply I hear some bad noises coming from the supply and then I smell burnt electronics.

So far I have only given it a cursory examine and I could not find anything that was bad. I am beginning to have doubts that I am going to be able to achieve my goal, however I have not yet given up all hope. Once I find out what failed, I will try to find a heavier replacement so I can give it another try.

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

Well I did the capacitor test for the snubber circuit again.

1 nF = 75nS (First attached picture)

560pF = 58 nS (Second attached picture)

I could not find any other values in between these two so I just went with the 1nF capacitor. I calculated the associated resistor should be 16 ohms. I had a 15 ohm resistor handy so that is what I used.

The rest of the scope shots (attached pictures 3-5) are with the snubber in place.

Not too much difference from the previous snubber but it does appear to be a slight improvement.

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

I was just looking over the graphs I posted in post #70 and realized that I did not place the second cursor in the wrong spot in the 1st, 3rd and 4th shot. Just eyeballing the first graph it looks like the 1nF capacitor more than doubled the original 34 nS so it looks like I am going to have to back up and start over with my snubber calculations. Oh well the results so far have been interesting, to me at least, so actually using the correct values for the snubber should only improve the results.

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

I found a 5.6 ohm resistor so I decided to use it along with the 3.3nF capacitor. All the scope shots are done with the same 3.2 ohm load used in the previous scope shots, except the last one which has a 2.0 ohm load. Increasing the output current did not seem to make much difference in the voltage spike with the snubber in place. Now I should be able to resume my load testing, at least until something else pops.

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

I did some testing with four different capacitor values to narrow to the best choice for a snubber. Since with no snubber I was getting a time of 34nS I was looking for something to give me about 68nS.

1nF=38nS
10nF=286nS
3.3nf=75nS
2.2nf=64nS

The the attached scope shots are in the same order as listed above.

I am not sure which of the last two values are the best choice or if it makes much difference.

If I did my calculations correctly, I need a resistor of approximately 5 ohms if I use the 3.3nF capacitor or a resistor of approximately 8 ohms if I use the 2.2nF capacitor. So far the closest values I have been able to find in my junk piles or stocks is a 1 ohm and 10 ohm resistor. I am thinking that unless I find something else, the 2.2nF and 10 ohm resistor may be my best option. I am not sure how important the it is for the resistor value too be dead on nor how altering it will affect result.

If I can find it, I think I would rather use a 4.7 ohm resistor with the 3.3nF capacitor.

Any constructive comments or input would be appreciated.

[ben7]

Re: Antec SL-400 5vsb Mod with NCP1200

Phew! I guess that means the transformer is still good. Sometimes shorts will burn out the thin wire in the small smps transformers.

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

I finally was able to put this supply back together and the 5vsb is working again. I did the first step in making a snubber circuit for the output diode by measuring the waveform time. It appears to be 34nS so I need to pick a capacitor that will double this time. I am not sure what values I have on hand but I will try something I have and see what happens. I will then adjust the value up or down from there. (This test was done with a 3.2 ohm load.)

[LDSisHere]

Re: Antec SL-400 5vsb Mod with NCP1200

If there is not pull down resistor for the mosfet, then yes, it most likely charged up by itself and turned ON.

Without the board I do not think there is a pull down resistor, which is something I will probably change.

Next time, use a lightbulb (~60 watts should be good) in series with the power supply, so a short circuit won't make things explode as badly!

What fun would that be? A good fireworks show every once in a while will keep you on you on your toes and will usually be a good teacher if you are smart enough to learn from it.

[ben7]

Re: Antec SL-400 5vsb Mod with NCP1200

I do not think the soldering was the problem but at this point who knows. That supply has not been in a case since I got it working many months ago and it was upside down when I powered it on so that I could probe around to see if I could find any problem areas. I think the FET may have been damaged and I did not catch it or the gate was "floating" on, but I have no evidence one way or the other at this point. All I can do now, is put Humpty back together and try not to push him back off the wall again as I did in this situation. I want to save that for the load tests.

If there is not pull down resistor for the mosfet, then yes, it most likely charged up by itself and turned ON.

Next time, use a lightbulb (~60 watts should be good) in series with the power supply, so a short circuit won't make things explode as badly!