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Built a 250W SMPS, possible to modify?
Built a 250W SMPS, possible to modify?
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Old 31st December 2006, 03:14 PM   #11
simingx is offline simingx  Singapore
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Of course not

If it's actually THAT simple then I don't see why I couldn't do a fullbridge converter...

But the transformer turns ratio will be different, correct?
At least according to the turns ratio calculator here...
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Old 31st December 2006, 03:15 PM   #12
N-Channel is offline N-Channel  United States
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Yup. Since the primary side of the main Xfmr will now see the full voltage (320-360V) you will double the number of primary turns.
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Old 2nd January 2007, 06:02 AM   #13
simingx is offline simingx  Singapore
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OK... I managed to pop the 2 IRF730's by (accidentally) shorting the output. Is there any way to protect against short circuits?
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Old 2nd January 2007, 09:26 AM   #14
luka is offline luka  Slovenia
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Built a 250W SMPS, possible to modify?
Yes there is. Short circuit protection . For that you will need to place current trafo somewere in primary and sence primary current.If you search this forum you will find a lot of shemas of what I mean.
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Old 2nd January 2007, 09:37 AM   #15
simingx is offline simingx  Singapore
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I did do that... but it seems not fast enough to protect the FETs against short circuit (it works for overload by sending the circuit into some sort of "current limit" mode, I'm not sure whether it's supposed to work like that?).

And I'm not very excited about trying that little "experiment" again..
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Old 2nd January 2007, 09:54 AM   #16
luka is offline luka  Slovenia
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Built a 250W SMPS, possible to modify?
Is not fast enough?! I think it is faster then fuse blowing up... I seen a lot of supply that have let say 105% current limiting, but if current goes to 130% then it shuts down.

Where did you put your current trafo in your design?
If it did work for overload, it must have been working for short circuit, since in both cases is intended to use more current then wanted.
There must have been any other type of error to blow up your mosfets or your current limiting didn't work.
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Old 2nd January 2007, 10:03 AM   #17
ifrythings is offline ifrythings  Canada
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I've been doing a lot of learning for something like this too, I have a schematic for a full bridge but not sure if it will work.

I was tring to desgin this so it can run off of the TL494, the SG3525 should work too, I have to premaid drive transformers for fets that I robed out of a cooked server PSU.
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Old 2nd January 2007, 10:19 AM   #18
luka is offline luka  Slovenia
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Built a 250W SMPS, possible to modify?
Looks good, but I would put fast turn of circuit for each mosfet its own, here you can see it for half bridge, or in your case one side of full bridge.
Click the image to open in full size.
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Old 2nd January 2007, 10:56 AM   #19
ifrythings is offline ifrythings  Canada
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May be a stupid question, but I'm still learning here, What have I put in?

I was tring to find some pictures Eva posted for different turn on and off circuit for Bipolars and fet but the search function on this fourm hates me today, do you know where they are?

Also what is more of a problem slow turn on or slow turn off? (or both)?

Thanks Dave.

BTW the schematic I posted is for 120V mains getting x2 by caps, remove link from bridge to center of caps for 240V operation.
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Old 2nd January 2007, 11:29 AM   #20
luka is offline luka  Slovenia
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Built a 250W SMPS, possible to modify?
Well as I understand that is both.Some what slower turn on is good for core saturation, since fast turn off is good all the time I think.What you have put is: For turn off mosfet gate is discharged through resistor and trafo.If there is leakage inductance in trafo, mosfet will not discharged fully, what could result in destruction of fets or high heat.But if you use that shema the fet is turnd on slowly(resistor), but turnd off fast with transistor.

As for pictures: Eva have deleted them from her server, and I have only that shema of what there was.
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