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Old 17th August 2003, 09:37 AM   #1
MaXiZ is offline MaXiZ  Italy
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Default SMPS problem

Hi to all.. i hope to get an answer here!
I've build the following schematic
Click the image to open in full size.
Now i've the following problem..
At the output rail i've measured +80 and -50 instead of +/-40 as expected and the IRF540 become very very hot... (one time they've unsoldered themeself).
Now i'm ansking myself... what's wrong? A ppl tell me that from schematic frequency is around 28Kh and he suggest me to set it to 500-700Khz...
I don't own an oscilloscope so i can't check directly sorry...
Maybe the problem IRF540? How i've wind the transformer?
"how i've wind the tranformer"
Please some one help me...
Thanx in advance to all.
mail:maxiz@inwind.it
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Old 17th August 2003, 10:41 AM   #2
MaXiZ is offline MaXiZ  Italy
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Default ...

Just a suggest is accepted too...
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Old 17th August 2003, 08:58 PM   #3
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Something much closer to 28 KHz is appropriate.
700 KHz for this circuit is from the moon.

If you don't have access to a scope, you will have
difficulty diagnosing your problem. Get one.
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Old 18th August 2003, 05:25 PM   #4
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Default SMPS problem

Where is the output inductor? I don't see an inductor in the output, after the transformer. Without an inductor, how is the energy per cycle being stored? Also, what is the purpose of the full wave bridge rectification on the output? This is obviously a push-pull topology, but the output inductor is missing, and the expected full wave center tapped output rectifier has been replaced with a full wave bridge rectifier. I'm puzzled. If you answer the questions above, I might be able to help you troubleshoot the SMPS. Best regards.
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Old 18th August 2003, 05:59 PM   #5
MaXiZ is offline MaXiZ  Italy
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Claude i'm a beginner so i've just build the schematic without know exactly what i should do o that's missing in the schematic.. sorry! Anyway
where should stay Inductor output? How to calculate it?
this SMPS will be used for an audio Amplifier...
Could u explain me the difference between:
full wave center tapped output rectifier and
full wave bridge rectifier.
I know that these question for u are stupid and a lost of time.. but if u could give an answer.. privately too..
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Old 21st August 2003, 03:52 AM   #6
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First of all put a heatsink on your mosfets. I believe the reason your mosfets are getting hot is because you don't have enough turns on your primary. Put about 10 turns and go from there. Also make sure your very careful about how you go about finding your center tap. this could be why your having such large offsets.
Once you have your primary figured out your secondary shouldn't be so hard to figure out.
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Old 21st August 2003, 05:03 AM   #7
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You should use output inductors (chokes). Don't worry too much about polarity on the chokes now. Also configure the rectification differently than you show. You may also have to add windings to the secondary after doing so to the primary. All the suggestions offered have been good. I would not even have seen the problem with your rectification if it had not been pointed out and I am surprised that I didn't think of the output chokes.
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Old 21st August 2003, 09:05 AM   #8
subwo1 is offline subwo1  United States
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Something else may help you. I wind my chokes air core. I take magnet wire and wrap it around a smooth, round stick form in two sections separated by a short length of magnet wire. Then I bend the connecting wire to make the two halves next to each other so that they will pass current in opposite directions. This method should cancel EMI at a distance.

Try different sized chokes after the rectifiers until you get best performance. I usually make them just large enough to increase the efficiency to over 95%. But you will not reach that kind of efficiency with your circuit.
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Old 22nd August 2003, 02:47 AM   #9
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Default Correction

It occurred to me that the bridge rectifier in the schematic MaXiZ posted is the same as the discrete component set-up I posted. Now I am thinking the problem is likely to have something to do with the transformer windings. I suggest you rewind your transformer, removing all the present turns and starting over. Make sure that you add more turns at least to the primary side than you had before. Try adding about 25% more turns and try it out. If the mosfets still overheat, try adding another 25% and do that up to 4 times, if needed, and let us know the results
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Old 22nd August 2003, 12:48 PM   #10
subwo1 is offline subwo1  United States
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MaXiZ

One important thing. I noticed that your diagram does not include pull-down resistors for the output pins of the SG3524 which go to the gates of the mosfets. These resistors should be 150 ohms. They need to be included to turn off each set of mosfets at the correct time.
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