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Old 23rd March 2010, 09:01 PM   #1
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Default spike problem !

hello!
Plese help me to eliminate this spike!
All measurements have been made with the gound as reference.
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Old 24th March 2010, 05:45 PM   #2
Mooly is offline Mooly  United Kingdom
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Looks like a commercial design from the way the circuit is drawn.
Why is the "spike" a problem ? Is the unit faulty in some way.
You have to be very careful in just what you call "ground" too, moving the connection to avoid it being influenced by circuit resistances etc as the very high currents have a real effect on measurements..
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Old 24th March 2010, 09:04 PM   #3
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Thank you for the reply. The spike increases the output voltage, causing problems. Can this spike be a ringing of the ferrite ?
Could the ferrite itself cause the spike ? Or is it the way the coil is wound ?
ferrite material N87 R 50.0 x 30.0 x 20.0 http://www.epcos.com/inf/80/db/fer_07/r_50.pdf
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Old 25th March 2010, 12:18 AM   #4
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I suspect layout problems. You must confine all high currents to the intended path. A stray amount of current can cause spikes.

How about a photo of the circuit with notes showing where the parts are...
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Old 25th March 2010, 07:13 AM   #5
Mooly is offline Mooly  United Kingdom
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Is this something you have made yourself ? or is it a faulty commercial product ?

When you say the "output voltage increases" what do you mean ? You haven't shown the secondary connections, feedback etc. Do you rectify the outputs ?
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Old 25th March 2010, 08:51 PM   #6
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i suspect sg3525!
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Old 26th March 2010, 07:21 PM   #7
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My problem is the large input capacitance of irfp2907.
I cannot find a good gate resistor value. (resistor + diods)
I have tested values between 22 and 100 ohms, all with bad results.
The lower value resistor I'm using, the more obvious the problems are.

Last edited by mgm2000ro; 26th March 2010 at 07:25 PM.
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Old 29th March 2010, 08:44 PM   #8
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Please help me, this problem is getting irritating. I'not able to drive irfp2907pbf.
The spike is the main issue because it causes an increase in voltage after the rectifying bridge and I don't want this.
I've tried gate resistors with values between 2 and 100 ohms + a diode for closing, I've even tried a PNP tranzistor for forced closing and still no help.
I've even modified the number of windings on the ferrite and still no result.
The mosfet opening / closing circuit seems to be ok but I get a ringing at 2 different frequencies. I can tame the 4Mhz ringing with a snubber but for the life of me I can't tame the 1.1Mhz ringing.
Please help, i'm on the edge of giving up with the project.
wave form on primary
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Old 29th March 2010, 09:14 PM   #9
luka is offline luka  Slovenia
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well try diffrent snubber.
Also when I had some load to smps, this was much better, idle was worse, and its not bad anyway... do you have snubbers on pri and sec windings? each to gnd?
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Old 29th March 2010, 09:15 PM   #10
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Sum ting wong. There must be a "secret" inductor somewhere. Can you get a much bigger shot of the board? Both sides? What about the rectifier circuit?

Last edited by Andrew Eckhardt; 29th March 2010 at 09:17 PM.
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