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Old 18th October 2005, 04:54 PM   #1
sss is offline sss  Israel
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Default problem - spikes!!

hello all

i'm making a new smps to run from a car battery - for amlifier

the pwm chip is uc2825 .

i've tested the transformer today and i get big spikes
i'm using snubbers but its not helping to supress 'em

its a push pull circuit , i'm testing only with one part of the traffo , maybe all will be ok when i will connect the second primery ?

or maybe my core is bad?
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Old 19th October 2005, 12:16 PM   #2
sss is offline sss  Israel
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partially fixed
i used too low gate resistors (20ohm), so when the mosfets were turning off there were a big spikes .increasing the value of the gate resistor solved the problem ,partially though .

now i got very small spikes when no load ,but a 10V-15V spike at the primary when load is connected , no spike at the secondary though.

my main problem now is 20-30V spikes doring turn on .i'm using mtp50n06 mosfets , so the max voltage is 60V and i get very close to that
also i didnt test with full load yet (gotta buy more bulbs ), but the spikes increase with the load , again , no spikes at the secondary

any help would be apprisiated
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Old 19th October 2005, 04:33 PM   #3
subwo1 is offline subwo1  United States
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Default Yikes! Spikes!

Hi, the spikes getting worse as the load is increased is caused by the coupled inductance kicking back harder. One way to lessen the problem is to add more turns to your transformer--both primary and secondary. I think the problem is minimized when the primary inductance is just high enough to get all the power you need and no more.

I am working on a quasi-ZVS push-pull idea which uses capacitor connected from the drain-source of the MOSFETs. A (Schottky?) diode reverse biases the capacitor when the MOSFETs turn on. Another pair of windings in phase and having an equal number of turns as the primaries pulls the capacitors back low a little after the primary is pulled down.
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Old 19th October 2005, 04:42 PM   #4
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Default Re: Yikes! Spikes!

Quote:
Originally posted by subwo1

I am working on a quasi-ZVS push-pull idea which uses capacitor connected from the drain-source of the MOSFETs. A (Schottky?) diode reverse biases the capacitor when the MOSFETs turn on. Another pair of windings in phase and having an equal number of turns as the primaries pulls the capacitors back low a little after the primary is pulled down.
can u show what u r talking about plz...
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Old 19th October 2005, 05:13 PM   #5
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so the spikes are when the mosfet is going to off mode ...
i was thinking the next circuit will help

Click the image to open in full size.

i'll make R1 "small" so the turn on time will be fast as normal and R2 "big" so the turn off will be slower .... hopefully this will supress the spikes , whatcha think?
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Old 19th October 2005, 05:37 PM   #6
subwo1 is offline subwo1  United States
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Here is the basic concept. It may be interesting to play around with, anyway.

thanks ImageShack
Click the image to open in full size.
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Old 19th October 2005, 05:40 PM   #7
subwo1 is offline subwo1  United States
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I think it would help. Normally people do it the other way for faster turn-off, for efficiency. Watch your MOSFETs to make sure they do not get too hot.
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Old 19th October 2005, 05:56 PM   #8
subwo1 is offline subwo1  United States
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Hi again, one thing which can help with trying out the quasi-ZVS is that the capacitor discharge windings can be much smaller gauge wire than the power windings. I have tried wire as small as 28AWG.
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Old 19th October 2005, 08:13 PM   #9
mzzj is offline mzzj  Finland
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I assume that your transformer winding is "standard" S-P-P-S, ie. split secondary and center-tapped primary between them?

Edit: And what kind of like snubbers you use? if they dont help there might be room for improvement in component values, layout or topology.

Picture of your project would tell a lot if you have done some "silly things"
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Old 19th October 2005, 08:27 PM   #10
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Quote:
Originally posted by mzzj
I assume that your transformer winding is "standard" S-P-P-S, ie. split secondary and center-tapped primary between them?

Edit: And what kind of like snubbers you use? if they dont help there might be room for improvement in component values, layout or topology.

Picture of your project would tell a lot if you have done some "silly things"
i'm using 68ohm +100nf snubbers
tomorrow i'll buy lower value resistors , like 56 or 47ohm...
the transformer is 2 primeries with CT and 2 secondaries withCT

also i'll post pics tomorrow if my brother will bring me back my camera



btw ,the circuit is on a prototype board , no pcb yet ...
i know , this can cause problems but i dont think thats the case here.

later...
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