SMPS Problem, push pull topology w/gs3525 - diyAudio
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Old 17th April 2004, 12:22 AM   #1
alexv is offline alexv  Romania
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Question SMPS Problem, push pull topology w/gs3525

Hello all,

I've been reading what all of you wrote about smps and their problems but I am facing one that has not been touched.

First of all, my project started with Elliott's project 89.
I used IRFZ44N as mosfets, BYV32 as rectify diodes, a toroidal 40mm diam (300kHz max freq).

Without load I have a beautiful square wave in the primary.
Using 2 PC PSU as source for my smps and a load of 5 12V (21W) bulbs in series, the wave on the primary is still square with no overshoots. When I increase the load the overshoots appear but the psu's can't take the load and shutdown.

I then changed the power source with a car battery and overshoots appear on the raising edge and and short overshoots on the falling edge.

The overshoots on the falling edge are gone because I put some inductors after the rectifiers (before the caps).

I don't think that overshoots are normal but I can't get rid of them.

Tomorrow I will pot some photos from scope.

I am not shore that they are called overshoots or rings or something. I hope I made myself understood despite my bad english.
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Old 17th April 2004, 01:32 PM   #2
alexv is offline alexv  Romania
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I have just posted some pictures that I promised.

Here they are:
http://ploter.home.ro/smpsprob/index.html


hope you can help me now.
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Old 17th April 2004, 01:46 PM   #3
bknauss is offline bknauss  United States
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Have you attempted to try the amp out using the wall outlet? (220V I'm assuming for you guys) Check it out that way and see if the problems are still around.
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Old 17th April 2004, 03:17 PM   #4
alexv is offline alexv  Romania
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My smps converts 12V to +-30V.
I don't have a 220-12V convertor with more then 15 Amps.
But the goal is to use it in the car.
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Old 18th April 2004, 01:11 AM   #5
Grucho is offline Grucho  Croatia
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Hi.

Are You using 56k+100nF on primary and on secondary ??

Also I can't see how big are overshots(spikes) on primary and on secondary ?

I have talked to some peoples and they say that spikes are normaly but they have to be reduced to lowest possible size using inductors, capacitors and good wounded transformer ...

byw: what shematics from ESP you are using first one with pwm regulation or second without pwm ?

Im for testing with loads using this amp http://valveaudio.tripod.com/images/...s/mosfet50.JPG , preaty simple for testing and very cheap mosfets on output .

bye
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Old 18th April 2004, 08:24 PM   #6
alexv is offline alexv  Romania
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no, I am using 56ohms on primary and secondary.

on primary the spike is 20-30V with 100W-200W and almost no spike with no load.


the schematic si the second without feedback. i also tried optoisolated feedback and the spike increases.


I changed the 100ohm resistor for deadtime with 500ohm and I saw that the spike happens when the mosfet opens, because with a big dead time the spike can be seen distinct before the next mosfet close.

Is it normal???


Thank you very much for your help.
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Old 19th April 2004, 10:18 AM   #7
Grucho is offline Grucho  Croatia
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Hi .

Well I'm not that experienced in smps so I could tell you how big spikes are normal, but I know this :

1) I saw on primary that some amps have instead 56+100n -> 2.2+100n or 4.7+100 . You can experement with that !

2) I checked my smps with friends scope with 2 different transformers and one haved greater spikes than other. Both winded with same number of turns only one was winded nicely .

In my smps (not finished fully) I tried with reducing 56 to less but resistors started to heat a lot.

And some peoples told me that there is no neeed for output inductors (one between fast-diodes and capacitors) when you are not using pwm .


Is yout 56ohm producing heat ??
How much current is your smps pulling without load ?


bye
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Old 19th April 2004, 05:12 PM   #8
alexv is offline alexv  Romania
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with no load tha ampermeter shows 200-300max mA so it works very well.
The eficiance is very good.


I will try to change the resistors.

does your mosfet's radiator heats up???
with heavy load ofcourse. 200W.
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Old 19th April 2004, 06:42 PM   #9
Grucho is offline Grucho  Croatia
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Hi.

I wounded 3 transformers .

With first wich have too little turns, radiators were very hot when smps loaded(90watt), but were warm with light load .
And I haved voltage drops at secondary side after capacitors ( at about 80watt).

With second and third transformer radiators are completly cold, and slightly warm when loaded about 150watts . My amplifier produce more heat than smps .
I haved no voltage drops at 150watt .


1. transformer -> 4x4/11x11 -> target voltage was 28x28 (bad)
2. & 3. transformer -> 4x4/19x19 -> target voltage was 35x35

Like I said I'm using pwm with optoisolator and I define voltage but zener diode connected to opto.

But i'm using PWM .

Does your 56ohm resistor heats up or is cold ?

Also what kind electrolitic capacitors you use ? Special or normal ?

bye
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Old 19th April 2004, 08:19 PM   #10
alexv is offline alexv  Romania
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they heat up a leattle but i changed them with 2,2 ohm and the spikes ar smaller.

with 56 ohm - 20V spikes
with 2,2 ohm - 7-8V spikes
same load.

I intend to do the same, to wind the transformer for a higher voltage and lower it with pwm. also with optoisolator.
but first I want to see how to solve this problem.
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