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Old 29th March 2006, 07:17 AM   #1
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Default Funny waveform!

Well I thought to start of with a basic smps I would make one that could take 12(13.8)v and make +-15V @100mA. I get the required voltage at no load drawing 107mA from my psu and the output device start to get warm. So I did a bit of poking with the scope and get this as an output (see picture 1). Now if I put a snubber on the primary of the transformer I get (see picture 2) and the supply draw over 200mA of current and the output devices get very hot.

I'm using Mr. Rod Elliott's desgin but added a pair of transistors in between the pwm and transformer.

Please any idea's can really help me.
P.S pwm is running at 54KHz and tranny is old driver trasnformer from PC power supply.
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Old 29th March 2006, 07:19 AM   #2
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I really wish you could put more than one picture in a post.

here's the other one.
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Old 29th March 2006, 07:32 AM   #3
Eva is offline Eva  Spain
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The first one looks quite ugly. The other a bit better, but switching devices should not get hot without a load in such a circuit.

How have you wired those additional transistors?

Could you post a picture of your transformer and tell how it was wound?
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Old 29th March 2006, 06:55 PM   #4
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Here's the switching transistors, the emitters go to the legs of the transformer.

As for the transformer, it was a pre-built one I pulled out of an old PC power supply, it was the driver transformer. I used it because it gave me 30VP-P which in a half bridge design gives a nice +-15V and I am asumming because it's a pre-built transformer I wouldn't have to worry about lage peaks from poor winding.
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Old 29th March 2006, 08:52 PM   #5
Eva is offline Eva  Spain
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If the transformer is the classic one with two 70T windings in one side and two 10T plus another 2T in the other side, it's probably not going to suit your needs.

The transformer is probably saturating. Place a 100 ohm resistor in series with the winding you are using as primary and check the waveforms again. Then try 10 ohm. It would be useful if you could post these waveforms also.
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Old 29th March 2006, 09:40 PM   #6
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Well I'll try putting a 100ohm res in when I get home, but the transformer I have only has 2 windings on it, it's been a while since I removed it from the unit but I think it was a flyback power supply with that transformer as the drive.
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Old 30th March 2006, 03:11 AM   #7
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Well I tried putting the 100 Ohm resistor in the primary but it didn't change anything . The file contains the pictures of the waveform, input 1 and output one are one channel and input 2 and output 2 are another. main output is the secondary and just for you Mr. Eva I took a picture of the transformer

Here's the pictures.
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Old 30th March 2006, 06:06 AM   #8
Eva is offline Eva  Spain
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Well, it looks like the magnetizing inductance of the transformer is in the low side and is playing games with the 1V deadband introduced by both your buffers and the buffers inside the IC. There are a total of four deadbands, that's why the output looks so full of stairs.

Also, the transformer is not saturating as the 100 ohm resistor would have changed things a lot otherwise, so remove that resistor. However, I recommend placing a non-polar 1uF capacitor in series with the primary winding just to be sure that saturation is not possible.

Try to place a load in the secondary side, either a single resistor directly across transformer terminals, or one resistor across each rectified ouput. Try to draw something like 50mA or +-25mA respectively. The waveforms should look much better with load as the interaction between magnetizing inductance and buffer voltage deadbands should be tamed.

Also, I recommend driving the transformer directly from the IC if your output current requirements are not too high. That would produce cleaner and higher output as two of the deadbands would be removed. According to the datasheet, the IC should be capable of providing 150mA reliably (something like +-50mA translated to the output).
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Old 30th March 2006, 03:00 PM   #9
mzzj is offline mzzj  Finland
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Default Re: Funny waveform!

Quote:
Originally posted by ifrythings
Now if I put a snubber on the primary of the transformer I get (see picture 2) and the supply draw over 200mA of current and the output devices get very hot.

What component values you used for snubber? Sounds like they are far away from optimal.
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Old 30th March 2006, 04:43 PM   #10
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Well I tried to run the transformer off of the IC but it made a nice audible sound and drawed over 600mA with no load (still), I put the 100 ohm resistor in series with it and made it stablize a bit (no more audible sounds) but it was still drawing a fairly high current.

I decieded to dismantal the transformer to see how it was wound, but as most pre-built transformers are glued, I broke the ferrite but I did check out the windings and it was one layer of windings for the secondary and two for the primary.(I also found out I had it backward, as the secondary was wound first)

so anyways I have another transformer I was able to get a part (by the way anyone know of a good way to dismantal pre-built transformers) so I was woundering how to wind it?

F is 54KHz, input is 12(13.8)V, output should be +-15V@100mA

should I wind it as 10 wraps for the primary, then 25 for the secondary and then another 10 on top in series for the primary.
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