Push-Pull SMPS overshoot problem - diyAudio
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Old 10th March 2008, 06:11 PM   #1
circuit is offline circuit  Lithuania
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Default Push-Pull SMPS overshoot problem

Hello from Lithuania

I'm designing a small SMPS for my huge graphic VFD from Noritake. This display needs DC 60V and AC 6V 100kHz. For even illumination, that AC supply signal must be symmetric. Also, PWM is needed.
This VFD is going to be used in my power amplifier, so supply should be really well built. It will be supplied from DC 5V and total power is about 5 Watts, so it is 1A.

Problem #1:
I can't find a SMPS controller chip, that could operate on 4.5V-5.5V. Something similar to TL494 would be OK. Only 1A will be drawn through power switches, so, in best cause, they could be integrated into that chip.
I have temporarily implemented it on small microcontroller ATtuny15.

Problem #2:
Simplified schematic looks somewhat like this:
Click the image to open in full size.
Notice signal on transformer's secondary. Well, this is what should be theoretically, but I get something a lot worse:
Click the image to open in full size.
I have marked pulses that shouldn't be there. If those diodes are removed, pulses go few times larger than supply voltage and fries mosfets.

This is caused because of high parasitic inductance in transformer's primary wingding. I have seen a lot of schematics and working power supplies of this type, but none of them had such problem.
What am I doing wrong? Does here matter ferrite core material?

Thanks in advance. Oh, and sorry for my English
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Old 10th March 2008, 07:20 PM   #2
circuit is offline circuit  Lithuania
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Some more info.
Here is an example of transformer current:
Click the image to open in full size.
Mine is running at 100kHz, that is, a maximum pulse width is 5µs. According to picture above, it goes to peak of 300mA. Is it OK?

I'm using EPCO's transformer ER 11/5 with N87 core. Also tried N49 and T38 - result the same, only with T38 that pulse is slightly smaller.


Anyway, I'm looking for a way to fix this.
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Old 11th March 2008, 01:21 AM   #3
HVYMETL is offline HVYMETL  United States
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The waveform that you're seeing is due to the magnetizing current (I-m) of your transformer. This current flows into the transistor which is ON, then when that transistor is turned off, I-m has to flow somewhere so it forces the drain high, and the opposite drain low until the reverse diode conducts. The integral of volt-seconds while the switch is ON has to equal the volt-seconds when the switch is OFF. That accounts for the part of the waveform which you circled. After this is finished, the resonant voltage you see is just the ringing of the natural parasitics of the transformer and capacitances.
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Old 11th March 2008, 01:48 AM   #4
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This output is for the VFD filament and the waveforms are taken unloaded, right? When connected to the filament the flyback pulses you see will go away or at least be attenuated.
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Old 12th March 2008, 07:47 PM   #5
circuit is offline circuit  Lithuania
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Quote:
Originally posted by megajocke
This output is for the VFD filament and the waveforms are taken unloaded, right? When connected to the filament the flyback pulses you see will go away or at least be attenuated.

Wow, thanks. Somehow I didn't realized that. Well, everything works fine now.
Will post some photos when my 256x64 grapchic VFD lights up
Thanks again.
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Old 15th March 2008, 11:33 AM   #6
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Default i have over shoot too

on my 12 to 24 volt full bridge i have "overshoot" too

it seems i have always had it when the transistor turns off i get a 90 to 100 volt pulse.

i wonder if i am measuring something wrong

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Old 15th March 2008, 12:16 PM   #7
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Full bridge or centertapped push-pull? Where are the 100V pulses? Primary?
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Old 16th March 2008, 12:38 AM   #8
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regular full bridge

on the primary


which i thought that they couldnt go much above the supply voltage in a full bridge


nothing special about 2 t of foil on an etd49 at 100khz with a series cap of 25uf
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Old 16th March 2008, 01:49 AM   #9
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I'd guess it has something to do with the layout then, if the inductance is a bit high in the supply leads this could happen. How far from the transistors are the power supply caps?
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Old 16th March 2008, 04:17 AM   #10
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mmmmm.
about 20 inches in the dc power supply that i am using to power the board.

should i have bulk caps near the transistor inputs?

thanks for your time
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