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:
[IMGDEAD]http://img215.imageshack.us/img215/1848/adsit9.png[/IMGDEAD]
Notice signal on transformer's secondary. Well, this is what should be theoretically, but I get something a lot worse:
[IMGDEAD]http://img80.imageshack.us/img80/7372/asdjq6.jpg[/IMGDEAD]
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 ;)
 
Some more info.
Here is an example of transformer current:
[IMGDEAD]http://img219.imageshack.us/img219/556/srov480bb3.jpg[/IMGDEAD]
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.
 

HVYMETL

Member
2008-03-03 6:41 pm
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.
 

jamesrnz

Member
2005-08-14 3:01 pm
this is a representation of the signal i have at the source drain of the upper and lower mosfets on either side of the transformer.

i dont have any bulk caps at the input

i have it hooked to a 10 ohm load at 18 volts output at 1.8 amps.

the input is 12 volts at about 3 amps

the transformer is an etd49 with 2 turns 1" wide foil .040, and the secondary is 5 turns foil.

i am running at 100khz transformer (200 controller)

the current mode control 6,7 are shorted to disable. and the current sense pin 9 is grounded. the voltage divider is a little different but feeding back fairly stable at 18 volts( i have a small pot in there)

it is on a circuit board i made that is fairly balanced in its layout. i have quieted it down a lot.

i may have some ringing on the rectifiers that i have to deal with.

fixed the phasing on the gate drive transformer so i dont create plasma

uhmmmmmm i am running it at low current while i sort out the problems before i load it heavier
 

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jamesrnz

Member
2005-08-14 3:01 pm
jamesrnz said:
this is a representation of the signal i have at the source drain of the upper and lower mosfets on either side of the transformer.

i dont have any bulk caps at the input

i have it hooked to a 10 ohm load at 18 volts output at 1.8 amps.

the input is 12 volts at about 3 amps

the transformer is an etd49 with 2 turns 1" wide foil .040, and the secondary is 5 turns foil.

i am running at 100khz transformer (200 controller)

the current mode control 6,7 are shorted to disable. and the current sense pin 9 is grounded. the voltage divider is a little different but feeding back fairly stable at 18 volts( i have a small pot in there)

it is on a circuit board i made that is fairly balanced in its layout. i have quieted it down a lot.

i may have some ringing on the rectifiers that i have to deal with.

fixed the phasing on the gate drive transformer so i dont create plasma

uhmmmmmm i am running it at low current while i sort out the problems before i load it heavier


any comments on this scope pattern or is this normal at this point of measurement when the transistors turn off?
 

Attachments

  • jimscope.pdf
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