Difficulties with DC/DC converter 12V to +/-40V, 8 amps

[vena2a]

Hi all,

I want to show you something. It's a push-pull converter from 12V DC to +/-40V, 8 amps (640W). I added here some pictures, schematics and efficiency graph. The converter looks quite nice, but there are some problems with stability . Could you please give me some advice.

Thanks

schematic:


Vds1 and Vds2:






Efficiency graph:

[warmen]

did you measured gate voltage on oscilloscope?

[vena2a]

of course I did ... there are gate voltage and drain voltage waveforms (unfortunately the gate voltage belongs to the second MOSFET, but the gate voltages are very similar)

[Mr Evil]

I can't help you with your problem, but I thought I would say that you should compress photographs to JPG instead of PNG. This will result in smaller file size with no appreciable loss of quality (or tiny file size and very low quality, depending on how high you set the compression). PNG is great for graphics with solid blocks of colour, but bad when it comes to gradients and noise, like photos.

[Eva]

Could you upload the pictures to a different server and add links to them? I have problems getting them, the transfer stalls before the image is completely loaded. I only managed to see the schematic.

[sivan_and]

EVA wrote :

Could you upload the pictures to a different server and add links to them? I have problems getting them, the transfer stalls before the image is completely loaded. I only managed to see the schematic.

same condition here also...even with a megabit net connection !!!

[vena2a]

OK, the images are now on the different server.

efficiency:


Vds1 and Vds2:


Vgs1 and Vds2:


shcematic:


smps pic 1:


smps pic 2:

[Eva]

Gate drive looks OK, but turn on is way too fast. The control IC can drive these MOSFET without buffers at the required speed, the 9A drivers are overkill.

Transformer leakage inductance looks high too, it can be improved with a properly wound toroid. E cores are a pain to wound for 12V push-pull.

Output inductors must be wound bifilar on the same core for proper coupling and control loop stability.

Frequency compensation is completely wrong (absent). If component values are right, you need a zero around 700Hz, and it must be implemented before the optocouplers. Forget about the zener and dual optocoupler approach.

Input EMI filter is missing.

Implementing a current transformer with an iron powder core is a very bad idea, it lacks precision, magnetizing inductance usually shunts most of the sensed current and is hard to predict and keep stable.

[richwalters]

Eva, OMO, looking at the circuit diagram I notice there aren't any small decoupling caps (smd) on the supply rails esp near driver ic's and push pull centre tap. Ringing in the duty cycle deadband..
with no small decoup caps I question the scope probe ground....on tram designs this has fooled me many times due to charge induction effects. If the waveform changes when moving the ground lead around then what is one looking at ? I can't quite make out if vias are on the ground plane to underside.
To me each half primary isn't symmetrically switching, one side toward hard saturation looks like current mode ain't working properly.
The primary snubbers 10R &10nF ? seem high to me. The waveform suggests one side is getting hotter than the other.

I question the efficiency figures. Above 0.8 great care has to be taken with each drop and measurement. Can one expect more from the topology with only a 12V supply ?

I have wound an E core for 12V p-p. Not recommended unless one has a 100% precise understanding of leakage inductance and is good with copper foil. The terminations are quite critical.
The planar is an expensive alternative but also good if one likes a niche for such designs. Again a copper exercise.

richy

[vena2a]

to Eva:

The current transformer is not made of iron powder core. I used ferrite toriod core (material 3C94 by Ferroxcube I think), so this should not cause any problems.

Could you give me some advice for the frequency compensation (e.g. some schematic or link on website). I would appreciate it, because the converter unstability really make me sad

Thanks