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Issue with 12V to 230V full bridge 1.5kW converter

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edgaras006:
Hello everyone,

I have been battling with 12V to 230V converter intermittently over a month now. It should take from 10.3V to 14.4V and convert it to 230VDC up to 6.5A. I know that layout is critical in this design. PCB is 6 layers 2Oz. Design uses local gnd and power planes as a reference plane along with lots of capacitors.

My issue is - it does not start up. I get 40V on the output, U14 conduct almost nothing, despite having over 20V (because of unloaded transformer which generates that isolated voltage) between cathode and anode, but just 0.4V on reference pin. What have I missed?

P.S. two components with || before their value were not in original schematic. Before I added them it didn't even attempted to start.

Thank you.

Edited:
Transformer is 1:31:31, primary inductance is 4.5uH, switching frequency 110KHz. Primary is rated for 300A with DC_R of 0.35mOhm. Leakge inductance about 10nH. I rated it between 1.5nH and 25nH, custom transformer was about 10nH.

T3sl4co1l:
R47 needs a pulldown near it. :)

Is supply switching really the best way to "enable" U13?  Surely you could pull up CS or something like that?

5Hx and 12Hx are absent, or at least I can't see where they are. Schematic isn't searchable, it's flattened graphics, well mostly, some text labels survive but not the important ones like labels and part numbers.

Also weird that none of the lines line up.  I assume everything is snapped to grid and connected, and that's some weird output generation glitch.  At least in my PDF reader.

Just two pins to that poor transformer? I'm very interested in how you intend to get up to 180A through that poor thing without it fusing open...  On the upside, leakage doesn't matter much, in the H-bridge configuration.

Supply, you probably want some electrolytics in there for damping, and/or TVS for transient limiting.  I'm guessing load dump isn't part of the requirement (but beware if that battery is charged by an alternator) but there's still inrush.  At least, I'm assuming since no precharge circuit is shown.

U11 should have a bias resistor in parallel with the diode, to supply D5+U14 leakage/bias currents.

C12 got turnt.

Why 1200V diodes for a 230V output? Is there a ton of transient voltage there? Why not FWB rectifier? Would snubbers help?

Tim

schmitt trigger:
Similar complaints here.
I am attempting to follow your schematic on a cellphone’s screen, and I can’t find U14, which apparently provides some important bias voltages.

T3sl4co1l:
TL431, right of the controller.

Not sure if you have text search on your cellphone, but yeah, hard to find things.

Schematic readability can be improved by separating and sectioning things better; all the stuff around the controller just kind of blobs together.  Doesn't have to be rigid hierarchical blocks or dividing lines, but just putting in a little space, is the direct schematic equivalent of gently spaced paragraphs. :)

(I think I found everything I needed to see?  I might not push back on the schematic layout in a formal design review, but the fact that it would take longer than usual, and be less accurate, would be worth noting on the review.)

Tim

edgaras006:
Yea, schematic is messed up. I noticed that only when I zoomed in. Something terribly wrong happened to DIPTrace as it used to work perfectly in the past.

Anyway, I added 4.7K bias resistor to U11 diode and got 53V on the output instead of 41V before. Good start, I was measuring what is happening on U14 (TL431) when board started buzzing and current hit limiter (3A). OK, I switched to 25A lab power supply. For a several seconds it stays at 5.5A, before it hit current limiter too. Q20 and Q4 warm up. I have massive planes so even if there were a puch through it would take a while to warm up reasonably. It takes about 15-20 minutes to heat board up to soldering temperatures with two heatguns...

To my surprise input voltage from power supply jumps by 2V (resonating power line?)

Anyway, something is happening.

T3sl4co1l expressed concerns about contacts of transformer primary, well it is 4 16mm^2 multi strand copper windings in parallel, so that should do it.

Thanks for all of your comments, up to this point it really helped. I had never done design this powerful before. I had few hundred watts step down from mains or step up designs, but all below 500W. Some of them were zero voltage switching full bridges, but this one is a beast for me which I need to tame.

Apparently I have a puch through even if gate channel resistance looks OK'ish, but channel is shorted Q20.

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