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Need help with current limiting.

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Patrick

Tue Jan 09 2018, 01:40AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

hen918 wrote ...

Where are you getting the 12V from? if it's on your input side you'll loose all isolation, and if it's on the output, what guarantees it's at 12V?

Yes i considered this witch is why i always want to post and get opinions. But i was thinking when the conventional secondary is unpowered the LED is dark, with this circuit the 12 or Vreg should be dark too, until start up, then the control segment brightens and reduces the control error signal. but i guess the LED current is dependent on whether its 12v is even present.

ok, back to simple TL431, with the shown OPTO, we can revisit this issue in a future version.

Im looking at that app note for what applies to my circuit, seems useful, though its for a Phase shifter.

Patrick

Thu Jan 18 2018, 05:57AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

SMPS for ATX power supply.

Another schematic.

PFC + SMPS schematic.

Heres the front end ive drawn so far, i meant to include te inrush current limiter... ill repost that change.

UPDATE:
logic and control

1516326116 2431 FT1630 Logic And Control

UPDATE :
the board will be 3.8 x 6.75 or smaller, im sending it out to a board shop.

Plasma

Fri Jan 19 2018, 08:11PM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

Patrick it looks like a inverter optoislotor try to have a pull up resistor of 10-30mA and feed off the high side.
when I designed a circuit like that it's setup_fallout2_2.0.0.10.part2.rar the way that you would think is noninverting but is not, had no end problem working that out

hen918

Fri Jan 19 2018, 11:09PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

hmmm, the issue with push pull at a high voltage / power is that the energy stored in the leakage inductance is dumped into the opposite MOSFET body diode on every cycle. At higher powers you end up with large currents flowing, so the energy increases, as does power dissipation, as you increase voltage you put many more turns on the transformer, which increases. Note that the designs you link to are forward converters, which whilst not being very space (or power) efficient, are able to handle a few hundred watts.
If you look at the datasheet of the uc28025 the example is of a 50V 50W. Which is probably the limit of the non-resonant push-pull.

Patrick

Sat Jan 20 2018, 04:06AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

hen918 wrote ...

If you look at the datasheet of the uc28025 the example is of a 50V 50W. Which is probably the limit of the non-resonant push-pull.

i was hoping for 300 watts.

Ok what about a two switch forward type ?

hen918

Sat Jan 20 2018, 01:05PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Nothing wrong with the two switch forward converter. You could make the design considerably simpler than the computer PSUs, throwing out the PFC, multiple output voltages and the shutdown controller.
I would recommend the half-bridge however, for it's simplicity and the full utilisation of the transformer. It would also be easier to change the transformer without having to change to many other components.

Patrick

Sat Jan 20 2018, 08:25PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

another site ive found that uses TI and Unitrode info.

Im thinking about how the two equalizing caps values are calculated and the high side turn on.

Plasma

Sat Jan 20 2018, 11:11PM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

Old circuit that shows a optoislotor driven a igbt

1516489918 61406 FT174221 Img 20180121 120328

Patrick

Sun Jan 21 2018, 06:44AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Plasma wrote ...

Old circuit that shows a optoislotor driven a igbt

thats great i plan t mock up a steady state example soon, can you and a schematic and part numbers ?

klugesmith

Sun Jan 21 2018, 05:59PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Nice work; please keep posting. Oh, and applause for nicely duplexing those utility boxes. Glad to see you abandoned the flyback topology in early part of this thread.

Making any mains-powered high frequency PSU will put you ahead of me. Have you considered, or already mentioned, hacking the secondary side of a computer PSU? Could even literally saw off the board near the isolation line.

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