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Old 10th June 2012, 12:38 AM   #11
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and it's better to have a low inductance for each primary or it's better to have high?
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Old 10th June 2012, 02:54 AM   #12
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Default DC-DC Converter Design

As mentioned earlier, I wouldn't worry too much about primary inductance. Inductance only becomes a big issue at much higher switching frequencies. Below 75KHz (where many high-power SMPS's run at) you should be fine. Therefor, I think you'd be fine up to 12 or so primary windings which will allow you to have a better window of regulation for your secondary winding's count.

Questions:
What frequency are you planning (and modeling) to run?
How tight do you need the secondary regulation?
What core material are you going to run?

Attached is a picture of a 500W DC/DC converter I designed and built. The core is a Ferroxcube 3C81 material running at ~23KHz. I use an SG3526 PWM IC and a dual FET driver. It converts 12V to +/- 40 for a car amplifier. I was able to get close to 800W from it, but that's beyond the continuous ratings of the secondary rectifiers, so it was only briefly. It's un-regulated, only relying on the transformer ratio of 4:12 (3:1 technically), but is surprisingly stable under varying load. That is, as long as the input voltage is stable. Of course, that also means as the input swings from 12 to 14.4VDC, the secondary fluctuates from +/-35 to +/-42 depending on load. Building it this was makes it's design very simple with almost no part being critical and it works great for a car audio amp.

I'm posting my design as an example because you can meet all of your needs with a similar design. I can draw up a schematic if you'd like to see the specifics. I hope this helps.
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File Type: jpg My 500W DC-DC.jpg (350.4 KB, 451 views)
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Old 10th June 2012, 04:12 AM   #13
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Wow! thanks you so much DCPreamp!

Answering your question, I'm using 50kHz switching frequency and for the output regulation I will use a opto (I didn't do it yet). I'm simulating it at LTspice to check that is working perfectly.

My principal problem is at the transformer, I don't know anything about the core material, cable thikness, primary and secundary inductances, series resistances, etc. So I'm a little bit scared about making the transformer by myself.
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Old 10th June 2012, 05:59 AM   #14
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For wire, pretty basic magnet wire, Litz wire (multi-strand wire), enameled wire, etc. will all do. It can be purchased from eBay, Digi-Key, Newerk, etc. Do some research on AWG of the wire, how paralleling strands handles more current, etc.

Next, for switching transformer cores, do some further research on their properties, losses, etc. But make sure to look at this guy: eBay My World - audiosector because he sells a number of different cores ideal for high-power switching transformers. For under $15 (if you're in the USA) you'll have a core good for 1KW. Like this one: 170799584797 (go to eBay and paste this number into a search box).

You still have some work to do, but that should eliminate a lot of it. Once you have the core & wire, actually wrapping the transformer is easy. Plus, a lot of the cores are pretty forgiving from 20 to 100KHz with the primary inductance only being a concern during simulation.
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Old 10th June 2012, 02:21 PM   #15
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And what about buying the transformer? it's a good idea or It's better to make it?
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Old 10th June 2012, 04:04 PM   #16
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I suggest winding your own over ETD44 or ETD49. You have to calculate the primary turns count basing and f and B, 0.15T at 100kHz or 0.2T at 50kHz are typical values.
Thickness- that's an easy one- choose the thickest that will fit into the winding window based on the bobbin and core dimensions, preferably several thinner wires in parallel.
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Old 10th June 2012, 06:42 PM   #17
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You'll have to make your transformer. It's highly unlikely you'll find an off-the-shelf piece that will meet all your requirements, and even of you did, there would be huge expense in minimum orders.

Where are you located? You can get everything easily in the USA, and pretty easy in most of Europe. Other, more distant locations, may be a problem.

But, again with the vendors I listed, plus some basic epoxy resin, you can wind your own custom, high-performance, switching transformer for under $40 (US). Don't get scared off by this proposal. Seriously, it's easy to do. a 2"/51mm toroid core, or an ETD39, ETD44, or ETD49 in a 3F3, 3C81, 3C85, N87, TSF7070 materials etc. will all work. Based on the winding ratios discussed earlier, wrap the windings on the core (look at my SMPS for an idea & Google for pics of transformers) counting the turns. The wire coating will prevent it from shorting when close together or overlapped. Once completed and verified, apply epoxy resin to keep the windings in place and to prevent them from getting worn because of vibration. Lastly, to solder to the magnet wire, you will need to use a blade to scrape the coating off the wire exposing copper.

You could also, if you live in some obscure corner of the world, or are totally broke, and with enough time, effort, and tenacity, tear apart a large PC power supply, remove the switching power transformer, and modify it for your needs. This is a lot of work, but is also a good learning experience on the proper ways to build transformers. You'd need a supply that meets your needs - a 650 to 1KW rating - and gut it. You'd probably need a couple of them because the likelihood of successfully unwinding a SMPS transformer without damaging it is slim. Check on this forum for how to reuse a transformer.

That's it! Really, only fear and doubt will stop you from building your supply!
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Old 10th June 2012, 10:57 PM   #18
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Ok, I'll try it whe I finish the design, thank you so much.

Now I'm triying to feedback, I'm doing something like the schematic of page 27 of this paper http://www.datasheetcatalog.org/data...cs/mXywysu.pdf but I don't have great results.
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Old 11th June 2012, 12:18 PM   #19
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I've found the problem!
I'm trying to use the SG3525 error amp as a differential amplifier, but it doesn't work, why is happening this?
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Old 12th June 2012, 03:23 AM   #20
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I've finished the feedback but.... I'm thinking about.... what about the input voltage compensation? the voltage of the battery will decrease with the time, it will be a good idea to compensate this variations also. Anybody knows about this?
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