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Thread: PSU 12V issue

  1. #1
    Low Bitrate
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    Mar 2002
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    Portugal - Cacém
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    PSU 12V issue

    After reading many posts here in the board i see that one of the main problems in making a power supply is to get the 12V line to give more power, well, how about the L296 Switching regulator from St (www.st.com) it can put out 4A with a Voltage input of 9 to 46V ( i know 9 is a bit high but with a tank circuit i think it would hold up when crancking up the engine). Currently i have (not yet but almost) a PSU that has:


    3.3V@10A
    12V@8A
    -12V@1A
    -5V@1A
    the 5 volt line has 30A but it also feeds the 12V, -12V and -5V line so
    (5*30) - ((12*8)+(12*1)+(5*1), this leaves 37W in the 5 volt line, 37W/5V thats about 7.4A remaining in the 5V line, but since the 12V line isn't always pulling 8A (nor does the negative ones pulling out 1A each) there is always a bit more than just 7.4A in the 5V line.

    You have to admit, these are pretty good specs for a home-made power supply. But my problem is SIZE, this power supply is 20cm by 10cm (figure out the inches), it is all MAXIM based, so you can imagine the enourmous number of choke's, schootky diodes, MOSFET, that i'm using.

    Thats when i got the idea of looking inside of a normal PCs PSU and i saw that many used ST chips, so i went to their website and found the L296 chip ( http://us.st.com/stonline/books/pdf/docs/1333.pdf ),if i use this chip i don't have to use 5V line to power the 12V one, i could just put the 5V line giving out 13.4A (67W), in order that 12W would go to the -12 line and 5W would go to the -5V line the leaves 50W in the line so 50W/5V = 10A, so the specs would be:

    5V@10A
    3.3V@10A
    -5V@1A
    -12V@1A
    12V@ at least 4A (could put two chips in paralell to have 8A).

    At the best of all, i could reduce DRASTICALLY the size of my PSU

    Tell me what you think, is it doable and does ST give out free samples?

    Sorry for the long post.
    Shades

  2. #2
    Raw Wave Rob Withey's Avatar
    Join Date
    Apr 2000
    Location
    Bedfordshire, UK
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    2,139
    I have been using the L296 for a long time for the 5v stage of my power supply. This regulator (afaik) is a step down switcher though. The datasheet says that the input voltage should be designed to fall about 4v above the output voltage. You might find this tricky when the output is 12v and the input is a car battery.

    The L296 is not difficult to find to buy. I would advise against not being a freeloader. Getting free "samples" just ****es the suppliers off (unless of course you are planning to actually produce a legitimate product).


    Rob
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

  3. #3
    Low Bitrate
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    Mar 2002
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    Portugal - Cacém
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    95

    Thanks

    Thanks Rob,
    Well, back on using the GIANT one.

    About the freeloader thing, don't take me wrong, it's not that i don't have the cash but to get, let's say, 3 of these chips, i'd have to pay the chips (no problem) but then i'd have to spend about the double or triple in shiping charges, not including of course customs, here in Portugal all they want is money money money, so if a package doesn't come from whitin the European Union, you're toasted. That's why i wanted the free sample.
    Shades

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