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+/-15VDC supply from 48VDC

Curt Gibson
Prodigy 190 points
Other Parts Discussed in Thread: LMZ35003, LMZ34002, LM5576

I am hoping to make a 48VDC to +/-15VDC power supply. The LMZ34003 does not say whether it is isolated. Is there some way to us use two of them, one for + 15 and the other for the -15? 

  • 0
    TI__Genius 11085 points

    I believe you meant LMZ35003.

    The LMZ35003 is not isolated. For the 48V to +15V, you can use this part.

    For the 48V to -15V, you need to find a buck converter with max Vin rating > (48+15) = 63V, then config it as inverting buck boost.

    regards,

    Yang

  • 0 in reply to YangZhang
    Prodigy 190 points

    Thank you Yang,

    Yes LMZ35003 is what I meant. You answered my question.

    Could you tell me the most common way for me to achieve +/- 15 - 25 VDC, 90 watts on a circuit board where I have just a 48V source? I am considering:

    1)LMZ34002 fed from the output of the LMZ35003?

    2) Virtual ground half way between the 48V source, which looks like it woudl work for +/- 24, but I tried that and the voltage varied. Maybe I am doing something wrong, or it is just a bad design. Not sure.

    2) Two isolated PTB48510CAH Power modules one providing +15 the other flipped to provide -15.

  • 0 in reply to Curt Gibson
    TI__Genius 10095 points

    Hi Curt,

     

    If you go with option 1. 

    The LMZ34002 fed from the LMZ35003, you will be supplying all of the current for the -15V rail from the +15.  The issue with this is that if you have large negative currents you lower the amount of available power from the positive rail.

    Instead of this you might want to use the LM5576 in an inverting configuration to create -15 from 48V.  It is not quite as simple to use as the modue but is rated for higher voltage.  The following app note explains how to create an inverting convfiguration out of a buck module but the concept is the same for any buck.

    www.ti.com/lit/an/snva425a/snva425a.pdf
     
    2)  In the inverting application above, the ground for the inverting application is referenced to the -15V so you would need to level shift enable signals to turn off the parts.  The circuitry for this is covered in the app note.
     
    3)  Two isolated modules would work but when I looked up this part it was listed under not recommended for new designs.  Not sure why.
     
    Regards,
    Marc
     
  • 0 in reply to Marc Davis-Marsh
    Prodigy 190 points

    Marc,

    I choose the LM5576. It seems perfect. 

    I appreciate your wisdom of using the LM5576. For the quantities I am thinking, and the inventories that I will need, this fits perfectly. But the problem is that I have no way to do any R&D with this chip, or any other little chip ( I cannot solder, or even see those little pins). I need to be able to make a full working circuit on a 0.100 X 0.100 protoboard. Yes you offer evaluation boards, but they are not my configuration. I need the chip you suggested soldered on to a board that I can work with it.

    If you could do me and many others a huge favor and relay to your product managers that if they were to simply offer chips soldered to boards that can be worked with on standard protoboards, they would dramatically increase adoption and market acceptance of the product. It is not big deal to go to a contract board manufacturer and ask them to provide TI with lets say 1000 of a chip soldered to a small circuitboard where the pins are broken out to a 0.100" pitch pins that slide into a protoboard. This may cost about a dollar per part. .

    If you have these boards, or some other work around, please let me know. Until then your advice is appreciated, but unable to be implemented. I think you know that I am saying this in a tone of appreciation for your product, but constructive criticism related to how the engineer works wit you.

    If you are not interested in providing these boards, I might even be interested in doing this and offering the chips on Ebay. But why would i do that? It should be my friends at TI to do that.

    Sincerely, and in a positive tone,
    Curt Gibson

    (209) 483-3910

  • 0 in reply to Curt Gibson
    Prodigy 190 points

    Hi, 

    Could someone from TI reply to my prior post? I am curious if I am missing something.

    Curt