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Galvanic isolation DC/DC

Discussion in 'General Electronics Discussion' started by johnny_cash, Aug 14, 2013.

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  1. johnny_cash

    johnny_cash

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    Aug 13, 2013
    Hello,

    I need to have a galvanic isolation between 2 supply lines. My input 'll be 5/40v DC, my output is 5v DC, but GNDs need to be different. The output will draw max 500mA.

    My first idea is to use a PCB transformer : http://www.murata-ps.com/data/magnetics/kmp_1605.pdf?sel=magnetics/kmp_1605.pdf then a voltage regulator BUT:
    - is such a transforme able to work from DC/DC ? in my thought, transformers were only usable in AC/AC

    I plan to use something similar to LTM8048: aka an LDO with isolation, is this a pretty good reference ?

    Thank you !
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    This transformer alone will not work. It requires AC.
    >ou want a DC/DC convert module similar to those here. But you'll have to dig a bit to find one suitable for 5V...40V input.
    The LTM8048 comes near, but does not quite fit the bill with Vinmax=32V
     
  3. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Most DC to DC converters are not isolated. You would need to find one that is, which will indeed use a transformer.

    Bob
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,513
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    Nov 17, 2011
    Traco, Recom, TDK to name only a few manufacturers all have isolated converters in their portfolio.
     
  5. johnny_cash

    johnny_cash

    39
    0
    Aug 13, 2013
    EXCELLENT Harald Kapp ! i found 2 suitables. Kudos !

    It is forbidden to chain DC/DCs ? Thank you !!!

    Notice 1st regulator is not isolated. Then i could use Vout1+ and Vout1- as VDD and VSS, right ? :)
     

    Attached Files:

  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    No, but remember that your losses multiply...

    If all the power supplies are 85% efficient, then for each watt out, you need 1/0.85 W in to a single stage DC-DC, but (1/0.85)*(1/0.85) if you have 2 stages. (That's 1.18 vs. 1.38)
     
  7. johnny_cash

    johnny_cash

    39
    0
    Aug 13, 2013
    Hello, i finally choose this 5VDC to 3.3VDC isolated converter: MEE1S0503C (DS: http://www.murata-ps.com/data/power/ncl/kdc_mee1.pdf )

    Now i have a trouble:

    1) no application notes. Does it mean this DC/DC is not requiring side components (such as Vout 10uF capa)
    2) there is a "minimum load" chapter in the DS (top of page 5). Is a 74AHCT126 a correct load or it is totally not suitable ?

    Thank you !
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Well, a single 74AHCT126 is unlikely to provide a load large enough to satisfy the minimum load, so the output voltage could rise to just over 4V.

    Considering this device is specced for a power supply between 2V and 5V, it's not a worry. Is there anything else that demands a voltage lower than 4V?
     
  9. johnny_cash

    johnny_cash

    39
    0
    Aug 13, 2013
    no, thank you for the answer. I simply want to display this line logical status (with resistor + led 7mA) without perturbing the input.
     
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