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Current and switching PS

Discussion in 'General Electronics Discussion' started by tempus, Mar 2, 2013.

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  1. tempus

    tempus

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    Feb 24, 2013
    Hey all;

    Suppose you have a switching supply like this one:

    http://www.suntekstore.com/goods-14002072-lm2596_dc-dc_step-down_adjustable_power_supply_module.html

    It's rated for 2A output. Does all the previous circuitry have to be rated for 2A as well? IOW, if you were powering this from a wall outlet, would the stepdown transformer also have to be rated at 2A, along with the rectification circuit?

    I assume the answer is yes, until I look at some other PS units. I've got a wall wart that supplies 1700mA at 9v, and there's no way there's a transformer big enough in there to deliver that kind of power. Or what about computer PS? I've cracked open a few of these, and I can't imagine that the dinky little transformers in there can source the 18A or whatever the supply demands.

    How are they getting away with using these small transformers?

    Thanks
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The size of the transformer is a function of the frequency. A typical mains operated transformer therefore is rather big. A switch mode power supply operates at much higher frequencies (>50kHz). The transformer therefore can be proportionally smaller.
     
  3. tempus

    tempus

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    Feb 24, 2013
    Thanks for your reply Harald.

    Does that mean that because the switching supply is operating at a high frequency the mains transformer can also be much smaller for the same current? I suppose the primary would be operating at 50/60Hz, and the secondary at say, 150KHz. Wouldn't the transformer still need to be physically large to source that much current? Also, how would you calculate the size of transformer needed based on the switching frequency? I'm picturing something like this:

    Transformer rated at 500mA, 12V out 50/60Hz.
    same transformer rated at 2000mA, 12V out at 150KHz.

    Is that kind of the idea?

    Thanks again
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    No, at 150kHz the transformer will be MUCH smaller for a given power
     
  5. tempus

    tempus

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    0
    Feb 24, 2013
    Thanks for your reply Steve.

    So is the reverse also true, i.e., will the same size transformer source a much greater current?

    Let me try to rephrase my question. If I have a transformer rated for 500mA at 12V, and I used the DC-DC converter linked in my original post, could I actually have higher current capability because of the high switching frequency? I'd like to try this test, but I want to make sure I'm not going to damage anything first. I've actually got a wall wart with the above specs, which has a 12VDC output. If I connect it to the DC-DC converter, will I now be able to source 2A instead of 500mA?

    Just to clarify, I understand that while transformers may be designed for higher or lower frequencies, they don't actually have any frequency. I guess where I'm getting lost is that in either case, the mains frequency is still 60Hz. I'm not seeing how changing the switching frequency on the secondary side would affect current handling of the transformer overall, especially if it's been rectified to DC already, as is the case in my scenario.

    I'm grateful for any clarification.

    Thanks
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    No, you need t use a different transformer at this much higher frequency.

    Even if you could use the same transformer, the current would not be increased significantly (the wire used was chosen for the current the transformer was rated for).

    The main issue is the iron core, It will not operate at the higher frequency without huge losses.
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Now how would that be possible? The input and output to a transformer both are at the same frequency. Read this explanation of the workings of a SMPS.
     
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