Power Supplies

Discussion in 'General Electronics Chat' started by mlkcampion, Nov 18, 2006.

  1. mlkcampion

    Thread Starter Active Member

    Nov 16, 2006
    Hey Forum

    If i hace a DC/DC converter and i am drawing 7amps at 24vDC on the
    output, ignoring the effeciency of the device with 300vDC at the input,
    how do i calculate the current that is been drawn by the DC/DC Converter.

    Another type of Power converter i came across recently, i was drawing between
    7-8amps at 50vDC on the output, but when i measured at the input to the power supply it was drawing over 10amps for 230vAC. Should you not
    be drawn less?

    How do you calculate these for such power supplies (switch mode)

    Cheers :)
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    It is generally the case that a DC/DC switcher draws slightly more power from the input power source than it delivers to the load. The factor "slightly" depends on how efficient your DC-to-DC converter happens to be under the specific load conditions to which it is being subjected.

    You have stated "ignoring the efficiency" in your post, so it is safe to say that the power drawn from the input power source will be no less that the power consumed by the load.

    Maybe someone else can address this issue.

  3. n9352527

    AAC Fanatic!

    Oct 14, 2005
    You just equate the power rating. According to energy conservation law:

    Pout = Pin - Pwasted

    In your example above, ignoring the efficiency:

    7A * 24V = Iin * 300V - 0W
    Iin = 0.56A

    If you draw 8A at 50V that's 400W at the output, while the input power is 10A * 230V = 2300W. You are missing 1900W! That's a space heater power :D If you couldn't feel any significant heat coming out of the converter or didn't see any smoke, then I think there's just something overlooked during the measurements. A more likely input power rating, assuming about 85% efficiency is around 470W.
  4. Gadget

    Distinguished Member

    Jan 10, 2006
    85% is about the norm from what I understand as well.
  5. pppptang

    New Member

    Nov 20, 2006
    You can check whether the power supply EMI effecting to you testing equipments.
  6. mlkcampion

    Thread Starter Active Member

    Nov 16, 2006
    Hey thanks to everybody for their replies, much appreciated and helpfull.
    Actually N9352527, there was a nice bit of heat coming of the power supply, also it was wired with .75 flex at the input which got warm. I can't understand where all the power was going, i mean some of the tracks on the board i thought surely couldn't handle that sort of current.
  7. n9352527

    AAC Fanatic!

    Oct 14, 2005
    Well... what kind of supply is it? How big is the supply? And how warm? Did it feel like a '1.9KW of power' warm?

    As a rough comparison, I have a 2KW kettle that boils close to 2 litres of water in just a few minutes. 1.9KW creates quite a significant amount of heat.

    I tend to think that there's something amiss during measurements instead. Probably interference to your instruments, using cheap digital meters (which are known to have issues with SMPS measurements), or the effect of not using true RMS meters when the quantities are not perfectly sinewaves and there are high harmonics contents, etc.