Switching regulator filter

Discussion in 'General Electronics Chat' started by Highlight, Jun 20, 2012.

  1. Highlight

    Thread Starter New Member

    Nov 26, 2010
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    Hi, I am studying a switching regulator circuit that steps down the 24V input to 5V. There is "filter" at the regulators input which i circled. Can anyone explain the purpose of this "filter" and why it is needed? I looked at the regulators schematic in datasheet and the filter is not there so im confused if its really needed. thanks.
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    It's to smooth out & reduce the current pulses switching into the switcher's input to contain the noise to the switcher itself.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    It also is there to stop EMI from being conducted back out the input lines to affect other circuits.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    A switching regulator typically draws square pulses of current form the input source voltage. If this is not supplied from a low impedance filter at the input, then these pulses will cause voltage spiking in the circuit due to stray circuit inductance and EMI due to the high frequency current pulse edges in the input wires to the raw DC power. Thus you need a low impedance filter at the input with short fat leads as close to the switching circuits as physically possible.
     
  5. Highlight

    Thread Starter New Member

    Nov 26, 2010
    28
    0
    If a linear regulator is the input to the switching regulator do i still need an EMI filter? My understanding is that the EMI is to keep the mains supply less nisy.
     
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