HV cap in EMC filtering for DC to DC

Discussion in 'General Electronics Chat' started by BrianH, May 19, 2011.

  1. BrianH

    Thread Starter Active Member

    Mar 21, 2007
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    Hi everyone,

    I am designing a DC to DC converter into a power supply circuit I'm working on. In the datasheet for the component it recommends some EMC filtering circuitry, please see attached.

    Can anyone tell me what is the function of the high voltage 470pF capacitor connected between +Vin and COM out?

    I am planning to connect the COM output and -VIN input together. This is because I do not require the electrical isolation that a DC to DC converter supplies, I only need it to generate a low current 15V dual rail supply from a 5V input. The 15V dual rail supply will feed some op-amp signal conditioning circuitry, and I want the output of this circuitry to be with reference to system ground, hence why I want to connect COM to -VIN.



    If anyone with more experience using these devices wants to tell me I'm going about it the wrong way and offer advice, please feel free it will be very welcome!

    Cheers,

    Brian
     
    Last edited: May 19, 2011
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    282
    That's the function of the 470pF cap.
    That will create a short and destroy the device.
    Tying the converter common to the circuit common will do what you want, unless your conditioning circuitry is something other than what I imagine. Can you post the schematic?
     
  3. BrianH

    Thread Starter Active Member

    Mar 21, 2007
    43
    0
    Hi beenthere, thanks for the response.

    Tying the COM output to -Vin will not "destroy the device". I think we have a misunderstanding here. The input to this device is +5V and GND. GND connects to the -Vin pin. So by tying the COM output to it, I am connecting the converter ground to my system ground. I lose my isolation that way, but I don't require electrical isolation so that's okay.

    Regarding the cap, yes I understand that it's part of the EMC filtering. My question is, in what way does this capacitor contribute to EMC filtering? And why does it need to be such high voltage?
    I'm not questioning the validity of the design, I am asking purely from a self-interest point of view.

    Thanks,

    Brian
     
  4. BrianH

    Thread Starter Active Member

    Mar 21, 2007
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    I had an inkling that this cap had something to do with the isolating properties of the circuit, because I've seen these kinds of capacitors connected between electrically isolated circuits before. This is the main reason why I'm asking about it, because by connecting the COM output to my system ground, I've effectively nullified the electrical isolation anyway, so I was wondering what purpose this capacitor would serve anymore...

    I confess that I don't fully understand what the capacitor is for in the first place, and hence my need to ask on here.

    Cheers,

    Brian
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Confused -Vin with -Vout. Not the same.

    As to filtering, the data sheet should have an explanation.
     
  6. BrianH

    Thread Starter Active Member

    Mar 21, 2007
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    Unfortunately the datasheet does not provide further information. It offers the EMC filter circuit only as an application guide, so they don't provide any further details of its operation.

    Thanks,

    Brian
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    The small filter capacitor effectively bridges the output isolation, so if the full converter voltage isolation rating is to be utilised, the capacitor must be rated for at least that value, with possibly some added margin.

    As to its purpose, I would guess that it helps reduce common-mode switching noise voltage which would otherwise turn up on the floating output due to coupling via parasitic capacitances. I am a little surprised that the capacitor returns to +in but perhaps someone better informed will come up with a better explanation?
     
    Last edited: May 19, 2011
    BrianH likes this.
  8. BrianH

    Thread Starter Active Member

    Mar 21, 2007
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    0
    Aha right, of course. So the reason a 4KV capacitor is required is because otherwise the output isolation is being compromised - that makes perfect sense.

    I still don't fully understand what the function of the capacitor is though.

    Thanks for your advice.

    Brian
     
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