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Thread: Need advice on common mode choke to filter out noise

  1. #1

    Need advice on common mode choke to filter out noise

    Hi All

    This is my first post here. I design my own hifi power amp and I want to use switching power supplies for +/-12V and +5V. Attached is the rough schematic. I wrote the current requirement of each supply which is 1A, 0.2A and 0.2A for +5, +12 and -12 resp. I am using 3 individual supply, I cannot find a triple supply that is thin enough to fit into the amp as space are very tight ( really too bad, I only have 1.2" gap, most are 1.4" or so thick). I am buying these on ebay from China, there is no spec with frequency they are running. These are what I am going to order:

    https://www.ebay.com/itm/10W-5V-Ultr...19.m1438.l2649

    https://www.ebay.com/itm/10W-12V-1A-...19.m1438.l2649

    I assume the frequency is about 50KHz.

    Also, I need to get one for the AC input also.

    Please advice what value I need to buy. Also, what range of values for those caps.

    Thanks

    Alan
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    Last edited by Alan0354; 05-24-2018 at 10:54 PM.

  2. #2
    .... Silvio's Avatar
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    The power supply you need looks like the ones they use for DVD players. It is a pity that you are going to buy 3 they will be very cumbersome. The voltages and current needed can be housed in one supply if wanted, Well if you are in a hurry then here is another alternative for you.

    Check these links

    https://www.aliexpress.com/item/5-pc...27424c4dkjVOdj

    https://www.aliexpress.com/item/SANM...ceBeautifyAB=0



    Regarding switching frequency they are rather in the 100khz mark.

    As far as the input and output filtering well the ones I suggested may not have but do not bother too much especially in the input as these draw very little current. Well if you like to make one then the total input current can be easily measured (one filter for 3 supplies) and a common mode choke that can handle the current can be chosen. regarding capacitor values for the input this has to be chosen according to the switching frequency of the smps.

    Output filter Well here you can add a further small inductor (47uH) and electrolytic capacitor. You can experiment a little and see what ripple you actually have with and without load. Be careful do not exaggerate with the output cap as this will present a dead short on start up and may blow your chip. At the current needed 100uf and 0.1uf ceramic will suffice

    I hope that helps

    Silvio

  3. #3
    Thanks Silvio

    I prefer supplies that is encased to block the magnetic field from spraying out. Just too bad I can't find a triple output power supply that has +/-12V and +5V. Not even +/-15V. Ebay used to have a lot of small tripled voltage supplies, I even have one tiny one. But I just cannot find it anymore.

    I'll come back with the coke and inductor selection, I am still reading about this, I never use these before.

    Thanks

  4. #4
    Hi

    This is my schematic. For this version, I use a single Triple voltage +15, -15 and +5V supply. I use two common mode choke, 1mH 0.3ohm is for low current +15 and -15V. I use the 500uH 0.11ohm resistance for the +5V.

    I use the same 1mH for input 115VAC as shown.

    I connect the 1mH as inductor as shown for +/-15V. The parts I used are:

    https://www.digikey.com/scripts/DkSe...35705906434693 1mH for low current.

    https://www.digikey.com/scripts/DkSe...35712571292432 500uH for +5V that use more current.

    Please take a look and comment

    Thanks

    Alan
    Attached Images Attached Images

  5. #5
    Hi

    This is my schematic. For this version, I use a single Triple voltage +15, -15 and +5V supply. I use two common mode choke, 1mH 0.3ohm is for low current +15 and -15V. I use the 500uH 0.11ohm resistance for the +5V.

    I connect the 1mH as inductor as shown for +/-15V as 2mH inductor. The parts I used are:

    https://www.digikey.com/scripts/DkSe...35705906434693 1mH for low current.

    https://www.digikey.com/scripts/DkSe...35712571292432 500uH for +5V that use more current.

    Please take a look and comment

    Thanks

    Alan
    Attached Images Attached Images

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