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    Modifiying an offline LED driver PCB with EMC components on "flying wires"

    Hello,
    We have done an offline, 240VAC PFC’d Flyback LED driver. We have a slight failure of conducted emissions at 30MHz, as attached. In order to combat this, we wish to use two common mode chokes instead of just one like we have at the moment. That is, we wish to have a 10mH common mode choke, and a 500uH common mode choke in cascade…we believe that the smaller one will help attenuate our 30MHz problem.
    The problem is that we currently only have one PCB and the scan session is on Monday. The PCB is very densely populated. So anyway, we cant layout a new PCB in time. …What about if we de-solder the existing common mode choke, and then use twisted pair “flying wires to “jump” over to a bit of stripboard, on which we will mount the two replacement common mode chokes as discussed?
    Will this modified setup create so many other EMC problems that it won’t be a valid and worthwhile test?

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    Re: Modifiying an offline LED driver PCB with EMC components on "flying wires"

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Another point is that we would like to experiment with different values of diff mode inductors. As such, how good (or bad) a practice would it be to have these diff mode inductors coming out on flying wires so that we can change value quickly and take a fresh conducted mode scan?
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Also, suppose we depopulated the entire EMC filter of the offline SMPS….(the section upstream of the mains rectifier bridge), and then made one using the same components, but on a piece of stripboard, and then wired this stripboard into the PCB, (so it again had a filter) how much different would the scan of the top post be, when dont with the "replacement" stripboard filter?
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    I think you can see what I am getting at here…….the situation of needing to vary the EMC filter components and then re-scan.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Having to make a whole new PCB every time an EMC filter component is changed, and then re-EMC-scanning, is not terribly practical.
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Also, I have often seen offline SMPS EMC filters, with 22pF (or so) ceramic capacitors across the diff mode inductors. The purpose of these is to have a component that you can vary in order to reduce particular peaks on the conducted EMC scan. What do you make of this practice?
    Also, at one company, they had done a 150W Boost BCM PFC stage, and across the boost inductor, they had a ~100pF ceramic capacitor in series with a ferrite bead. When asked about it, they simply said it failed if that was not there, so they were leaving it in. This was at a very large successful multinational TV company.
    Last edited by treez; 27th July 2019 at 16:43.



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    Re: Modifiying an offline LED driver PCB with EMC components on "flying wires"

    Another point is that many isolated offline SMPS’s have Y Capacitors across the isolation barrier on both sides of the transformer. Do you agree that this acts to further reduce common mode emissions compared to having just one Y capacitor on one side of the transformer?



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    Re: Modifiying an offline LED driver PCB with EMC components on "flying wires"

    Not ideal but under the circumstances I think I would consider SHORT flying wires to a small protoboard so you can easily attach various filter components to try them. The wires themselves will obviously have some effect on EMC so they need to be as short as practical to reach the board.

    I'm not sure how two caps across the isolation barrier helps unless it is to do with the physical layout rather than capacitance per se.

    Brian.
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    Re: Modifiying an offline LED driver PCB with EMC components on "flying wires"

    Thanks, being able to wire in a proto AC filter board to the main pcb, in order to try and trial out different filter components is of immense value.......so i think we will do a trial where we compare the scan using the pcb's own AC filter, and compare it with another scan taken with the filter taken off , but put on an external protoboard, and then wired back into the main pcb.

    the main PCB is so densely populated that components simply cant be quickly wired in and out of it for emc scan trials.....we need it out on a separate more spacious proto board.

    ...Any ideas on how they will compare, and at what frequencys the differences will mostly occur?



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