+ Post New Thread
Results 1 to 6 of 6
  1. #1
    Advanced Member level 5
    Points: 30,455, Level: 42
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    6,367
    Helped
    467 / 467
    Points
    30,455
    Level
    42

    60W Offline Flyback LED driver fails Conducted and Radiated EMC

    Hello,
    We recently took our 60W Offine , isolated, PFC’d Flyback LED driver (schematic attached) for Conducted and Radiated EMC scans.
    The conducted EMC scan fails at a few spot frequencies as the attached shows.
    We also failed Domestic Radiated emissions (CISPR F). –But we passed on Industrial Radiated emissions (CISPR B).


    The problem is, that we have no more room on the PCB for any more filter components. As it was, we had to use SMD Y2 capacitors on the bottom of the board because there is no room on the PCB for radial ones.
    I believe we need a second common mode choke, but there is no room on the board.
    Are there any filter tricks we can pull off here?...unfortunately we aren’t allowed to make the board any bigger.

    The ceramic Y2 caps were 3.3nF….but as you know, ceramic X7R capacitors are unfortunately only around 10% of nominal value when operated at rated voltage.

    The 60W Flyback’s FET and Diode had small TO220 heatsinks directly connected to them..The FET heatsink is then obviously connected to the drain tab of the FET.…..however, do you agree that this would have made the Radiated scan result worse? (…I believe we should have had an insulating pad between FET heatsink and the FET itself, do you agree?)

    During the scans, we had a 60W LED load that we “hacked” together connected……Unfortunately, due to the “hacked nature”, much of the wiring in the LED load bank was not twisted and so there were wide area current loops. The LEDs were on bits of MCPCB, and these were not connected to Earth. The secondary 0V was just “floating”.


    (We passed Mains Harmonic Emissions and Power Factor)


    VJ2220 SMD X2 and Y2 capacitors
    https://www.vishay.com/docs/45020/vjsafetycapsx7r.pdf

    ELC09D inductors
    www.farnell.com/datasheets/2245397.pdf

    Wurth 7448640415 Common Mode Choke
    http://katalog.we-online.com/pbs/dat...7448640415.pdf

    •   AltAdvertisment

        
       

  2. #2
    Super Moderator
    Points: 66,847, Level: 63
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    13,644
    Helped
    3122 / 3122
    Points
    66,847
    Level
    63

    Re: 60W Offline Flyback LED driver fails Conducted and Radiated EMC

    Hi,

    Are there any filter tricks we can pull off here?
    Yes, a proper PCB layout.
    The right order and the placemant of the components, a slot in the GND plane may improve EMI signifcantly without additinal parts and without additional board space.

    Check your noisy signals in combination with their return paths.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.


    1 members found this post helpful.

    •   AltAdvertisment

        
       

  3. #3
    Advanced Member level 5
    Points: 30,455, Level: 42
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    6,367
    Helped
    467 / 467
    Points
    30,455
    Level
    42

    Re: 60W Offline Flyback LED driver fails Conducted and Radiated EMC

    Thanks, i did make all power switching current loops as low in area as possible, and was sure not to have power switching current flowing through lengths of control ground...also, we kept switching nodes as minimal net size as possible, and away from sensitive tracks and components. So i think there's very little extra we can do in terms of the PCB layout.
    I suppose we could put in some 10nF ceramics right up close in to the switching loops to make them even lower in area....but there really isn't that much extra room on this PCB.

    As far as Radiated emissions are concerned i suppose we could look into Shield windings in the transformers, and flux bands around the transformers.

    I also wondered if we should have "cheated" and used a little voltage regulator based dummy load…so that there weren’t wide area current loops in the load….this would have worsened our radiated scan I believe?
    Last edited by treez; 25th August 2018 at 15:04.



    •   AltAdvertisment

        
       

  4. #4
    Advanced Member level 4
    Points: 7,220, Level: 20

    Join Date
    Aug 2015
    Posts
    1,125
    Helped
    432 / 432
    Points
    7,220
    Level
    20

    Re: 60W Offline Flyback LED driver fails Conducted and Radiated EMC

    The 10n cap C76 forms a 100kHz resonant ckt with the 220uH choke - so make it 220nF like the others...

    The CM choke has a broad hi-Z at 300kHz - 1Mhz so well chosen.

    Your snubber should be more aggressive on D8 - this will help with radiated emissions, a small snubber across your main switch will not hurt either.

    From the graphs on the data sheet fr the CM coke the winding c = 8.2pF per wdg, thus the DM value of the choke (leakage) = 15.8uH per side

    Thus you have a resonance at 191kHz with the 22n cap C23, and a further res of 731kHz with your Y caps ( 3n3)

    the CM resonance is 10mH and 3n3 = 27.7kHz - and there is a rise on the graph around there....

    Put a damping resistor 10E in series with C3.

    good luck ...


    1 members found this post helpful.

  5. #5
    Advanced Member level 5
    Points: 30,455, Level: 42
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    6,367
    Helped
    467 / 467
    Points
    30,455
    Level
    42

    Re: 60W Offline Flyback LED driver fails Conducted and Radiated EMC

    Thanks, woops ive realised i drew it incorrectly above with both Y caps connecting to live...(it wasnt like this in the real circuit).
    The corrected schem is as in the attached.
    (I believe most people would have seen it as Y caps connected to Live and Neutral anyway)



    •   AltAdvertisment

        
       

  6. #6
    Advanced Member level 5
    Points: 30,455, Level: 42
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    6,367
    Helped
    467 / 467
    Points
    30,455
    Level
    42

    Re: 60W Offline Flyback LED driver fails Conducted and Radiated EMC

    Thanks Easy Peasy. As you kindly discuss, we intend to increase those X2 capacitors when we have more room on the next board revision. Also, we will add the 10R damping resistor and snub the secondary diode more heavily.

    In the meantime, we did another quick Conducted EMC scan, this time with the attached EMC filter. This performed much worse than the first EMC filter (shown in the top post). This worse conducted EMC scan (on 8th November) is also shown attached here. (the better scan of 22nd August is also attached here, for ease of reference)
    We believe that the worsening is due to the fact that we no longer have a Y capacitor from primary to secondary of the 500mW bias supply transformer. We also believe that the change to using Radial Y capacitors in the AC part of the filter, as opposed to SMD ones as used in the previous scan, has also had an impact. –This being due to the increased stray inductance of the radial Y capacitors.
    Also, in the worse scan, our LED load was sitting on a much bigger heatsink. Also, in the worse scan, the Power NFET was screwed to a bigger metal heatsink. (the tab of the FET being the switching node means that this made EMC worse).
    So do you agree that we need to have Y capacitors between primary and secondary of both transformers in order to improve things?

    I have also attached the (better) previous scan of 22nd August here. (Also the EMC filter schematic is also atttached)



--[[ ]]--