+ Post New Thread
Results 1 to 19 of 19
  1. #1
    Full Member level 3
    Points: 2,195, Level: 10
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    153
    Helped
    5 / 5
    Points
    2,195
    Level
    10

    Ringing on PSFB secondary switches

    Dear All,

    I have designed a PSFB DC-DC converter designed for with input voltage range of 105 to 135 V and output voltage range of 48 V to 90 V with power rated at 750W. The transformer turns ratio is 1:1.5:1.5 (Centre tapped). The resonant inductor value is calculated at 4uH. However when I am testing , even at 30V input and 20 V output heavy ringing is observed at secondary side reaching upto 350V peak. I am attaching the waveforms for your reference. I have put a snubber of 1nf and 50ohms across output switches. What should be done for rectifying this ringing issue.

    https://imgur.com/hvR5g4n
    https://imgur.com/41pA6Nn

    The first image shows Transformer current and Voltage across switches in one leg of primary. The second image shows secondary Transformer current and voltage across switches in secondary. Please let me know what else can be done to avoid ringing.

  2. #2
    Advanced Member level 5
    Points: 20,393, Level: 34
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    3,352
    Helped
    1192 / 1192
    Points
    20,393
    Level
    34

    Re: Ringing on PSFB secondary switches

    Post your full schematic, with component values. 4uH is very small for a resonant inductor. Do you know what the transformer's leakage inductance is?



    •   AltAdvertisment

        
       

  3. #3
    Full Member level 3
    Points: 2,195, Level: 10
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    153
    Helped
    5 / 5
    Points
    2,195
    Level
    10

    Re: Ringing on PSFB secondary switches

    Thanks Mtweig for the reply,

    The transformers leakage inductance is 0.85micro Henry at primary side. Since Primary current is around 10 -15 Amps, based on the output capacitance of MOSFET IRF4768PBF, I am getting calculated value of 4 uH. The magnetizing inductance is 376uH when seen from primary. I have attached schematic.
    https://imgur.com/NlDBdrs



  4. #4
    Advanced Member level 5
    Points: 20,393, Level: 34
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    3,352
    Helped
    1192 / 1192
    Points
    20,393
    Level
    34

    Re: Ringing on PSFB secondary switches

    At max load, yes 4uH should be enough to provide ZVS on the FETs. Are you testing at max load?

    Are you actually using that capacitor in series with the primary? It might be ringing with your leakage inductance...



  5. #5
    Super Moderator
    Points: 255,114, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    44,445
    Helped
    13540 / 13540
    Points
    255,114
    Level
    100

    Re: Ringing on PSFB secondary switches

    I wonder if the primary voltage spikes are measurement artefacts (bad probe circuit) or real (bad layout).



  6. #6
    Full Member level 3
    Points: 2,195, Level: 10
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    153
    Helped
    5 / 5
    Points
    2,195
    Level
    10

    Re: Ringing on PSFB secondary switches

    Hi mtweig,

    The capacitor was used to prevent saturation of Transformer, due to imbalance. It was mentioned in application note. I am not able to test at desired power level and voltage level due to the severe spikes. I am also wondering whether PSFB is better than Full Bridge with SiC Devices. Please suggest how should trouble shooting be approached. The snubber (secondary side) is also getting warm quickly. Is any snubber required at primary side.

    - - - Updated - - -

    Quote Originally Posted by FvM View Post
    I wonder if the primary voltage spikes are measurement artefacts (bad probe circuit) or real (bad layout).
    Hi FvM,

    The secondary spikes are sever, come upto 380V when output voltage is only 20 V. The snubber is also getting warm. Is PSFB advantageous over FB when turns ratio is nearly 1. I have seen that most application notes use PSFB for 400V to 12 V step down or 400V to 48 V step down. My application requires (105 to 120V input to 48 to 90V). Are there any other alternative topology for 750W. How about full bridge with SiC switches and synchronous rectification on secondary.

    - - - Updated - - -

    The second image shows Transformer current and Voltage across switches in one leg of primary. The first image shows secondary Transformer current and voltage across switches in secondary. Please let me know what else can be done to avoid ringing.[/QUOTE]



    •   AltAdvertisment

        
       

  7. #7
    Full Member level 3
    Points: 2,195, Level: 10
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    153
    Helped
    5 / 5
    Points
    2,195
    Level
    10

    Re: Ringing on PSFB secondary switches

    Hi all,

    Any suggestions regarding ringing in PSFB. Does magnetizing inductance play a role in the ringing. Say we if we have a Lm of 300uH as compared to 1mH, what could be the effect.



  8. #8
    Advanced Member level 5
    Points: 20,393, Level: 34
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    3,352
    Helped
    1192 / 1192
    Points
    20,393
    Level
    34

    Re: Ringing on PSFB secondary switches

    Quote Originally Posted by sabu31 View Post
    Hi mtweig,

    The capacitor was used to prevent saturation of Transformer, due to imbalance.
    Typically the PSFB is done with current mode control, in which case this capacitor is not needed...
    I am also wondering whether PSFB is better than Full Bridge with SiC Devices. Please suggest how should trouble shooting be approached. The snubber (secondary side) is also getting warm quickly. Is any snubber required at primary side.
    I wouldn't expect a hard switched full bridge to outperform a PSFB in an equivalent application. Though the hard switched bridge may be easier for you debug.

    The secondary spikes are sever, come upto 380V when output voltage is only 20 V.
    380V spikes on 20V pulses would indicate some major issue with parasitic inductance and/or improper driving of the FETs. But the waveforms you posted don't show this, at least not on the secondary side. In fact the secondary waveforms (pink and green) suggest that the synchronous rectifiers aren't being controlled properly, and that their body diodes aren't even conducting. Putting a light load on the output might fix that...



  9. #9
    Super Moderator
    Points: 255,114, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    44,445
    Helped
    13540 / 13540
    Points
    255,114
    Level
    100

    Re: Ringing on PSFB secondary switches

    The secondary spikes are sever, come upto 380V when output voltage is only 20 V.
    You didn't yet post secondary waveforms with 380V ringing. Where do you see it? How is it exactly measured?



    •   AltAdvertisment

        
       

  10. #10
    Full Member level 3
    Points: 2,195, Level: 10
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    153
    Helped
    5 / 5
    Points
    2,195
    Level
    10

    Re: Ringing on PSFB secondary switches

    Quote Originally Posted by FvM View Post
    You didn't yet post secondary waveforms with 380V ringing. Where do you see it? How is it exactly measured?
    Hi FVM,

    I am attaching secondary waveforms and transformer current in secondary

    Click image for larger version. 

Name:	hvR5g4n.jpg 
Views:	4 
Size:	64.1 KB 
ID:	152594

    [found at: i. imgur. com/ hvR5g4n. jpg]

    Its measured using a differential voltage probe across switch legs. Also what do you thing of suitability of PSFB vs FB with SiC when Turns ratio of transformer is nearly unity with (input voltage 105 to 120 and output nearly 48 to 90) . In case of PSFB i need to use Np:Ns upto 1:2 due to duty cycle loss.
    Last edited by BradtheRad; 25th April 2019 at 21:26. Reason: Image converted to Edaboard attachment



  11. #11
    Advanced Member level 4
    Points: 8,301, Level: 21

    Join Date
    Aug 2015
    Posts
    1,314
    Helped
    492 / 492
    Points
    8,301
    Level
    21

    Re: Ringing on PSFB secondary switches

    How are you achieving the half turn on the secondaries? if not done right this can introduce significant leakage on the sec side and cause the ringing

    Check that the ringing observed is not simply CM effect, put the probe and the gnd lead on the same point and see if you get a flat line of pick up the "ringing"

    - - - Updated - - -

    what is the reverse recovery of your output fets? if they are not turned off soon enough - you will get these problems ...



  12. #12
    Full Member level 3
    Points: 2,195, Level: 10
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    153
    Helped
    5 / 5
    Points
    2,195
    Level
    10

    Re: Ringing on PSFB secondary switches

    Quote Originally Posted by Easy peasy View Post
    How are you achieving the half turn on the secondaries? if not done right this can introduce significant leakage on the sec side and cause the ringing

    Check that the ringing observed is not simply CM effect, put the probe and the gnd lead on the same point and see if you get a flat line of pick up the "ringing"

    - - - Updated - - -

    what is the reverse recovery of your output fets? if they are not turned off soon enough - you will get these problems ...
    HI Easy peasy,

    I am using primary turns of 7 and secondary turns of 11 and 11 (Centre Tap with each winding 11). Which gives approximately 1:1.5 turns ratio.
    The secondary diode reverse recovery is 250ns app (IPW60R031CFD7). The switching frequency is 60Khz. While calculating I am getting core size of ETD59 which is looking high for a 750W converter. Any other alternative converter for this power range as I have less time for trouble shooting.
    Last edited by sabu31; 26th April 2019 at 09:38.



  13. #13
    Super Moderator
    Points: 255,114, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    44,445
    Helped
    13540 / 13540
    Points
    255,114
    Level
    100

    Re: Ringing on PSFB secondary switches

    I am attaching secondary waveforms and transformer current in secondary
    Turns out that the waveforms in post #1 are flipped (first image secondary, second image primary). Should be mentioned earlier.

    Measurement "across switch legs" means Vds? But what's the gate waveform?



  14. #14
    Advanced Member level 4
    Points: 8,301, Level: 21

    Join Date
    Aug 2015
    Posts
    1,314
    Helped
    492 / 492
    Points
    8,301
    Level
    21

    Re: Ringing on PSFB secondary switches

    hi there, assuming dead time on the output fets, their internal diodes will carry current - they will have to reverse recover when the primary power pulse is applied - assuming continuous current in o/p choke - this is the source of the spiking/ringing on the output

    - - - Updated - - -

    lack of good solid bus de-coupling on the pri side could well be the cause of volt spiking on the pri side - please show picture of layout and sch.

    - - - Updated - - -

    at light load snubbing will be needed - and it is a good idea to have it anyway for RFI ... esp on o/p diodes/fets


    1 members found this post helpful.

    •   AltAdvertisment

        
       

  15. #15
    Advanced Member level 5
    Points: 31,819, Level: 43
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    6,641
    Helped
    483 / 483
    Points
    31,819
    Level
    43

    Re: Ringing on PSFB secondary switches

    The secondary diode reverse recovery is 250ns app
    Yes i second that from EasyPeasy, 250ns is a long trr for this kind of service.

    - - - Updated - - -

    The following might be worth reading , and shows an adjustment to the PSFB, so as to mitigate the probelm of ringing on the secondary diodes of the PSFB....

    https://www.researchgate.net/publica...Secondary_Side

    - - - Updated - - -

    Here is a relevant thread on sec ringing for PSFB
    https://www.edaboard.com/showthread....ely+over-hyped

    ....this used to be (maybe still is?), the most highly read thread on edaboard.


    1 members found this post helpful.

  16. #16
    Full Member level 3
    Points: 2,195, Level: 10
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    153
    Helped
    5 / 5
    Points
    2,195
    Level
    10

    Re: Ringing on PSFB secondary switches

    Hi all,

    Thansk Easy Peasy and treez. I think that could be the cause of ringing along with layout . I have a query whether, the PSFB is a good topology for variable output from 48 V to 90V or is there any other alternative topology.



  17. #17
    Advanced Member level 5
    Points: 31,819, Level: 43
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    6,641
    Helped
    483 / 483
    Points
    31,819
    Level
    43

    Re: Ringing on PSFB secondary switches

    Comparing Phase shift full bridge converter with LLC converter.............

    Page 7 of the pdf linked here says...
    …………………………………………………………………………….
    “Design considerations of the power MOSFET in the primary side LLC resonant converter are relatively simple compared to a phase-shifted full-bridge converter, as it is not affected by leading or lagging waveforms with regards to the power MOSFET.”
    …………………………………………………………………………….

    The document is called “Primary Side MOSFET Selection for LLC Topology”....
    http://www.infineon.com/dgdl/Infineo...47d3430e927e5d

    .....................
    LLC is probably worse than PSFB for your range of vin and vout....since you are only at 750W, i believe a straight full bridge with sync rects is the way to go for you.
    .................................................. ..................

    - - - Updated - - -

    The PSFB has some pretty nasty failure modes if not mitigated.....the attached folder has docs on this..

    - - - Updated - - -

    By the way, if you wish, here is a link to my free SMPS course.
    This is the link from my google drive
    (It has a folder on PSFB, which i cant attache here as its too big)

    https://drive.google.com/open?id=1cm...8ZimpEp7HCMIcY

    Please tell me if it doesnt work.


    2 members found this post helpful.

  18. #18
    Full Member level 3
    Points: 2,195, Level: 10
    Achievements:
    7 years registered

    Join Date
    Jan 2011
    Posts
    153
    Helped
    5 / 5
    Points
    2,195
    Level
    10

    Re: Ringing on PSFB secondary switches

    Quote Originally Posted by treez View Post
    Comparing Phase shift full bridge converter with LLC converter.............

    Page 7 of the pdf linked here says...
    …………………………………………………………………………….
    “Design considerations of the power MOSFET in the primary side LLC resonant converter are relatively simple compared to a phase-shifted full-bridge converter, as it is not affected by leading or lagging waveforms with regards to the power MOSFET.”
    …………………………………………………………………………….

    The document is called “Primary Side MOSFET Selection for LLC Topology”....
    http://www.infineon.com/dgdl/Infineo...47d3430e927e5d

    .....................
    LLC is probably worse than PSFB for your range of vin and vout....since you are only at 750W, i believe a straight full bridge with sync rects is the way to go for you.
    .................................................. ..................

    - - - Updated - - -

    The PSFB has some pretty nasty failure modes if not mitigated.....the attached folder has docs on this..

    - - - Updated - - -

    By the way, if you wish, here is a link to my free SMPS course.
    This is the link from my google drive
    (It has a folder on PSFB, which i cant attache here as its too big)

    https://drive.google.com/open?id=1cm...8ZimpEp7HCMIcY

    Please tell me if it doesnt work.
    Hi Treez,

    Thank you for the suggestion.



  19. #19
    Advanced Member level 4
    Points: 8,301, Level: 21

    Join Date
    Aug 2015
    Posts
    1,314
    Helped
    492 / 492
    Points
    8,301
    Level
    21

    Re: Ringing on PSFB secondary switches

    There is no doubt that phase shift full bridge can be made to work - the slow o/p diodes in this specific case mean that a lot of o/p snubbing is required. Changing dead time timing with load for the passive ( lagging) leg is important, see UCC3879 data sheet and app notes, for some apps it is usual to have a separate choke in series with the Tx and diodes from the other side of this choke to rail, relatively low leakage in the main Tx is helpful.


    1 members found this post helpful.

--[[ ]]--