+ Post New Thread
Results 1 to 5 of 5
  1. #1
    Advanced Member level 5
    Points: 33,876, Level: 44
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    7,038
    Helped
    511 / 511
    Points
    33,876
    Level
    44

    Half Bridge SMPS in Current Mode but with slope compensation is OK?

    Hi,
    Page 122 of “power supply cookbook” (2nd Ed) by Marty Brown details the design of a 280W offline Half Bridge Converter in Current mode with slope compensation.
    Current Mode is in theory not advised for Half Bridge converters due to potential imbalance of the rail splitting input capacitors.

    However, can we actually say that as long as significant slope compensation is used, then Current mode is actually fine for Half Bridge SMPS’s?

    The attached two LTspice simulations show two otherwise identical current mode Half Bridge converters. One is unstable, but the one with slope compensation is fine.

    •   AltAdvertisement

        
       

  2. #2
    Advanced Member level 5
    Points: 12,398, Level: 26

    Join Date
    Aug 2015
    Location
    Melbourne
    Posts
    2,023
    Helped
    744 / 744
    Points
    12,398
    Level
    26

    Re: Half Bridge SMPS in Current Mode but with slope compensation is OK?

    if you add a lot of slope comp, technically the ckt is no longer just current mode - but tends to be closer to voltage mode - in its control characteristics - however the amount of slope comp to allow an half bridge to operate without cap imbalance has never been computed - to my knowledge ...

    - - - Updated - - -

    in any event the control loop will need to be a lot slower than for optimised peak current mode ...


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  3. #3
    Super Moderator
    Points: 263,400, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,022
    Helped
    13996 / 13996
    Points
    263,400
    Level
    100

    Re: Half Bridge SMPS in Current Mode but with slope compensation is OK?

    I see a misunderstanding involved with the 280W Half Bridge Converter design example. MC34025 is no current mode controller. It's a voltage mode controller with additional current limit and shutdown function. The difference is obvious by the fact, that the current sense input is compared against a fixed threshold rather than the error amplifier output, see diagram below. Slope compensation may be nevertheless necessary for stable current limit operation.

    Click image for larger version. 

Name:	mc34025.PNG 
Views:	8 
Size:	87.3 KB 
ID:	156894


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  4. #4
    Advanced Member level 5
    Points: 33,876, Level: 44
    Achievements:
    7 years registered

    Join Date
    Sep 2008
    Location
    cambridge
    Posts
    7,038
    Helped
    511 / 511
    Points
    33,876
    Level
    44

    Re: Half Bridge SMPS in Current Mode but with slope compensation is OK?

    Thanks, yes, on pg 127, Brown states that the controller MC34025P can be configured as either voltage mode or current mode, but Brown specifically states he is putting it into current mode.

    On page 132, Brown gives the schematic…..he has a current sense transformer….the resistor at the output of this is fed to pin9, and also to pin 7 ……he also feeds in a slope comp ramp into pin 7, which he drives from the oscillator (and he buffers it with a PNP).
    As such, Brown is indeed using it as a current mode controller with added slope



  5. #5
    Super Moderator
    Points: 263,400, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,022
    Helped
    13996 / 13996
    Points
    263,400
    Level
    100

    Re: Half Bridge SMPS in Current Mode but with slope compensation is OK?

    Yes, you are right. Current mode is established by also connecting pin 7 to the current sense signal.

    The reservations about usefulness of this mode haven't been dispelled though.


    1 members found this post helpful.

--[[ ]]--