+ Post New Thread
Results 1 to 5 of 5
  1. #1
    Advanced Member level 4
    Points: 7,345, Level: 20

    Join Date
    Dec 2015
    Location
    Madrid, Spain
    Posts
    1,275
    Helped
    314 / 314
    Points
    7,345
    Level
    20

    Buck with FET in the low side

    Hello everyone,

    The attached buck converter uses the MOSFET in the low side, however the drawback is the floating output. This makes it easy to drive the MOSFET and even can be used as buck converter for input voltage as high as rectified mains because no high voltage capacitor is needed for the bootstrap.

    1) Why isn't this topology as popular as the usual one ? Only because of the floating output ? Is there any issue I do not see ?
    2) I do not see any problem in changing the output filter inductor at the cathode of the diode, am I right ?

    Thank you for your time !

    Click image for larger version. 

Name:	Buck with low side switch.png 
Views:	9 
Size:	10.5 KB 
ID:	140645

    •   AltAdvertisement

        
       

  2. #2
    Super Moderator
    Points: 77,827, Level: 68
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    15,771
    Helped
    3590 / 3590
    Points
    77,827
    Level
    68

    Re: Buck with FET in the low side

    Hi,

    I think there are some issues with your idea:
    * Cin needs to be high voltage
    * no bootstrap capacitor needs to be high voltage
    * this circuit shows a typical buck converter circuit. The only difference is the reversed polarity. It has the same advantages and disadvantages than a positive buck converter.

    Your assumption 2) is wrong.

    Klaus


    1 members found this post helpful.

    •   AltAdvertisement

        
       

  3. #3
    Advanced Member level 4
    Points: 7,345, Level: 20

    Join Date
    Dec 2015
    Location
    Madrid, Spain
    Posts
    1,275
    Helped
    314 / 314
    Points
    7,345
    Level
    20

    Re: Buck with FET in the low side

    Yes, I made a mistake in the original post regarding the bootstrap capactior that it needs to be high voltage.

    Anyway, this circuit does not need a high side driver, it can go along with a low side driver for the FET, that is an advantage.

    Probably I have explained wrongly assumption 2). I meant to say moving the output inductor at the cathode of the diode but still in the same mesh as in the circuit in the original post, NOT in series with the diode. In other words, the output inductor would have been connected between the cathode of the diode and the Vout(+) (positive plate of the cap).

    If Synchronous Buck, then it has no advantage whatsoever.
    Last edited by CataM; 20th August 2017 at 17:03.



    •   AltAdvertisement

        
       

  4. #4
    Advanced Member level 5
    Points: 14,971, Level: 29
    schmitt trigger's Avatar
    Join Date
    Apr 2013
    Posts
    2,488
    Helped
    805 / 805
    Points
    14,971
    Level
    29

    Re: Buck with FET in the low side

    I've seen his circuit mostly in LED drive circuits, since in those applications it does not matter that the output negative rail is not common with the input's negative rail.

    Feedback may be achieved with common optoisolator techniques.
    My batteries are recharged by "Helpful Post" ratings.
    If you feel that I've helped you, please indicate it as a Helpful Post


    1 members found this post helpful.

  5. #5
    Super Moderator
    Points: 260,240, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    45,443
    Helped
    13827 / 13827
    Points
    260,240
    Level
    100

    Re: Buck with FET in the low side

    Are you asking about this topology?

    Click image for larger version. 

Name:	Buck with low side switch2.png 
Views:	2 
Size:	13.3 KB 
ID:	140673

    It's really bad related to generated common mode noise.

    As already stated, the low side buck switch is an option for floating (not ground referred) loads. Need to check your application requirements if it's suitable.


    1 members found this post helpful.

--[[ ]]--