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TPS54160EVM-230: Freewheeling diode forward voltage

David Dean
Expert 2305 points
Part Number: TPS54160EVM-230

I have noticed that the forward voltage of the freewheeling diode used on the eval board (Vishay B260) is about 525mV @ 1A.  There are a good number of similar diodes available that are at least 100mV less.  Is there any parameter of the B260 that I may be overlooking?  Would an alternative like PMEG6020ER or B260S1F-7 be acceptable (these are smaller footprints as am space constrained).  I assume that a lower Vf means higher efficiency but I don't want to overlook any negatives of these parts.

  • 0
    TI__Mastermind 31390 points
    Hello David,

    Selecting a schottky rectifier is a good choice for a rectifying diode as do not have a reverse recovery time. Selecting a diode with a VRRM>Vin max and an IF=>Iout.

    With that said the other parameter you need scrutinize is the power dissipation of the diode. This is where the VF needs to be considered. Once this is calculated you need to check the temp rise of the device. If for example you have a 1A out, look at the Vfwd.

    Advise using the Hot Vfwd. from the Datasheet of say the B260S1F the hot VFWD is ~0.5V. The power dissipation is calculated by taking the average current and multiplying with the Vfwd.

    The average current is, IFAVE= D*Iout.

    Where D = Vout/Vin for a buck.

    Once IFAVE is determined multiply with VFWD.

    So for a 12V in 6V out; D= 0.5.

    Iout = 1A, say.

    IFAVE = 0.5A,

    Dloss = 0.5A*0.5V = 0.25W.

    Now look up the RJA for the diode (thermal Resistance Junction to Ambient).

    According to the Datasheet it is 100C/W

    Therefore the temp rise for the example given is 25C above ambient.

    You need to ensure that under worse case input voltage, output current and ambient temperature, the temp rise of the junction (Tj) is not exceed 150C, I would keep Tj<125C.

    Hope this helps?

    David.
  • 0 in reply to David Baba
    Expert 2305 points
    Hi David,
    Very helpful, thanks! So basically, if I select a Schottky diode then I don't need to worry about reverse recovery time, and then I just handle all the
    'normal' things like forward voltage and power dissipation? So if an alternate diode like the PMEG6020ER has a lower Vf, and all other parameters check out OK, then this should be OK to use and, due to the lower Vf, potentially more efficient than the B260?

    On the issue of a Schottky diode - do you have any experience with the Super Barrier Diodes from Diodes Inc? I was looking at SBR2U60S1F-7 for this application, as it has an even lower Vf. The diode application notes mention suitability for use in a SMPS, but I'm unclear if using this part would require worrying about the reverse recovery time, or if this part would be as suitable as a normal Schottky diode?
  • 0 in reply to David Dean
    TI__Mastermind 31390 points
    Hello David,

    No need to worry about Trr when using a schottky diode. As mentioned, as long as the VRRM>Vin max, IF => Iout, more importantly the power dissipation is one factor needing closest scrutiny after the VRRM. The SBR2U60S1F-7 looks suitable from a Trr and VFWD perspective. Hope this helps?

    David.
  • 0 in reply to David Baba
    Expert 2305 points
    How were you able to determine Trr for the SBR2U60S1F-7? Or do you consider it to have the reverse characteristics of a Schottky for this application?

    Related question - another diode I am looking at is Panasonic DB2W60400L. Despite being a Schottky diode they list a reverse recovery time, and it is only 12nS. Any concerns about that part?
  • 0 in reply to David Dean
    TI__Mastermind 31390 points
    Hello David,

    The fact that the SBR2U60S1F-7 does not have a Trr specification, the same as most Schotkys, they don't specify this spec as this is none.

    The DB2W60400L is a schottky and it does specify a Trr = 12ns, which is very fast, which is a good thing. ,

    David,
  • 0 in reply to David Baba
    Expert 2305 points
    I guess my concern about the SBR2U60S1F-7 is, IS this part a Schottky? Is 'Super Barrier' something different, or just marketing?
  • 0 in reply to David Dean
    TI__Mastermind 31390 points
    Hello David,


    I would go ahead and try this device if the power dissipation check out. It looks suitable from a switching perspective.

    See link below that discusses the benefits of an SBR.

    www.diodes.com/.../

    Hope this helps?

    David.
  • 0 in reply to David Baba
    Expert 2305 points
    I saw the Diodes Inc info and it looked good but I wanted to hear it from someone else too.

    Based on past experience with similar regulators and diodes of this size I'm pretty confident that power dissipation will not be a problem. I'll try to get hold of a human being over there and ask them the same questions :)