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UCC28950

K.Yaita
Guru 29720 points
Other Parts Discussed in Thread: UCC28950, UCC1895

Hi,

I'd like to ask about UCC28950.

1) equation about Tmin

It seems mismatch between equation8(Tmin=5.92xRtmin, datasheet P.18) and Figure.10.

In UCC28950 excel design tool, it indicate "Tmin=6.6xRtmin+15". Is equation8 incorrect?

2)about Burst mode operation

In datasheet P.9, there is timing chart about burst mode operation.

I want to know the first pulse(L to H) timing about "out D".

In my customer's board, out D pulse is too thin in burst mode operation.(Tmin=91kΩ.)

Yaita

UCC28950Burst.xlsx
  • 0
    TI__Guru* 86286 points

    Hello Yaita

    The data sheet was revised in October 2011.  The excel design tool was created to go along with the following application note which the revision date is earlier. http://www.ti.com/litv/pdf/slua560c  It looks like they changed the equation for Rtmin since the application note was written.

    The IC/Systems engineer who defined this part had the gate driver signals behaive this way to stop the transformer from saturating.  Figure 25 in the User's guide show the same behavior that you are seeing.  The following link will get  you to the User's guide.  http://www.ti.com/litv/pdf/sluu421a

    The only thing I found when designing with this device is if you are using gate drive transformers.  The uneven pulses can cause gate drive transformer saturation if DC blocking capacitors are not used in the design.  Please refer to the User's guide for the gate drive transformer configuration.

    Regards,

    Mike

     

  • 0 in reply to Mike O'
    Guru 29720 points

    Hi, Mike

    Thank you for reply.

    About question 1), Is that mean equation"Tmin=5.92xRtmin(ns)"is correct, and Figure10 is uncorrect?

    In datasheet P.18, equation(8) and Figure10 is mismatched.

    Regards,

    Yaita

     

     

  • 0 in reply to K.Yaita
    TI__Guru* 86286 points

    Hello Yaita,

    I don't know why the data sheet got messed up equation 8 does not match figure 10.  The equation in the excel design tool and MathCAD design file match figure 10.

    Equation 140 in the application note calculates Rtmin and in the denominator should be 6.6ns instead of 6.6s.  I will work on getting the data sheet and application note corrected.  The excel file and MathCAD file calculated the correct values that match figure 10.

    Regards,

    Mike

  • 0 in reply to Mike O'
    Guru 29720 points

    Hi,Mike

    OK, I understand.Thank you for your support.

    If you have any update(which is correct),Please let me know.I will get back to my customer.

    Regards,

    Yaita

  • 0 in reply to Mike O'
    Intellectual 855 points

    Mike-san

    question:

    we are using the older UCC1895 & it has the single core triple 1:1:1 gate drive xmfr for A-B & another for C-D legs.  Can this still be used  with UCC28950?

  • 0 in reply to robin gangopadhya
    TI__Guru* 86286 points

     Hello Yaita,

    You should still be able to use the 1:1:1 gate driver configuration with the UCC28950.

    Regards,

    Mike