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UCC28950: Output E and F not generate pulses.

Andrej Dobrovic
Prodigy 140 points
Part Number: UCC28950

Tool/software:

Hello.  I o design new power system from battery with webench help. I create prototype board. Design from webench is included. Output is regulated well with low load (0.1A), but increasing load not starts output pulses in OUTE and OUTF pins. Then output FETs not switching, only internal FETs diodes rectify output.

webench_ucc28950_design_forum.pdf

Here are outputs of Vout (yellow), Gate Mosfet E (blue), Gate mosfet F (green) and transformer output connected to Drain mosfet F. Output OUTE and OUTF not starts to switching.

The same output as OUTE and OUTF is in CS pin.

When I try to calculate values with https://dr-download.ti.com/design-tools-simulation/calculation-tool/MD-rlvLC7vkDZ/01.00.00.0E/SLUC222E.xlsx I got other values for some resistors. But there is big different between webench and SLUC222E - dividers ADEL and ADELEF are connected to CS pin (in webench and datasheet) but in SLUCC222E is connected to VREF. What is correct? I think here is any problem, why outputs E and F are not switched.

  • 0
    TI__Guru* 86016 points

    Hello,

     

    Do to high volumes your inquiry will be answered in the order that it was received.

     

    Regards,

  • 0 in reply to Mike O'
    Prodigy 140 points

    I found part of problem with pulses in OUTE and OUTF - resistor between DCIM and ground is too hight. When I connect DCIM direct to ground, OUTE and OUTF generate pulses. But my  question:There is big different between webench and SLUC222E - dividers ADEL and ADELEF are connected to CS pin (in webench and datasheet) but in SLUCC222E is connected to VREF. What is correct?

  • 0 in reply to Andrej Dobrovic
    TI__Guru* 86016 points

    Hello,

    When resistor dividers are corrected to from CS to ADEL and ADELEF that is to use adaptive delay.

    If ADEL and ADELEF are connected to ground or resistor divider this is to use a fixed delay approach which is used in application slua560d which can be found at the following link.  It is much easier than using the adaptive delay approach.  98% of our customers use this design technique.

    https://www.ti.com/lit/an/slua560d/slua560d.pdf

     

    Regards,