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UCC28950: How the magnetic inductance affect the diode mode?

Bubble machine
Prodigy 30 points
Part Number: UCC28950

Tool/software:

Hi, team

I use a transformer with smaller magnetic inductance than the design recommendation.

I have problem going into synchronous rectification mode on the secondary side. Driving signals QE and QF are always low no matter what load conditions.

Thus, the questions are:

Will the magnetic inductance prevent the controller generating the QE and QF? as the magnetic inductance is small, the current is always DCM through whole load range in my application.

There is difference between the figures in Page 39 and Page 55 regarding the resistor divider connection with ADEL and ADELEF:

Figure on the left is voltage divided from CS only, on the right is voltage divided from VREF (CS seems disconnected).

Best regards

  • 0
    TI__Guru* 86266 points

    Hello,

    If the voltage that is seen at the CS pins in less than the voltage set by the DCM comparator OUT E and OUT F will not produce a gate drive.  The following link will bring you to an application note that goes through the step by step design process of a PSFB using the UCC28950.  There is a section on how to setup the DCM comparator.

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

    The ADEL and ADELEF can be used in several ways.  They can be both setup with resistor dividers of the reference as one of those schematics shows.  This technique is fixed delay approach that is described in the above application note.  Also note you could short ADEL and ADELF to ground to the fix delay approach as well.  98% of the designers that design PSFB converters used the fixed delay approach.

    Setting the ADEL and ADELEF is an adaptive delay approach that is used to remove body diode conduction and it is a lot of trial an error.  For this reason, this approach is not that popular.

    Regards,