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UCC28950 / clamp diodes in reference schematic

K.Yaita
Guru 29690 points
Other Parts Discussed in Thread: UCC28950
Hi Team,
 
I'd like to ask about UCC28950.
In the following application report, there is clamp diodes(Db, Dc) in reference schematic.
 
According to the following E2E thread, it seems that these diodes protect the synchronous rectifiers(Qe, Qf) from an over voltage.
So, I have 3 questions about this .
 
(1) Could you explain why the difference between the current of Ls and the current of T1 could be occured?
(2) If the current difference is occured, why the voltage across the T1 could ring up to a high voltage?
(3) My customer adopted these clamp diodes. However, he confirmed that the efficiency degrades in light load due to these clamp diodes. Is there any method to improve efficiency?
 
Best Regards,
Yaita / Japan disty
  • 0
    Intellectual 375 points

    Ls + leakage inductance of the transformer, with the parassitic capacitance form a ressonant circuit.

     I advise to check with an oscilloscope if you have ocillations in the point between Ls and T1.

    Many years ago I designed a 3 kW full bridge phase shift converter and found that adding a damping resistor in series with to the diodes (you need only one resistor)  was much better than the damping diodes alone. Even better,  you may add an RC damping network (keeping the diodes) to the primary of the transformer.

  • 0
    Intellectual 375 points

    Ls + leakage inductance of the transformer, with the parassitic capacitance form a ressonant circuit.

     I advise to check with an oscilloscope if you have ocillations in the point between Ls and T1.

    Many years ago I designed a 3 kW full bridge phase shift converter and found that adding a damping resistor in series with  the diodes (you need only one resistor)  was much better than the damping diodes alone. Even better,  you may add an RC damping network (keeping the diodes) to the primary of the transformer.