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UCC24612: Efficiency question

Vincent Chen64988
Expert 4480 points
Part Number: UCC24612
Other Parts Discussed in Thread: LM51551,

Hi Team,

My customer is evaluating the performance of flyback controller (LM51551)+Synchronous Rectifier Controller and the SCH is as below.

The efficiency test result shown below, it seems no difference with or without SR controller. The efficiency result is better even using diode instead. Is there any methods to improve the efficiency?

Q9

Q10

snubber

Efficiency(%)  (V=22.6V)

Iout= 0.3A

Iout=0.5A

Iout=1.0A

Iout=1.5A

Iout=2.0A

Iout=2.5A

MOS FDMS86252

DIODE SMPD5100-C

ON

77.26

79.81

83.98

86.37

85.15

85.55

MOS FDMS86252

DIODE SMPD5100-C

NC

81.69

85.61

85.42

87.26

87.3

86.04

MOS FDMS86252

MOS FDMS86252

ON

77.36

74.76

82.55

85.42

83.64

84.22

MOS FDMS86252

MOS FDMS86252

NC

77.29

82.81

82.51

84.95

83.26

83.95

Thanks a lot.

Vincent Chen

  • 0
    TI__Mastermind 32370 points

    Hello Vincent,

    Thank you for your interest in the UCC24612 SR controller.

    From the test results, it is evident that the SR MOSFET is not turning on, it is rectifying using body-diode only.  That is why the external diode has higher efficiency.

    The customer can verify this my probing gate-to-source on the SR-Fet to see if there is a good Vgs waveform or not.

    Assuming there is none, we look for reasons for bad Vgs.  From my examination of the schematic diagram, I see that C30 which filters VDD is only 0.1uF. 

    I believe that C30 is too low to provide the cycle-by-cycle gate charge needed to drive the Fet while keeping VDD above UVLO.
    Please increase it to the normal value of 1.0uF (or higher).  Check for proper gate-drive and check VDD for sufficient margin to its UVLO threshold.

    Regards,
    Ulrich