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UCC24630 : The high current issue on the primary-side FET

Yoshwa
Expert 4050 points
Other Parts Discussed in Thread: UCC24630, UCC24636

Hi, team,

Customer is developing an quasi-resonant fly-back power application using UCC24630.

UCC24630 was changed from UCC24636, because of same high current issue, about one month ago.

The high current of over 10 A still occurs irregularly at the red elliptic circle timing of the attached waveform in the power up period using UCC24630.

Waveforms and schematics are attached.

UCC24630 waveforms.pdf

UCC24630 schematics.pdf

VIN=170 Vac, Vout=5 V, Iout=16 A,

Transformer : 420 uH, Np=31 T, Ns=2 T.

 

What kinds of problems are considered?

How can we get rid of this high over current issue?

  • 0
    TI__Genius 9130 points
    Yoshwa-san,

    What control IC is the customer using on the primary side? Can the primary controller allow CCM operation, or does it guarantee QR/valley-switching/DCM operation?

    The UCC24636 can ONLY be used with a primary controller that guarantees no CCM operation.


    However, from looking at your pdf waveforms, it looks like the flyback is going into CCM for sure.

    The first 3 or 4 cycles look like they are QR. But the 5th cycle before the red circle is definitely CCM. As a result, the 6th cycle ends up terminating early - it's really hard to see under the red circle, is there a short pulse of high current, or is the cycle actually skipped altogether?

    Since the 6th cycle is effectively skipped, there is a long time for the secondary current to decay, so the 7th cycle starts DCM again. But the 8th cycle and beyond are CCM again.


    If the primary controller can go CCM, then the UCC24636 should not be used - the UCC24630 should be used instead, since it has internal mechanisms to help deal with CCM operation.


    Thanks,
    Bernard
  • 0 in reply to Bernard Keogh
    TI__Expert 4050 points

    Thanks your prompt response.

    The used power IC is UCC24630 for CCM operation, at this time not UCC24636.

    I have missed to revise the schematics.

    And resister parameters of the schematic have been also changed to the attached UCC24630_Calc data.

    The zoomed waveform of the red circle potion is attached.

    The control IC used on the primary is asking to customer.

    I have directly sent the customer primary schematics to you by a mail for customer’s requirement.

    UCC24630 Iprim VSC VPC VDD.pdf

    24630_Calc.xlsx

  • 0 in reply to Yoshwa
    TI__Genius 9130 points
    Yoshwa-san,

    Can you include the SR DRV pin waveform in the 2 scope plots you already attached?


    Did you send me an email with the schematic? If so I did not receive it. My email address is bernardkeogh@ti.com.


    Thanks,
    Bernard