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UCC24630 / About DRV and VPC.

ACGUY
Guru 11170 points
Other Parts Discussed in Thread: UCC24630

Hi all,
Could you let me know about UCC24630?
Our customer is evaluating UCC24630 now.
However, it does not work as intended.

Normally, when the primary side switched ON, the secondary side switched OFF.
However, when the operation was changed from CDM to CCM, DRV-pin didn't become Low.

Please let me know the cause of this phenomenon.

Power supply specifications are as followings;
- Vin :   AC90 - AC264.
- Turn ratio :   7.4:1.
- Pout :   5V or 15V (resistance switching by the switch, the same substrate) 2A.
- Architecture :   Fly-back & Secondary synchronous rectification.
- fsw :   100kHz.
 
I'll attach the following documents;
- Secondary side schematic.
- The measured waveform. (DRV-pin & Secondary FET's Vds)

Best regards,
Nagai


Schem_Waveform.pptx

  • 0
    TI__Mastermind 34465 points

    UCC24630_VF.pdfHi Nagai san,

    I see that you have disconnected the gate drive resistor R38 for these tests .

    This was probably to aid in troubleshooting and to prevent failure of the power supply I think.

    On the attached document, I copied the waveforms and measured the switching period.

    You can see that the period changes from 15uSecs to 10uSecs.

    So the power supply is operating in a variable frequency condition when the UCC24630 misbehaves.

    According to the data sheet the UCC24630 is compatible with CCM conditions  only when the power supply is operated in fixed frequency mode.

    (see section 9) and I think that's what the issue here is.

    The UCC24630 is not suitable for this application

    John Griffin

  • 0 in reply to John Griffin1
    Guru 11170 points

    John san,

    Thank you for your reply.
    I was convinced in your advice.

    May I question again ?
    Why DRV-pin doesn't become Low in variable frequency and CCM ?

    Best regard,
    Nagai

  • 0 in reply to ACGUY
    TI__Mastermind 34465 points
    Hi Nagai san,
    The UCC24630 does not monitor the secondary side current but it uses the previous cycles timing to calculate when to generate the DRV signal.
    In CCM this causes the part to misbehave. In DCM this is not a problem because the secondary current is zero.
    Regards,
    John
  • 0
    TI__Mastermind 23015 points

    Hello Nagai-san,

    John Griffin responded with the correct summary of the UCC24630 operation in CCM limits. The period shift of ~15us to 10us, and going into CCM operation exceeds the CCM dead time capability.

    Can you provide the primary controller part number that is used in the power supply?

    Richard Herring.

  • 0 in reply to Richard Herring
    Guru 11170 points

    Hi Herring-san,

    I could not get the part number.
    However, The controller we expect that it is manufactured by Rohm.

    Many Thanks,
    Nagai