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UCC256404: AC high line operate in ambient 90 degreeC IC RVCC unstable

Chao Fu Chen
Intellectual 1170 points
Part Number: UCC256404

Tool/software:

Hi Expert ,

As topic, my customer test  this part find in that condition RVCC became unstable then their system can not open.

but the IC in AC low line the RVCC is really good and system was safe. please refer to the attachment picture.

could you have any comment or IC internal root cause to explain this situation ? 

Eddie

  • 0
    TI__Mastermind 40580 points

    Hello Eddie, 

    There is no AUX pin on the UCC256404 device, but I will assume it refers to the VCC pin, is that correct? 

    In the 90Vac case, Vaux (Vvcc?) rises linearly to ~26 V and RVCC rises to ~13V regulation and everything starts up okay.

    In the 264Vac case, Vaux rises in staggers, and it looks like there is a load to pull it down after a brief interval of rising. 
    When it finally reaches 26V, RVCC attempts to rise to regulation, but is stopped after rising a few volts, then it decreases as if under a load. 
    I think that for the 264Vac case, there is a high load on VCC and/or on RVCC that is not there when starting at 90Vac. 

    Please check for that.

    I don't think that this is RVCC protection, I think RVCC stops rising because the voltage at VCC has dropped to 9.65V, possibly because of insufficient capacitance on the VCC pin.  Please check for that, too.   At 4s/div sweep speed, you may not see any drop in VCC on the oscilloscope. 

    Regards,
    Ulrich