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UCC28051: UCC28051

Part Number: UCC28051


I have a simple question about the input undervoltage protection (page-3 of schematic). Can anyone please advise. 


B+ is around 400V (output of PFC). If you see the opamp U9B on page-3 , it will disable the VSNS pin of the PFC IC : UCC28051D which is good. LLC IC is disabled by monitoring the B+ voltage and once pfc is disabled, the LLC IC will be disabled as well. But, lets say the input voltage is around 70Vac or 60Vac, the PFC+LLC stages are both disabled, but the auxiliary supply is still ON, the reason is whatever the Vac input is, it appears right at the B+ output even though pfc, LLC are disabled ( rectified voltage at B+ = 70Vac * 1.414), this means the auxiliary power supply will still be working ( drawing few mW). Shouldn't the complete power supply be turned OFF under brownout condition? Please advise.

Best Regards,


  • Usually brown-out means when line voltage is at brown-out point, the output(s) of the converter turn off. As per inside of the converter, it depends on the specs. The main purpose of brown-out is to make protection on the primary side components of those with high current not exceeding each designed rating, or otherwise would need to design them with more margin.

    In this case, UCC2889D associated circuit won't need over design so it can still be alive after main power stage in brown out - which is also ok since no specs ask to turn it off. But if you want to turn off UCC2889D as well when main power stage in brown-out, you need to add additional circuit to do so.

    Most PFC + down-stream converters require a bias always alive, i.e., no brown-out when line is above certain level, from what we know. So it sounds your requirement is special.