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UCC28180: Ucc28180 startup issue

Jiahui Gao
Intellectual 2025 points
Part Number: UCC28180
Other Parts Discussed in Thread: UC3842

Tool/software:

Hi team，

I am currently developing a boost PFC+flyback switching power supply. The PFC chip uses UCC28180, and the flyback uses UC3842.

1. The power supply for UCC28180 uses the auxiliary winding of the flyback transformer, while UC3842 uses a high-voltage bus and a starting resistor. After starting, the auxiliary winding of the flyback transformer is used for power supply. Is this appropriate for handling?
2. If this is done, is it because the flyback works first and then the flyback output supplies power to the PFC chip before the PFC can work?
The flyback is actually connected to the front stage Vac100-246v first, and the flyback output supplies power to the PFC. After the PFC works, the input of the flyback becomes Vdc400v, right？

Thanks & Best regards，

Jiahui

  • 0
    TI__Mastermind 32500 points

    Hello Jiahui,

    For Q1:  Yes, that is a typical way to implement bias power for the PFC controller.  It is appropriate. 

    FOr Q2:  Yes, the flyback works first, then provides bias to the PFC and then the PFC works.   Usually, the flyback input is connected to the PFC output even when the PFC is not working, since the PFC output will be the same rectified line voltage as if the flyback was connected to the front stage Vac100-264.  
    So just connect the flyback to the PFC output. 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    TI__Intellectual 2025 points

    Hi Ulrich

    Thanks for your quick reply.

    Generally speaking, when designing a flyback switching power supply, it is usually designed according to the default input of AC85-265V. The technical documents about TI also design the flyback switching power supply according to this input range, such as the example in the datasheet of uc3842. However, if the front stage of the flyback switching power supply is a PFC circuit, the initial input of the flyback switching power supply is AC85-265V, but when the PFC gets power from the auxiliary winding of the flyback switching power supply, the PFC will output 390V when it works normally, then the input of the flyback switching power supply is DC-390V. How should the flyback switching power supply be designed at this time? Just increase the derating? Is the minimum voltage input still considered to be AC85V?

  • 0 in reply to Jiahui Gao
    TI__Mastermind 32500 points

    Hello Jiahui,

    You are correct, the flyback supply must be designed to from the lowest non-boosted voltage up to the maximum PFC output voltage (typically 390~400Vdc) including overshoots. 

    At start-up the flyback is not expected to be heavily loaded, so the peak of low-line can be the lowest Vin.  
    Higher loading on the flyback output should be delayed until the PFC output come into or close to regulation. 
    Thermally, the flyback should be designed for operation at the lowest steady-state PFC voltage expected. 

    If the PFC is expected to be disabled for any reason, make sure that the loading on the flyback is reduced enough to operate reliably (no excess thermal stress) at the low-line condition, including the rectified ripple valley voltage (lower than the peak of low-line). 

    Regards,
    Ulrich