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UCC28050: Flyback startup network.

Part Number: UCC28050
Other Parts Discussed in Thread: PMP11155,

Dear Team,

I would like to know the working of startup network in PMP11155.

Working voltage of Vcc is 20V but as per resistor network Vcc net will have 41V while input voltage is 90Vac.

Once the IC starts operating, how does Vcc change over from HV net to bias winding.

Thanks in advance.

Warm Regards.

  • Initially, the current though R1 & R5 is enough to raise Vcc up to the UCC28050 startup threshold.  Once this threshold is reached, the output starts regulating.  At this point, the current provided through R1+R5 is too small to power Vcc, whereby C11 provides current until the regulator formed by Q1, D9, R7 takes over.  

    Q1, D9 and R7 regulate Vcc to ~15V-Vbe, so the 20V AbsMax is never violated.