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UC3845: UC3845 Webench Flyback Circuit Components

Behtan Umut Mercan
Prodigy 60 points
Part Number: UC3845
Other Parts Discussed in Thread: TL431

Hello,

I have designed a flyback circuit with the webench schematic. I have used UC3845 for the following configuration input 12-18 V. Output 48 V 1 A. I attached the pdf file below.

Could you please tell me what is the purpose of QSC and Q1 transistors. Can the circuit work without them?

Thank you in advance.

Sincerely,

Behtan Umut Mercan

 WBSchematicUC3845N.pdf

  • +1
    TI__Guru 73215 points

    Hi,

    QSC is for current slope compensation. It can be skipped since UC3845 max duty < 50%.

    Q1 and Dz for an LDO. Your output voltage 48V, the VR (TL431) cannot connect to that high voltage. To avoid that happen you need to have this LDO.