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UC3843: UC3843

VIJAY GANGADEEN
Prodigy 40 points
Part Number: UC3843

Hi, Please refer to the application note SLUS223E for IC UC3843. Please refer to figure 25 on page 22.

I am designing a DC-DC converter. The DC input  voltage will be 18-25 VDC and output will be 15V, 1A. 

Now, the supply voltage of IC UC3843 is 12-28 VDC. Hence, in my opinion, I do not need the primary auxiliary winding NA, of the flyback transformer nor the components Rstart, Vvcc, Dbias, Rvcc. Rstart can be replaced by a jumper. 

Please advise if this will be ok and I am not missing anything.

Thanks.

  • 0
    TI__Expert 6080 points

    Hello Vijay,

    The RStart resistor provides the controller with the initial startup voltage and current to charge the Cvcc cap. Once the startup is achieved the schematic uses the circuit from the Aux winding to Rvcc to Dbias to Vcc in order to supply for the VCC bias current. The reason for doing this is to maintain efficiency by reducing the power dissipation across the Rstart.

    It is recommended to have the Rstart to supply for the 1mA startup current to charge the Cvcc cap as a protection from the main voltage supply. The rest of the circuit is basically to maintain the efficiency numbers.

    Regards,

    Sonal

  • 0 in reply to Sonal Singh
    Prodigy 40 points

    Hi Sonal,

    Thank you very much for your response. 

    In my design, I will not be using AC mains and I will not be using the resistor Rstart. My input will be batteries whose voltage will be 20-25VDC. Hence, I can power the IC directly with the batteries. Do you agree with this ?

    The reason I have been inquiring on this is because it is a real pain to find a transformer with primary auxiliary winding having voltage higher than 12V. I will have to custom make one and this can be expensive.

    Thanks. 

    Vijay

     

  • 0 in reply to VIJAY GANGADEEN
    Prodigy 40 points

    Hi Sonal, 

    Any news on this ?

    Thanks.

    Vijay 

  • 0 in reply to VIJAY GANGADEEN
    TI__Expert 6080 points

    Hello Vijay,

    If the input if from a 25V battery, I think power dissipation across that resistor is not a worry. You should be fine.

    Regards,

    Sonal