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UC3843: RTL bias

Carlo
Expert 7715 points
Part Number: UC3843
Other Parts Discussed in Thread: TL431

Hi team,

Good day.

Our customer would like to know how the RTLbias was calculated. As shown in Figure 25, the RTLbias is 1kohms. 

Also, if our customer would like to achieve an output voltage of 15V instead of 12V, what are the changes needed to be made in the design sample schematic.

Thank you in advance for your help.

Regards,
Carlo

  • 0
    TI__Genius 13660 points

    Hi Carlo,

    The recommend Cathode current of TL431 is 1mA~100mA.

    RTLbias is used to provide the minimum cathode current of TL431 even when there is no primary current of opto-coupler, assume its forward voltage is ~ 1V.

    So you don't need to change the value when output is 15V. 

    Regards,

    Teng

  • 0 in reply to Teng Feng
    TI__Expert 7715 points

    Hi Teng,

    Thank you for your response.

    Our customer would like to confirm if his calculation of RTLbias  is correct, as seen below.

    RTLbias= (12V-2.5V)/10mA = 950 ohm. The next higher standard resistance value being 1Kohm.  

    Please confirm what is the correct calculation. Also, what is the other purpose of the 10V Zener diode connected to RTLbias aside from providing a regulated voltage of 10V?


    Regards,
    Carlo

  • 0 in reply to Carlo
    TI__Expert 7715 points

    Hi Teng,

    Good day.

    Any update on this? Thank you very much for your support.

    Regards,
    Carlo

  • 0 in reply to Carlo
    TI__Genius 13660 points

    Hi Carlo,

    Correct my explanation:

    You are right, Output voltage provide the operating current for TL431 through RTLbias, the cathode of TL431 is 2~3V, so here the TL431 current is ~12mV.  RTLbias can be regulated as the target output voltage changes.     You don't have to change the RTLbias value when output is 15V，or increase RTLbias properly.

    The 10V Zener diode is used to clamp the maximum voltage of cathode of TL431 at 10V,