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UC3843A: Design materials and tools for resistor Ra and capacitor Ca

R.Fukunaga
Expert 2455 points
Part Number: UC3843A

Tool/software:

Hi all,

My customer consider Flyback circuit is configured and used as shown in Fig. 7-1 of the datasheet.

I would like to know how to design resistor Ra and capacitor Ca connected between COMP (1pin) and FB (2pin).
Also, do you have design materials and tools for resistor Ra and capacitor Ca?
When I connected an electronic load to the output and increased the load, the ON duty of the FET became unstable after a certain amount of load.
When I changed the resistor Ra from the current 150 kΩ to 10 kΩ, there was an improvement trend, so I am thinking of reviewing the peripheral circuit design.

UCx84xA Current-Mode PWM Controller datasheet (Rev. H)

Best Regards,

Ryusuke

  • 0
    TI__Prodigy 740 points

    Hi,

    Ra and Ca are used for pole compensation to introduce a pole at ESR zero, where the frequency of the zero is given by:

    and the frequency of the compensator is given by:

    Here, only the RC product of Ra and Ca matters for setting the pole frequency. By rearranging both the equations, we get the following formula:

    I would recommend you try changing Ra to 10 kΩ just as the datasheet recommends and re-calculate Ca with the formula above. The datasheet recommends this value of Ra, because it avoids small Ca capacitance value, which can be dominated by parasitic capacitances.

    In this circuit, since Ra is too large (150 kΩ), Ca is so small enough to be dominated by parasitic capacitances (0.1 nF). 

    Hope this helped.

    Thanks,

    Best,

    Adhvaidh