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UC3843: Slope Compensation

J
Expert 1045 points
Part Number: UC3843

I watch this paper and I have some questions, in this figure the Vramp is current slope add  slope compensation, and use superposition can get  this formula, 

1.) in the Simplis simulation, when I design the current mode control I just need to generate the triangle wave and then add with inductor current.

using the UC384x I need to use this method to get the slope compensation but the inductor slope would reduce R2/(R1+R2)，is this situation ok?  why can reduce the inductor current?

2.) If the inductor current peak value is 2A the Rsense is 0.1 so the voltage through the Rsense is 2*0.1=0.2V that mean the voltage command is 0.2V, but if I use below equation I need to time R2/(R1+R2)?
0.2V*R2/(R1+R2) is this correct?

 

1.)

  • 0
    TI__Mastermind 28015 points

    Hi,

    1.) in the Simplis simulation, when I design the current mode control I just need to generate the triangle wave and then add with inductor current.

    using the UC384x I need to use this method to get the slope compensation but the inductor slope would reduce R2/(R1+R2)，is this situation ok?  why can reduce the inductor current?

    [TI]: Yes, it is acceptable. There is a slight reduction of the CS waveform and a greater reduction of the RT, CT waveform. For effective and stable peak CMC, the two are summed together such that the contributed amount from the sensed inductor waveform should be much greater than the contributed amount coming from the oscillator waveform. If there is too much contributed amount coming from the oscillator, the control method will be more like VMC. The ratio of resistpr based on CS filtering and oscillator RAMP contribution must be adjusted accordingly.

    2.) If the inductor current peak value is 2A the Rsense is 0.1 so the voltage through the Rsense is 2*0.1=0.2V that mean the voltage command is 0.2V, but if I use below equation I need to time R2/(R1+R2)?
    0.2V*R2/(R1+R2) is this correct?

    [TI]: Correct

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    Expert 1045 points

    OK I got it

  • 0 in reply to Steven Mappus1
    Expert 1045 points

    Excuse me, Sir one more question

    The example you give me before, the turn off slope of inductor  and you said it max value is about 60%, but in the paper you give me they usually use down slope of inductor current , why I can always use turn on slope .

  • 0 in reply to J
    TI__Mastermind 28015 points

    Hi

    If you use the inductor downslope you will always assure enough slope compensation even for duty cycles >> 50%

    Regards,

    Steve M

  • 0 in reply to Steven Mappus1
    Expert 1045 points

    I still not get it why the downslope can assure the enough slope compensation but upslope can't