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UC2843: query for UC2843_ IL is abnormal

Harry Ma
Intellectual 2250 points
Part Number: UC2843

Hi Team

Customer is using UC2843 as a buck controller. the waveform was shown as folloing when Vin-410V, and Vout=380V, Pout=1000W. and control circuit is shown as the second picture.

Can you help to show your insight in this phenomenon? Why this happen?Duty cycle and IL are not stable looks like.

CH1:DRV；CH2:comp；CH3:IL；CH4: Visense；math:(COMP-1.4)*(1/3)

  • 0
    TI__Mastermind 29755 points

    Harry,

    If the purple signal is the CS, it doesn't look so good. Maybe try cleaning up the CS signal and measure directly across C207 with tip and barrel scope probe technique. Also, it looks like maybe there is not enough slope compensation. What happens if you decrease R219 (adds more slope comp)? How much will the power stage allow the input voltage to increase without damage because as you increase VIN, you decrease the duty cycle and require less slope comp - This would be a quick test just to see if slope comp is the issue? Also, check the gate drive directly at VGS and make sure the drive edges look good. 380V/410V is 93% duty - check the voltage across the low side buch diode and make sure there is no reverse recovery problems.

    Regards,

    Steve

  • 0 in reply to Steven Mappus1
    TI__Intellectual 2250 points

    Hi Steven

    • Customer try to increase the Vin, and find this phenomenon is disappeared.  
    • They are using Sic Diode, so there is no reverse recovery current.

    Looks like the reason is slop compensation to cause this result,  beside decrease R219, is there any other way to help increase compensation?

  • +1 in reply to Harry Ma
    TI__Mastermind 29755 points

    Harry,

    The time-varying inductor current is di/dt=V/L, so to overcome instability due to subharmonic oscillation in peak current mode control, you can either:

    1. Increase the voltage (as you just reported)
    2. decrease the inductance
    3. increase the amount of artificial ramp (slope comp) 

    Regards,

    Steve