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UCC28180: UCC28180

Pravin Iyengar
Prodigy 190 points
Part Number: UCC28180

Hello,

I am trying to understand how the Soft over current limiting works on the UCC28180. I am building a 3kW PFC but the I_SOCcurrent is limited to 30A at the moment.

When I apply an overload at the output, I see that the PFC current starts to lose control and you can see the bump in the waveform below. This is also seen in simulation. I would have expect it to start clipping at the point of the Soft over current limit. Could anyone explain why this happens and how I can control it. 

CH1: VIN CH2:I_IN, CH3: V_boost, MATH: P_IN

Applied load: 35 Ohms

Thank you.

Regards,

Pravin

  • 0
    TI__Mastermind 26225 points

    Hi Pravin,

    The soft over current feature is when the voltage at the ISENSE pin falls below the -0.285V threshold. This does not immediately shut off the gate driver however. Instead an internal 4kΩ resistor is from VCOMP to gnd which will discharge the VCOMP pin (may take some time to fully discharge). You should see the duty cycle decrease as VCOMP discharges. The waveforms you are seeing are likely due to the modified VCOMP voltage which is used to set the current amplifier gain and the PWM ramp slope.

    If you are looking for a sharper overcurrent threshold, you could use the peak current limit feature of the device which will immediately terminate the active switching cycle.

    Best Regards,

    Ben Lough

  • 0
    TI__Expert 6680 points
    Hi Pravin,

    Soft Overcurrent (SOC) limits the input current during over load condition. SOC is activated when the current sense voltage on ISENSE reaches –0.285 V. This is a soft over current control as it does not directly switch off the gate driver. Instead a 4-kΩ resistor connects VCOMP to ground to discharge VCOMP and the control loop is adjusted to reduce the PWM duty cycle. This is why you see a distorted current waveform.

    Regards,
    Bosheng
  • 0 in reply to Benjamin Lough
    Prodigy 190 points

    Hello Ben, Thank you for the information. This distorted current due to the modified Vcomp voltage, will it clip when it hits the Peak current limit, or is it likely to go beyond the limit. In the simulation, for a given 84A peak current limit, the current peak seems to rise in excess of 120A . I am not sure if this is due to the limitations of the simulation model or if I should expect that in practice.

    Thanks

    Pravin

  • 0 in reply to Pravin Iyengar
    TI__Expert 6680 points
    Hi Pravin,

    There are two current limits: peak current limit, this is a cycle by cycle peak current protection, the PWM will be chopped if current hit the limit. the other one is soft over current protection (SOC). Is the 84A for peak current limit or soft current limit? When SOC is triggered, Vcomp will drop, there is no clamp on current, its peak value may still go up, however, PFC output voltage will decrease, so the total output power will be limited. This limitation is a soft limitation, the output power will not be precisely limited to a fixed value.

    Regards,
    Bosheng
  • 0 in reply to Bosheng Sun
    Prodigy 190 points
    Hi Bosheng,
    Yes the 84A is the peak current limit, SOC is at 50A. In my test setup, the load is located on the secondary side of a LLC resonant converter. Thanks for your explanation, its making more sense now.
    This is how I understand it. Please correct me if I'm wrong. If I apply an overload at the output, the input current should rise to the SOC initially and the duty cycle should start to reduce. However, in this region there is no clamp on current, so the input current may still continue to rise until it reaches the peak current limit, upon which the controller should go into cycle-by-cycle current limiting, in this case, 84Apk.
  • 0 in reply to Pravin Iyengar
    TI__Mastermind 26225 points
    Hi Pravin,

    Yes, your understanding is correct.

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Prodigy 190 points
    Great, Thank you very much for clarifying that.

    Best regards,
    Pravin