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TMS320F28379D: 28379D CMPSS cycle-by-cycle current trip

Kaiyu Wang
Prodigy 100 points
Part Number: TMS320F28379D


Hi,

I am using 28379D CMPSS module to run the peak current control for a buck converter. The comparator positive is from a LEM sensor output, and the comparator negative is from the internal DAC. The LEM sensor outputs 2.5V when zero current flow through it, then this signal passes a voltage divider to feed 1.5V to comparator positive. Although the LEM sensor can measure bi-directional current flow, we only use it to measure the one-direction current in our application, so the 28379D comparator positive measures a voltage from 1.5V to 2V when the buck converter current changes from 0A to 50A. 

In order to test the peak current control, I set the internal DAC of 28379D comparator negative bus slightly higher than 1.5V, and recorded the buck current (light green) and PWM (blue) waveform when the buck converter starts.

I can see two issues from the waveform 

(1) The trip threshold is not consistent. The first trip happens at higher threshold than the others although I keep the DAC value same.

(2) There seems a huge hysteresis when the comparator change from 1 to 0. The current cannot maintain a relative stable value.  (I set the comparator hysteresis to 0)   

The PWM is at 50kHz and LEM sensor bandwidth is at 250 kHz. I understood that I lost some measurement resolution when the zero current is set at 1.5V instead of 0V.  

What could be the possible root cause of such behavior? 

  

  • +1
    TI__Mastermind 28275 points

    Hi Kaiyu,

    A few questions:

    1. You mention in your description that the buck converter current changes from 0A to 50A and the comparator positive input will see this change as 1.5V to 2V. Your scope shot however is showing current going from 0 to 0.8V. Can you clarify this?

    2. Do you have filtering enabled?

    3. Can you provide your CMPSS configuration code?

  • +1 in reply to Frank Ankapong
    Prodigy 100 points

    Hi Frank,

    I think we found the issue. The corner frequency of our external low pass filter which is connected to the comparator positive bus is too low. After I changed the filter capacitance, the inductor current now can be maintained at certan threshold.

    I just have a quick question about the low pass filter design for this CMPSS comparator. There are different R and C combinations to make the same corner frequency. Is there a design guideline to pick the optimal range of R and C?

    Thanks,

    Kaiyu

      

  • +1 in reply to Kaiyu Wang
    TI__Mastermind 28275 points

    Hi Kaiyu,

    Glad to hear you resolved the issue! In terms of picking the components, this really depends on the frequencies you are trying to filter out.

    However you typically want to consider the source and load impedances. The comparator input is high impedance so practically doesn't affect the circuit. At attenuation frequencies, the R of the RC network will become the load seen by the source (+ the source's own impedance) since the capacitor effectively shorts to ground. You just need to make sure the R is not too high of a load for your source. Generally you don't want the R to be too low.