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Question about UCC28950 CS pin waveform

Go
Genius 4220 points
Other Parts Discussed in Thread: UCC28950

Hi,

I'd like ask about the waveforms in the attached file,

Why does the VCS voltage become GND level before the V_r201 voltage becomes GND level?

Is this correct behavior?

Thanks

Go

UCC28950 Schematic and Waveform_pptx.pdf

  • 0
    Intellectual 385 points

    I think that the CS pin is shorted to ground when the output of PWM comparator changes state when the CS voltage reaches de EA output value. This is not shown in the  datasheet but I found it experimentally.

    By the way, It seems  that your filter in the CS has a a too high time constant. D201 is not needed. The value of the RSUM resistor seems extremely low. 120K to 130K looks more reasonable.

  • 0
    Prodigy 20 points
    Did you have any problem monitoring CS pin? I have blown up the FET just placing the probe on the CS pin. Seems CS pin is very sensitive. I was wondering if you had any similar issues...thanks
  • 0 in reply to Udi Jay
    Intellectual 385 points
    I have put the scope probe many times in the CS pin without any problem, even at full load. (Probe in the x10 position. Ground of the oscilloscope power supply disconnected).
    The CS pin has a high impedance but because it is Connected to C251 and R310, the node has quite a low impedance. Check that the tracks from the pin to R201 are short and close to the ground track. I assume you are working at about 60 kHz and I think that 1 K and 470 pF would be a better choice for R310 and C251.
  • 0 in reply to Francesc Casanellas
    Prodigy 20 points
    Thanks for the reply. I'm working with the UCC28950 demo board. I changed the FETs to SiC FETS and A/B and C/D delay times are fixed. got a bit better efficiency. running at 100KHz. Seems I just can't touch the CS pin at all. Once scope was floating and once it was grounded. Either way FETs blow up. I changed out the Current sense transformer to different low profile one, but still blew up. I just can't seems to understand why CS pin would make FET's blow up. Always the left leg FETs. I have to go back to original FET's and see if this problem still there.