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UCC28950: oscillation current DC / DC full bridge

yelsing torres
Intellectual 340 points
Part Number: UCC28950

Hi.
I am using the ucc28950 for a DC / DC source. I am using the current sensor B82801C2245A200. As you can see in the image below (blue signal), the current oscillates a lot. I do not think it's ringing due to poor grounding, because I made a single board for the current sensor, and I'm not connected to the ucc28950 CS signal. I have done simulations, and it is apparently by the parasitic inductor of the transformer, but I am not sure. What else can cause this oscillation in the current, and how can I avoid it?
Thank you.

el 

  • 0
    TI__Mastermind 27135 points
    Hi Yelsing torres,

    Thank you for your interest in UCC28950. The ringing could be caused by parasitic inductance and parasitic capacitance creating a resonance. Could you try replacing your current sense transformer with a sense resistor and see if the ringing is still present?

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Intellectual 340 points

    Hello benjamin, thanks for your answer.
    It is difficult for me to use a shunt resistor to measure current. I have tested my current sensor, and I am pretty sure that it is measuring well and the sensor has a very low impedance return. What is your suspicion to measure with a shunt instead of the transformer current sensor?
    I have made a reflow of my board and partially solved the problem of oscillation in the current. In the image below, in blue color you can see the current signal. The current continues to oscillate, but at a more tolerable level. Is it possible to eliminate this oscillation even more?
    The oscillation period is 280ns and I have put an RC filter at the output of the current sensor of 1.8MHz

  • 0 in reply to yelsing torres
    TI__Mastermind 27135 points
    Hi Yelsing,

    It would be ideal to identify which components in your design is contributing to the parasitic inductance and capacitance. I suspect the primary source of the parasitic inductance is one of the magnetic components, either TX1 or TX2. My suggestion is to (temporarily) replace the current sense transformer with a sense resistor and see if the ringing is still significant.

    Best Regards,
    Ben Lough
  • 0 in reply to Benjamin Lough
    Intellectual 340 points
    For me, oscillation in the current is due to the capacitance of the rectifying diodes and the parasitic inductance of the main transformer. It can be seen in the yellow signal, which corresponds to the voltage in the transformer, an oscillation occurs with the same frequency as in the current signal. I suppose that osilcaion in the voltage, is followed by the current. I did a test, removing the rectifiers and connecting a resistive load to the output of the transformer. The current signal was totally clean. I think that if I can reduce the oscillation caused by the rectifiers, the oscillation in the input current would decrease.
    The next week I'm going to look for a good module to measure current with shunt, or I'll borrow a current probe, and I'll tell you how it was.
    Thanks