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Original question:

UCC2897A: Design is sometimes shutting down leaving the control circuit consuming a lot of power.

UCC2897A: Power is shutting down while probing the pins which are controlling the gate signals.

Navin Kumar2
Intellectual 380 points
Part Number: UCC2897A

Hello,

I am facing an issue, in which my output is shutting down, whenever I am touching the pins LINEUV, LINEOV, CS, SLOPE ( Only one pin of the multimeter is connected at that point). All these pins are controlling the gate signals and are connected to a current source generated from the Vref. But when I am connecting capacitors across the LINEUV and LINEOV, then this is not happening at these two pins only(as shown in the schematic). Why is this happening?  How can I solve this? 

  • 0
    TI__Mastermind 34465 points

    Hello ,

    Connecting a multimeter to any of these nodes will introduce noise to the high impedance input 
    which will cause it to misbehave in an unpredictable behaviour.

    Connecting a cap from any of these pins to GND filters out any noise on the pins.

    I recommend that you do not connect a multimeter to these pins when the system is operating.

    Regards

    John

  • 0 in reply to John Griffin1
    Intellectual 380 points

    Hello John,

    I am getting this issue even when I am touching only one probe at those pins, so there cannot be a closed-loop current path. This shows that these pins are very sensitive. I am worried that this problem can persist if I encapsulate(conformal coating/potting) my circuit.

    Also, since you are recommending not to use multimeter at these pins, how do I characterize my circuit? 

  • 0 in reply to Navin Kumar2
    TI__Mastermind 34465 points

    Hello Navin,

    The long lead on the multimeter will add noise to any pin on the controller that you touch and it is just not possible
    to use such an instrument with a switching power supply.
    If you want to measure voltages and look at waveforms then you will need to use an oscilloscope with suitable
    high frequency probes.
    A conformal coating on the pcb or encapsulating the entire power supply will not do this.
    It will only replace the dielectric constant and magnetic permeability of air with the properties of your coating.

    Suitable coating and potting materials are designed so as not to interfere with the electrical signals of the power supply.

    Regards

    John

  • 0 in reply to John Griffin1
    Intellectual 380 points

    Thank you, John!

    I tried to probe these points using the TPP0201 Tektronix oscilloscope. Now the voltage is not dipping except for the SLOPE point, where the output voltage is going to zero if probed. This point is connected to a current source Islope. I tried to find the value of this current in datasheet and application note, but I could not. Please help me with the value of this current. 

  • 0 in reply to Navin Kumar2
    TI__Mastermind 34465 points

    Hi Navin,

    This is a new question so you should close out this post and open a new post relating to this new issue.

    Regards

    John