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OPA2197: Why does the resistance of the 5th pin of the opamp decrease compared to the reference point

TAHA CANKAYA
Prodigy 20 points
Part Number: OPA2197

Schematic Design with OPA2197IDR 

Notes:

Vpilot+: +12V

Vpilot-: -12V

The OUT PILOT line is a PLC (power line communication) line. A signal in the frequency range of 2 to 30 MHz is superimposed on this line. But these signals have no effect on the operating state of the opamps.

Hi,

The circuit above is included in the card we use in the electric vehicle charging station. These cards fail after working for an average of 1-2 months.

+12V is produced on the 1st pin of the 1st channel of the opamp. We also measure the voltage produced in the second channel of the opamp (from the 5th pin of U6B). The resistance value between the 5th pin and the 4th pin (Vpilot-) of the U6B component is 1k ohm (Resistance measurements were made by removing the opamp from the card).

Since the resistance value has decreased so much, the +12V voltage produced (produced by the U6A component) is measured at -10V level. But the value I should read is +12V. That's why the card goes into failure state.

My question is this; Why does the resistance of the 5th pin of the opamp decrease compared to the reference point? Thank you in advance for your support in this matter. 

TC

  • 0
    Guru 287360 points

    The P4SMAJ12CA has a stand-off voltage of 12 V, but at currents that exceed the OPA2197's limit, i.e., 10 mA, its breakdown voltage is between 13.3 V and 14.8 V. So I suspect that the input got damaged by voltage spikes (probably generated by L3, or coming from OUT_PILOT or QCA_PILOT).

    It might also be possible that voltage spikes from PILOT_PWM cause damage. Is there any protection on that line?

    Put a resistor of more than 250 Ω in front of pin 5 to ensure that most of the clamping current goes through D1.

  • 0 in reply to Clemens Ladisch
    TI__Guru 54996 points

    Hi Clemens,

    Thank you,

    HI Taha,

    As Clemens have suggested, in addition to the P4SMAJ12CA you will need to improve the protection adding series resistor, to limit the current in to the amplifiers inputs to less than ±10mA during any fault and will also need to add protection the non-inverting input pin (3) of U6A.

    I agree with Clemens comments, the amplifier is likely damaged, and the op-amp inputs need to be protected to ensure the absolute maximum ratings of the op-amp are not exceeded at any point in time:

    Please review the TI Precision Lab Session explaining overstress or EOS protection:

    https://www.ti.com/video/series/precision-labs/ti-precision-labs-op-amps.html

    To find the session on EOS protection, look into "Topics" and browse the bottom of the page, you will find 4-sessions on op-amp:

     

    Here is also a short article that discusses the op-amp input protection using Schottky diodes protecting an op-amp.

    https://www.ednasia.com/provide-robust-input-overvoltage-protection-for-amplifier-analog-input-modules/

    Let us know if you have any questions,

    Thank you and Regards,

    Thank you and Kind Regards,

    Luis

  • 0
    Prodigy 20 points

    Thank you very much Clemens and Luis,

    There is only TVS diode for protection but it also activates at minimum 13,3V. This may cause the opamp to be damaged.

    I will try your suggestions as soon as possible and share the results with you.

    Regards

  • 0 in reply to TAHA CANKAYA
    TI__Guru 54996 points

    Thank you Taha,

    Kind Regards,

    Luis