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OPA192: OPA192 output voltage trigger issue

Vincent Li00
Intellectual 2035 points
Part Number: OPA192
Other Parts Discussed in Thread: TINA-TI, TL431, TLV431

Hi Guys

our customer is using our OPA192 as high side current sense. common mode voltage is 12V. They set the output current protection 75A. But when we test this current sense circuit. 

If we step by step test and keep the static current stable, we can get the ideal result. Protection current is ~76A;

But if we test the inrush current or current transient, we found the protection current will go up to 90A. It is very dengerous for the whole system.

Could you help us analyse what lead to the protction current change?

CH2: Iin_det (Iinrush_det)

CH4: current

Thanks

-Pengfei

  • 0
    TI__Guru* 93105 points

    Hi Vincent,

    We will require more information to assist with the issue:

    1. What are the VCCO and VCCO_RTN values?
    2. The graph details are not easily seen and interpreted. It would be helpful if more explanation could be provided regarding each DSO's trace meaning. 
    3. Do you expect there is an issue with the way the OPA192 is functioning in the application? It looks like from the schematic that the maximum OPA192 output should be about close to 0 V for Ish = 0 Amps, 2.3 V for Ish ~ 75 Amps, and about 2.8 V for Ish ~ 90 Amps. The green DSO curve looks to be okay?

    If the circuit performs as expected when operated statically and doesn't during a transient condition, then there is likely a dynamic response issue with the design.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to Thomas Kuehl
    TI__Intellectual 2035 points

    Hi Thomas

    1.VCCO=16V.   VCCO_RTN=GND. single end power supply;

    3. sense resistor is 0.000536 ohm. R612 R613 should be 808 ohm. So the real Gain is about 61.88. Their protection circuit design the protection point is 2.5V target to 75A. But real test result is 89A protection just as waveform shows.

    Thanks

    -Pengfei

  • 0 in reply to Thomas Kuehl
    TI__Intellectual 2035 points

    One more question: if we want to sample 5A/us at 0.5m ohm resistor at 62.5x Gain. what is the bandwidth we need? 

    Thanks

    -Pengfei

  • 0 in reply to Vincent Li00
    TI__Guru* 93105 points

    Hello Pengfei,

    For reference purposes note that the OPA192 slew rate and unity-gain bandwidth are typically 20 V/us and 10 MHz, respectively. If we use the 5 A/us rate of current change through the 500 micro-Ohm shunt resistor, the voltage rate of change applied to the OPA192 inputs would be 25 mV/us. Since the gain is 62.5 V/V, the OPA192 output voltage would be required to slew at 62.5 V/V(25 mV/us), or 1.56 V/us.

    Op amp slew rate is equal to SR = 2 pi (f) (Vpk). This can be rearranged to solve for "f". The bandwidth required, can be derived from:

    f = SR / [2 pi (Vpk)] 

    Using a voltage change at the op amp output of 3 V (0 to 95 Amps at the OPA192 input)

    f = 1.56 V/us / [6.28 (3 V)] = 1.56e6 V/s /[6.28 (3 V)] = 82.8 kHz

    Therefore, the OPA192 has plenty of bandwidth for the application. I put together a TINA-TI simulation and a dc sweep of the input current produced the expected results. It looks like the OPA192 is working as expected. Dynamically, the behavior may be more difficult to capture. It depends on the ac accuracy of the rest of the models used in the circuit. The OPA192 has a very good dc/ac/noise model.

    My concern is the way the TL431 shunt voltage regulator is being shut down when the current trip level is reached. Since Precision Amplifiers doesn't cover that device we have no information about how it behaves when applied in that manner. It may behave differently in static and dynamic modes when used in this manner.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

     

  • 0 in reply to Thomas Kuehl
    TI__Intellectual 2035 points

    Thanks Thomas. Could you share this simulation model to us? I will evaluate by different value.

    Thanks

    -Pengfei

  • 0 in reply to Thomas Kuehl
    TI__Intellectual 2035 points

    Hi Thomas

    at your simulation model, what is the current protection point? 90A? or 75A? could you have a try to add this input current slew rate to 1A/us to check whether this system can support 75A proctection function?

    Thanks

    -Pengfei

  • 0 in reply to Vincent Li00
    TI__Guru* 93105 points

    Hello Pengfei,

    The TINA dc or static simulation of the circuit shows the current trip occurs at about 80.8 Amperes:

    I then set up the circuit for a 1 A/us current change through the shunt resistor. For a 90 Amp current step having a 1 A/us slew the duration would be 90 us. The results are as follows:

    I am confident of the OPA192 ac results and believe the transistors are good too, but I don't know enough about the TL431 results because it belongs to another TI division. Nevertheless, if the simulation is close there is about a 22 us delay after the input pulse reached 90 Amps until the output of the TLV431 (VM2) changes state. Then on the decreasing side of the current pulse there is a similar delay and the VM2 changes state when the input current drops to about 63 Amps. Hopefully, this is useful in helping you understand how the circuit functions.

    I am including my TINA simulation circuit for your examination. Note that the PBHV8115T simulation model I found on line does appear to be functioning correctly so I substituted another HV bipolar transistor at the output stage (2N3440). That should have very little effect on the overall simulation results.

    OPA192_Hi_Cur_Sh_01.TSC

    Regards, Thomas

    Precision Amplifiers Applications Engineering 

  • 0 in reply to Thomas Kuehl
    TI__Intellectual 2035 points

    Thanks Thomas. I will talk about it with customer. Seems good for OPA192 device.

    -Pengfei

  • 0 in reply to Vincent Li00
    TI__Guru* 93105 points

    Hi Pengfei,

    Since we have done our best to address your OPA192 questions would it be possible for you to close this e2e inquiry now?

    Thanks. Thomas

    Precision Amplifiers Applications Engineering