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DRV8702-Q1: Abnormal current trip before OCP threshold

Amirul Kamaruzaman
Prodigy 40 points
Part Number: DRV8702-Q1

Tool/software:

Hi,

I have recently received my prototype board for the H-bridge DC motor driver circuit using the DRV8702-Q1.

Schematic can be referred in this thread: DRV8701EVM: Queries on the output current limit - Motor drivers forum - Motor drivers - TI E2E support forums

The design is to current chop 23A during startup, and 40A OCP with the 3mohm Rdson FETs.

However, during my testing I observe some abnormality where the board starts to trip whenever I load more than 20A.

When I further investigate on this, I see that the current spike to 80A and cause the board to trip. But my eload is only set to 20A.

I have scoped all related points for your reference as below:

My test setup are:

2x PLZ334W E-load, 24V Battery, and a MSOS404A mixed signal oscilloscope (differential probe for voltage, current probe for current).

All time is set to 20us, and the yellow graph (10V/div) is voltage respective to each probe point and red graph (20A/div) is the current from the eload (-ve) to the terminal P42 of the board.

For this test only one direction flow from right to left so it is expected only Q10&Q13 is ON.

It is observed that before the current spike, Vgs GL1 (Q14) is suddenly turned to high and Vgs (Q10) GH1 is turned to low. I am unclear why this happen and need your advice how to overcome this issue.

Is this behavior only happening because of using e-load to simulate the motor? Would using a DC motor eliminate this issue? I am still waiting for the DC motor to arrive.

I did also try to connect the eload directly to the battery and no problem found to load more than 20A directly from the battery. 

Let me know if you require additional information or any other points to probe, I can provide you the graph.

Appreciate your help on this.

Best Regards,

Amirul

  • 0
    TI__Expert 6700 points

    Hi,

    Thanks for your question. Our device expert will look into this and get back to you.

    Thanks,

    Ibinu

  • 0 in reply to Ibinu Alaudeen Nadeera
    TI__Mastermind 35745 points

    Hi Amirul,

    Thank you for your questions.

    First suggestion is not to use E-load (PLZ334W) for this type of testing. I suggest to use actual motor. E-Load sometimes shows a little strange behavior. It confuses us. Eventually we may spend time for debug of E-load, not DRV.

    When we want to do some test while motor is not on our hand, normally we use Inductor (~10mH) + Power resister(~3ohm).

    regards

    Shinya

  • 0 in reply to Shinya Morita
    Prodigy 40 points

    Hi Shinya,

    Thank you for the response, I have tested the circuit with DC Motor and summary of the results is shared below.

    Detail of the test also can be referred to this google sheet link here.

    Test Procedure:

    1. Turn on switch signal to start motor in FWD direction (P32 +ve to P42 -ve)

    2. Run for 3s, and turn OFF switch signal to stop the motor

    3. Capture the graph

    4. Change critical points to probe voltage (yellow) and repeat steps above.

    Test Equipment:

    1. Motor Driver PCBA

    2. 24V 12Ah Battery (LPL12-12T2) 2S1P

    3. DC 24V Motor 0.5HP (BA3628-7012-9-48B)

    4. Mixed signal scope. Yellow (differential probe) 10V/div to measure voltage on critical points. Red (current probe) 5A/div to measure motor current.

    Summary Test Results:

    Observation & queries of the test results are:

    1. Running with motor produce different results compared to e-load.

    2. Current chopping is functioning at around ~23A during startup. (Good as per designed)

    3. The Vgs of the FETs during current chopping goes to -ve value. Is this normal?

    4. The Vds of the FETs have sudden transient spike up&down during motor shutdown. Is this normal?

    5. The voltage across motor during shutdown also transient spike up&down during motor shutdown. Is this normal?

    6. Since e-load is producing abnormal behavior, any idea how may I test the OCP that I set to 40A without e-load?

    I would appreciate it very much if you can go through the detail test results and highlight for any abnormalities.

    Best Regards,

    Amirul

  • 0 in reply to Amirul Kamaruzaman
    TI__Mastermind 35745 points

    Hi Amirul,

    Thank you for your questions. I will feedback to you in a day or two.

    regards

    Shinya Morita

  • 0 in reply to Shinya Morita
    Prodigy 40 points

    Hi Shinya,

    I have also tested the REV direction and the result is the same where during motor shutdown, there is a spike across the Vds of each FET.

    The spike maximum is equivalent to battery voltage (~25V). However for the spike across motor terminal will be doubled the battery voltage (~50V).

    Reverse direction summary results are shared as per below:

    Hope to get your reply soon!

    Best Regards,

    Amirul

  • 0 in reply to Amirul Kamaruzaman
    TI__Mastermind 35745 points

    Hi,

    Thank you for your questions.

    Unfortunately your attached pic is low resolution. However, I guess your cap and something yellow box may impact it. I suggest to remove it to try.

    regards

    Shinya 

  • 0 in reply to Shinya Morita
    Prodigy 40 points

    Hi Shinya,

    Are you able to open the google sheet that i provided in earlier post? In there I have zoomed version of the scope image captured.

    The yellow box is actually a placeholder for diode and is already DNP in the board.

    The cap across each drain-source FET is following the reference design, is it not required?

    Best Regards,

    Amirul

  • 0 in reply to Amirul Kamaruzaman
    TI__Mastermind 35745 points

    Hi Amirul,

    I can see only low-resolution waveform like bellow. So I can see 2 waveforms but not sure which one is what. And cannot read time and voltage scale. Can you attach with Microsoft power point or word?

    regards

    Shinya

  • 0 in reply to Shinya Morita
    Prodigy 40 points

    Hi Shinya,

    As mentioned in the test equipment, yellow is the voltage probe at each critical points using differential probe and red is the motor current going out from P32.

    Attached is the excel and power point FYR. Hope this can provide you a clearer image of the captured waveforms.

    . Motor Test Results.xlsx

    Motor Operation FWD & REV.pptx

    Best Regards,

    Amirul

  • 0 in reply to Amirul Kamaruzaman
    TI__Mastermind 35745 points

    Hi,

    Thank you for your question. I noticed your waveform is 1sec/div. Normally we use ~10us range for spike. Your spike meant longer period.

    I cannot review your all waveform and cannot comment about your system related behavior. 

    Let me try to answer these questions.

    If your goal is to avoid VDS over current shut down, you may try higher VDS or disabled.

    regards

    Shinya