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BQ79656EVM-021: BQ79656 EVM-021 Current Measurement Issue

Ahmet Kaan Büyükkaya
Prodigy 20 points
Part Number: BQ79656EVM-021

Hello,

I am using this BQ79656 EVM-021 in simulator mode and I can read current, voltage and temperatures so everything seems fine. But when there is no flow there is voltage swing around 1 mV in ADC reading. I tried to short circui SRP_s and SRN_s test points using jumpers so that I can see 0 amperes but it burned the circuit and IC. I changed all capacitors and resistors and worked with the module again and everything was ok. I thought that the first issue might not be realted to the current sense and put a 0 ohm resistor at R17. Apparently it was related to the current sense because after a while I saw that Cell 12 and Cell 13 voltages started to change so my guess was that the capacitors are being damaged. I powered down the circuit and Changed the filter capacitors and other 2 capacitors( C36-38-40-42). Now everything is ok again.

When I used power supply to simulate shunt resistor at SRP_s and SRN_s test points. I tried 25 mV, 50 mV, 75 mV and 100 mV and I clearly got the right results.

I don't use special current sense ADC for now. I only use main ADC. I used LPF to see if that would solve the fluctations but it didn't work.

Why does that happen and is there anything that I should be careful while testing it with a shunt resistor? I wonder if I would have such an issue. I thought that since it is a board designed for multiple ICs, there might be an underlying layout mistake.

  • 0
    TI__Expert 3760 points

    Hi,

    This is odd. A few preliminary questions:

    1. When you say "no flow", are you referring to when there is no shunt resistor? 

    2. When you changed the resistors and capacitors, do you mean those relevant to the SRN/P pins?

    3. What is your grounding scheme for this board? This is important for ADC accuracy. 

    There may be some delay in subsequent responses because of the holiday. Thanks for understanding. 

    Best,

    Nancy

  • 0 in reply to Nancy Hidalgo
    Prodigy 20 points

    1. Yeah exactly. When they are left unconnected.

    2. Nope. Not at all. You can see them on schematics they are filter capacitors for Cell ADC circuit.

    3. My connections are as told at the "5.2 Power Connections" section. I use 48V power supply. PWR test point is connected to the 48V and Cell0 test point is connected to the ground. J14 and J16 jumpered.

    Here you can see the schematics of the board to understand components I am talking about: 

    BQ79616-Q1, BQ75614-Q1, BQ79656-Q1 Evaluation Module User's Guide (Rev. B)

    Also, SRP_s and SRN_s pins are also BBP and BBN pins. But all the Bus Bar connections are configured as dnp therefore I don't expect shorting these 2 pins to affect anything. This is why the first thing that came to my mind was a forgotten layout path or something like that.

    Best Regards.

  • 0 in reply to Ahmet Kaan Büyükkaya
    TI__Genius 17370 points

    Ahmet,

    We are entering into US Thanksgiving Holiday, so please expect delays.

    Regards,

    Taylor 

  • 0 in reply to Taylor Vogt
    TI__Genius 17370 points

    Ahmet,

    If I understand correctly, everything is working properly now? My guess is perhaps you shorted one of the NPNB or LDOIN test points to BBp/BBN or GND and that caused the first issue since they are all nearby each other. I would say just be very careful and let us know if it happens again.

    Regards,

    Taylor