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BQ76952: Questions about Short Circuit

Tonino
Prodigy 95 points
Part Number: BQ76952

Hello TI Team,

while testing a BMS for a 7S battery, we have a question about the result of our short circuit test.

Set-up description:
On the BMS we use two CHG and DSG FETs each in series mode (high side).
On the low side we use a 0.5mOhm shunt.

To measure the short-circuit current, we have connected another 100µOhm shunt externally to PACK-.

BQ Setting:

SCD threshold = 2 (40mV)
SCD Delay = 15µs
SCD Recover = 30s

The following image was captured with the oscilloscope:

Ch1 (yellow) = PACK+
Ch2 (green) = Voltage over 100µOhm Shunt (Short-Circuit current) 
Ch3 (blue)   = Drain Voltage of DSG FET
Ch4 (red)     = Gate Voltage of DSG FET 

After the drain voltage goes high when the FETs are switched off, the FETs seem to switch on and off for a short time.

Can you please help me?

  • 0
    Prodigy 95 points

    Oscilloscope Image:

  • 0 in reply to Tonino
    TI__Guru* 81034 points

    Hi Tonino,

    From the waveforms it looks like the FETs themselves are oscillating. This can be due to the individual gate resistors values of the FETs - can you share the portion of the schematic that has your FET circuit?

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 95 points

    Hi Matt,

    thanks for you reply.
    Here I have the portion of the schematic for you:

    Best regards,

    Tonino

  • 0 in reply to Tonino
    TI__Guru* 81034 points

    Hi Tonino,

    I think the problem is that there are no individual gate resistors for each of the parallel FETs. Parallel FETs can oscillate when switching without a resistor or ferrite bead for each FET gate. This is described in more detail of Section 4 of this document: https://www.ti.com/lit/an/sluaa09/sluaa09.pdf

    Also, the resistor used on the DSG pin is small which may result in too fast of a turn off. Section 6 of the same document goes into some detail describing the issues this may cause. 

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 95 points

    Hi Matt,

    I have already read the article about multiple FETs.
    However, I think that the result is less related to it.
    I have now performed a short circuit test with 60V FETs.
    The result looks much better:

    Ch1 (yellow) = PACK+
    Ch2 (green) = Voltage over 100µOhm Shunt (Short-Circuit current) 
    Ch3 (blue)   = Drain Voltage of DSG FET
    Ch4 (red)     = Gate Voltage of DSG FET

    As you can see on the image, I now have no oscillations.
    Could it be that the discharge FET is overloaded at the drain source voltage when switching off the short circuit?

    In the first short circuit test the SIR638ADP-T1-RE3 was on it.
    Now I have tested with the BSC010N04LS6.

    Best regards,

    Tonino

  • 0 in reply to Tonino
    TI__Guru* 81034 points

    Hi Tonino,

    That is great that these FETs do not show the oscillation. This type of issue is very FET dependent. Regarding individual gate resistors, you should check with the FET supplier because some are concerned about this while others are not. The FET supplier will be able to advise best on how to ensure this oscillation does not occur for their device.

    Best regards,

    Matt

  • 0 in reply to Matt Sunna
    Prodigy 95 points

    Hi Matt,

    thanks for your support.
    I will check this with the FET supplier.

    Best regards,

    Tonino