BQ76952: Charge MOSFET Failure during High charge Currents

Hello Mounish,

You can send your schematic privately through a private message! I will send a friend request.

Could you describe in more detail how the CHG FET is behaving? What are you seeing happening to the CHG FET? Does the BQ76952 raise any alert flags?

Best Regards,

Luis Hernandez Salomon

Hi Luis I did send a private Message Please check

To be specific, when I put over charge current protection, I can see that Charge FET is turned off with commands but Battery Pack is still taking load and Over Voltage Protection is not observed

Hello Mounish,

From your schematic, the reverse charge polarity circuit should be referenced to PACK-, not Vss. Although I don't believe this would be causing the issue.

If you turn-off the CHG FET prior to 25-A, is there still a charging current? If the CHG FETs are OFF then there should be no charging current. Unless there is some leakage that could be bypassing the FETs. Were the FETs placed correctly? 

Is there any waveform showing the behavior? What protection faults are triggered by the BQ76952 according to its battery status?

Best Regards,

Luis Hernandez Salomon

Over Charge Current protection is triggered, FET is damaged/shorted and it is allowing current. Is Gate resistors ok?

Mounish,

Is there any waveform showing the event?

There could be different reasons that the FET was damaged, if the turn-off was too fast voltage transients may be able to damage it. If it is too slow, the FET could potentially also get damaged. The following FAQ (Which discusses turn-off of FET during short-circuit) may provide some insight: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1054353/faq-bq769142-mosfet-short-after-short-circuit-test---voltage-now-always-on/3900591#3900591

Best Regards,

Luis Hernandez Salomon

Ill try to get Waveform . But is there a way to calculate Chg FET resistor to choose correct rise time/fall time?

Mounish,

We do not have a good way to calculate the rise/fall times as these can be non-linear. The fall/rise time will depend on the amount of FET capacitance there is.

However, we do have some information that may give you some idea, you can see t_(CHGFETOFF)/t_(DSGFETOFF) under Section 7.25 High-side NFET Drivers in the datasheet. Additionally you can also read the Multiple FETs with the BQ76952, BQ76942 Battery Monitors app note for some more idea on this topic, which is discussed. It also includes a table for different fall times of the DSG FET with different FET capacitances.

Best Regards,

Luis Hernandez Salomon

Luis H.S,

Please find wave forms ( Fall times of FETs under load and no load conditions in Personal message )

I would like to inform that

1. Device is safe when Battery charging/Discharging  Interruption is made by heating Temperature sensor (NTC)

   Note: Charging is done with CC-CV charger and Discharging is done with resistive loads

2. When Battery Charging/ Discharging Process is interrupted by heating NTC, I see Discharge FET failure in one machine and charge FET failure in another machine

 Note: Here charging and Discharging process is performed a Test machine  ( https://en.neware.com.cn/ )

3. What is your comment on my Fall times? Fall times should be faster or slower in my case .. Kindly Please let me know

Hi Mounish,

Luis is out of office right now for the holiday, he will be back soon and will respond, based on the documents you provided to him personally.

Thanks,

Terry

Hello Mounish,

It would be more useful to see the DSG/CHG fall time in relation to the PACK/BAT voltage respectively during the fault condition. Are there high transients during the turn-off at a fault? I want to know if the FETs are damaged due to heat or due to voltage transients.

By how much are the FETs heating up? I am surprised these are heating up that much with 6 FETs in parallel. Are the FETs not able to handle above 25-A?

Best Regards,

Luis Hernandez Salomon

Luis H.S,

150Ah Battery Pack and Testing equipment Model A:

 I will send Fall times with scope ground at PACK - , I personally feel this is not a heat failure because, if the fault is not triggered they are able to handle 40Amps easily, only when fault is triggered and discharge process is started again ( after fault is recovered ) i see smoke from  Discharge FET . Where as in charging process FETs are safe.

40Ah Battery Pack and Testing equipment Model B:

In this Pack and Testing equipment Discharge FET is safe but I see charge FET failure but I doubt Testing equipment producing voltage peaks in this case, I will try to send waveform for this too..

So,

In first case do you think failure is due to Battery Transients ? , If so should i slowdown FET off time? 

Thanks & Regards,

Mounish

Hello Mounish,

I see, so you see the smoke after the FETs start again. Is the load still applied when the FETs start on? It could be possible that the turn-on is too slow while a large load is applied, if it is too slow and the FET remains in the resistive region for too long, it could be possible for the FET to heat-up and burn. 

• Do you see the FET burning if the load is removed before recovery? 
• I see the DSG FET turn-on resistor is 10-kOhm, which is pretty large, could you lower this and see if there's an improvement?

Regarding the 40-Ah battery, is there any additional information as to when/how this happens? Does it also occur after recovery?

Best Regards,

Luis Hernandez Salomon