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BQ76940: Charge path of Bq76940 datasheet Figure 7-3 and its components

smash Micheal
Intellectual 1000 points
Part Number: BQ76940

Dear All,

Am working on Figure 7.3 of Bq76940 datasheet for Charge Path Fall Time & Rise Time @ Gate to Source pins of Mosfet.

Am obtaining 2.5msec fall time @ Gate to Source Pin of Mosfet over oscilloscope..Below is the circuitry i have used.

 

I wanted to understand the significance of reducing R2 @ Charge Path circuitry.

I Mounted following :-

  1. R1-1 mega Ohm Fall TIme Obtained is 2.5   mili second.
  2. R1-100K Ohm     Fall TIme Obtained is 1.25 mili second.
  3. R1-10K Ohm       Fall TIme Obtained is 500  micro second.
  4. R- 1K Ohm          Fall TIme Obtained is 50    micro second.

And on the Discharge path with 1Mega Pull Down, Both Rise & Fall time are same approx 2.5 micro seconds.

Above readings which, i have observed with MCP1416t-ET Driver instead of AFE CHG & DSG Driver pins(Because when I Use AFE Driver Pins, I get following )

  1. Discharge path Rise Time – 300 micro seconds
  2. Discharge path Fall Time – 120 micro seconds
  3. Charge path Rise Time – 300 micro seconds
  4. Charge path Fall Time – 4 milli seconds

 

, means i have Bypassed CHG & DSG pins of AFE to trigger Mosfets, and instead of it i have used MCP1416t-ET Driver to Drive Mosfets.

Also i have placed single Mosfet only in parallel and 2 mosfets in series with Common Drain Configuration  .

So please help me to understand, why if i use Gate-Source Pull Down of 1 M Ohm, than Fall time are not equal ,  for both (Charge & Discharge paths)(Discharge Path 2.5micro sec Fall time & Charge Path 2.5 milli second Fall time @ Respective Gate-Source Pins of a Mosfet?)

Also help me to understand scenario , when Pack- will be lower than Bat- or VSS or Ground? Am unable to visualize it.

Also share use of Keeping R1,R2 at CHG path @1M Ohm . Can it be increased?

Rohit

 

  • 0 in reply to smash Micheal
    TI__Expert 7035 points

    Hi,

    In the electrical characteristics table you can observe the FET rise times, t_FET_on, which seems to be about correct. The CHG and DSG FETs have different rise and fall times. In the same table, you can find the value for tDSG_OFF. We mention in section 10.3.1.3.1 of the datasheet that "DSG uses a fast pull-down to VSS when disabled, while CHG utilizes a high impedance (nominally 1 MΩ) pull-down path when disabled." This means that the CHG fall time will be slower, as you observed.

    The values of R1 and R2 can be adjusted to change the rise/fall time. We recommend the value of 1MOhm.

    PACK- should be approximately the same value as BAT-/VSS.

  • 0 in reply to Shawn Hinkle
    Intellectual 1000 points

    Dear Shawn,

    What is the purpose that you recommend R1,R2 to 1M ohm?

    What will happen, if i change it to some lower value or some higher value?

    Also in Datasheet it is not given t_FET_off for CHG mosfet path , why is it so?

    Also as i can see different pull down applied to DSG & CHG path, which will lead to different OFF timing of Mosfets, and because of which CHG mosfet will remain ON for more time compared to DSG mosfet when Turned OFF, which will put stress on DSG mosfet.

    In above scenario think of the Case of Mosfet connected in Vehicle and Re-generation current will come to Mosfet and consider Mosfet is facing Over Temp, and hence it will be OFF , so in this case DSG Mosfet OFF first and later CHG Mosfet will be OFF ( Due to their different Turn OFF timings ) , which will put Stress on DSG mosfet in terms of Vds of DSG mosfet - which may go till 200Vdc and each Mosfet is rated just for Vds of 100v only.

    So both Mosfets must Turn OFF in nearly same time Interval, else Mosfets will become Week sooner and in some time it will damage also.

    Also for PACK- in Datasheet Figure 7.3 it says as below:-

    Q3 is a low-cost PCH FET and is used to keep CHG away from any voltages below VSS. When CHG is not being
    pulled high, PACK– being pulled below VSS will not be seen by CHG as Q2 does not turn on. Q3 also allows R2 to
    keep Q1 OFF, since all voltages below this FET can
    Q3 "follow" PACK– as it goes below VSS.

    Above it is saying that their will be a case when Pack- will be lower than VSS, i wanted to Know which will be that case? 

  • 0 in reply to smash Micheal
    TI__Expert 7035 points

    Hi ,

    The R1 and R2 values are calculated to determine the rise and fall time for the FETs. A larger value will result in a longer time, and a smaller value will result in a shorter time. 

    I am not exactly sure why we did not include t_fet_off for the CHG FET. 

    The recommended values in the datasheet have been thoroughly tested to ensure that they work properly. I would not expect any issues to come from the circuits described. But if you are concerned about the fall time of the CHG FET, you can adjust the R1 and R2 values to meet your requirements.

    Which document are you referring to? The most up to date version of the BQ76940 datasheet does not have a figure 7.3 or mention the paragraph you quoted.

  • 0 in reply to Shawn Hinkle
    Intellectual 1000 points

    Dear Shawn,

    R1 which is connected across Diode in Fig - 7.3, is the one responsible to control timing of Charge path Mosfet Fall time.

    If i change R1 from 1 mega ohm  to 250 ohm valued resistor , timing obtained is 2.50 micro seconds, which is lesser than earlier timing of 2.5 m sec. 

    But here a serious issue is observed of Battery getting discharged even when Mosfets are Off . Here Zener Diode which is connected across R2 resistor gets forward biased & through R1  &  P channel Q3 Mosfet it goes to Driver pin , which is grounded internally, hence battery will keep on discharging for long time & result in Deep Discharge of Battery after long unused timing.

    So according to me R1 & R2 should not be Disturbed and should keep it as 1 mega only. Please correct me if i am wrong.

    Where as R2 is not contributing in reducing timing of Charge Mosfet Fall Time, which i had practically verified.

    So please suggest how to reduce fall time of Charging Mosfet without changing R1  & R2 resistors.

    Rohit

  • 0 in reply to smash Micheal
    TI__Expert 7035 points

    Hi,

    The battery will naturally discharge very, very slowly when not connected to anything, but the behavior you are seeing is concerning. I would make sure to investigate what path the discharge current is taking. 

    Changing these resistor values are the only way to manipulate the rise and fall time of the FETs. The programming only determines if the pin is high or low, but now how quickly.