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BQ76200: BQ76200 - separate Charge/Discharge path Bidirectional Back to Back NFETs in Common Source configuration

Padmanathan Jeganathan
Prodigy 205 points
Part Number: BQ76200
Other Parts Discussed in Thread: UCC27517A-Q1, UCC27517A

Hi,

           Currently, we are using the BQ76200 driving the separate Discharge, Charge and Pre-discharge FETs for High Side Switch path in a 54V EV BMS application;

         - please help to get response urgently for the below clarification;

               -  + Bat to Load+ (Discharge path)- switched using Bidirectional (Series Back to Back) NFETs in Common Source configuration;

                         - controlled by DSG signal; 

                         - Common Source to Gate a 10M resistor Placed;

               -  + Bat to Charger+ (Charge path) - switched using Bidirectional (Series Back to Back) NFETs in Common Source configuration;

                         - controlled by CHG signal; 

                         - Common Source to Gate a 10M resistor Placed;

               -  + Bat to Load+ (Predischarge path) - switched using Bidirectional (Series Back to Back) P-FETs in Common Drain configuration;

                         - controlled by PCHG signal; 

                         - a 10M resistor Placed between both side Sources to Gate;

Note: Charge and Discharge paths are independent; only one Path need to Turn ON at a time;

Please confirm Bi-direction Switch implementation for separate Charge and Discharge paths are OK when Gates are driven from BQ76200;

Can the Discharge Gate drive work properly in the above configuration, if the Load+ connected to the PACK pin  and +BAT connected to the BAT pin of BQ76200 respectively;

                      - The Charge pump for DSG FET drive can work/Turn-ON the N-FETs (in Bidirectional Common Source config) even when the Load+ is at 0V? 

Thanks & Regards,

Padmanathan J

  • 0
    TI__Guru 54715 points

    Hi Padmanathan,

    The discharge should switch effectively.  The PACK pin should follow the Load+ terminal.

    The charge will not switch effectively, the range of CHG is from BAT to the charge pump voltage.  If the Charger+ pulls down when the FETs are off the battery side FET will come on again in a source follower mode.  See  http://www.ti.com/lit/slua952 for considerations using a blocking FET with a CHG pin like the BQ76200.

    The common drain P-ch will not work.  The drop of one side will turn on the other through the 10M resistors.  Perhaps with diodes in the  individual gate drives. 

    Yes the charge pump can turn on the DSG for the common source N-ch FETs when the load is at 0V, the battery side FET is the conventional discharge FET, the other FET acts as a diode when it is off. 

  • 0 in reply to WM5295
    Prodigy 205 points

    Hi,

              Thanks for the response.

    Considering the Discharge Path,

                 - we assume it is ok, as you confirmed with NFETs in Common source back to back configuration;

                 - Just need to confirm, if the Source pin is not required to be connected to any pin of the BQ76200; only LOAD+ output connected to the PACK pin of BQ76200; and in this config, FETs can Turn ON and OFF with just DSG pin drive; 

    Considering Charge Path;

             - we followed Common source considering the Diodes facing outward for safety;

             - please help confirm, if we Can follow Common drain Configuration in this separate Charge path to make it work, along with the Blocking FET circuit shown in Figure-10 or Figure-15 of  http://www.ti.com/lit/slua952 with BQ76200; Also, the Charge + Input from Charger cannot be connected to the PACK pin of BQ76200 as Load (Discharge) and Charge are separate paths;

            - Also, Worst case, if we short the Battery side FET for the Charge Path in Common source configuration, it should work as intended right;

    Considering the Pre-discharge path;

               - to make it work with Bidirectional FETs,

                          - can we use a Common Source configuration;
                          - or, as you stated, please let know on using diodes in individual gates;

    Please help to confirm urgently;

    Thanks & Regards,

    Padmanathan J

  • 0 in reply to Padmanathan Jeganathan
    TI__Guru 54715 points

    Hi Padmanathan,

    For DSG, no connection to the source is needed.  The DSG is driven with respect to the PACK pin and the CP pin.

    For CHG there is not an ideal connection for back to back FETs.  Select a method which fits your system needs. You are correct, the PACK pin of the BQ76200 is used for the LOAD+ connection in your system description, no connection is made for the Charge+ from the charger.   If you short the battery side common source connected FET's gate to source it will be off.The CHG pin will pull up to BAT, you will need to limit current from this pin somehow.  You might see the application note on shutdown of the part.

    For pre-discharge, either the diodes in the gate pull down or a common source connection of the P-ch may work. 

  • 0 in reply to WM5295
    Prodigy 205 points

    Hi,

              Thanks for the confirmation.

    For the Pre-discharge path FETs in Discharge configuration;

                  - when using Common Drain PFET Configuration, could you please explain or share example/reference (on the connection nodes of diode in the each FET gate;

                  - when using Common source PFET configuration, it is assumed that the PCHG voltage always depends on the highest value between PACK or BAT;

        • PCHG = [MAX (BAT, PACK)], when OFF; and PCHG = [MAX (BAT,PACK) - 12V] when ON;

                                                       - So that in Common Source PFET configuration, FET turns ON and OFF properly; please confirm;

        •  Note: the Series Current limit resistor is positioned between the Pre-Discharge FETs and the Load+ node;

    For the Charge FETs;

                - currently, we plan to use the Common drain dual NFET (Blocking FET configuration) same as given in Figure-10 of http://www.ti.com/lit/slua952 (BQ769x2); to avoid leakage from battery to the charger side loads (which occurs when using a single FET);

                             - please confirm urgently, if this circuit works fine with the BQ76200 also, for the separate Charge path Switch using CHG drive signal;

    And also lastly, apart from all these above stated High Side Switches; Low side redundant switch path;

    - we are using a UCC27517A-Q1 driver (supplied by 12V) for switching a bidirectional (back to back) Common Source NFET Switches on the Low Side (between BATT- and LOAD -);  just a 10M resistor (in parallel with a Zener) is placed between Common Source to Gate; the GND of the driver is connected to BATT-; OUT pin is used to drive the Gate

                   - please confirm if this part is enough to drive this low side bidirectional FET configuration;

    Thanks & Regards,

    Padmanathan J

  • 0 in reply to Padmanathan Jeganathan
    TI__Guru 54715 points

    Hi Padmanathan,

    For predischarge I don't know of a diagram.  We don't commonly recommend such a back to back connection.  Add the diodes to the gate path, check the current flow to avoid leakage.  If the connection is for pre-discharge the BAT should be the high potential when the pre-discharge is turned on.    When pre-discharge is off if a charger is on the LOAD+ pin the PCHG pin should follow the PACK pin up with LOAD+ to keep the FET(s) off.

    For charge the blocking diode can avoid leakage.  If you need to turn it off fast it will provide a leakage path as noted in the application note, design appropriately.  If you need to provide short circuit protection on the charge path provide adequate gate drive for fast turn off.  Design for your system needs and check for performance.

    The UCC27517A is a very capable drive and should be able to control your back to back FETs.  You may notice it has no connection to the sources of the FETs as it drives to GND.  The UCC27517A will drive faster than the BQ76200, slow the switching as needed with a gate resistor to avoid damaging transients from the battery inductance.