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BQ77PL157A4225: MOSFET orientation shown in the datasheet

George Hug
Genius 4350 points
Part Number: BQ77PL157A4225

It appears that Figure 1 of the datasheet has the orientation of the MOSFET backwards.  It has the drain at ground and the source at the negative battery terminal.  As I understand it, this chip performs overvoltge protection during charging.  If that's the case, then when the MOSFET is off, conventional current would continue to flow out of the negative terminal throught the body diode to ground, and there would be no protection.  But perhaps I misunderstand what this chip is supposed to do.  Could I get some clarification on this?

  • 0
    TI__Guru 54715 points

    Hi George,

    Your understanding of the intended function is correct.  The FET symbol in figure 1 does not include the body diode symbol. The source is at PACK-.  When the FET is off and a charger is present greater than the battery voltage the PACK- will go negative with respect to GND.  The body diode will block current flow from the charger (counter clockwise in the figure).  With a load present on the PACK terminals the current could flow clockwise in the diagram through the body diode, PACK- would be the body diode voltage above GND.

  • 0 in reply to WM5295
    Genius 4350 points

    WM5295 said:

    Hi George,

    Your understanding of the intended function is correct.  The FET symbol in figure 1 does not include the body diode symbol. The source is at PACK-.  When the FET is off and a charger is present greater than the battery voltage the PACK- will go negative with respect to GND.  The body diode will block current flow from the charger (counter clockwise in the figure).  With a load present on the PACK terminals the current could flow clockwise in the diagram through the body diode, PACK- would be the body diode voltage above GND.

    Thanks for the reply, but I don't think I understand at all.  If a charger is present and charging, I assume it is connected between PACK+ and GND.  The conventional current flow would then be from the charger's plus terminal into PACK+, through the pack to PACK-, through the mosfet to GND.  That would be the case anytime the charger is charging the pack, and is clockwise in the figure.  If that's right, then PACK- is not negative with respect to GND.  It is positive.

    Then if the voltage at some point gets too high and the BQ77 wants to shut down charging, it would turn off the mosfet.  But the body diode would continue to allow charging current to flow from PACK- to GND, and in effect there is no shutdown and no protection.  If the mosfet were oriented the other way, then the body diode would block all charging current when the mosfet is off.

    I admit to not being at all expert on battery charging, but where is my analysis wrong?

  • 0 in reply to George Hug
    TI__Guru 54715 points

    Hi George,

    Charge current flows in the opposite direction from what you envision.  It flows counter clockwise, out the charger + terminal into PACK+ of the battery and through the cells.  

    Clockwise current in the figure would be discharge current, out the PACK+ terminal of the battery.  The BQ77PL157 cannot protect against discharge since it has no control and the body diode of the FET allows the current.

  • 0 in reply to WM5295
    Genius 4350 points

    Well perhaps we have conventional current vs electron flow.  In any case, I've revised Figure 1 to show the body diode as well as my assumption about the connection of a charger to the circuit.  Does this look correct?

    Edit:   This is completely wrong.  See next post.

  • 0 in reply to George Hug
    Genius 4350 points

    Ok, nevermind.  I was interpreting the drawing completely backwards.  PACK is where the charger would be, and the mosfet is correct.  Apologies for wasting your time.