Reverse charger protection

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Aug 12, 2016.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    349
    6
    1. I have to give reverse charger protection for battery management system in which N-channel mosfets are in ground path.

    2. Attached is the ckt for bq40z50 in which N-channel are on +ve path i.e Q2 & Q3.
    Battery is coonected across Bat+ & bat- & charger is connected across Pack+ & pack-.
    In this circuit Q4 is for reverse charger connection protection. Suppose charger is connected in opposite direction then there is +ve voltage at gate of Q4 & ground at its source. It turns on the Q4 & it then it shorts gate & source of Q3, so protecting any current to flow.

    3. Now i have a circuit in which n-channel are on ground path, I have to add reverse charger polarity in it also. Current Circuit without protection is attached with name actual

    4. I have attached modified circuit. three components added are R1,Q4 & R2. Its P channel mpsfet. When charger is connected in correct configuration then gate at +ve & source at -ve, it turns off the mosfet Q4, but if reverse charger is connected then gate become negative & source become +ve, it turn on Q4 which in turn connect gate of Q2 & its source, effectively turn off the mosfet & save any current in/out of battery.


    Query:
    1. Is the modified circuit is ok?
    Bat volatage in 1S configuration is 2.5V to 4.5V & charger voltage is +4.5V max
    Bat volatage in 2S configuration is 5V to 9V & charger voltage is +9V max

    2. Can Q4 mosfet be replaced by any n channel mosfet as in original circuit as I have 2n7002 in use.

    actual-1.gif

    modified-1.gif



    Mod note:
    You used the wrong file type, so it caused the file size is too big, now the file type all changed to the gif, it could reducing the file size from 5 times to 10 times, the png file type almost using in simple horizontal and vertical lines.
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,951
    387
    1. No, this won't work. When the charger is reversed, the body diode in Q2 will be forward biased and conducting so removing its gate voltage will be ineffective. It is Q1 which needs to be turned off if the charger is reversed. I think this needs an extra MOSFET across Q1 gate source with its gate connected to Q4 drain and R1 moved to Q1 gate.

    2. No, it must be P channel
     
  3. MrAl

    Well-Known Member

    Jun 17, 2014
    2,438
    492
    Hi,

    The only way i have found so far to protect a charger or power supply when the battery is reversed is to use a high current diode in anti parallel with the power supply output, and a fuse inline to the 'load'. Since the load can be a battery if it is reversed the diode conducts and blows the fuse.

    Of course there are more sophisticated ways like using an SCR based crowbar circuit, but that's a lot more complicated. Advantage is it can be reset.

    It is also interesting to know that a diode in series with the load does not help to prevent damage when the load (battery) is reversed because the series diode just goes right on conducting :)
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,236
    Since the voltage is indeterminate when the charger is reversed, depending upon the relative current ability of the battery versus the charger, the only way I see to electronically protect against reverse connection is to add an overcurrent circuit to open the connection when an overcurrent is detected.
     
  5. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,951
    387
    I think this version works:
    upload_2016-8-12_17-41-42.png
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,236
    That circuit depends upon the voltage across the terminals, and that voltage is indeterminate with a reverse connection, depending upon the relative impedances of the battery and the charger.
     
  7. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    349
    6
    Thanks @AlbertHall for replying.

    @crutschow voltage at which terminals at output(pack) or input(bat)? Why it is inderminate?
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,236
    It's indeterminate because you don't know which is more powerful (has a lower impedance) -- the battery or the reverse connected charger.
    I'm betting it's the battery.
     
  9. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    349
    6
    battery voltage is always higher then charging voltage until full charge. Impedence of battery is in milliohms
     
  10. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,951
    387
    That is true if Q1 is switched on when the charger is reverse connected - otherwise there is no connection between battery and charger. So it all depends on the state of DSG at the time the charger is connected.
     
  11. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    349
    6
    Do you mean that the circuit works only when DSG fet is off otherwise it will not work?
     
  12. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,951
    387
    Yes. If Q1 is already on when the charger is reverse connected then as Crutschow says the battery will likely determine the voltage, not the charger, and the circuit to turn off Q1 will not work.
     
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