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Original question:

BQ25798: Reverse polarity protection for charger input

BQ25798: Reverse polarity protection

Ivan Beck
Prodigy 30 points
Part Number: BQ25798
Other Parts Discussed in Thread: TPS552892,

Tool/software:

Following up on a closed thread regarding the issue. What can be done to make the IC withstand reverse polarity events? 

  • 0
    TI__Guru**** 166626 points

    Hi Ivan,

    The simplest method for reverse polarity is using keyed hardware connectors to ensure it is physically impossible to connect in reverse. 

    If current is only flowing in one direction, i.e. input to a charger IC that will not used in reverse/OTG mode, a FET can be used as explained at https://www.ti.com/lit/an/slva139/slva139.pdf.

    If current is flowing in both directions, e.g. a battery getting charged then discharged, protection circuits get more complicated.  Unfortunately, we do not have a good circuit based solution for this.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    My issue is I am using the part for an automotive application. So no keyed connector there. The idea is to have an iot car tracker that controls remotely the vehicle and has a battery backup. The UPS feature in conjunction with the Li-ion controller was perfect for my application, in which the IC automatically boosts the battery when the supply is not available. It can also measure and change two input supplies, which is useful. I think my best solution is two diodes, one for each supply in series.

  • 0 in reply to Ivan Beck
    TI__Guru**** 166626 points

    Hi Ivan,

    Two diodes might work but will be lossy.  TI has ideal diode controllers that might work.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    I think I can manage lossy using a good Schottky diode. My circuit won't consume more than 15W, around 1.3A at 12V, so not many losses at the diode. The other option would be to combine something like the TPS552892 with a simple lithium controller and some comparators to automatically switch the input to battery backup when disconnected. I am not really sure what would be the best option.

  • 0 in reply to Jeff F
    Prodigy 30 points

    Can you have a series diode to VAC input? From what I understand, VAC inputs are the only ones that would not be protected in case of a Reverse polarity event. A small voltage drop in VAC shouldn't interfere in normal operation, and some functionality can be compensated via software. 

    Is my reasoning correct, or am I missing something?

  • 0 in reply to Ivan Beck
    TI__Guru**** 166626 points

    Hi Ivan,

    VACx is high impedance voltage sense pin.  I haven't tried it (and can't easily test on the EVM) but I see no reason why their couldn't be a Schottky diode in series.  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    Perfect. Would that make the IC reverse polarity protected? Or am I missing something? Assuming I have a bidirectional switch on both inputs.

    I am going to try in my design.

    Thanks for all the help Jeff

    Regards, Ivan.

  • 0 in reply to Ivan Beck
    TI__Guru**** 166626 points

    Hi Ivan,

    That might work.  Let me know.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    I think I also need to protect ACDRV pin, as the first mosfet would allow current to flow in that direction. 

  • 0 in reply to Jeff F
    Prodigy 30 points

    The Zener/TVS diode should block the negative voltage back feeding to ACDRV pin, but I am worried that the capacitor between Gate and Source might provide a path for the negative voltage during a transient event and damage the input with the short negative pulse. 

  • 0 in reply to Ivan Beck
    TI__Guru**** 166626 points

    Hi Ivan,

    That is a concern.  I found a reverse polarity protection circuit for input voltage for an older device in https://www.ti.com/lit/ug/sluu462a/sluu462a.pdf.  It doesn't protect the VACx lines however.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 30 points

    Jeff, sorry for the lack of expertise/understanding, but simulating the circuit in spice did not yield any protection, i am not clear on how it works.

    Regards, Ivan.

  • 0 in reply to Ivan Beck
    TI__Guru**** 166626 points

    HI Ivan,

    With reverse input, I think Q1 turns on, shorting Q2 and Q3 gate to sources (and IC CMSRC to ACDRV).  Any reverse current flows through this FET and Q1 diode. This circuit may not work with BQ25798 because it does not have the CMSRC pin and has the VACx pins instead.

    Regards,

    Jeff