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LM5149: Power supply and battery charger: reverse current thru LM5149

Federico Battaglin
Prodigy 80 points
Part Number: LM5149

Tool/software:

Dear All,

we are designing a power supply with 24V PB battery charger. The mains voltage is from a toroidal transformer + active bridge.
So, the power supply Vout is connected to batteris and to the output of LM5149. The microcontroller modify the LM5149 output voltage in order to manage the batteries charger.

In case of mains blackout, the batteries powers the LM5149 from the output because the LM5149 Vin falls to zero.
In this situation, Does LM5149 damage or not?

Is better to add just after theLM5149 but before the batteries a reverse polarity protection, such as "reverse battery protection ideal diodecontroller LM74700D"?


Thank You and Best Regards
Federico Battaglin

  • 0
    TI__Mastermind 33030 points

    Hello Federico,

    I suggest using reverse protection.  current will flow through the Body diode of the HS FET and if you are using VCCX reverse current will flow through the device.  If the HS FET BD is rated for such reverse currents, you may be able to get away with just placing a blocking diode from power supply in, to the VIN of the device.

    Hope this helps,

    David.

  • 0 in reply to David Baba
    Prodigy 80 points

    Dear David,

    thank You for your reply!

    About Mosfet, we are evaluating to use NVMFS6H858NLT1G (your demo board) or SIR124 (from Vishay)... I think the active sync rectifier doesn't want current from output wirhout any alrterante current at the input...

    The Vccx is powered by +5V generate from a small dc-dc powered by the power supply output, in this case, from batteries... ok, if I understand well, I have to cut this connection, when the mains falls down... is it so?

    How much reverse current flows thru LM5149?

    So I think it's better to use a reverse polarity circuit after LM5149 but before batteries... are You agree?
    Does LM74700D is a good choice?

    Thank You and Best Regards
    Federico

  • +1 in reply to Federico Battaglin
    TI__Mastermind 33030 points

    Hello Federico,

    Yes, mustprevent reverse current flow.

    The amount of current, will depend on the input capacitance and the output capacitance the output voltage and the impedance in between.  If VIN<VCCX and reverse current flows, it typically damages the device.  I would definitely suggest protection to prevent reverse current flowing through the device.

    Ideal diode device as you mentioned above will prevent reverse current flow and damage to your device.

    Hope this helps,

    David.