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LM74700-Q1: load sharing from parallel lithium ion batteries

dugan yoon
Prodigy 130 points
Part Number: LM74700-Q1
Other Parts Discussed in Thread: LM7480-Q1, LM5050-1

I want to put multiple LM74700 ideal diodes in parallel sourced by Lithium Ion batteries to give me longer holdup time, not more output current. I would prefer to have all batteries share the load equally, but would be willing to let the load share move around between the batteries. My question is this, will the load sharing get to a linear point of equilibrium where more than one ideal diode is conducting or will the ideal diodes be banging from one source being the master at a time and then jumping to another as each battery discharges and its voltage drops? Won't the soft output characteristic of each battery want to equalize the load sharing when all the batteries are at a similar state of charge?

  • 0
    TI__Guru 57737 points

    Hi Dugan,

    LM74700-Q1 has reverse current blocking functionality. When input voltage is less than output voltage by V(AK REV) = 11mV, the LM74700-Q1 turns OFF the FET to block reverse current flow. 

    So, the Lithium Ion battery with highest voltage will voltage will supply the load (as the LM74700-Q1 in the path of other batteries will turn OFF the FETs in their path to block reverse current) unless the battery voltages are almost equal within 11mV of each others voltage. 

  • 0 in reply to Praveen GD
    Prodigy 130 points

    So if I understand this correctly there will always be an on or off behavior and never linear behavior so when all the batteries are at same state of charge the FETs will be chattering between on and off states. Have you ever configured a system in a similar way and observed chattering behavior? What are the consequences of the FETs chattering on and off?

  • 0 in reply to dugan yoon
    TI__Guru 57737 points

    Hi Dugan,

    There will be no ON/OFF behavior due to linear gate control and the the forward turn ON threshold required for the gate to turn ON.

    Once the GATE is turned OFF to block reverse current, it will be turned ON again only after the forward voltage drop of around 177mV is seen across ANODE and CATHODE. Hence there would be no shattering.

    The value of 177mV (V(AC_FWD)) is taken from LM7480-Q1 datasheet and it should be similar for LM74700-Q1 as well. 

  • 0 in reply to Praveen GD
    Prodigy 130 points

    Are you saying this with conviction or guessing what the behavior will be.  Since we are dealing with batteries and not supplies with tight output regulation I am guessing that as a battery becomes master and takes over the full load it will experience a voltage drop due to internal impedance. Hence the chattering between different batteries becoming master.  A follow on question is if the LM5050 will have the same or different behavior in this configuration?

  • 0 in reply to dugan yoon
    TI__Guru 57737 points

    Hi Dugan,

    This is actual device behavior. You can find ORing test report for LM74810-Q1attached.

    The behavior for LM5050-1, LM74700-Q1 and LM7480x-Q1 will be the similar as they have similar architecture. 

    LM74810-Q1 ORing tests.pptx

    . 

  • 0 in reply to Praveen GD
    Prodigy 130 points

    Was this data taken using tightly regulated power supplies or with soft unregulated outputs like Lithium Ion batteries? I don't see the input voltages being affected when their corresponding ideal diodes turn on and start drawing current, as would happen with a Lithium Ion battery.  The test shows what happens when low impedance supply voltages cross each other in both directions but it doesn't necessarily show what happens when two Lithium Ion battery outputs at approximately the same state of charge are presented to two different ideal diode controllers. Would you agree or am I wrong in these assertions?

  • 0 in reply to dugan yoon
    TI__Guru 57737 points

    Hi Dugan,

    The waveforms were captured on tightly regulated power supplies and not on Lithium Ion batteries. 

    The behavior of the ORing can differ for Lithium Ion batteries as the voltage may drop while loading. The point to understand here is that there is hysteresis of  around 177mV between the GATE OFF and ON which will avoid ON/OFF behavior of GATE when the input power supplies are very close to each other. 

    For exact behavior of the controller for Li-ion batteries as power supplies, you will have to test with LM74700EVMs and Li-ion batteries. 

  • 0 in reply to Praveen GD
    Prodigy 130 points

    OK, thanks for the support. My guess is that the FETs will toggle between each other as the hysteresis voltage is met. For heavy loads this could be fairly quickly and for light loads it may take battery capacity to fall sufficiently to reach the threshold. And it further may depend on SOC of the batteries, age of the batteries, and maybe even temperature (all the things that affect internal impedance). I think you are correct that testing will tell.