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Using LMZ23608's in parallel as redundant supplies

Jamaal_Charles
Genius 3245 points
Other Parts Discussed in Thread: LMZ23608, TPS2113A, LMZ13608, TPS3700

Hello,

We are considering using two LMZ23608's in parallel (5V out), but are doing so to use them as redundant supplies (safety critical). 

 In the design notes the slave supply doesn't get a feedback as that is how the slave to master relation is established.  We are wondering if we can make the slave operate in a "master failed mode" by adding the FB or sense resistors to make sure the slave can regulate with out the master operating?  Is there an issue using the supplies in this matter?  Also, can you define the failure modes?  Do they fail open or shorted?

Thank you!

  • 0
    TI__Guru 71536 points

    Hi Jamaal,

      You might consider using the TPS2113A Power MUX: http://www.ti.com/product/tps2113a , or something similar (I don't know the load requirements in your design).  It has an autoswitch function, so if/when the primary supply fails, the secondary is automatically connected.

    ~Leonard 

      

  • 0 in reply to LeonardEllis
    Genius 3245 points

    Hi Leonard,

    We considered something like that, or a diode OR, but the problem that we are trying to avoid is a single point of failure, so this doesn't really help. 

    Will the LMZ23608 not work the way we are describing?

    Thanks,

    Jamaal

  • 0
    TI__Genius 10095 points

    Hi Jamaal,

    When the LMZ23608 starts up if the FB pin is floating the part assumes that it is the slave and disconnects its share connection so that it will not fight with the master supply.  The part will then regulate based upon the Voltage at the share pin which is provided by the master.  If you connect the FB pins as you describe then essentially both parts are trying to regulate the current and the output voltage will double when both parts are operational.

    Let me think about it and see if I can come up with a way to have it operate as you are looking at.

    Marc

  • 0 in reply to Marc Davis-Marsh
    Genius 3245 points

    Hi Marc,

    Do you happen to come up with a solution?

    Thank you for your help!

  • 0 in reply to Jamaal_Charles
    TI__Genius 10095 points

    I've been thinking about this but there are a lot of variables.

    Here are several possible failure modes for the LMZ13608.

    1)  If an over voltage event occurs on Vin then the high side mosfet and boot circuitry can fail.  These can fail open or as a short to ground depending on the severity of the damage.

    2) If a feedback resistor divider fails.  The resistors tend to fail open.  The output voltage of the regulator will drop to either Vref (bottom resistor failure) or rise VIN (top resistor failure).  Also if the FB pin is shorted to ground for any reason the output voltage will rise to Vin.

    3) There is solder internal to the module that can short pins together during the reflow process.  I will attach the recommended reflow profile  This shorting can be intermittent.  This usually shows up as Vout or another pin shorted to ground.

    7563.Texas_Instruments_English_LMZ1_LMZ2_PowerModules_SMT-Design_Guide-SNAA214_REV_H_Lock.pdf

    4) If the devices are connected in a sharing connection and the master control fails both supples would function incorrectly.

    5) If an electrolytic capacitor fails because of thermal cycles drying out the electrolytic, you could see it fail as a short.

    Based upon these possible failure modes.   You would want circuitry to switch in the second supply if the output voltage fails either low or high.  To avoid down time you would need to have the second supply up and running but unloaded until the first supply is disconnected.

    For instance you could have both supplies running and connect them through an oring controller such as the LM5050 (there might be better choices this is off the top of my head).  The second supply would be disconnected from the load unless the output voltage from the first supply was out of spec either high or low. This could be controlled using some logic or a supervisor circuit such as the TPS3700.

     

    Regards,

    Marc