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SN6505B: Two in parallel to increase power

Prodigy 70 points

Replies: 14

Views: 124

Part Number: SN6505B

Is it possible to have two SN6505B parts in parallel (One on top of the other one)?  They will be sync with the same CLK.   We want to do this because the current limiting sometimes kicks in.

Thank you,

Pablo

  • Hi Pablo,

    Welcome to TI E2E Forum!

    Did you mean to ask if the outputs of two SN6505B based power supply solutions can be tied together to increase the power? Or did you mean to ask if two SN6505B devices can be connected in parallel to a single transformer to increase input current support?

    The answer is NO for both of these questions. The devices do not have any features to allow them to be connected in parallel and the external clock option is only to help customer choose the switching frequency to the value of their interest to aid them in managing emissions better. Thanks.

    Regards,
    Koteshwar Rao

  • In reply to Koteshwar Rao:

    I'm not too sure what is the difference between the two  option questions you give me.  The second question seem closer to what I want to do.  Having two SN6505B devices, I want to connect D1 of device 1 to D1 of device 2 and D2 of device 1 with D2 of device 2. and both D1 and D2 each go to the transformer.

    If the answer is NO, then can I drive external FETs with a single SN6505B (D1 going to a gate of FET1 and D2 going to a gate of FET2) or is there another way to increase the power?

  • In reply to Pablo Gonzalez1:

    Hi Pablo,

    As Koteshwar mentioned, connecting the D1 and D2 pins of two SN6505 (or SN6501) devices is not recommended because they are not designed to divide current loads between multiple devices. The part-to-part variation in switching characteristics could lead to damage of any two devices connected in parallel.

    External FETs can be included as shown below to increase power capability of the isolated supply, however this solution increases voltage capacity on the primary side, not current-sinking capability:

    For the solution above to boost current, Vin would need to be greater than Vout and the transformer would need to have a turns ratio < 1 (step-down transformer). What is the maximum load current this solution needs to supply?


    Thank you,
    Manuel Chavez

  • In reply to Manuel Chavez:

    Hi Manuel,

    We connect the SN6505 to a battery pack which can go as low as 2.9V and our Vout after the LDO 2 in your schematic needs to be kept at 3.1V.

    Is there a way to boost the Iout current at Vout when Vin is lower than Vout?

    We will be okay at the Vout side, if we can get 150mA more current from our existing design when Vin is 2.9V .  Our existing design looks exactly as your schematic, just substitute the LDO1 with a 4.2V battery pack and connect the D1 & D2 directly into the transformer without the HV FETs.  The LDO 2 outputs 3.1V.

    Thank you,

    Pablo

  • In reply to Pablo Gonzalez1:

    Hi Pablo,

    Thank you for providing the additional design information. Our team is continuing to discuss potential solutions, and I expect to follow up on that early next week. Meanwhile, please confirm whether an external CLK will be used for this SN6505 system or if the internal CLK of SN6505 will be used.

    Respectfully,
    Manuel Chavez

  • In reply to Manuel Chavez:

    We prefer not to supply an external CLK but if it is necessary we could add it.

    Thank you,

    Pablo

  • In reply to Pablo Gonzalez1:

    Hi Pablo,

    Thank you for your flexibility here. Our team had considered connecting SN6505 devices in parallel with an external CLK, however even then there is a concern for unintended D1 / D2 short-circuits.

    A final idea we are considering is using SN6505 to drive the gates of external FETs capable of sinking >1A. Please allow us until the end of this week to conclude this conversation and share our findings.


    Let us know if there is additional support we can provide meanwhile.


    Thank you,
    Manuel Chavez

  • In reply to Manuel Chavez:

    Hi Pablo,

    The topology we've considered to drive >1A using SN6505 is shown below:



    V1 can be operated by 3V, but the external N-channel FETs will need to have a Vgth much lower than this and be capable of sinking >1A. How much current will be needed is unclear, but a FET like SSM3K329R should work.

    This design does come with the tradeoff that some features of SN6505 will be lost on the external FETs, like current limit and thermal shutdown.


    Please let me know if this is a solution for you to consider, otherwise the the isolated load could be divided between two independent SN6505 power supply circuits.


    Thank you, and have a great weekend,
    Manuel Chavez

  • In reply to Manuel Chavez:

    Thank you for your response.  I'm building a test board to test both configurations.  This one (FET Driven) and the parallel (Two SN6505B in parallel). Should I connect diodes from the drain of the FETs to the center of the transformer to reduce EMI?  Or the diodes are going to screw up the transformer performance? Like this: 

  • In reply to Pablo Gonzalez1: