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EMB1499Q: Active Balancing Solution & Contradicting Component Selection Guidelines - Need clarification!

DeeJay
Intellectual 840 points
Part Number: EMB1499Q


Hi,

My query relates to the EMB1499/EMB1428 active balancing solution.

Specifically, the EMB1499 bidirectional forward converter.

I’m having doubts as to the component selection guidelines. The reference design for this solution has component choices that contradict the guidelines in the datasheet.

Links to relevant documentation below:

Reference design schematics: http://www.ti.com/lit/pdf/tidrn59

Reference design BOM: http://www.ti.com/lit/pdf/tidrn60

EMB1499 datasheet: http://www.ti.com/lit/gpn/emb1499q

The first issue relates to the forward transformer selection.

The datasheet specifies the following:

-          Turns ratio should be n:1 where n = 0.25 * num_cells

-          Primary side inductance minimum 190uH

However the reference design uses this Pulse part (T1 on schematic and BOM):

https://www.digikey.com/product-detail/en/pulse-electronics-power/PA3856.005NLT/553-2139-1-ND/3906266

which is only 128uH and the max turns ratio achievable is 3.14. Note that this is for a 16 cell design, so the ratio should be 4:1.

Your thoughts?

Thanks.

DJ

  • 0
    TI__Genius 11480 points
    Hi Deejay,

    Thank you for pointing this out - you certainly are correct.

    This app note may be helpful in understanding the component selection for the active clamp topology: www.ti.com/.../slua535.pdf
    .The EMB1499 is an active clamp controller with some bells and whistles, essentially). The main difference between the EMB1499 active clamp and the one i linked is that the EMB1499 is bi-directional.

    I will always recommend following the datasheet for best performance, but it seems that the creator of the reference design found this transformer acceptable (and it does indeed work - it may just not be optimal)
  • 0 in reply to David Wiest Jr
    Intellectual 840 points
    Hi David,

    Thank you for the clarification and confirmation of the errata in component selection guideline.

    We do have more questions on the EMB1499 and hope you could assist here.

    From a safety perspective, we are considering a condition where the stack-side of the EMB1499 transformer sees an open circuit rather than a battery stack. Do we need to add some kind of over-voltage protection?

    Is there an unsafe condition if the voltage on the stack-side of the converter is too low? We might have a use case where the stack voltage is about half the designed/nominal voltage.

    Thank you again.
    Best regards,
    DJ
  • 0 in reply to DeeJay
    TI__Genius 11480 points
    Hi DeeJay,

    I apologize for the delayed response.

    By Stack Side - do you mean the side that goes through the switch matrix, or the side that goes to an external 12V/24V or to the top of the module?

    If you are working something safety- critical, i can't argue against adding redundancy. There are OVP and UVP limits on the cell side of the emb1499.

    Off of the top of my head, i can't think of an unsafe condition. I'd recommend double checking to make sure you don't get crazy things like 150% duty cycle or unreasonable currents at those voltages.