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Original question:

[FAQ] What type of bridge do I need for my PoE PD application?

TPS2372: Understanding the various diode bridge front ends on different EVMs.

Jonathan Baiardo
Prodigy 40 points
Other Parts Discussed in Thread: TPS2373, TPS2373-4EVM-758, TPS2372-3EVM-757, TPS2372-4EVM-006

Tool/software:

We have been working on a high power POE design and have looked at all 3 eval kits for the TPS2373.  Each one has a slightly different front end, and each has its own efficiency benefits.

TPS2373-4EVM-758 - discrete full bridge Ideal Diode FET.  Best efficiency compared to diode-only bridge.

TPS2372-3EVM-757 - discrete full bridge with Ideal Diode FETs only on low side, high side uses Diodes.  Moderate efficiency

TPS2372-4EVM-006 - Uses the bridge ideal diode chip. Moderate efficiency.

We implemented our own stripped-down version of the 4EVM-758 where we don't have BJTs on the gates of all of the FETs.  We also built just a "half" version where we weren't concerned about input voltage polarity reversal.  Unfortunately, we have some backfeed issues if we have a pre-biased load on the output.  It's probably due to our half version, so we understand that.

What we are wondering is when we're modeling our front-end after these circuits, why do the 4EVM-758 and 4EVM-006 use BJTs in the gate drive circuitry rather than FETs?

can we use small signal FETs in place of the various BJTs?  

when replacing the bridge w/ the FDMQ8205A, could we just replace the control BJTs w/ FETs?

Thanks

  • +1
    TI__Mastermind 37071 points

    Hi Jonathan,

    Thanks for reaching out!

    For 4EVM-758, my feeling is that you can use NMOS circuit to replace the upper BJT circuit and use PMOS circuit to replace the lower BJT circuit. But not sure is MOSFETs are more helpful than BJTs or not. For surge conditions, MOSFET's Vgs may be easier to couple with voltage overshoot  to mis-trigger, and MOSFETs are harder to be controlled by a single circuit, while BJTs current loop can be cut off by Q6 easier.  

    FDMQ8205A is recommended to use BJTs as its datasheet showing. 

    Best regards,

    Diang

  • 0 in reply to Diang Xing
    Prodigy 40 points

    Thank you. 

    I think the

    "For surge conditions, MOSFET's Vgs may be easier to couple with voltage overshoot  to mis-trigger, and MOSFETs are harder to be controlled by a single circuit, while BJTs current loop can be cut off by Q6 easier.  " 
    is the answer I was looking for.
    It seems like BJTs are the solution if we are duplicating the circuit and we just manage the PCBA part count as best as we can.