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THVD8000: Power Line Inductor

didier gibert
Prodigy 10 points
Part Number: THVD8000


Tool/software:

Hi,

I´m using the THVD8000 to connect between 5~10 nodes with 12-24VDC and I use 2 wires to carry the data and power at the same time.

Each node use filtering (2xC, 2xL, rectifier & LDO) to get 5VDC with 0.2Amax and comunicate at 19200bds.

For now, I'm using 2 independant inductors like SRR1208-121YL, but for space reasons, I would like to use a dual (un)coulped inductor in single package like SRF0905A-102Y, SRF1006-121Y (used in filter line) or MSD7342-683MLC (use in SEPIC schematic) or other.

Can this type of dual inductor work?

What are the C and L values that you can advise me for my solution?

Please help me to clarify or any suggestions are welcome.

  • 0
    TI__Guru* 77981 points

    Hi Didier,

    What is your max distance you're trying to support?

    Do you have a block diagram of the system so I have a better picture of how it looks? (I think you're following the datasheet suggestion but want to make sure my understanding of your written description isn't wrong).

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 10 points

    Hi Bobby,

    Yes I'm following the datasheet suggestion and the max distance is 5m for all my 5~10 nodes (1 master node with Power supply, and all others node with rectifier & LDO).

    Didier

  • 0 in reply to didier gibert
    TI__Guru* 77981 points

    Hi Didier,

    5m is pretty short so we can try to max out the carrier frequency to 5MHz.

    Using equation 5 from datasheet for Capacitor value:

    Cseries = 6.37nF// assumes 5MHz and 5 ohms (I picked an arbitrary value I thought was small, can choose lower if you want but I wouldn't go over 10). So just choose a value larger than 7nF and it should be fine.

    For the inductor equation 3:

    For 10 nodes if the target impedance is 375 (this comes from the RS485 standard as the min impedance allowed since this device is built using an RS485 PHY), then the Z needs to be 3750 ohms (3750 in parallel with 10x gets you an equivalent impedance of 375)

    Ln = 119uH // Ln = (10 nodes x 375)/(2xpix5MHz)

    So your inductors should be sized to be larger than 120uH. 

    Above assumes you terminate your bus with 120 ohms on both sides. If you don't intend to do this, then you could reduce the 375 ohm number in the Ln equation to become 60 ohms. 

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 10 points

    Dear Bobby,

    Thanks for the quick response, I will try this new values.

    For my principal question "I would like to use a dual (un)coulped inductor in single package like SRF0905A-102Y, SRF1006-121Y (normaly used in filter line scheme) or MSD7342-683MLC (use in SEPIC schematic) ", WHAT DO YOU THINK this will work? if Not do you know 2 inductors in a single package that will work for for my solution?

    Didier

  • 0 in reply to didier gibert
    TI__Guru* 77981 points

    Hi Didier,

    I'm not too much of an expert on inductors so I don't have too much guidance on this point.

    My understanding with the inductors are that they should be sized for the peak current load you expect and include some room for tolerance since the actual inductance can shift. (The MSD7342 for example has a char that plots inductance moving with current).

    My last piece of advice is to use a differential mode choke configuration and not a common mode configuration.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 10 points

    Hi Bobby and thank you for everything.