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THVD8000EVM: various questions (data rate, inductor sizing, capacitor values, termination, diode recommendation)

Kevin Lai
Intellectual 2916 points
Part Number: THVD8000EVM
Other Parts Discussed in Thread: THVD8000

Hi team,

please help with the below customer questions.

I was able to evaluate the boards yesterday. It seemed to work with no issues even at 2A for a short period of time.

I have a couple of questions that I want to ask before I begin to potentially embark on a design.

  1. My data rate is 250 KB/s, I used 5MHz as the modulation frequency as the next step down, 2MHz is less than 10x my data rate… Is there any issue or peril with being more than 10X the data rate?

  2. For a given system, I’ll have a power supply module which will be capable of delivering up to 100W of power at approximately 48V (2.1A). For a given load, I’m suspecting that the inductors need to be the same inductance, but only sized for current capabilities based on the load rating of the device (For example, a 40W load consumes less than an amp). Is this the case?

  3. I anticipate that I’ll have no more than 5 devices (power source and loads)… I based the inductance value on 5 X 375 ohm impedance mentioned in the design guide; is this correct?

  4. The evaluation board uses 1uF, 100V capacitors for the isolation between the bus and the transceiver; the design guide indicates that ~30nF is all the larger that the capacitor needs to be. I’m presuming that the evaluation board value is to accommodate the lowest modulation speed, but is still 3X the value shown for the lowest frequency in Figure 7… Would a 33nF capacitance be adequate and/or appropriate for this system?

  5. The discussion of terminating the end of the line is not very clear… Apart from the one for the evaluation module, there seems to be no schematic that shows it. In our proposed system, we will be using CAT5e cable with all of the solid color wires paralleled and all of the striped wires paralleled; I don’t know what this would do to the characteristic impedance of the cable; it seems that it would increase the capacitance per ft by a minimum factor of 4 and maybe decrease the inductance by 4. I’ve found nothing on the subject via web search…any input here?

  6. On the schematic for the EVM, there is a dual diode (D5), but there is no BOM that says what might be recommended. Do you know what the recommended part is for that component?

  7. Is there a similar product or concept for this over an AC power line? 

Thanks,

Kevin

  • 0
    TI__Mastermind 25705 points

    Kevin,

    I'm glad THVD8000 works for your application. Here are my answers for the questions:

    1) The major drawback with a higher carrier frequency is less communication distance.

    2) Your understanding is correct. The inductor keeps the same inductance with different current rating.

    3) The first order estimation is correct. However the impact of the inductor also depends on the resistance and cable impedance. Please refer to this post for some discussion.

    4) Yes, smaller cap can be used since you choose a high carrier frequency.

    5) The common cat5 wire has the impedance around 100Ohm. The purpose of adding the termination is for better signal integrity. You can start testing without the termination, then compare the results with the termination.

    6) You can use SM712 for the diode, which is a common choice in RS485. But since THVD8000 doesn't have to work with wide common mode range, the other TVS should work too.

    7) Actually THVD8000 works with AC powerline. So far 24VAC has been tested. We're still investigating the challenges like noise and coupling from higher AC voltage.

    Please let me know if you any further questions.

    Regards,

    Hao

  • 0 in reply to Hao L
    TI__Intellectual 2916 points

    Hi Hao,

    see questions/comments inline below. 

    1) The major drawback with a higher carrier frequency is less communication distance. We are looking at maybe 400 ft maximum at 50W or so; 200 ft at 100W…

    2) Your understanding is correct. The inductor keeps the same inductance with different current rating. OK…

    3) The first order estimation is correct. However the impact of the inductor also depends on the resistance and cable impedance. Please refer to this post for some discussion. At the lengths that I’m considering; which will be dominant? The cable or the inductors on the board?

    4) Yes, smaller cap can be used since you choose a high carrier frequency. OK.

    5) The common cat5 wire has the impedance around 100Ohm. The purpose of adding the termination is for better signal integrity. You can start testing without the termination, then compare the results with the termination. In this case, my thought is to parallel the twisted pairs in the CAT5e cable; does this affect the impedance of the cable at all? One thought is that it makes the impedance ¼ of its usual value; another thought (looking at what goes into the impedance calculation) is that it doesn’t affect it… Any thoughts as to which one is correct?

    6) You can use SM712 for the diode, which is a common choice in RS485. But since THVD8000 doesn't have to work with wide common mode range, the other TVS should work too. That is a part that I’ve used in the past.

    7) Actually THVD8000 works with AC powerline. So far 24VAC has been tested. We're still investigating the challenges like noise and coupling from higher AC voltage.  We would be looking for something that would work on a 120V, 20A branch circuit. I’m guessing that there would be some kind of network between the power line (circuit breaker) and the load wires. Do you know where a basic schematic would be along with a design guide for AC applications? I didn’t see it in the EVM book…

     

    Thanks,

    Kevin

  • 0 in reply to Kevin Lai
    TI__Mastermind 25705 points

    Kevin,

    1. The communication distance depends on the cable loss at the carrier frequency.

    3. The inductor's impact on the network's impedance is complicated. Other than the inductor, the total impedance includes the termination resistance and cable parasitics. Inductor impedance affects the load more than the cable impedance.

    5. There is some difference on the impedance between twisted and untwisted wires. But I don't think the delta is significant. You can roughly treat them same in the application. After all, the pure resistive termination doesn't perfectly match the cable. Please refer to the section 7 of the app note.

    7. The implementation of THVD8000 for DC or AC power is the same. The app note for AC part is still under work.

    Regards,

    Hao