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I am trying to design a communication system that will send data signals over a 400V DC voltage line by using line transceivers. I attached the LTspice schematics below. The transformer is for providing isolation between 400VDC and the transceivers.

The problem is the size of the inductor shown inside the red circle. If I decrease the inductance value, the square wave coming from the A-B input loses its shape and peak voltages occur. This causes false high voltages at the RO output of the receiver side. However, since the DC current is 3.5A at the power line. The required inductor size increase enormously with the inductance value in order to prevent saturation. In the attached plot the simulation results are shown while using 200 uH inductor. You can see the problem. So, how can I calculate the required inductance and is there any other configuration for this system that works with a smaller inductor?

simulation results for L=200uH

LT spice simulation

The "DK" input shown in the left of the schematic is similar to the picture I attached below. I did not show the generation part of that "DK" input at the question for decreasing the picture size.

The LTC2862 transceiver outputs the modulated signal (second plot of the below picture) from its A-B outputs. That modulated signal travels through the transformers and 400V DC line then comes to the A-B input of transceiver at the right side of the schematic. RO output of this gives a modulated signal (but only 0V and 5V, like the green plot at the picture I attached at the question) according to the voltage at the A-B inputs. Then low pass filter removes the 80kHz carrier signal and the data sequence is formed and received just like the first plot at the below picture. As I know this is amplitude modulation but If I am wrong please correct me. enter image description here

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This answer was produced before the OP explicitly revealed that he was proposing to use amplitude modulation rather than try and push raw data onto the power line.

is there any other configuration for this system that works with a smaller inductor?

An effective powerline communication system (whether it's DC or AC power) should adopt a data modulation scheme to ensure that the required spectrum at the receiver is as band-limited as it can be. This, in its basic form usually means FSK (frequency shift keying) and band-limiting with a band-pass filter before transmitting to the line. The receiver would also use a similar band-pass filter before amplifying and demodulating/detecting the data. In other words, make your spectral occupancy as small as possible given the FSK frequencies and your data bandwidth: -

enter image description here

Picture from here. I've added red blocks where band pass filtering is needed to avoid load-power interference whilst simultaneously allowing the communication signals to effectively interface with the power line. In many respects it boils-down to a radio system using a "wire".

It looks like you are trying to feed digital data directly to the line using an RS485/RS422 transceiver and this is going to produce problems in that low frequencies are going to be significantly attenuated by the DC pass inductor. Also, the bandwidth of the transmitted data will be naturally large and this means that any receiver has to have an open door for distortions produced by the load when drawing an irregular or cyclic DC current.

Regarding that inductor, you can make it AC resonant at the FSK frequencies and this will act as a band-limited block to those frequencies and this, in turn allows a smaller inductance value because, parallel tuning it with a capacitor will give a high blocking impedance to the FSK.

Do yourself a favour and use an FSK regime. What data rate are you contemplating by the way?

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  • \$\begingroup\$ Data rate is 1.2 Kbps. As I understand by your answer that I should not use Amplitude modulation which is the method I was intending to do. \$\endgroup\$
    – Gökhan
    Commented Aug 20, 2020 at 10:07
  • \$\begingroup\$ @Gökhan your schematic doesn't suggest any form of amplitude modulation so what do you mean? \$\endgroup\$
    – Andy aka
    Commented Aug 20, 2020 at 10:09
  • \$\begingroup\$ I tried to explain the circuit in answer below. \$\endgroup\$
    – Gökhan
    Commented Aug 20, 2020 at 13:34
  • \$\begingroup\$ @Gökhan your schematic doesn't suggest any form of amplitude modulation. Where in your schematic (in your question) is the amplitude modulation part? I see your answer below (a place reserved for answer so please delete it) but what has that got to do with the schematic in your question. You asked a question and I answered use FSK - what is the answer you have given got to do with my answer and your original question (especially the schematic)? \$\endgroup\$
    – Andy aka
    Commented Aug 20, 2020 at 13:42
  • \$\begingroup\$ You asked me what I mean by amplitude modulation according to the schematics so I tried to explain. I post it as an answer because I can't post picture at the comment. Also as I said I did not show the modulation part since when I do the picture become too big. But the modulation is just a logic AND operation of data sequence and 80 Khz carrier signal. The modulated signal is the "DK" input as you can see at the schematic \$\endgroup\$
    – Gökhan
    Commented Aug 20, 2020 at 14:25

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