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TCAN1042-Q1: When to use separate power supply?

Dejan Hrovatin1
Intellectual 380 points
Part Number: TCAN1042-Q1
Other Parts Discussed in Thread: ISO1042, SN6501

Tool/software:

Hi,

I'm thinking about using TCAN1042 transceiver in my project to connect two boards in between. There will no be GND connection in between these two boards, however boards are still not galvanic isolated because supplies are connected "together" over anplifiers of both converters. When do you suggest to use isolated power supplies in CAN connections?

Many thanks, Kind regards, Dejan.

  • 0
    TI__Mastermind 28310 points

    Hi Dejan,

    It depends on if you have emission (power source can generate a lot of noise) or short circuit concerns. Our ISO team makes an isolated version of TCAN1042, which is named ISO1042. I'll transfer this thread to ISO and let them comment on your isolated CAN application.

    Regards,

    Sean

  • 0 in reply to Sean Guo
    TI__Mastermind 29375 points

    Hello Dejan, 

    The short answer is that Isolated CAN will help to make a robust and reliable design if you are expecting ground potential differences (GPD) between the CAN GND and the logic grounds. GPD can cause damage to equipment, sensitive circuits or risk of electric shock. 

    The following application report goes into detail regarding how to design for isolated CAN. 

    Top Design Questions about Isolating CAN Nodes Bus Design (Rev. A)

    Best,
    Andrew

  • 0 in reply to Andrew J
    Intellectual 380 points

    Hello Sean, Andrew,

    many thanks for both replies.
    I'm thinking about specific situation, which is clearly visualized by the picture below:


    Two remote devices are supplied from the same power supply and communication type between these two devices is CAN. Do you suggest to use CAN isolation in that example? I'm worry that some kind of loop would be created with such connection, because power supplies are still connected (are not galvanic isolated) over CAN interface amplifiers, although there is no ground (and also power supply wire) between devices.

    What do you think?

    Many thanks,
    Kind regards,
    Dejan.

  • 0 in reply to Dejan Hrovatin1
    TI__Mastermind 29375 points

    Hello Dejan, 

    Thank you for the added details, this helps a lot. Both isolated transceivers will function in this configuration, however there will be no isolation. Side 1 and side to of the isolators must also have isolated power supplies in order to benefit from the full isolation capability. 

    You can use our push-pull transformer driver, SN6501, in order to generate the isolated power for the CAN bus.

    Best,
    Andrew 

  • 0 in reply to Andrew J
    Intellectual 380 points

    Hello Andrew,

    thanks for your reply. 

    What do you mean with "in order to benefit from the full isolation capability"? What can happen if I don't use isolated power supply in my application? Please don't forget that I would not use GND and power wire between devices within CAN cable - there would only be CANH and CANL connection for SDA and SCL separately, so 4 wires. However, devices will be supplied from the same power supply externaly (as shown on the picture in my previous post) but because CAN cable would not have power and GND wire, I don't see any chance for loop, or am I wrong?

    Many thanks, Dejan.

  • 0 in reply to Dejan Hrovatin1
    TI__Mastermind 29375 points

    Hello Dejan, 

    A digital isolator or isolated transceiver must have isolated power and grounds in order for the isolation barrier to be effective. The purpose of isolation is to transmit signal across the isolation barrier and to eliminate the effects of ground potential differences. A ground potential difference can still happen if power is applied over the same ground lines. 

    Isolated power is needed to for proper isolation performance. The following document should provide additional details How to Isolate Signal and Power in Isolated CAN Systems (Rev. C) (ti.com).

    Best,
    Andrew

  • 0 in reply to Andrew J
    Intellectual 380 points

    Hello Andrew,

    it looks that TCAN1042 is not isolated transceiver by default, so it should work as it is without isolation. However, since I have two devices supplied from the same power supply, even there is no CAN GND and CAN POWER wire within CAN cable, it is better to isolate transcievers (isolated on one side should be enough) in my application to be sure that there will be no any ground difference between these two devices which can exceeds maximum common mode voltage of TCAN1042. Did I understand you right?

    Many thanks, Kind regards, Dejan.

  • +1 in reply to Dejan Hrovatin1
    TI__Mastermind 29375 points

    Hello Dejan, 

    You are correct. TCAN1042 is not an isolated device. Please note that the isolated version (ISO1042) will operate even with the GND1 and GND2 pins together and with VCC1 and VCC2 sharing the same supply. However, the device will not galvanically isolate the signal unless there is also isolated power and ground. 

    Allow me to clarify the next point. The purpose of isolation is to protect the sensitive logic circuits connected to side 1 of the isolator from possible damaging transients on the ground bus. The ISO1042 would not stop a transient on the CAN bus.

    That being said, you could use one power supply to power the CAN side of the device (side 2) if you used two ISO1042's. Side 1 of each device would need to be supplied power from an isolated source. 

    Does this help?

    Best,
    Andrew