What am i missing - lack of power supply to isolated CAN Transceiver?? - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: January 27, 2017, 05:23:58 pm
content: I'm just looking at a control module that has a CAN output, and that output is galvanically isolated from the ecu's power/gnd. Ok, nothing surprising so far, here's a picture:On the left (U2), there is a digital isolator, an Analogue Devices ADUM1401, and on the right (U3), the CAN transceiver, a Microchip MCP2551http://www.analog.com/media/en/technical-documentation/data-sheets/ADuM1400_1401_1402.pdfhttp://ww1.microchip.com/downloads/en/DeviceDoc/21667f.pdfAs there is no power supply (regulator, or external power input) i expected the ADUM1401 to include a power transfer across the isolation barrier to power the CAN transceiver, but it doesn't seem to have that capability! So what the heck is powering the isolated side??I removed the components to check for any sneaky vias hidden under the parts and there are none:The CAN High and Low signal traces come in from the rhs, to pins 6 & 7 as expected, the CAN slew rate control pin (pin8) is grounded (no slew rate limiting - terrible for EMC........) and the transceiver gets it's power (pin3) decoupled by C6. The power trace continues left, to pin 10 of the ADUM1401 (output enable pin) and then runs between pins 11 and 12, under the isolator, then runs up and to pin 16, where it's decoupled by C3 and C4. BUT, where does the power come from?? I'm beginning to think this device is a prototype, and the CAN bus has never worked due to a schematic screw up and forgetting to power the isolated side? The ground for the isolated side is connected to the ground plane around these devices by a via (visible just above + label for C38), and then a track to the output connector, so that CAN ground is coupled to the output side.Have i missed anything obvious, because i just can't see how this is mean't to work

author: David Hess
author_rank: Super Contributor
timestamp: January 27, 2017, 05:34:54 pm
content: They might use a charge pump to provide power across the isolation barrier but how many pins does the CAN connection have? Maybe power is provided through the cable side.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: January 27, 2017, 06:02:09 pm
content: Small chance there is enough leakage from the CAN signals (typically around 2.5V) to charge the VCC capacitor.But indeed more likely it never worked, certainly not reliably. Perhaps there was supposed to be another isolator with IsoPower capability, like ADUM6400 family.

author: nctnico
author_rank: Super Contributor
timestamp: January 27, 2017, 06:06:34 pm
content: I don't see the point of putting an isolator in this circuit because from the pictures it seems the primary and secondary side are connected to the same ground! TVS diodes and series diodes would have done a much better job (cheaper and more rugged!). Without secondary power the isolator isn't going to work anyway so this might be a prototype with a screwed up CAN interface.

author: JPortici
author_rank: Super Contributor
timestamp: January 27, 2017, 06:09:15 pm
content: i thought the same but if you look closely on the bottom right corner you see that the other ground plane is following the white silkscreen rectangle

author: JPortici
author_rank: Super Contributor
timestamp: January 27, 2017, 06:17:16 pm
content: Vdd is between pin 10 16 of the isolator and pin 3 of transceiverbut no obvious voltage sorurce.i wonder... could it be that they are using the bus to power the transceiver?like... diode is a dual diode and on common cathode there is a buried via to vdd2have you monitored VDD2 with the bus connected/disconnected?

author: Jeroen3
author_rank: Super Contributor
timestamp: January 27, 2017, 06:53:11 pm
content: Quote from: JPortici on January 27, 2017, 06:09:15 pmIt features about 50v common mode isolation with those clearances.AD has some isolators that have an internal power supply, maybe those are needed?

author: max_torque
author_rank: Super Contributor
timestamp: January 27, 2017, 06:54:24 pm
content: I checked every I/O pin on the connector, none is connected to the power bus, and there is no obvious trace to do that anyway (no trace on top, no via to any other trace on bottom side etc). The ground IS connected, because typically a CAN bus shares a common ground, which is why they will want the isolation, but i just can't see how the power is transferred! All i can think of is that there must be a digital isolator in this same footprint from Analogue that includes a charge pump, and outputs power to the isolated side??

author: Jeroen3
author_rank: Super Contributor
timestamp: January 27, 2017, 07:19:25 pm
content: I've looked at it on my PC.My conclusion: this design will not work reliably, or not at all.There are 3 required wires in CAN bus:- High- Low- GNDOf which it seems GND is missing, since it's not going to the connector. CAN bus will suffer severe common mode issues and it will probably fry the tranceiver on the first transient.

author: Simon
author_rank: Unknown
timestamp: January 27, 2017, 07:36:49 pm
content: Quote from: Jeroen3 on January 27, 2017, 07:19:25 pmUm isn't that the point of it being g isolated? No ground reference.Sent from my phone so mind the autocorrect.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 27, 2017, 07:59:21 pm
content: Can still needs ground to function properly.

author: max_torque
author_rank: Super Contributor
timestamp: January 27, 2017, 10:02:53 pm
content: There IS a ground! It runs from the i/o connector to the via just above the "+" test on C38, where it connects to the isolated ground plane!!What i can't find is the power equivalent..........

author: DBecker
author_rank: Frequent Contributor
timestamp: January 28, 2017, 03:39:45 am
content: Are you sure that wasn't something like a ADUM5401, with two isolated channels and a supply? It's not quite within specs for supply, but would work if you kept transmit duty cycle to under 50%.CAN transceivers cannot operate off of "parasitic" power. They suck down 70mA at 5V, and can only expect to see a 2V swing from which they can draw negligible current.I can't say I like the layout. You pay for great isolation and give it all up with token creepage and clearance. You might not think you need the isolation, but getting it has no material cost.

author: nctnico
author_rank: Super Contributor
timestamp: January 28, 2017, 12:25:53 pm
content: Quote from: Jeroen3 on January 27, 2017, 07:59:21 pmNo. CAN will work just fine with only CAN_H and CAN_L. It is designed to work with any of the 3 connections missing but with one wire missing from the start there is no redundancy. I've seen products designed that way and they do work (but I strongly recommend against it). As long as there are TVS diodes in place to protect the transceiver (which usually are extremely rugged by themselves) not much can go wrong.

author: tatus1969
author_rank: Super Contributor
timestamp: January 28, 2017, 04:14:46 pm
content: the large tantal is a indication that the design indeed does use parasitic supply, provided by the transceiver's protection diodes. can idle state has zero differential volts, so the other communication partner would need to drive the bus active for some time to charge up the capacitor, before communication can take place. but that would be non standard, strange.

author: David Hess
author_rank: Super Contributor
timestamp: January 28, 2017, 08:10:12 pm
content: Quote from: tatus1969 on January 28, 2017, 04:14:46 pmUnfortunately there are no details (of course) in the Microchip datasheet for the output protection structures of the MCP2551 but I think you are correct; D4 is the negative side of the rectifier and the positive side is using the protection circuits inside the MCP2551. I would have included the other side of the rectifier externally and protection from over voltage whether it worked using the MCP2551 like that or not.I also notice that no CAN bus termination is included so I assume it is meant as either a diagnostic port or it connects in the middle of a CAN bus between two other devices.

author: bktemp
author_rank: Super Contributor
timestamp: January 28, 2017, 08:29:17 pm
content: Quote from: David Hess on January 28, 2017, 08:10:12 pmI doubt that: If you look at the specs, MCP2551 allows +/-42V on the CAN lines. Therefore there can't be any diodes from CAN inputs to the supply rail. Also CAN transceivers must not load the bus when unpowered.D4 is most likely an ESD protection diode (something like PESD2CAN).

author: tatus1969
author_rank: Super Contributor
timestamp: January 28, 2017, 09:11:32 pm
content: Quote from: bktemp on January 28, 2017, 08:29:17 pmgood point, that invalidates my idea. is it possible that the ad iso chip is wrong and should be an adum5401 instead?

author: max_torque
author_rank: Super Contributor
timestamp: January 28, 2017, 09:51:45 pm
content: The pinout fits for an ADUM5401. But it's not an ADUM5401 that is fitted!! I wonder if someone screwed up

author: David Hess
author_rank: Super Contributor
timestamp: January 28, 2017, 11:36:00 pm
content: Quote from: bktemp on January 28, 2017, 08:29:17 pmThat is why I said it is unfortunate that there are no details showing the I/O output circuits. The input protection no doubt includes a high series impedance but that does not mean that there are no protection diodes going to ground and Vcc behind it. The datasheet does not specify input resistance or current when the inputs exceed ground or Vcc.D4 is not consistent with input protection because there is no series resistance. A surge would destroy D4 likely shorting it killing the device anyway. The MCP2551 already has suitable input protection up to +/-42 volts.

author: bktemp
author_rank: Super Contributor
timestamp: January 29, 2017, 06:26:35 am
content: Quote from: David Hess on January 28, 2017, 11:36:00 pmEven if there are diodes after some high value resistors, the datesheet specifies <150uA leakage current for Vdd<3.4V. In standby mode (receiver+transmitter turned off) the supply current can be as high as 365uA. So without any external power supply, the CAN transceiver can't work currectly. Unless there is something hidden somewhere, the layout looks like a complete screw up.QuoteIt is unusual to have a series resistor at the output, at least I have never seen it. Here is one example:http://www.onsemi.com/pub_link/Collateral/AND8169-D.PDFThe protection diode is connected directly at the output to the CAN line.

author: DBecker
author_rank: Frequent Contributor
timestamp: January 29, 2017, 01:46:59 pm
content: Quote from: tatus1969 on January 28, 2017, 04:14:46 pmThe larger capacitor would be consistent with an ADUM5401 powering the transceiver. It's amazing what you get in the package, and really convenient to design with, but it's not especially powerful or efficient. The capacitor needs to store enough energy to transmit a frame without too much voltage sag, and to avoid the internal PWM regulation from interacting with the transmit bits.Did this device ever work? If it's really a ADUM1401 in place (I can't tell from the picture), presumably not.

author: max_torque
author_rank: Super Contributor
timestamp: January 29, 2017, 05:53:44 pm
content: Yes, it's an ADuM1401, or at least that is what the screenprinting on the device says!I've powered it up, and i get 0.6V on the isolated side with a CAN bus connected (but without the transceiver active) which isn't enough volts for anything......I've no idea if this device has ever worked, it was lent to me to reverse engineer to see how it does certain things. I suspect that perhaps, given the 5401 costs roughly double that of the 1401, did the device assemblers try to cut some corners and "save" a bit of money, without checking things out properly??

author: Ian.M
author_rank: Super Contributor
timestamp: January 29, 2017, 06:04:30 pm
content: Patch in a 5V output isolated DC-DC converter and see if the device exhibits working CAN functionality. That would be strong evidence for corner cutting iin production. If it doesn't make any difference (i.e the firmware's CAN support is broken or missing) it would point to a FUBARed design.