SN65HVD234: CANH and/or CANL pins are shorted to GND

Hi Gabriele,

It's definitely possible that a DC short on one of the CAN lines has damaged the TVS diode. This particular diode appears to have a lower clamping voltage than the absolute max tolerance of the transceivers CAN pins. This is good protection design and will allow the TVS to engage before the conditions cause the transveier to be damaged. However, it sounds like this damages the TVS, possibly because the fault persists and causes the TVS to heat to the point of failure (short to GND). 

A different TVS diode with higher clamp voltage would allow the system to tolerate higher DC faults. Choosing a TVS with a clamp voltage closer to the absolute maximum rating of the transceiver's CAN pins would allow a greater range here, and take advantage of the higher bus fault tolerance of these transceivers. Alternatively, a thermistor device such as a PTC fuse could be included on the fault path before the TVS diode. Such a device would limit the current through the shunting device during a DC fault and relieve some of the thermal dissipative requirements of the diode. 

To confirm your suspicions first, I would recommend removing the TVS diodes from the affected boards to ensure that this is the source of the short. If the short persists, it may be that the transceiver is damaged or some other component on the bus. If it appears that the TVS diodes is the cause of the short, you may consider use of a different diode or the inclusion of a blocking component such as a PTC fuse. 

Let me know if you have any more questions.

Regards,
Eric Schott

Thanks for your answer, Eric.

It's difficult to do some tests on the board because the board is completly coated with two-component resin.

But if I measure on the CANL and CANH cable I find a short to GND that I describe in the previous message.

Before trying to remove the resin I would have liked to understand how CANL and CANH shorted to GND.

I think that the problem regard a customer miswiring, because the transceiver are protected from ESD events till +/-12kV and the TVS protect CANH and CANL pins until +/-30kV. So I thought the cause of that fault is the customer miswiring or a DC voltage above +8V.

 What is the max voltage on CANH or CANL pins meet the standard ISO11898? Is it possible to have a DC voltage above +8 on CAN line meets to ISO11898?

BR,

Gabriele.

Hi Gabriele,

Both of these CAN transceivers have bus fault protection specifications that could withstand all CAN bus miswirings in 12V or 24V systems. SN65HVD234 and SN65HVD257 have Max ratings on the CAN pins of +-36V and -27 to +40V respectively. These transceivers would tolerate any improper connection of the CAN pins to Vbat or GND (depending on the battery potential of the system). 

Based on the TVS diode selection, it doesn't sound like this is the case for the whole system. A short to Vbat across such a diode would result in a large current flowing through it, causing it to overheat and be damaged if the current is not limited. 
Do you have a schematic for how the TVS diode is implemented? It appears this particular device has separate terminals for each of it's two diodes. This means that one IC should be used for CANH and one used for CANL, connecting pins 1 and 2 to CAN bus and GND and leaving pin 3 floating to ensure protection from both fault polarities. Does this sound consistent with the current implementation?

Based on the information you've provided so far, I suspect that the short in the system is due to the TVS diode or other bus component (if any others, please share). If possible to do so through the resin, I would recommend removing the TVS diode to verify that it is damaged and confirm that the rest of the board remains operational. After this, we may be able to recommend a different protection device in future units to increase resilience to such fault cases. 

Regards,
Eric Schott

Hi Eric.

The TVS GSOT08C is connected as follows:

- pin 1 to CANL line

- pin 2 to CANH line

- pin 3 to GND line

What do you think?

Hi Gabriele,

This configuration protects the CAN bus from voltages that exceed the reverse breakdown voltage with respect to ground. However, if a negative voltage is applied to the CAN bus, the TVS will engage when the negative voltage exceeds the forward voltage of the diode. To protect against both positive and negative potentials, two diodes facing opposite directions can be placed in series. This is what I described as pin 1 to a CAN line and pin 2 to GND (pin 3 floating). 

With this configuration, it sounds probable that a miswiring that caused a DC negative voltage (or positive voltage that exceeded the reverse breakdown of the diode) caused the TVS to sink more than it's maximum rated current, causing a temperature failure and the resulting short to GND. 

Let me know what you are able to find out after modifying the board. Best of luck with the resin, I know that can make things quite difficult. 

Regards,
Eric Schott