CAN BUS protection.

The CAN interface is the most common BUS inside a car. Some years back, I was introduced to the CAN bus by a good friend of mine from Argentina, who came to the UK for a few years and needed an electronic engineer for his company. I am so grateful to Roman Di Lullo for coming to the UK and creating a lifetime friendship.

Bringing this post back to its subject, here are a few tips for you on how to protect the CAN bus.

In some ways, the CAN BUS and the RS485 are very similar. Basically, it is a bidirectional bus that can be used for either listening or transmitting. In most cases, the line is terminated at both ends with a 120 ohm termination resistor. Hence, it is very similar in many ways to RS485.

CAN BUS cannot fail now.

Consider what could happen if the CANH and CANL pins get shorted together due to a failure on the CAN BUS, which is a shared bus.

This will result in all CAN devices on that branch not working. A hazard like this cannot be tolerated.

CAN BUS should therefore be protected to the greatest extent possible.

On the CANNH and CANL lines, the CAN BUS can accept voltages between 3V and 32V, but it needs to withstand transients between -150V and +100V.

It is important to note that I wrote fast transients, not surges (this is not always true, as we sometimes need to protect those lines by surges as well). Let's assume for now that we only want to protect against transients.

From a PCB layout perspective, Fast Transients need to be treated as high-speed signals, and TVS tracking is crucial. If you do the tracking wrong, the TVS will not work! So be careful here! In this post, I won't discuss PCB layout.

For protection of CANH and CANL buses, we will either need some bidirectional TVS diodes, which are very fast, but their power capabilities are usually limited, or we will need some varistor units.

Varistors have the problem that their breakdown voltage gradually decreases over time, while TVS then are damaged they fail as a short usually. As you can see, there is no magic solution that works for all, and choosing the right components depends on the situation.

The CANH and CANL lines have been protected against transients with this solution.

Do you think this is enough?

No way!

Our EUT (Equipment Under Test) needs to emit as little noise as possible during EMC testing. In general, I use common mode chokes with a 1 mH inductor, which give me a common mode rejection impedance between 600 ohm and 4 kohm between 100 kHz and 30 MHz.

In some cases, it is also a good idea to add 10 pF capacitors on both sides of the common mode filter.

One last thing to keep in mind.

During the start up of an electric vehicle, the 12V can surge above 500V! Unless you have taken precautions on the Vbattery, this can be extremely dangerous. Many people consider 12V batteries to be ideal, which is a mistake I have seen many times. Unfortunately, even on the car 12V line, there can be surges, and if we power up our CAN using this voltage, we will still have a problem.

As a recap:

1. For CANH and CANL lines, use TVS or varistors

2. Between the CANH and CANL and the OBD2 ports, use a CM filter and 10 pF caps

3. You need to protect your electronic equipment from surges caused by the battery and remember that those surges can be very powerful, so the components may be very large. Thanks a lot, and happy CAN busing!