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Growing importance of high-voltage fault protection for CAN transceivers

Editor’s note: I am happy to present this month’s author for this Signal Chain Basics blog, John P. Griffith, who is a systems engineer for the Interface group at TI.

This article focuses on the increasing need for high-voltage bus pin fault protection for controller area network (CAN) transceivers across transportation industries. High-voltage fault protection prevents damage to a transceiver during bus faults, such as when a CAN bus wire is shorted to the power supply. CAN transceiver data sheets will specify bus pin fault protection voltages as an absolute maximum DC voltage that the pins can withstand without damage. With a growing number of applications transitioning to higher voltage DC supplies, this parameter is becoming increasingly important and applications are requiring higher and higher fault protection.

The need for higher voltage DC supplies, and thus the need for increased high-voltage fault protection, is growing in multiple industry segments. Two prominent markets are passenger vehicles and heavy duty vehicles like tractors and forklifts.

Tightening fuel economy requirements around the world are contributing to the need for increasing fault protection in the passenger vehicle space. One popular way to increase fuel economy is by reducing the number of auxiliary loads that are connected to the engine. Historically, systems like power steering, air conditioning, water circulation, and others have been belt-driven systems, and therefore place a load on the engine at all times. By transitioning these systems to electric power, and reducing the load on the engine, the engine can run more efficiently, decreasing both fuel consumption and CO2 emissions.

Unfortunately, since some of these loads, like electric power steering, require a large amount of power it can be difficult and costly to run them on a 12-V supply. It is for this reason that manufacturers are developing and standardizing 48-V systems for vehicles. For a 3-kW load, a 12-V system would draw 250 amps of current, but a 48-V system would only require 62.5 amps of current. Because current, not voltage, dictates the gauge of wire needed, this large drop in load current enables much smaller and lighter wires to be used. Lighter wires shave off weight in the overall vehicle, and the lighter the vehicle, the more efficiently it will run. The current LV 148 standard states that the required fault protection voltage for 48-V systems is a minimum of 60 V for dynamic overvoltage events that can occur. Thus, communication wires that run in the same wire harnesses must be able to handle these voltages as well.

Moving to the heavy-duty vehicle space, farming tools like seeding attachments are beginning to transition from being largely mechanical to utilizing electrically-controlled, motor-driven systems. These systems provide better control over, and consistency in, their results. For large combines that seed 16+ rows of produce at a time, the electrical load requirements become quite large, sometimes in the range of 10-20 kW for the seeding attachment. For these applications, 12-24-V supplies would struggle to deliver this amount of power. Therefore, total system-level power consumption is driving the need for higher voltage battery supplies. As with automotive applications, the adjacent wiring needs to be able to handle direct shorts to these battery voltage supplies with margin for overshoot and maximum charge voltages.

One additional factor to consider when designing systems with high-voltage supplies is how much margin is needed for components that may be subject to these short-circuit events. Figure 1 shows an example of a 55 V DC supply being shorted to a CAN bus wire through a 1-meter cable. As is evident, the peak voltage due to inductive ringing is in excess of 80 V! Short circuit impedance, along with protection and filtering components, play a large roll in reducing these transients.

Figure 1

Transient overshoot for high-voltage short

Transient overshoot for high-voltage short

While higher voltage systems have a major benefit of being able to deliver more power, they come with the system-level tradeoff of needing higher fault protection devices. With the growing number of applications transitioning to these higher voltage levels, the need for transceivers to handle these voltages will follow. Examples from Texas Instruments include the TCAN1042 and TCAN1051.

Join us next time where we will discuss the basics of 4-20 mA sensor transmitters.

Additional reading

More information from TI about CAN and RS485.

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