Design In TVS Diode Protection to Enhance CAN Bus Reliability

By Steven Keeping

Contributed By Digi-Key's North American Editors

The Controller Area Network (CAN) bus, a popular bus standard for automotive and other applications, has a high degree of built-in immunity to overvoltages and overcurrents. But with up to 70 electronic control units (ECUs) as part of a premium vehicle’s electronic network, designers are becoming increasingly concerned about preventing damage from electrical transients such as those caused by lightning and electrostatic discharge (ESD). Such sensitivity increases the risk of failure and threatens to undermine vehicle reliability.

Although many transient voltage suppression (TVS) devices are available, automotive applications are typically constrained by tough cost, weight and reliability specifications. These restrictions eliminate many of the larger, more complex TVS devices. But recently, manufacturers have introduced automotive grade versions of the modest TVS diode which offer an inexpensive, compact and highly reliable circuit protection option. Further, unlike some alternatives, TVS diodes increase the noise immunity of CAN transceivers and have negligible impact on high frequency communication signal integrity.

This article describes how TVS diodes can provide a high level of inexpensive protection for sensitive CAN bus implementations. The article will explain why it’s important to select not only an automotive grade device, but also to carefully consider the peak voltage and current, capacitance, leakage current, and clamping voltage to fully protect sensitive ECUs and CAN transceivers. The article will then introduce suitable TVS solutions from Texas Instruments, ON Semiconductor, Bourns, and Semtech and explain how to properly apply them.

An introduction to CAN

CAN was born out of a need to add more electronics to vehicles without multiplying the complexity and weight of the wiring harness. The CAN standard specifies a robust peer-to-peer network supporting several physical layers (PHY), but the most common PHY is the high-speed version (a two-wire implementation enabling raw data rates up to 1 megabit per second (Mbit/s). The network allows communication between multiple CAN devices such as ECUs. Connected ECUs only require a single CAN interface (rather than multiple analog and digital I/Os) to connect to every other device on the network, eliminating complex and expensive wiring.

A typical CAN bus differential (CAN H/CAN L) scheme comprises transceivers communicating on a serial bus. A twisted pair cable with a nominal characteristic impedance of 120 ohms (Ω) is used to transmit the signal between nodes on the bus. A split termination topography is often used to improve electromagnetic interference (EMI) immunity (Figure 1).

Diagram of CAN bus uses a differential communication scheme

Figure 1: CAN bus uses a differential communication scheme enabling transceivers to communicate reliably across a serial bus. (Image source: Bourns)

While CAN ECUs and transceivers comprise inherently fragile silicon, they are expected to withstand challenging operating conditions. For example, most vehicle manufacturers demand qualification to AEC-Q100, a failure mechanism stress test for vehicle electronics. Major auto makers also require compliance to recent international standards (ISO 7637 and IEC 61000-4-5). These standards specify test electrical transients designed to simulate electrical disturbances from conduction and coupling during vehicle operation.

Some chip vendors’ offerings meet these specifications. Texas Instruments’ SN65HVD1050DRG4 CAN transceiver, for example, features cross-wire, overvoltage, and loss-of-ground protection from –27 volts to 40 volts and overtemperature shutdown. The chip can also withstand the -200 volt to 200 volt transients defined in ISO 7637.

One downside of a high specification device is cost, a critical consideration during vehicle design. Second, while a hardened device might be able to withstand electrical transients for a period of time, repeated exposure risks damage. Third, lightning and ESD can expose the auto’s electronics to voltages and currents beyond that demanded for compliance to some standards. Additional protection, whereby electrical transients are diverted to ground and hence away from sensitive silicon, is worthwhile for car makers striving for greater reliability.

Transient voltage suppression using diodes

There are several established techniques for implementing electrical transient protection. These can be generally categorized as blocking, suppression, or isolation. In simple terms, blocking uses fuses and circuit breakers, suppression employs TVS devices such as TVS diodes and metal oxide varistors (MOVs), and isolation depends on employing isolating devices such as optocouplers and transformers.

Blocking is effective and inexpensive. The downside is that after the devices have activated, they need to be replaced or reset, which is very inconvenient in automotive applications. At the other end of the scale, isolating devices are completely effective and don’t have to be replaced or reset, but they are bulky, complex, and expensive. TVS devices fall in the middle. They are generally effective, compact and are mid-priced.

TVS devices come in a range of types including TVS diodes (and TVS diode arrays), MOVs and proprietary transient current suppression devices. While TVS diodes aren’t the highest performing TVS devices, they are inexpensive and hardy (especially when combined with CAN nodes that meet AEC-Q100 and ISO 7637 standards), making them a good choice for circuit protection in space and cost-constrained automotive applications.

A TVS diode is a p-n device specifically designed with a large junction cross-sectional area to absorb high electrical transient currents. While the voltage/current characteristic of a TVS diode is similar to that of a zener diode, the devices are designed for voltage suppression rather than voltage regulation. A key advantage of a TVS diode is its rapid response (typically within nanoseconds) to electrical transients—diverting the energy of the transient safely to ground while maintaining a constant clamping voltage—compared to other suppression devices.

Theoretically, the protection mechanism is straightforward. Under normal operating conditions, the TVS diode presents a high impedance to the protected circuit, but when the safe operating voltage of the protected circuit is exceeded, the TVS diode operates in an avalanche mode providing a low impedance path to ground for the transient current. The maximum voltage to which the protected circuit is subject is typically modest and limited to the diode’s clamping voltage. The TVS device returns to a high impedance state after the electrical transient current subsides (Figure 2).

Diagram of TVS diodes protect circuits by providing a path to ground

Figure 2: TVS diodes protect circuits by providing a path to ground for electrical transients while clamping voltages to safe levels. (Image source: Semtech)

In practice, the protection circuitry for CAN implementations is more complex because the network supplies not only power but also data, carried via a differential signaling scheme

TVS diode selection for CAN applications

TVS diodes are available in two types, unidirectional and bidirectional. While each provides protection for both positive and negative surges, the key difference is the breakdown voltage (the voltage at which the device starts to conduct in avalanche mode and hence exhibits low impedance). The bidirectional device offers the same breakdown voltage in both directions, while the unidirectional device has a much lower breakdown voltage (equal to the diode’s forward bias voltage) for negative transient voltage spikes.

While unidirectional and bidirectional devices can be used for the same application, there are some applications where the different breakdown voltage characteristics of each provide an advantage. For example, if the CAN transceiver serves a digital logic IC, the unidirectional TVS diode’s low breakdown voltage to negative surges offers superior protection.

Among the key advantages of bidirectional TVS devices are that they solve the common-mode offset voltage problem. This occurs because CAN transceivers must be able to function with a signal line voltage that can be offset by as much as 2.0 volts from the nominal voltage level. Because bidirectional TVS devices feature a large clamping voltage in positive and negative directions, they will not clamp due the influence of a signal line offset. Additionally, bidirectional TVS diodes can be dropped in as a direct replacement for inherently bidirectional MOVs.

There are several alternative topologies for CAN bus protection. The simplest uses a TVS diode arrangement comprising two bidirectional diodes, one across the CAN_H (or DATA_H) line and ground, and the other across the CAN_L (or DATA_L) line and ground. The alternative arrangement swaps the bidirectional TVS diodes for unidirectional devices (Figure 3).

Diagram of bidirectional (left) or unidirectional (right) TVS diodes

Figure 3: Depending on the application, either bidirectional (left) or unidirectional (right), TVS diodes can be used. Manufacturers often offer solutions that integrate the two diodes into a single package. (Image source: ON Semiconductor)

It is possible to use individual TVS diodes to protect each CAN data line, but many manufacturers offer packages that integrate both diodes into a single package. For example, ON Semiconductor supplies the NUP2105LT1G TVS diode which provides bidirectional protection for each CAN data line with a single compact SOT−23 package. The device can handle a peak power dissipation of 350 watts. The NUP1105LT1G is the unidirectional equivalent.

Once a designer has settled on a topology, the performance of the circuit is determined by careful selection of a TVS diode with operational characteristics that match the needs of the application.

The key device parameters of bidirectional TVS diodes include:

  • Reverse working voltage (VRWM) - which is the maximum DC operating voltage. At this voltage the diode is in a non−conducting state and acts as like a high impedance capacitor.
  • Reverse breakdown voltage (VBR) – which is the point (typically measured at 1 milliamp (mA)) where the device conducts in avalanche mode and changes to low impedance.
  • Peak pulse current (IPP) - which is the maximum surge current specified for the device.
  • Maximum clamping voltage (VC) - which is the maximum voltage drop across the diode at IPP.
  • Reverse leakage current (IR) – which is the current measured at VRWM.
  • Test current (IT) – which is the current at VBR (Figure 4).

Diagram of voltage/current characteristic for bidirectional TVS diodes

Figure 4: Voltage/current characteristic for bidirectional TVS diodes illustrating the key device parameters. (Image source: ON Semiconductor)

The CAN specification details the critical transceiver characteristics, which in turn determine the characteristics of the TVS diodes selected to provide electrical transient protection. Key parameters include:

  • -3.0/16 volt min/max bus voltage (12 volt system)
  • -2.0/2.5 volt min/nom CAN_L common-mode bus voltage
  • 2.5/7.0 volt nom/max CAN_H common-mode bus voltage
  • Recommended ≥ ±8.0 kilovolt (kV) (contact) ESD
  • ISO 7673-3/IEC 61000-4-5 surge current pulse immunity

The first parameters the developer should consider are VRWM and VBR. These should be sufficient such that during normal operation, the TVS diode presents as a high impedance, but not so high that the device doesn’t start conducting until the CAN transceiver has been exposed to a dangerously high voltage. Note, that while auto electrical systems generally run off a 12 volt battery, most are designed to be jump started from a 24 volt supply in the event of an emergency. The TVS diode choice should take this into account.

For example, the ON Semiconductor NU2105L has a VRWM of 24 volts and a VBR of 26.2 volts at 1 milliamp (mA). The Bourns CDSOT23-T24CAN CAN bus Protector, a SOT-23 packaged dual bidirectional TVS diode, features an identical specification.

Next, the developer should check the maximum capacitance of the TVS diode. Large capacitance undermines signal integrity. The faster the data rate, the lower the capacitance should be. A rule of thumb is for a maximum capacitance between signal lines and ground of 100 picofarads (pF) at a data rate of 125 kilobits per second (kbits/s) and 35 pF at 1 Mbits/s. Note that some data sheets express the capacitance at 0 volts, while others express it at the average voltage of CAN transceivers, which is 2.5 volts. In addition, the capacitance of the two differential signals should be matched to maintain pulse width integrity in the amplifier’s output signal.

At 0 volts and 1 Mbit/s, the Bourns CDSOT23-T24CAN, for example, has a capacitance of 22 pF between signal line and ground. Semtech’s UCLAMP2492SQTCT, a SOT-23 package housing two bidirectional TVS diodes and specifically designed for CAN bus surge immunity, features a capacitance of 15 pF (at 0 volts and 1 Mbit/s) between signal lines and ground.

It also makes sense to choose a device with low reverse leakage current (IR) to maximize system efficiency. Note that IR rises with temperature, so operational conditions should be taken into account when selecting a device. The NUP2105L, for example, has an IR of 0.1 microamps (µA) at 25°C, while the UCLAMP2492SQTCT device has an IR of 0.2 µA at 25°C and 0.35 µA at 125°C.

Finally, the developer should ensure that the TVS diode can dissipate the energy of a non-repetitive electrical transient surge without damage, and that the clamping voltage at the electrical transient peak current will not damage the CAN transceiver.

IEC 61000-4-5, the IEC’s standard specifying how to test for surge immunity, details the typical surge waveform used to determine a TVS diode’s capability. The waveform reaches 90% of its peak value in 8 microseconds (µs) and decays to 50% of the peak value in 20 µs. Data sheets often refer to this as the “8/20 µs waveform” (Figure 5).

Graph of example of the waveform parameters (“8/20 µs”)

Figure 5: Example of the waveform parameters (“8/20 µs”) specified in IEC 61000-4-5 to test for a TVS diode’s surge immunity. (Image source: Bourns)

The response of the Bourns CDSOT23-T24CAN TVS device to an 11 amp (A) 8/20 µs waveform is shown in Figure 6. The manufacturer cites a maximum clamping voltage of 36 volts for a 5 A surge and 40 volts for an 8 A surge. The equivalent figures for the ON Semiconductor NUP2105L are 40 and 44 volts with a peak power dissipation of 350 W, and for the Semtech UCLAMP2492SQTCT, 44 volts at 5 A.

Graph of Bourns CDSOT23-T24CAN response to an 11 A 8/20 µs waveform

Figure 6: Bourns CDSOT23-T24CAN response to an 11 A 8/20 µs waveform. Note the rapid response of the TVS diode package to the surge current transient and clamping voltage peak of 36.4 volts. (Image source: Bourns)

Once the developer has selected the appropriate TVS diodes for the job, careful consideration should be made as to the best pc board layout for optimum performance. The overriding principle is that once activated by an overvoltage, potentially damaging surges are directed away from the CAN transceiver by the TVS diode(s) and safely dissipated in the ground plane.

Bourns, for example, advises the SOT-23 device should be placed as close to the bus connector as possible with short traces to the signal lines. The company advises that a standard 10 mil, 1 ounce copper trace is more than adequate to handle the peak current level from typical electrical transients. The ground pin of the device should be connected to the PCB ground plane using a short trace and a via. Finally, if there is a ground plane on the signal side of the near the TVS diode, the component should be connected directly to the ground plane (Figure 7).

Diagram of recommended pc board layout for Bourns’ CDSOT23-T24CAN

Figure 7: Recommended pc board layout for Bourns’ CDSOT23-T24CAN. The SOT-23 housing TVS diodes should be placed as close as possible to the CAN bus connector. (Image source: Bourns)

Conclusion

Cost, space, and weight restrictions limit the range of solutions to protect CAN bus devices from extreme events such as lightning and ESD. However, TVS diodes offer an acceptable trade-off between these constraints and protection performance. The key to a successful implementation is careful matching of the TVS diode’s electrical characteristics to the application such that protection is assured while normal operation of the CAN bus is not compromised.

The recent introduction of compact (SOT-23) solutions designed specifically for CAN automotive applications and integrating either unidirectional or bidirectional TVS devices, eases not only component selection but also design complexity and space requirements.

References

  1. Circuit Configuration Options for TVS Diodes, AND8231/D, ON Semiconductor, March 2017.
  2. TVS Diode Selection Guidelines for the CAN, AND8181/D, ON Semiconductor, August 2004.

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About this author

Steven Keeping

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Digi-Key's North American Editors