Protecting data and power lines in automotive electronic systems

Preface

Today, automotive electronic devices account for a large proportion of the entire vehicle. Although these electronic modules bring comfort and safety to car users, at the same time, from the perspective of the reliability of electronic modules in the automotive environment, some problems that cannot be ignored have also arisen.

Because electronic modules are very sensitive to electromagnetic interference (EMI), electrostatic discharge (ESD) and other electrical interference (the car itself is the source of these hazards), careful consideration must be given when using electronic modules in an automotive environment. In response to the common electrical hazards in today's automobiles, the International Organization for Standardization has issued several sets of electrical protection standards. Automotive manufacturers and suppliers must consider these standards, and only by adding protection components to electronic modules can they fulfill the main responsibilities specified in these standards. For example, automotive electronic modules must withstand electromagnetic interference and ESD transient voltages specified in the two major international standards ISO7637-2 and ISO10605.

Electrical Hazards in the Automotive Environment

The automotive environment itself includes many sources of electrical hazards. Various automotive components, such as ignition switches, relay contacts, alternators, fuel injectors, etc., will generate electrical interference such as electromagnetic interference and electrostatic discharge. Conductive hazards appear directly in the wiring harness; radiated hazards directly affect electronic modules. These electrical hazards generated by the automotive environment will affect electronic modules in two aspects: data lines and power lines.

Transmission of Electrical Hazards

Transient events generated in the automotive environment may vary from low energy levels to high energy levels, or from high energy levels to low energy levels, with extremely high voltage slopes dV/dt. The main standards for these electrical hazards are ISO7637-2 and ISO10605, and sometimes the IEC61000-4-2 standard is referenced because some manufacturers have been using ESD standards before the ISO10605 standard was released.

ISO7637-2 is the standard related to power line applications; ISO10605 (some manufacturers use IEC61000-4-2) is the standard for all electronic module accessories such as potentiometers, LCD displays, buttons and data line connectors.

Figure 1 briefly describes ISO7637-2 and the main surges on the automotive power line.

Figure 1 ISO7637-2 and main surges on automotive power lines

Data line and power line applications

The constraints of data lines and power lines are different. Data line applications are usually limited by low line capacitance, which is required for high-speed data line functions. At the same time, data lines also require high ESD protection performance. Therefore, it is difficult to choose a protection circuit that takes both low capacitance and high ESD protection performance into account.

On the other hand, although low capacitance is not required for power line applications, it is not easy to find the most suitable protection components because of the various pulses imposed by the ISO7637-2 standard. For example, different protection components are required for load dump surges and transient events generated by relay switching.

Figure 2 How to protect high-speed data lines

Data line protection

Let's take the USB accessories of car audio equipment as an example to discuss USB protection. The contact part of the USB interface may be affected by ESD transient events, so a dedicated protection component must be used for USB data. Because the data transmission rate of the USB data line is usually very high (480Mb/s), in order not to reduce the strength of the normal working signal, a low-capacitance protection component must be used, and it must be used to suppress the ±25kV ESD ESD surge (ISO10605). Using the topology of Figure 2 can find a balance between low capacitance and high-efficiency ESD suppression capability.

The essence of this method is to connect a protection component near the USB interface. It consists of several diodes in a rail-to-rail structure, which has low capacitance characteristics. The internal clamp device provides ±25kV ESD (ISO10605) protection for the two data lines and protection for the Vbus power line.

Assuming the capacitance of each line of the rail-to-rail protection device is 2.5pF, the predicted frequency response of this protection topology is shown in Figure 3.

The cutoff frequency in Figure 3 is about 5 GHz, far away from the normal operating frequency of the USB 480 Mb/s transceiver. Therefore, the USB transceiver can work safely.

Figure 3 Frequency response

Another way to check the impact of this rail-to-rail protection component on the normal operating mode of the USB protocol is to analyze the integrity of the signal data bits through the eye diagram response of Figure 4.

It is not difficult to see from Figure 4 that the integrity of the USB2.0 signal is not greatly affected, so its transmission is safe.

Figure 4 Eye diagram response and USB 2.0 template

Another benefit of a rail-to-rail protection solution is the ability to suppress ESD surges that occur on external interfaces when plugging in external accessories, such as a car audio system.

If we continue to use this rail-to-rail protection component with 2.5pF internal capacitance, and assume that the internal clamping voltage (Vbr breakdown voltage) is 6V, we can predict the air discharge response ESD ±25kV ISO10605 (150pF/330). When a +25kV ESD surge is applied, the residual overvoltage on the electronic side of the module is approximately +35V; when a -25kV ESD surge is applied, the residual overvoltage on the electronic side of the module is approximately 30V. [page]

Power line protection

Power line protection of on-board modules is another issue that needs to be paid attention to in the automotive environment, which involves the ISO7637-2 standard.

For example, to meet the ISO7637-2 standard, especially the pulse requirements of 1, 2, 3a and 3b in Figure 1, only one transient voltage suppressor (TVS) can be used to effectively protect the vehicle module.

The power line protection topology is shown in Figure 5.

Figure 5 Electronic module power line protection topology

When subjected to the pulses 1, 2, 3a and 3b in FIG. 1, the characteristics of the protection circuit are shown in FIGS. 6, 7, 8, 9 and 10.

Figure 6: Voltage and current across the TVS during the ISO7637-2 Pulse 1 test; Figure 7: Voltage and current across the TVS at different time bases during the ISO7637-2 Pulse 1 test

Figure 8: Voltage and current across the TVS during the ISO7637-2 Pulse 2 test; Figure 9: Voltage and current across the TVS during the ISO7637-2 Pulse 3a test

Figure 10 Voltage and current across the TVS during ISO7637-2 pulse 3b test

The problem we need to solve is to determine the parameters of the protection components according to the surge to be eliminated. Taking ISO7637-2 Pulse 2 in Figure 8 as an example, it is not difficult to see that the clamping voltage on the suppressor is about 30V, the peak pulse current (Ip) is about 12A, and the current pulse duration (tp) measured at the Ip/2 point is 4μs. Therefore, it can be determined that the TVS must dissipate a maximum pulse power (Pp) of 360W. All of these Pp, Ip, and tp values ​​are useful because they can help compare the peak pulse power versus exponential pulse duration curve in the TVS datasheet to ensure that the TVS is within the application limits.

in conclusion

Taking SMAJ24A as an example, these parameters are extracted from the product data sheet and compared with the curve in Figure 11. It is found that SMAJ24A meets the requirements of ISO7637-2 Pulse 2 test and can withstand 360 W exponential pulses up to 1.2ms. This value is much higher than the 4μs duration of ISO7637-2 Pulse 2 surge force.

Figure 11 Peak pulse power versus exponential pulse duration (Tj initial = 25°C)

The automotive environment is the main source of electrical hazards. Electronic devices are increasingly susceptible to interference, so electrical hazards must be carefully considered when designing electronic modules. In order to ensure the safety of electronic modules and systems and protect against various electrical hazards generated inside the car, the use of protection components has become a common method.