|
Edited by Rick DeMeis
Electronic equipment represents a large part of the automobiles of today. Although these electronic modules bring much more comfort and security for the vehicle user, they also bring significant concerns in terms of reliability regarding the automobile environment.
Because electronic modules are sensitive to electromagnetic interference (EMI), electrostatic discharge (ESD), and other electrical disturbances (and the automobile itself is the source of many such hazards), caution must be taken wherever electronic modules are used in the automotive environment.
Several standards have been produced to model the electrical hazards that are currently found in automobiles. As a result, manufacturers and suppliers have to consider these standards and have to add protection devices to their modules to fulfill the major obligations imposed by these standards.
For these reasons, electronic onboard modules must withstand immunity and ESD transients defined respectively by the international ISO 7637-2 and ISO 10605 standards.
Electrical hazards in the automotive environment
The automotive environment itself is the source of many electrical hazards. These hazards, such as electromagnetic interference, electrostatic discharge, and other electrical disturbances are generated by various devices such as the ignition, relay contacts, alternator, fuel injectors, and other accessories. These hazards can occur directly in the wiring harness in case of conducted hazards, or be applied indirectly to the electronic modules by radiation. These generated hazards can impact the electronics in two ways; either on the data lines or on the supply rail wires, depending on the environment.
Propagation of electrical hazards
Transients that are generated in the automotive environment can range from severe low level/high energy, to high level/low energy with, in some cases, high dV/dt. These mainly concern ISO 7637-2 and ISO 10605 standards and sometimes the IEC 61000-4-2 as some manufacturers used to rely on this ESD standard before the ISO 10605 came out.
ISO 7637-2 concerns the power rail applications; ISO 10605 (and IEC61000-4-2) concern all electronic modules' accessible parts like potentiometers, LCD screens, knobs, and data line connectors.
Figures 1, 2, and 3 illustrate a simple representation of the form of major EMI and ESD transients in the automotive environment.
View full-size images
Data line and power rail applications
Data line and power rail applications do not have the same constraints compared to each other.
Data line applications often bring a constraint of low line capacitance due to high-speed data bus capability, and at the same time require a high ESD level immunity. Thus, choosing good protection with the correct trade-off between low capacitance and ESD robustness is not so easy.
On the other hand, power rail applications do not require low capacitance, but due to the variety of pulses imposed by the ISO 7637-2 standard, it may be tricky to find the most appropriate protection device. For instance, protection from a load-dump surge (battery disconnect) does not require the same device as for transients coming from relay switching.
Data line protection
If we consider the USB accessories that figure in automotive radios, for instance, the accessible part of this USB connector may be subject to ESD transient. For this reason, USB data lines must be protected with a dedicated device. As the USB data lines are generally fast transmission data rate (480 Mb/s), a low capacitance protection device is required in order not to degrade the normal operating signals and in the same time provide protection capable of suppressing ±25 kV ESD surge (ISO 10605). This trade-off between low capacitance and ESD high-efficiency suppression can be achieved using the topology of Figure 4 below.
View a full-size image
Here the idea consists in using a protection device located close to the USB connector. This protection device is made of several diodes in a rail-to-rail configuration which intrinsically brings low capacitance characteristics. An internal clamping device ensures both data line ±25 kV ESD (ISO 10605) and Vbus protection.
Considering a rail-to-rail protection device with 2.5 pF per line capacitance, we can expect a frequency response of this protection topology as given by the graph of Figure 5 here.
The cut-off frequency shown in Figure 5 is around 5 GHz—far from the normal operating frequency of the USB 480 Mb/s transceiver—allowing it to operate safely.
Another way to check the impact of this rail-to-rail protection on the normal operating mode of USB protocol is to analyze the signal bit integrity via the eye-diagram response as shown in Figure 6 below.
View a full-size image
We can see in this eye diagram that the USB 2.0 signal integrity is not degraded too much, making the USB 2.0 transmission safe.
Among the benefits this rail-to-rail protection solution brings is also the ability to suppress ESD surges occurring on external connector when plugging in external accessories (i.e. MP3 players).
If we carry on with this 2.5 pF internal capacitor rail-to-rail protection, and if we consider it has an internal clamping capability of 6V (Vbr breakdown voltage), we can expect an ESD ±25-kV ISO-10605 (150 pF/ 330Ω) air-discharge response as shown in Figures 7 and 8.
View full-size images
Figures 7 and 8 show the possible ESD response of the rail-to-rail solution. The remaining overvoltage at the electronic side of the module is roughly +35V when +25-kV ESD surge is applied and roughly -30V when a -25-kV ESD surge is applied.
|
