ST AN2689 Application note
AN2689
Application note
Protection of automotive electronics from electrical hazards,
guidelines for design and component selection
Introduction
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 disturbances (EMI),
electrostatic discharges (ESD) and other electrical disturbances (and automobiles are 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.
The objective of this document is to help electronic module designers with a protection
design approach for selecting the most suitable devices for typical applications depending
on the protection standard the electronic module has to meet.
Section 1 describes the electrical hazards considered in this document. Section 2 presents
a list of parameters to be taken into account before selecting possible protection devices.
Section 3 and Section 4 provide worked design examples, with design calculations, for
several protection solutions. Section 5 provides recommendations for the design of PCB
layout for improved solutions.
Transil is a trademark of STMicroelectonics.
March 2010 Doc ID 14310 Rev 2 1/41
www.st.com
AN2689
Contents
1 Electrical hazards in the automotive environment . . . . . . . . . . . . . . . . . 4
1.1 Source of hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1 Conducted hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.2 Radiated hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Propagation of electrical hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1 Propagation on the data lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.2 Hazards on the supply rail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3 Standards for the protection of automotive electronics . . . . . . . . . . . . . . . 7
2 Parameters to consider in selecting protection devices . . . . . . . . . . . . 8
3 Data line protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1 Protection topologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1 Clamping topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.2 Rail-to-rail topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2 Data line protection example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3 Design calculation example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3.1 Determination of Rd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3.2 Power dissipation determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4 Supply rail protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1 Protection topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1.1 Clamping topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.2 Supply rail protection example 1: pulse 2 ISO 7637-2 . . . . . . . . . . . . . . . 21
4.3 Calculations for example 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.3.1 Determination of R
4.3.2 Power dissipation determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.3.3 Junction temperature determination . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.4 Supply rail protection example 2: pulse 5a load dump ISO 7637-2 . . . . . 28
4.5 Calculations for example 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.5.1 Determination of Rd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.5.2 Power dissipation determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5 PCB layout recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
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AN2689
5.1 Parasitic inductances of the Transil and the PCB tracks . . . . . . . . . . . . . 33
5.1.1 Parasitic inductance from Transil wiring . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.1.2 Capacitive and inductive coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.1.3 Parasitic coupling due to the loop-effect . . . . . . . . . . . . . . . . . . . . . . . . 39
6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Doc ID 14310 Rev 2 3/41
Electrical hazards in the automotive environment AN2689
1 Electrical hazards in the automotive environment
The automotive environment is the source of many electrical hazards. These hazards, such
as electromagnetic interference, electrostatic discharges and other electrical disturbances
are generated by various accessories like ignition, relay contacts, alternator, injectors,
SMPS (i.e. HID front lights) 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.
1.1 Source of hazards
1.1.1 Conducted hazards
These hazards occur directly in the cable harness. They are generated by inductive loads
like electro-valves, solenoids, alternators, etc.
The schematic in Figure 1 shows a typical configuration.
Figure 1. Conducted hazards
1.1.2 Radiated hazards
These hazards are generated by high current switching like relay contact, high current MOS
or IGBT switches, ignition systems, etc. The electromagnetic field generated by these
circuits directly affects lines or modules near the source of the electromagnetic radiation.
The schematic diagram in Figure 2 indicates how electromagnetic radiation creates such
hazards as electromagnetic interference in electronic modules.
Source of
distrubances
Alternator
Equipment
needing protection
ECU
Battery
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AN2689 Electrical hazards in the automotive environment
Figure 2. Electromagnetic radiation in the automotive environment
Equipment
needing protection
Ignition coil
Spark
plug
1.2 Propagation of electrical hazards
1.2.1 Propagation on the data lines
Transients that are generated on data lines are mainly ESD surges which are low energy but
very high dv/dt and can generate a very strong electromagnetic field. These mainly concern
ISO 10605 and IEC 61000-4-2 standards.
ECU
Battery
ABS
CAR
RADIO
...
The data lines concerned are communication lines like media transfer lines, data buses,
sensor data lines and so on.
Figure 3 shows surge forms of hazards that can be found on data lines.
Figure 3. Kinds of surges on data lines
±25 kV (1)
ISO 10605
±15 kV (1)
IEC 61000-4-2
±8 kV (2)
ISO 10605
Nominal
V dataline
0V
±8 kV (2)
IEC 61000-4-2
(1): Air discharge
(2): Contact discharge
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Electrical hazards in the automotive environment AN2689
The ISO 10605 ESD surge test is applied to a complete system. This is simulating the ESD
occurring on an electronic module in its environment due to human body contact.
1.2.2 Hazards on the supply rail
Transients that are generated on the supply rail range from severe low level-high energy, to
high level-low energy with sometimes high dv/dt. These mainly concern ISO 7637-2 and ISO
10605 standards.
Figure 4 shows a simple representation of the form of major supply rail transients.
Figure 4. Kinds of surges on power rail
±25 kV
ESD spikes
87 V
Load dump
Nominal
14 V
0V
6 VCrank
+100/-150V
Spikes
24 VJump start
Reverse battery
The most energetic transients are those resulting from load-dump and jump start. But all
other hazards may affect the normal operation of electronic modules.
The “6 V crank” is caused by the starting of the car. The energy necessary to crank the
engine makes the power voltage drop to 6 V.
The load-dump is caused by the discharged battery being disconnected from the alternator
while the alternator is generating charging current. This transient can last 400 ms and the
equivalent generator internal resistance is specified as 0.5 Ω minimum to 4 Ω maximum.
The “+100/-150 V Spikes” are due to the ignition system that is necessary to ignite the
gasoline mixture. The frequency of the spikes depends on the engine rotation speed and the
number of cylinders. These generate electromagnetic radiation.
The “24 V jump start” results from the temporary application of an over voltage in excess of
the battery voltage. The circuit power supply may be subjected to a temporary over voltage
condition due to the regulator failing or deliberately generated when it is necessary to boost
start the car. In such condition some repair vehicles use 24 V battery jump to start the car.
Automotive specifications call for the support of this over voltage application for up to 5
minutes.
The “reverse battery” is the result of battery inversion by mistake. Thus accessories have
their power termination polarized in the wrong way.
The “25 kV ESD spike” is the result of electrostatic discharge (ESD)
All these events may affect the electronic environment as conducted hazards or as radiated
hazards.
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AN2689 Electrical hazards in the automotive environment
1.3 Standards for the protection of automotive electronics
All the hazards indicated above are described by several standards bodies such as the
Society of Automobile Engineers (SAE), the Automotive Electronic Council (AEC) and the
International Standard Organization (ISO ).
Because the ISO 10605
(a)
and the ISO 7637
standards regarding electrical hazards, this document mainly concerns the cases
considering such standards.
(b)
are the most important automotive
a. ISO 10605: standard for “Electrostatic discharges” due to human body discharging inside a vehicle applied to a
complete system
b. ISO 7637 standard for “Electrical disturbances from conduction and coupling
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Parameters to consider in selecting protection devices AN2689
2 Parameters to consider in selecting protection
devices
To make the best choice in protection device and considering Figure 5, it is necessary to
consider several parameters such as:
● Nominal voltage the electronic module runs with (V
● Maximum voltage this electronic module can support (V
● Kind of surge that the electronic module may be called upon to support
– If the surge shape is exponential:
What is the surge maximum voltage (V
)?
s
What is the surge duration and at what level is it measured (t
What is the surge generator impedance (R
s
What is the number of cycles in the surge (1/f)?
– If the surge is a dc surge:
What is the dc voltage level (V
What is the duration of the surge (t
What is the surge generator impedance (R
● What is the ambient temperature (T
● What kind of protection package is preferred?
● Is the electronic module a simple dc module or a digital one?
dc
amb
)?
)?
)?
p
s
– If digital:
What is the signal frequency (F)?
What are the rise and fall time of the surge signal (t
What is the maximum line capacitance (C
)?
l
)?
)?
nom
)
max
, tf)?
r
)
)?
p
Figure 5. Surge application topology
Surge
Generator
Vs
Rs
Td
F
(*) Transil
8/41 Doc ID 14310 Rev 2
Protection
device (*)
or rail-to-rail protection device
Module to protect
Vcl
Vnom
Vmax
AN2689 Data line protection
3 Data line protection
3.1 Protection topologies
3.1.1 Clamping topology
Various protection topologies can be chosen for data line protection. There is the usual
topology that consists of using a clamping device as shown in Figure 6. The action of the
suppressor upon positive and negative surge occurrence is shown respectively in Figure 7
and Figure 8.
Figure 6. Data line protection using Transils
Module to protect
Sensitiveline
Vnom
Vmax
Accessory Connector
Transil
When a positive surge occurs, the over voltage is suppressed by the Transil as the voltage
passes the breakdown voltage (Vbr). Thus the current is diverted to ground. The remaining
voltage on the data line is limited to the clamping voltage (V
).
cl
Figure 7. Positive surge suppression
Vcc
Vs
Module to protect
Vnom
Sensitivedatalines
Accessory Connector
I
Vcl
Vmax
For the negative surge, the Transil is now in forward mode and the over voltage is eliminated
as the surge voltage passes the forward voltage drop of the protection device (V
remaining voltage is limited to the peak forward voltage of the Transil (V
).
fp
). The
f
Figure 8. Negative surge suppression
Vcc
Module to protect
Vnom
Sensitivedatalines
Vs
Accessory Connector
I
Vfp
Vmax
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Data line protection AN2689
This topology is easy to manage. There should be as many protection devices as there are
lines to protect. The protection device in fact can be several single Transils but there is a
possibility to use protection devices in an array package so that one device protects each
data line as shown in Figure 9.
Figure 9. Data line protection using a diode array
Vcc
Transilarray
Module to protect
Sensitiveline
Accessory Connector
Vnom
Vmax
This clamping topology is good when the clamping voltage of each Transil is close to the
nominal voltage of the data lines to be protected. For example, if the nominal voltage is 5 V,
an ESDA6V1xx (V
= 6.1 V) protection device is ideal. The Transil offers a fixed clamping
br
voltage which does not require external power supply as in the rail-to-rail configuration but
for some cases it is more convenient to use the rail-to-rail topology as described below.
3.1.2 Rail-to-rail topology
The rail-to-rail topology, shown in Figure 10, is achieved using simple regular diodes. In that
case the clamping level is no longer fixed but, instead, depends on the power supply voltage
V
. As soon as a surge occurs, all the voltage over Vcc is diverted to the power supply as
cc
shown in Figure 11.
In this case the remaining over voltage that the data line is exposed to is very low.
Figure 10. Rail-to-rail protection topology
Connector
Accessory
Vcc
lines
Module to protect
Sensi t i ve dat a
Vnom
Vmax
10/41 Doc ID 14310 Rev 2
AN2689 Data line protection
For a positive surge, shown in Figure 11, as the over voltage reaches the power supply
voltage V
plus the forward voltage drop of the upper diode, the surge current is diverted
cc
into the power supply line. To prevent this power supply line oscillating or being raised too
much, a capacitor (47 nF suggested) is needed close to the rail-to-rail protection device.
The remaining voltage V at the module data line is limited to Vcc plus the forward voltage
drop V
of the upper diode.
f
Figure 11. Positive surge suppression
Isurge
Vs
Sensitivedata lines
Accessory Connector
V
Vcc
Module to protect
Vnom
Vmax
For the negative surge case (Figure 12), the surge suppression is the same as described in
Figure 8.
Figure 12. Negative surge suppression
Vcc
Module to protect
Isurge
Sensitivedata lines
Vs
Accessory Connector
V
Vnom
Vmax
In the same approach as for the previous topology, there is a possibility to manage this railto-rail protection topology using as many single devices as there are data lines to protect or
one diode array device that fulfills all line protection needs as shown in Figure 13.
Figure 13. Rail-to-rail diode array
Vcc
lines
Connector
Sensi t i ve dat a
Module to protect
Vnom
Vmax
Accessory
DiodeArray
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Data line protection AN2689
This solution requires that the Vcc voltage track be accessible and a decoupling capacitor is
required close to the diode array device. On the other hand this topology is suitable for high
speed data lines that often requires low parasitic line capacitance.
3.2 Data line protection example
Let’s consider the ISO 10605 standard.
The ESD current waveform, shown in Figure 14, has the corresponding generator circuit
given in Figure 15 when the generator output is in short circuit.
This surge is specified for contact or air-discharge as shown in Ta bl e 1 and maximum
voltage occurring is a 25 KV (air-discharge)
Figure 14. ESD current waveform with generator output in short circuit
tr 1 ns≤
Table 1. Surge voltage levels for contact and air discharge
Contact discharge Air discharge
Level Test voltage (kV) Level Test voltage (kV)
1±4 1±8
2±62±15
3±83±25
Figure 15. Equivalent circuit schematic (occupant discharge model)
R (MΩ )2 kΩ
High voltage
supply
330 pf
Device
under test
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AN2689 Data line protection
If we consider the +25 kV air discharge surge test, then, regarding the suggested
application given in Figure 16: the worked example below may be used as a guideline for
protection solution selection.
Figure 16. ISO 10605 ESD test set-up
Surge
Generator
Vs = 25 kV contact
= 2 kΩ
Rs
tr
≤
1 ns
Vcl
Module to protect
= 13.5 V
Vnom
Vmax
= 45 V
Tmax
= 85 °C
Identification of the best protection (Transil)
About the module to protect:
● The protection shall be “transparent” for the normal operating conditions of the module,
in this case 13.5 V.
● The maximum voltage the module can withstand is 45 V so the Transil will not offer a
higher voltage than 45 V when acting.
● The max temperature is 85 °C.
About the surge to suppress:
● The maximum voltage (V
● The surge time constant duration τ is 660 ns (R*C).
● The maximum repetition is one strike (t1).
● The internal series resistor of the generator is 2 kΩ.
About the Transil:
● The suppressor has the following electrical characteristics (Figure 17):
) of the surge is 25 kV.
pp
Figure 17. Transil diode electrical characteristics
I
Ipp
I
V
Irm
Vcl
Vrm
Vbr
If
Vf
V
Where:
V
is the stand-off voltage measured @Irm. This corresponds to the nominal voltage of the
rm
application V
≥ 13.5 V.
rm
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