Employing Reliable Protection Methods
for Automotive Electronics
WHITE PAPER
BACKGROUNDPWR Series
Power Resistors

4800P Series
Thick Film Networks

Sparctube™ ST Series
Switching Spark Gap Devices

SRP Series
SMD Power Inductors

Multifuse® PPTC
Resettable Fuses

ChipGuard®
ESD Protectors

9/13 • e/K1333

Automotive systems continue to become more sophisticated with the introduction ofnew, modified and improved features every year. Each feature must be powered, protected
and connected to its respective interface within the overall automotive system. Direct
current (DC) to DC converters are necessary to convert the main 12 V automotive battery
supply to a range of voltage levels required for operation of these various features. DC-DC
converters are used to provide DC power to heavy loads in hybrid and electric vehicles, andfor lighting, displays, infotainment and any other electronics in all types of vehicles. Once
powered and operating, the automotive electronics are subject to various transient eventsand must be tested to ensure they comply with standards such as ISO 7637. With protectionin place, the electronics can be connected to the system using various standard interfaces
such as CANbus. It is important to ensure the system reliability of these interfaces interms of Electromagnetic Compatibility (EMC) and Common Mode Noise (CMN). The
protection of the system has to be re-examined after all interfaces are in place to ensure
adequate isolation and protection. Finally, Electrostatic Discharge (ESD) considerations
must be made since the electronics are subject to ESD events beginning as early as circuitassembly and continuing throughout the lifetime of the vehicle.
Effective and reliable circuit protection can be achieved by implementing a variety ofresistors and magnetics for DC-DC converters, Polymer Positive Temperature Coefficient(PPTC) devices for transient protection, common mode chokes and chip varistors for EMC
and interfaces, and multilayer varistors and clamp devices for ESD protection. This paper
will describe the power supply, transient, interface, and ESD requirements for components
in the automotive environment. It also will introduce the various technologies and productfamilies that have been found to be effective in protecting automotive electronics.

Employing Reliable Protection Methods for Automotive Electronics
TOUGH REQUIREMENTS FOR AUTOMOTIVE SYSTEMSPWR Series
Power Resistors

4800P Series
Thick Film Networks

Sparctube™ ST Series
Switching Spark Gap Devices

SRP Series
SMD Power Inductors

Multifuse® PPTC
Resettable Fuses

ChipGuard®
ESD Protectors

Electronics are replacing mechanical couplings in a broad range of systems such asengine controllers, safety systems, chassis control, diagnostic systems, infotainment, andmany more. A diverse set of network interfaces including CAN, Flexray and Ethernet
connects each Electronics Sub-Assembly (ESA) to the system and other ESAs, whichthen are susceptible to interference or reliability issues caused by other ESAs. Given therange of possibilities it seems on the surface there are few commonalities, yet regardlessof the function the electronics all must be powered from a single automotive battery
and withstand well-defined tests, surges and performance requirements throughout the
lifetime of the vehicle. Along with a robust power conversion design, circuit protection isnecessary to ensure that the electronics are not affected by outside surges, other electronics
in the system or human interaction. Designing a stable and reliable system begins
with understanding the needs and specifications for conversion, interfaces and threats.
Components then can be selected to address the requirements and meet or exceed the levelof performance and protection in each part of the automotive system.

Stabilizing Voltage Levels for Power Supplies
DC-DC converters are used in electronics to convert between voltage levels and provide astable and accurate supply. The nominal voltage in a car that runs on a 12 V battery is 14 V.
Some infotainment circuits require a reduction to voltages of 5 V or 3.3 V. Conversely,
LED lighting strings require a boost to 60 V, and permanent magnet DC motors like thoseused in hybrid and full electric vehicles present larger loads of several hundred volts. For
these conversions, the power supply circuit typically will include a MOSFET, inductor (L),resistor (R), and capacitor (C). A switch, as shown in figure 1, can represent the MOSFET.
In this circuit, the parasitic inductance of the wiring combined with capacitance of the
MOSFET can interfere with the normal operation of the circuit. Components must be
chosen or inserted to minimize the risk of this interference and maintain stable operationof the converter and the system, regardless of whether or not the converter design boosts or
reduces the voltage supplied to the load.

L
MOSFET

C
R

V
9/13 • e/K1333

Figure 1.

Equivalent Circuit of Power Supply

2

Employing Reliable Protection Methods for Automotive Electronics
TOUGH REQUIREMENTS FOR AUTOMOTIVE SYSTEMS (Continued)
Transient Event Protection
PWR Series
Power Resistors

4800P Series
Thick Film Networks

A great deal of consideration is given to protecting automotive electronics from varioustransient events. The transient surges that threaten automotive electronics are well-known andclassified by ISO 7637 according to the type of event. These events can last up to several seconds
and generate several hundred amps. Table 1 details the ISO 7637 test specifications, where Ri
indicates the internal resistance of the voltage generator. Each ESA must be designed to passtests according to ISO 7637. Designers need to comply with the requisite level of protection byadding components to limit the surge or clamp the voltage.

Table 1.

Sparctube™ ST Series
Switching Spark Gap Devices

SRP Series
SMD Power Inductors

Multifuse® PPTC
Resettable Fuses

Test

ISO 7637 Test Specifications

Simulation ofPeak Voltage (12 V) Peak Energy (24 V)

1

Disconnection of battery from power supply

2aSurge induced by wiring harness after current switched off-100 V (Ri = 10 Ω)+50 V (Ri = 2 Ω)-600 V (Ri = 50 Ω)+50 V (Ri = 2 Ω)

2b

DC motors acting as generators after ignition switched off+10 V (Ri = 0.05 Ω)+20 V (Ri = 0.05 Ω)

3a, 3bInduced currents through inductance in wiring-150 V (Ri = 50 Ω)-200 V (Ri = 50 Ω)

4Surge caused by energizing starting motor-7 V (Ri = 0.05 Ω)-16 V (Ri = 0.05 Ω)

5a, 5bDisconnecting discharged battery while the alternator isgenerating charging current (load dump)

87 V (Ri = 0.5 Ω)

174 V (Ri = 1 Ω)

System Interfaces Bring Additional Challenges
ChipGuard®
ESD Protectors

Adequate protection as a standalone unit is the starting point for introducing an ESA to theautomotive system. Once connected to the system through an interface, an ESA may contributeto a rise in the network voltage. For instance, most ESAs connected to a CANbus network willcontain a common mode inductor to limit unwanted high-frequency noise on the communicationbus, improve the immunity of the transceiver to EMC, and reduce Electromagnetic Interference(EMI). With all its benefits, the common mode inductor also can induce a high voltage across the
CANbus transceiver, as seen in the disconnection from the supply in ISO 7637 Test 1. This voltage
then needs to be clamped by a suppressor such as an automotive varistor to protect the ESA fromdamage that the overvoltage condition could cause.
The type of core, wire and construction of the chosen inductor as well as the amount of current
will have a bearing on the electromagnetic emissions from the ESA. The emissions must remain
within the specified guidelines of the vehicle manufacturer. On some occasions, the guidelines canbe tougher than the international EN55022 Class B specification, for which the emissions level is
30 dB (mV/m) over the frequency range 0 - 230 MHz, and 37 dB (mV/m) for 230 MHz - 1 GHz.

9/13 • e/K1333

3

Employing Reliable Protection Methods for Automotive Electronics
TOUGH REQUIREMENTS FOR AUTOMOTIVE SYSTEMS (Continued)
Protection Against Electrostatic Discharge EventsPWR Series
Power Resistors

4800P Series
Thick Film Networks

Sparctube™ ST Series
Switching Spark Gap Devices

Finally, automotive electronics must be protected from ESD damage. The risk of ESD events isincreasing with the availability of the Universal Serial Bus (USB) for infotainment. There arethree categories for the classification of malfunctions caused by the current and voltage surgeof an ESD pulse: (i) temporary errors corrected automatically by the circuit or its operating
software; (ii) temporary errors requiring operator intervention to recover and resume normal
operation; and (iii) permanent damage requiring partial repair or replacement entirely.
As with transient compliance, the automotive industry uses a well-known standard for ESD
protection guidelines. This Human Body Model (HBM) characterizes the current versus time
curve and electrostatic surge as shown in table 2. It simulates the energy transmitted by contact
with people during handling on the assembly line. In testing, a surge generator can replicate
the HBM curve by a connecting a voltage source to a 1.5 kΩ resistor and 100 pF capacitor, as
shown in figure 2. These tests will help designers to include the appropriate voltage suppression
in their designs, such as Bourns® ChipGuard® chip varistors.

SRP Series
SMD Power Inductors
Table 2. Tests for Human Body Model
Multifuse® PPTC
Resettable Fuses

ChipGuard®
ESD Protectors

Applied Voltage (kV)Peak current (A)Human Body Model

2

1.33

4

2.67

6

4.00

8

5.33

10

6.67

Vgen

1 MΩ

DUT

high supply
voltage
100 pF

Figure 2.

9/13 • e/K1333

1.5 kΩ

V

Test Circuit for Human Body Model

4

Employing Reliable Protection Methods for Automotive Electronics
SOLUTIONS THAT HELP ENSURE RELIABILITY
Resistors in Power SuppliesPWR Series
Power Resistors

4800P Series
Thick Film Networks

Sparctube™ ST Series
Switching Spark Gap Devices

SRP Series
SMD Power Inductors

Multifuse® PPTC
Resettable Fuses

ChipGuard®
ESD Protectors

9/13 • e/K1333

The first component to be inserted in the power supply for protection is a resistor. If the MOSFET
1
is switched voltage is induced across the capacitor: V - V * cos(ωt) where ω= LC. The peak
amplitude of this sinusoidal wave is twice the supply voltage, and this could damage the MOSFET
transistor. Adding a Resistor R in series with the MOSFET dampens these oscillations and protects
the MOSFET. The type of resistor used depends on the output power of the converter. A resistor’ssuitability for a design can be determined from several factors. First, its average power dissipation
Tpulsecan be calculated from the following equation: Pav = Ppk* D, where D = Tperiod; Tpulse is the pulse
length of the surge; and Tperiod is the time between surges. The total thermal resistance of the resistor
and board is given by: Rth = Rth(Resistor) + Rth(Circuit Board). The temperature rise for the circuit
is given by: ΔT = Pav*Rth. Finally, a graph of the energy that the resistor can handle in Joules versus
surge time. To calculate the energy in the surge, the peak power of the surge is multiplied by the
duration of the surge and by the relative area of the surge compared to a square pulse.
Bourns’ fixed resistor product portfolio includes AEC-Q200 qualified PWR series power resistors
in a variety of packages. These thick film non-inductive resistors are electrically isolated from theback plate and available with power ratings up to 50 W. Bourns® Models PWR163 and PWR263
have a low inductance, making them ideal for high frequency applications, and their excellent pulse
characteristics support current limiting or capacitor discharge applications. Bourns® Model 4816P
SOIC resistor network is available with isolated, bussed, and standard termination options with atolerance as low as 0.5 %.
As an example of evaluating a resistor for use in the power supply circuit, assume Pav = 9.2 W
and the operating temperature is 90 ˚C. Using Bourns® Model PWR263S-35 for the power resistor
and a typical high performance copper-clad circuit board with heat exchanger, the respective
thermal resistances of 3.5 ˚C/W and 2.5 ˚C/W total 6 ˚C/W. The temperature rise is 55.2 ˚C. The
overall temperature of the component is given by the sum of the temperature rise and operating
temperature. In this example, the resistor temperature would be 145 ˚C, which is less than the
maximum temperature of 155 ˚C on the Bourns® Model PWR263S-35 data sheet. The time versus
energy curve available on the data sheet will indicate whether the part is capable of handlingthe instantaneous power of the surge. If the resistor meets all of these specifications, it can beincorporated in the design.

5Employing Reliable Protection Methods for Automotive Electronics
SOLUTIONS THAT HELP ENSURE RELIABILITY (Continued)
High Intensity Discharge Lamps
PWR Series
Power Resistors

4800P Series
Thick Film Networks

Sparctube™ ST Series
Switching Spark Gap Devices

SRP Series
SMD Power Inductors

Another application for DC-DC converters is to supply the necessary lighting circuits inthe car. High Intensity Discharge (HID) lamps require a very high ignition voltage pulse of
several thousand volts followed by a constant voltage. A transformer coil and a spark tube
create the high ignition voltage as illustrated in figure 3. Bourns produces a range of highvoltage spark tubes called Sparctube™ Switching Spark Gap Devices for HID automotive
lamp circuits. Bourns® Sparctube™ ST Series is designed specifically for voltage controlledswitching of capacitive discharge circuits where high energy, low loss and a fast rate ofswitching are required. This series performs in high temperatures and offers long-term
reliability. Most commonly used in automotive are the 800 V and 840 V parts, Bourns®
Model ST-0800-Bxx-STD and Bourns® Model ST-0840-Bxx-STD, respectively.

R
V

C

Bourns® Sparctube™
Switching Spark Gap

Multifuse® PPTC
Resettable Fuses

ChipGuard®
ESD Protectors

9/13 • e/K1333

LOAD

Figure 3.

Application of Bourns® Sparctube™ Switching Spark Gap

6

Employing Reliable Protection Methods for Automotive Electronics
SOLUTIONS THAT HELP ENSURE RELIABILITY (Continued)
PWR Series
Power Resistors

4800P Series
Thick Film Networks

Sparctube™ ST Series
Switching Spark Gap Devices

SRP Series
SMD Power Inductors

Multifuse® PPTC
Resettable Fuses

Implementing Effective Transient Protection
Bourns® Multifuse® PPTC devices are ideal for transient protection. A Bourns® Multifuse®
device is a thermistor with a nonlinear resistance-versus-temperature characteristic.
Selection of a Bourns® Multifuse® PPTC device depends on the operating voltage,
which typically is 16 V for a nominal voltage of 14 V, the operating current for the ESA
application and the time-to-trip defined in the specification. During normal operation,
the device has an extremely low resistance and remains essentially invisible to the circuit.
When the PPTC operating current reaches the trip current, the device temperature
increases and the resistance jumps quickly to a very high value.
Different polymer blends during component manufacturing can be used to produce
specific features in the device such as high current capability (low resistance), highoperating voltage or high operating temperature. While the PPTC devices typically have amaximum operating temperature of 85 ˚C, some Bourns® Multifuse® devices are rated to125 ˚C. Bourns® components allow for a more stable resistance over a wider temperature
range. This is a great advantage in situations where the resistance of the thermistor
plays a role in the circuit. One such example is the differential line of a camera module.
The bandwidth of the module can be affected by changes in the PPTC resistance, so it
is important to select a component that will not interfere with normal operation of thecircuit or trip prematurely.

ChipGuard®
ESD Protectors

9/13 • e/K1333

7

Employing Reliable Protection Methods for Automotive Electronics
SOLUTIONS THAT HELP ENSURE RELIABILITY (Continued)
PWR Series
Power Resistors

4800P Series
Thick Film Networks

Sparctube™ ST Series
Switching Spark Gap Devices

SRP Series
SMD Power Inductors

Multifuse® PPTC
Resettable Fuses

ChipGuard®
ESD Protectors

Achieving Low EMI Levels with Common Mode Inductors
In a Common Mode Noise (CMN) choke, each inductor winding is connected in series withan input line. The connections and phasing of the inductor windings are arranged so that theflux created by each winding appears to cancel the flux of the other winding. The insertion
impedance of the inductor to the input signal is thus zero, except for small losses in theleakage reactance and the DC resistance of the windings.
Instantaneous current is shown in figure 4 as proceeding in one direction; in one input line
and returning through the other input line. The opposing flux generated in the core ensuresthat virtually no voltage is induced in either winding, so the input current can pass throughthe common mode choke with almost no power loss. Unwanted high frequency current called
Common Mode Noise (CMN), can appear in one or both input lines as seen in figure 5. The
CMN current returns to the noise source through the ground connection. Since it is notcancelled by an opposing return, this current sees the full impedance of either one or bothwindings of the CMN inductor. The CMN voltages must be attenuated in the windings of thecommon mode inductor to reduce the unwanted noise on the lines.
Inductors used in the power supply interface and at the CAN transceiver should beshielded surface mount components capable of handling high current. Bourns offers a large
selection of inductors in its SRP family. Their shielded housing helps achieve the low EMI
levels required of automotive design. Bourns® SRP family of inductors combines flat wire
technology and a powdered iron core to achieve a compact, efficient package. The flat wire has
increased conductivity due to maximizing the use of its surface area, and thus is capable ofhandling high frequency switching.

Figure 4.

9/13 • e/K1333

CG0603MLA-26KE

CG0603MLA-26KE

CG0603MLA-26KE

CG0603MLA-26KE

Flux Generated by Signals in
Common Mode Choke

Figure 5.

Flux Generated by Signal andCommon Mode Noise8

Employing Reliable Protection Methods for Automotive Electronics
SOLUTIONS THAT HELP ENSURE RELIABILITY (Continued)
PWR Series
Power Resistors

4800P Series
Thick Film Networks

Sparctube™ ST Series
Switching Spark Gap Devices

Avoiding Overvoltage Damage
In addition to performing as clamping devices in other areas of the automotive
system, Bourns® ChipGuard® multilayer varistors are designed to provide optimal
ESD protection. With low capacitance and leakage characteristics, these devices are
virtually invisible to the circuit during normal operation and can withstand multiple
ESD events. When an overvoltage condition arises, the Bourns® ChipGuard® varistor
clamps the voltage to a safe level to protect the ESA from overvoltage damage.
The suppressor shown in figure 6 is connected across the terminals of the ESA toprotect the electronics from ESD occurring during board mount or throughout its
lifetime. AEC-Q200 qualified varistors such as Bourns® Model CG0603MLC are
qualified using tests 1-3b of ISO 7637. Selecting the correct Bourns® ChipGuard®
device depends on the specific characteristics of the ESA, so that the voltage and trip
characteristics of the part meet the requirements of the application.

SRP Series
SMD Power Inductors

PPTC
Multifuse® PPTC
Resettable Fuses

V

ChipGuard®
ESD Protectors

Suppressor
Figure 6.

9/13 • e/K1333

ESA

Representation of Protection Circuit

9

Employing Reliable Protection Methods for Automotive Electronics
PROVEN COMPONENTS FOR AUTOMOTIVE ELECTRONICSPWR Series
Power Resistors

4800P Series
Thick Film Networks

Sparctube™ ST Series
Switching Spark Gap Devices

Bourns has been a trusted supplier of electronics components for decades. Their
resistive, magnetic, PPTC and fuse components meet the needs of automotive
electronics designers in DC-DC converter design, transient and ESD protection,and interface and system stability. All Bourns® components that are approved forautomotive applications are manufactured in facilities that are fully certified to TS16949.
The components have been tested and qualified using AEC-Q200, a well-known
specification for passive components. As the components are integrated into automotive
electronics designs, Bourns works with its automotive customers throughout theapproval process, providing the necessary documentation to obtain approvals, while
continuing to work toward meeting quality targets. With a proven track record for
quality, customer service, and innovation, Bourns helps to ensure that designers canmaintain focus on the design.

SRP Series
SMD Power Inductors

Multifuse® PPTC
Resettable Fuses

ChipGuard®
ESD Protectors

ADDITIONAL RESOURCES
For more information about Bourns’ complete line of circuit protection solutions,please visit:www.bourns.com

COPYRIGHT© 2013 • BOURNS, INC. • 9/13 • e/K1333“Sparctube” is a registered trademark of Bourns, Inc. in the European Community, with pending trademark registration applications in the U.S. and China.
“Bourns”, “Multifuse” and “ChipGuard” are registered trademarks of Bourns, Inc. in the U.S. and other countries.

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