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Suppression of Transients in an Automotive

Environment

Application Note

July 1999

AN9312.5

[ /Title

(AN93

12)

/Sub-

ject

(Sup-

pres-

sion of

Tran-

sients

in an

Auto-

motive

Envi-

ron-

ment)

/Autho

r ()

/Key-

words

(TVS,

Tran-

sient

Sup-

pres-

sion,

Protec-

tion,

Auto-

motive,

Load

Dump,

Alter-

nator

Field

The initial stage of solid state electronics into the automobile

began with discrete power devices and IC components.

These were to be found in the alternator rectiﬁer, the

electronic ignition system and the voltage regulator. This

was followed by digital ICs and microprocessors, which are

common in engine controls and trip computers. The usage of

intelligent power devices and memories is common,

beneﬁting improved electronic controls and shared visual

displays. With the extensive use of electronic modules in

today’s vehicles, protection from transient overvoltages is

essential to ensure reliable operation.

event of a battery disconnect while the alternator is still

generating charging current with other loads remaining on

the alternator circuit at the time of battery disconnect. The

load dump amplitude depends on the alternator speed and

the level of the alternator ﬁeld excitation at the moment of

battery disconnection. A load dump may result from a

battery disconnect resulting from cable corrosion, poor

connection or an intentional battery disconnect while the car

is still running.

Independent studies by the Society of Automotive Engineers

(SAE) have shown that voltage spikes from 25V to 125V can

easily be generated [1], and they may last anywhere from

40ms to 400ms. The internal resistance of an alternator is

mainly a function of the alternator rotational speed and

excitation current. This resistance is typically between 0.5Ω

and 4Ω (Figure 2).

90%

Transient Environment

As the control circuitry in the automobile continues to

develop, there is a greater need to consider the capability of

new technology in terms of survivability to the commonly

encountered transients in the automotive environment. The

circuit designer must ensure reliable circuit operation in this

severe transient environment. The transients on the

automobile power supply range form the severe, high

energy, transients generated by the alternator/regulator

system to the low-level “noise” generated by the ignition

system and various accessories. A standard automotive

electrical system has all of these elements necessary to

generate undesirable transients (Figure 1).

10%

120V

LOAD

DUMP

85V

NOISE

= 25V to 125V

= 14V

T = 40ms to 400ms

= 5ms to 10ms

R = 0.5Ω to 4Ω

NOMINAL

14V

6V CRANK

24V JUMP START

FIGURE 2. LOAD DUMP TRANSIENT

Jump Start

REVERSE

BATTERY

FIGURE 1. TYPICAL AUTOMOTIVE TRANSIENTS

Unlike other transient environments where external

inﬂuences have the greatest impact, the transient

environment of the automobile is one of the best understood.

The severest transients result from either a load dump

condition or a jump start overvoltage condition. Other

transients may also result from relays and solenoids

switching on and off, and from fuses opening.

The jump start transient results from the temporary

application of an overvoltage in excess of the rated battery

voltage. The circuit power supply may be subjected to a

temporary overvoltage condition due to the voltage

regulator failing or it may be deliberately generated when it

becomes necessary to boost start the car. Unfortunately,

under such an application, the majority of repair vehicles use

24V “battery” jump to start the car. Automotive speciﬁcations

call out an extreme condition of jump start overvoltage

application of up to 5 minutes.

The Society of Automotive Engineers (SAE) has deﬁned the

automotive power supply transients which are present in the

system.

Table 1 shows some sources, amplitudes, polarity, and

energy levels of the generated transients found in the

automotive electrical system [4].

1-800-4-HARRIS or 407-727-9207

Load Dump

The load dump overvoltage is the most formidable transient

encountered in the automotive environment. It is an

exponentially decaying positive voltage which occurs in the

10-49

Harris Corporation 1999

Application Note 9312

TABLE 1. TYPICAL AUTOMOTIVE TRANSIENTS

ENERGY

CAPABILITY

LENGTH OF

TRANSIENT

Steady State

CAUSE

Failed voltage

regulator

Jump starts with

24V battery

Load dump;

disconnection of

battery while at

high charging

Inductive-load

switching

transient

Alternator field

decay

Ignition pulse,

battery

disconnected

Mutual coupling

in harness

Ignition pulse,

normal

Accessory noise

Transceiver

feedback

ESD

VOLTAGE

AMPLITUDE

•

+18V

•

±24V

>10J

<125V

Infrequent

FREQUENCY

OCCURRENCE

Infrequent

5 Minutes

200ms to

400ms

The load dump energy available to the central suppressor in the

worst case depends on variables such as the alternator size,

the response of the sampled-data regulator system, and the

loads that share the surge current and energy. Each application

therefore tends to be somewhat different. However, by

combining several applications, it is possible to construct a

representative example. The key fact is the alternator surge

power available to be dissipated in the suppressor. Figure 3A is

suggested as a starting point for analysis. Since a peak surge

power of 1600W is available, a suppressor with a clamping

voltage of 40V would draw a peak current of 40A. The surge

energy rating needed for the suppressor can be found by taking

the integral of the surge power over time, resulting in

approximately 85J. A jump-start rating of 24V is also needed.

Evaluating central suppressor devices can be simplified with

the aid of a load dump simulator as shown in Figure 3B. The

inductor L, which simulates the alternator inductance, slows

the surge rise time but does not materially affect the analysis.

In the absence of a suppressor or load, the output waveform

will be similar to that of Figure 1B. If a suppressor is inserted,

the operating characteristics can be estimated as follows:

Assume V

= 40V, then I

= (80 -40V)/R

= 40A

< 320µs

<1J

300V to +80V

<1J

-100V to -40V

<0.5J

<75V

<1J

<200V

<0.001J

<1.5V

Often

200ms

Each Turn-Off

90ms

< 500Hz

Several Times

in Vehicle Life

Often

1ms

15µs

< 500Hz

Continuous

50Hz to 10kHz

R.F.

Infrequent/Ran

dom

The energy W dissipated in the varistor may be estimated

by: W = 1.4V

(see AN9771 on Energy). The impulse

duration

τ,

of the surge current (see AN9767, Figure 21) can

be estimated from the delay time as:

= 0.7RC

Burst

<50ns

≈

20mV

<10mJ

15kV

where R is the series-parallel combination of the effective

resistance of the varistor and simulator components R

and

. To facilitate this calculation, assume that the effective

resistance is given by V

/ 0.7 I

= 1.4Ω. The delay time

constant with the suppressor in the circuit then becomes:

2.4 x 7



-----------------

 (

0.03

)

= 0.054s



2.4 + 7



The achievement of maximum transient protection involves

many factors. First, consequences of a failure should be

determined. Current limiting impedances and noise

immunities need to be considered. The state of the circuit

under transient conditions (on, off, unknown) and the

availability of low cost components capable of withstanding

the transients are other factors. Furthermore, the interaction

of other parts of the automotive electrical system with the

circuit under transient conditions may require deﬁnition.

and the surge impulse duration:

= 0.7 RC

= 0.038s

The deposited energy now can be estimated by:

W = 1.4 V

= (1.4)(40)(40)(.038) = 85J

Protection by a Central Suppressor

A central suppressor was the principal transient suppression

device in a motor vehicle. As such, it is connected directly

across the main power supply line without any intervening

load resistance. It must absorb the entire available load

dump energy, and withstand the full jump-start voltage. To be

cost effective, it usually is best located in the most critical

electronic module. In newer applications additional

suppressors may be placed at other sites for further

suppression and to control locally-generated transients.

Hence, the simulator produces unprotected and protected

circuit conditions similar to those expected in the vehicle

itself.

A suppressor with the needed high energy capability has

been developed and already is in use. This improved Harris

Varistor model V24ZA50 has a load dump rating of 100J. A

narrow-tolerance selection can satisfy the clamping

requirement of 40V maximum at 40A, with a jumpstart rating

of 24V. The protective performance of this suppressor can

be measured conveniently using the simulator circuit shown

in Figure 3B.

10-50

Application Note 9312

2000

SURGE POWER (W)

1500

OUTPUT

BEFORE

1000

LOAD

DUMP

500

CHARGE

TO 80V

0.03F

SUPPRESSOR

UNDER

TEST

-0.08 -0.04

0.04 0.06 0.12

TIME (s)

0.16

0.2

FIGURE 2A. ALTERNATOR POWER OUTPUT INTO A

CENTRAL SUPPRESSOR

FIGURE 2B. LOAD DUMP SIMULATOR CIRCUIT

100

STANDBY CURRENT AT 12V (mA)

THRESHOLD OF THERMAL RUNWAY >100mA

PROPOSED END-POINT QUALIFICATION

CLAMPING VOLTAGE CHANGE (% AT 20A)

N=8

(mA) 0.1

0.01

JUMP

START 24V

0.001

10 DUMPS

AT 100J

1 DUMP

AT 200J

JUMP

START

24V

(%) 0

10 DUMPS

AT 100J

-5

1 DUMP

AT 200J

N=8

FIGURE 2C. STABILITY OF CLAMPING VOLTAGE

FIGURE 2D. STABILITY OF STANDBY CURRENT

Suppressor Applications

[3]

The sensitive electronics of the automobile need to be

protected from both repetitive and random transients. In an

environment of random transients, the dominating

constraints are energy and clamping voltage vs standby

power dissipation. For repetitive transients, transient power

dissipation places an additional constraint on the choice of

suppression device.

It must also be noted that the worst case transient scenarios,

load dump and jump start, place conflicting constraints on the

automotive suppressor. The high energy content of the load

dump transient must be clamped to a worst case voltage of

40V, while the leakage current/power dissipation drawn under

a jump start condition must also be kept to a minimum.

A centrally located suppressor is the principal transient

suppression device used in most automobiles. It is

connected directly across the main power supply line without

any intervening load resistance. It must be capable of

absorbing the entire available load dump energy, and must

also withstand the full jump start voltage. To be cost

effective, it is usually located in the most critical electronic

module. Additional secondary suppression is also employed

at other locations in the system for further suppression and

to control locally generated transients.

As previously mentioned, the maximum load dump energy

available to the central suppressor depends on a

combination of the alternator size and the loads that share

the surge current and energy which are thus generated. It

must be remembered that there are many different

automotive electronic conﬁgurations which result in a variety

of diverse load dumps.

Multilayer Transient Voltage Surge Suppressor

(AUML)

[4, 5]

The new automotive multilayer (AUML) transient voltage

suppressor is a voltage dependent, nonlinear device. It has

an electrical behavior similar to that of a back-to-back zener

diodes and it is inherently bidirectional. It offers protection

from transients in both the forward and reverse directions.

When exposed to high voltage transients, the AUML

undergoes a nonlinear impedance change which is many

orders of magnitude, from approximately 10

to 10Ω.

The crystalline structure of the AUML transient voltage

suppressor consists of a matrix of ﬁne, conductive grains

separated by uniform grain boundaries, forming P-N

10-51

Application Note 9312

junctions (Figure 4). These boundaries are responsible for

blocking conduction at low voltages, and are the source of

the nonlinear electrical conduction at higher voltages.

Conduction of the transient energy takes place between the

millions of P-N junctions present in the device. The uniform

crystalline grains act as heat sinks for the energy absorbed

by the device under a transient condition, and ensures an

even distribution of the transient energy (heat) throughout

the device. This even distribution results in enhanced

transient energy capability and long term reliability.

FIRED CERAMIC

DIELECTRIC

The AUML surge suppressor is a surface mountable device

that is much smaller in size than the components it is designed

to protect. The present size offerings for suppression in the

automotive environment are “1210” (0.120 x 0.100 inches),

“1812” (0.180 x 0.120 inches) and “2220” (0.220 x 0.200

inches). The correct device to use depends on the location of

the suppressor in the overall electronics system.

Device Ratings and Characteristics

Package Outline

The present size offerings of the AUML series are the industry

2220, 1812 and 1210 standard form factors. Since the AUML

device is inherently bidirectional, symmetrical orientation for

placement on a printed circuit board is not a concern. Its

robust construction makes it ideally suitable to endure the

thermal stresses involved in the soldering, assembling and

manufacturing steps involved in surface mount technology.

The AUML device is inherently passivated by means of the

fired ceramic material. They will not support combustion and

are thus immune to the risk of flammability which may be

present in the plastic or epoxy molded diode devices.

METAL

ELECTRODES

Load Dump Energy Capability

DEPLETION

REGION

GRAINS

FIGURE 3. AUML TRANSIENT VOLTAGE SUPPRESSOR

The AUML is constructed by forming a combination of

alternating electrode layers and semiconducting ceramic layers

into a rectangular block. Each alternate layer of electrode

material, separated by ceramic semiconducting material, is

connected to opposite end terminations of the device.

The paralleled arrangement of the inner electrode layers

represents signiﬁcantly more active surface area than the

small outline of the package may suggest (Figure 5). This

increased active surface area, combined with an

interdigitated block formation, results in proportionally higher

peak energy capability.

SEMICONDUCTING

CERAMIC

INNER

ELECTRODES

END

TERMINATION

The most damaging classiﬁcation of transients an

automobile must survive is a load dump discharge

occurrence. A load dump transient occurs when the

alternator load in the automobile is abruptly reduced and the

battery clamping effect is thus removed. The worst case

scenario of the load dump occurs when the battery is

disconnected while operating at full rated load. The resultant

load dump energy handling capability serves as an excellent

ﬁgure of merit for the AUML suppressor.

Standard load dump speciﬁcations require a device

capability of 10 pulses at rated energy, across a temperature

range of -40

C to 125

C. This capability requirement is well

within the ratings of all of the AUML series.

Due to the assortment of electronic applications in an

automotive circuit, there is a need for a wide range of surge

suppressors. The transient environment can generally be

divided into three distinct sections and there will be a need for a

different type of suppressor within each section. The 2220 size

was designed for operation in the primary transient area, i.e.,

directly across the alternator. The 1812 size for secondary

protection and the 1210 size for tertiary protection. A typically

load dump transient results in an energy discharge of

approximately 100J (depending on the size of the alternator).

The deciding factor in the selection of the correct size

suppressor is the amount of energy which is dissipated in the

series and parallel loads in the circuit. The higher the

impedance between the battery and the system requiring

suppression, the smaller is the suppressor required.

Random samples of the 1210, 1812 and 2220 devices were

subjected to repetitive load dump pulses at their rated energy

level. This testing was performed across a temperature

FIGURE 4. AUML INNER CONSTRUCTION

10-52

Application Note 9312

spectrum from -40

C to 125

C. This temperature range

simulates both passenger compartment and under the hood

operation. There was virtually no change in the device

characteristics of any of the units tested (Figure 6).

Unlike equivalent rated silicon TVS diodes, all of the AUML

device package is available to act as an effective, uniform

heat sink. Hence, the peak temperatures generated by the

load dump transient are evenly dissipated throughout the

complete device. This even energy dissipation ensures that

there are lower peak temperatures generated at the P-N

grain boundaries of the AUML suppressor.

Experience has shown that while the effects of a load dump

transient are of real concern, its frequency of occurrence is

much less than that of localized low energy inductive spikes.

Such low energy spikes may be generated as a result of

motors turning on and off, from ESD occurrences, or from

any number of other sources. It is essential that the

suppression technology selected also has the capability to

suppress such transients. Testing on the V18AUMLA2220

has shown that after being subjected to a repetitive energy

pulse of 2J, over 6000 times, no characteristic changes have

occurred (Figure 8).

2220 = 25J

1218 = 6J

1210 = 3J

VOLTAGE (V) AT 10mA

NUMBER OF LOAD DUMP PULSES

FIGURE 5. LOAD DUMP PULSING OVER A TEMPERATURE

RANGE OF - 55

C TO 125

Further testing on the AUML series has resulted in the

extension the number of load dump pulses, at rated energy,

which are applied to the devices. The reliability information

thus generated gives an indication of the inherent capability

of the series of devices. The V18AUMLA1210 sample has

been subjected to over 2000 pulses at its rated energy of 3J;

the V18AUMLA1812 sample over 1000 times at 6J. The

V18AUMLA2220 sample has been pulsed at 25J over 300

times (Figure 7). In all cases there has been little or no

change in the device characteristics.

VOLTAGE (V) AT 10mA

2 JOULES

1000

2000

3000

4000

5000

NUMBER OF PULSES

6000

VOLTAGE (V) AT 10mA

FIGURE 7. REPETITIVE ENERGY TESTING OF THE

V18AUMLA2220 AT LOW ENERGY LEVELS

25J

Clamping Voltage

120

150

180

210

240

270

300

NUMBER OF LOAD DUMP PULSES

FIGURE 6. REPETITIVE LOAD DUMP PULSING AT RATED

LOAD DUMP ENERGY

The clamping voltage of a suppressor is the peak voltage

appearing across the device when measured under

conditions of a speciﬁed current pulse waveform. The

industry recommended waveform for clamping voltage is the

8/20µs pulse which has been endorsed by UL, IEEE and

ANSI. The maximum clamping voltage of the AUML should be

below the system or component failure level. Shunt type

suppressors like the AUML are used in parallel to the systems

they protect. Their effectiveness can be increased by

understanding the important influence that source and line

impedance play in the overall system (Figure 9).

As previously discussed, the very high energy absorption

capability of the AUML suppressor series is achieved by

means of a new, highly controlled manufacturing process.

This new multilayer technology ensures that a large volume

of suppressor material, with an interdigitated layer

construction, is available in an extremely small package.

10-53

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