ST AN3361 Application note
AN3361
Application note
Schottky diode avalanche performance in automotive applications
Introduction
Electronic modules connected to automotive power rails may be affected by polarity
inversion due to poor battery handling and load-dump surges when the battery is
disconnected while the alternator is still charging. To protect against these phenomena,
module manufacturers add reverse-battery protection, usually using diodes.
Schottky diodes are preferred over bipolar ones because of their higher performance in
direct conduction. Schottky diodes feature a low forward voltage drop, and are able to
withstand the pulses defined in ISO 7637-2.
However, the diode needs a breakdown voltage higher than 150 V in order to pass the tests
for negative pulses 1 and 3a, whereas this tends to lower the forward performances. For
Schottky diodes, the intrinsic trade-off obeys the rule: the higher the breakdown voltage, the
higher the forward voltage drop.
There is a way to reconcile these conditions. Some Schottky diodes (depends on the
technology) have the ability to dissipate some power in reverse condition. This concerns the
P
parameter (Repetitive Peak Avalanche Power). For instance a 100 V breakdown
ARM
voltage Schottky diode may on the one hand support the negative pulse 1 and pulse 3a of
the ISO 7637-2 standard and on the other hand offer a very good performance in forward
voltage drop.
This Application note explains how to choose the best Schottky diode trade off in automotive
applications in order to preserve the low forward voltage drop performance and the ability to
pass the ISO 7637-2 pulses.
September 2011 Doc ID 018589 Rev 1 1/15
www.st.com
Definition of the electrical transients and tests AN3361
1 Definition of the electrical transients and tests
Two ISO standards are applicable to this situation.
● ISO 16750
● ISO 7637-2
The ISO 16750 standard defines the variations that automotive power rails may undergo. A
reverse battery connection due to poor maintenance is described as a key condition to be
considered. Electronic modules thus usually have a reverse battery protection device to
guard against this condition. Most of the time this protection consists of a diode in series
that prevents negative current from flowing if the battery connection is reversed (see
Figure 1).
This solution involves a voltage drop across the diode and therefore some power
dissipation. This is why a Schottky diode is preferred as its forward voltage drop is less than
that of a conventional bipolar diode.
Figure 1. typical schematic of a powered automotive module using a Schottky
diode as reverse battery protection
Battery reverse protection
I
F
V
F
+
Transient protection
Electronic module
ISO 7637-2 specifies the methods and procedures to test for compatibility with conducted
electrical transients of equipment installed on passenger cars and commercial vehicles fitted
with 12 V or 24 V electrical systems, whatever the propulsion system (spark ignition or
diesel engine, electric motor). The standard describes bench tests for both the injection and
measurement of transients.
The bench tests consist in applying positive or negative pulses to the modules. The test is
successful if there is no damage on the device. Each pulse models an abnormal behavior.
The most sever cases are given in Table 1.
2/15 Doc ID 018589 Rev 1
AN3361 Definition of the electrical transients and tests
Table 1. ISO 7637-2 main surge pulses
12V system
V
peak
t
p
Pulse Origin
Pulse
polarity
N° 1 Supply disconnection from inductive loads Negative -100 V 2 ms
The sudden interruption of current through a
N° 2a
device connected in parallel with the device
under test (DUT) due to the inductance of the
Positive +50 V 50 µs
wiring harness
N° 2b
DC motor acting as a generator after the
ignition is switched off
Positive 10 V 2 s
N° 3a Occur as a result of the switching processes Negative -150 V 100 µs
N° 3b Occur as a result of the switching processes Positive 100 V 200 µs
N° 4
Voltage reduction caused by energizing the
starter-motor of internal combustion engines
Negative -7 V 40 ms
Load-dump transient occurring in the event
N° 5b
of a discharged battery being disconnected
while the alternator is generating charging
Positive 87 V
Application
dependant
current, case with auto-protected alternator
The most severe positive pulse is pulse 5b (Figure 2). Its voltage range commonly varies
from +24 V to +48 V with a pulse duration up to 400 ms and a minimum series resistance
that can be as low as 0.5 Ω.
Figure 2. ISO 7637-2 pulse 5b clamped load-dump
t
U
0.1xU
S
Table 2. Parameter values for test pulse 5b
Parameter 12 V system
U
S
* As specified by customer
U
S
t
d
R
i
d
U
65 V to 87 V
40 ms to 400 ms
0.5 to 4 Ω
U
S
*
S
t
Doc ID 018589 Rev 1 3/15
Definition of the electrical transients and tests AN3361
The most severe negative pulse is pulse 1 (Figure 3). It can reach -100 V during 2 ms and a
peak current of 10 A in shorted conditions.
Figure 3. ISO 7637-2 pulse 1
t
U
t
3
2
0.1xU
S
U
0.9xU
S
t
r
t
d
t
1
Table 3. Parameter values for test pulse 1
S
t
Parameter 12 V system
U
s
R
i
t
d
t
r
(1)
t
1
t
2
(2)
t
3
1. Period t1 shall be chosen such that the DUT is correctly initialized before the application of the next pulse.
2. Period t3 is the smallest possible time necessary between this disconnection of the supply source and the
application of the pulse.
-75 V to -100 V
10 Ω
2 ms
1 µs
0.5 s to 5 s
200 ms
<100 µs
4/15 Doc ID 018589 Rev 1
AN3361 Definition of the electrical transients and tests
Pulse 3a (Figure 4) is specified at -150 V but with 50 Ω series resistor and 100 ns duration
which is far less energy than for pulse 1. This means that, if the Schottky diode specification
is compliant with pulse 1, pulse 3a will be covered as well.
Figure 4. ISO 7637-2 pulse 3a
0.1xU
0.9xU
t
5
U
S
S
S
t
r
t
U
t
4
1
t
Table 4. Parameter values for test pulse 3a
Parameter 12 V system
U
s
R
i
t
d
t
r
t
1
t
4
t
5
U
d
-112 V to -150 V
t
S
50 Ω
0.1 µs
5 ns
100 µs
10 ms
90 ms
Doc ID 018589 Rev 1 5/15
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