Schottky diode avalanche performance in automotive applications

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
PARM parameter (Repetitive Peak Avalanche Power). For instance a 100 V breakdown
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
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Definition of the electrical transients and tests
1
AN3361
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
VF
+
Transient protection
IF
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.
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AN3361
Definition of the electrical transients and tests
Table 1.
ISO 7637-2 main surge pulses
Pulse
12V system
Pulse
polarity
Origin
Vpeak
tp
N° 1
Supply disconnection from inductive loads
Negative
-100 V
2 ms
N° 2a
The sudden interruption of current through a
device connected in parallel with the device
under test (DUT) due to the inductance of the
wiring harness
Positive
+50 V
50 µs
N° 2b
DC motor acting as a generator after the
ignition is switched off
Positive
10 V
2s
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
N° 5b
Load-dump transient occurring in the event
of a discharged battery being disconnected
while the alternator is generating charging
current, case with auto-protected alternator
Positive
87 V
Application
dependant
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
td
U
US*
US
0.1xUS
t
Table 2.
Parameter values for test pulse 5b
Parameter
12 V system
US
65 V to 87 V
US *
As specified by customer
td
40 ms to 400 ms
Ri
0.5 to 4 Ω
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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
t2
U
t3
t
0.1xU S
US
0.9xU S
tr
td
t1
Table 3.
Parameter values for test pulse 1
Parameter
12 V system
Us
-75 V to -100 V
Ri
10 Ω
td
2 ms
tr
1 µs
t1(1)
0.5 s to 5 s
t2
200 ms
t3(2)
<100 µs
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.
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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
t4
U
t5
t
US
t1
0.1xU S
US
0.9xU S
tr
td
Table 4.
Parameter values for test pulse 3a
Parameter
12 V system
Us
-112 V to -150 V
Ri
50 Ω
td
0.1 µs
tr
5 ns
t1
100 µs
t4
10 ms
t5
90 ms
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Choosing the appropriate Schottky diode
2
AN3361
Choosing the appropriate Schottky diode
Schottky diode choice for reverse battery protection is determined by the electronic module
normal operating current on the one hand, and the need to pass the ISO 7637-2 pulse tests
on the other. Each module has its own normal operating current, which is defined by its
characteristics. So here we will consider only the method to choose an appropriate Schottky
diode to meet the ISO 7637-2 requirements.
2.1
Load-dump surge compatibility criteria
The first criterion is the compatibility between surge current and IFSM specified in the diode
datasheet.
2.1.1
Load-dump peak current calculation
Figure 6 shows the current shape through the Schottky diode during a load-dump surge
according to the schematic described in Figure 5.
Figure 5.
Pulse 5b surge test schematic
Schottky diode
Ip
Ri = 0.5 Ω
Pulse 5b:
Vbat =13.5 V
Vg = [24-48 V]
tps = 300 ms
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VF
Vcl
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Electronic module
AN3361
Choosing the appropriate Schottky diode
Figure 6.
Current and voltage at the transient suppressor side (with a 24 V Vbr
clamping device and Vg = 36V)
Vcl
max
C1
max
Ip
C2
Measure
value
status
10.0 V/div
-30.20 V ofst
P1:max(C1)
29.9 V
P2:max(C2)
35.9 V
P3:mean(F1)
P4:rise@Iv(C3)
10.0 V/div
-30.00 V ofst
P5:---
50.0 kS
50.0 ms/div
100 kS/s
P6:---
Stop
Edge
16.8 V
Positive
The equations below apply to the circuit shown in Figure 5.
Equation 1
Vsurge = Vg + Vbat
Vsurge = Vcl + VF (Ip ) + RiIp
VF (Ip ) = VT0 + Rd ·Ip
The calculation of VT0 and Rd is explained in the application note AN604: “Calculation of
conduction losses in a power rectifier”. Values are provided in the datasheets.
Then:
Equation 2
Ip =
Vsurge − Vcl − VT0
Rd + Ri
In the example presented in Figure 6 the generator surge voltage (Vg) is 36 V, its internal
series resistor is 0.5 Ω, the battery voltage is 12 V and the protection voltage clamping level
of the protection device is 29.9 V. The diode dynamic resistor Rd is 0.009 Ω.
As VT0<< Vsurge - Vcl the above relation can be simplified to:
Equation 3
Ip =
(36 + 12) − 29.9
0.009 + 0.5
So the peak current Ip is equal to 35.56 A.
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Choosing the appropriate Schottky diode
2.1.2
AN3361
Method to compare Ip and IFSM
IFSM is the maximum peak current of a sinusoidal waveform pulse during 10 ms. The loaddump peak current can be approximated with a constant and an exponential waveform
pulse. To compare both peak currents, IFSM and load-dump peak current, one method is to
calculate the equivalent sinusoidal surface of the exponential waveform in order to deduce
the equivalent pulse duration.
The surge load-dump surface is modeled using the following equation:
Equation 4
∞
t1
∫
∫
Ssurge = I0 ·dt + I0 ·e
−(t − t1)
τ dt
t1
0
Where I0 is the maximum load-dump current. The equivalent sinusoidal waveform is:
Equation 5
t sin
2ð
∫ I ·sin( 2·t
Ssin =
0
0
t)dt
sin
The equivalent pulse duration is tsin, since:
Equation 6
Ssin = Ssurge
Then:
Equation 7
π
(τ + t1)
2
t sin =
Where
Equation 8
t I0 − t1
τ=
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AN3361
Choosing the appropriate Schottky diode
Figure 7.
Equivalent sinusoidal surface of clamped load-dump surge surface
I0
t1
40
Isurge (t)30
Isin (ti)I0/2
20
10
tsin
0
0
tI0/2
0.1
t, ti
0.2
In our example the equivalent sinusoidal waveform pulse time duration tsin is 140 ms.
2.1.3
IFSM value versus pulse time
Using the equations:
Equation 9
I4 x t = Cste for tsin>10 ms
I3 x t = Cste for 20 µs <tsin>10 ms
I2 x t = Cste for tsin<20 ms
In the example for the STPS20L60C in Figure 7, as the pulse duration tsin is 140 ms, the
following law from Equation 9 can be used:
Equation 10
IF4 S M @ ts in × t s in = IF4 S M × 1 0 ·1 0 − 3
Where:
IFSM is the non repetitive forward surge current given in the data sheet.
IFSM @ t sin is the non repetitive forward surge current for a pulse duration tsin.
For the example of Figure 6, the IFSM = 220 A
Equation 11
IFSM@140ms = 4
10·10 −3 × I4FSM
t sin
IFSM@140ms = 4
10·10 − 3 × 220 4
140·10 − 3
The equivalent peak current is IFSM@140 ms = 113.73 A.
The peak current delivered by the test system is Ip = 35.56 A and it is less than
IFSM @140 ms. So the STPS20L60C meets the ISO 7637-2 requirements.
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Choosing the appropriate Schottky diode
AN3361
Table 5 gives a matrix of which Schottky diode is compatible with load-dump surge (pulse
5b) depending on surge voltage level and with the conditions: Vbat = 13.5 V, Ri = 0.5 Ω and
with load-dump surge duration of 300 ms.
Table 5.
2.2
Which Schottky diodes are good for which load-dump surge level
Pulse 5b load-dump surge voltage (Vg)
24
30
36
42
48
STPS160AY
Yes
STPS3L60SY
Yes
Yes
STPS20L60CGY
Yes
Yes
Yes
Yes
Yes
STPS1H100UY
Yes
STPS2H100UY
Yes
Yes
STPS5H100BY
Yes
Yes
Yes
STPS8H100GY
Yes
Yes
Yes
Yes
Yes
Most severe negative surge compatibility criteria
Now if we consider pulse 1 as shown in Figure 3, things are different since the Schottky
diode is reverse polarized.
For instance, the voltage applied on a diode with a maximum repetitive reverse voltage
(VRRM) of 100 V will be VR = -113.5 V (VR = Vsurge + Vc due to the charge of the capacitor).
Figure 8.
Example of application with pulse 1 using an STPS5H100BY
STPS5H100BY
I
Pulse 1:
Vbat = 13.5 V
Vsurge = -100
Ri = 10 Ω
tp = 2 ms
10/15
VRRM
Vc = 13.5 V
Electronic module
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AN3361
Choosing the appropriate Schottky diode
A Pspice simulation shows the power involved in an STPS5H100BY, for example, as shown
in Figure 10, according to the schematic of Figure 9.
Figure 9.
Pspice model of Pulse 1 surge test using STPS5H100BY Schottky diode
STPS5H100 Pspice Model
D3
V3
-
+
100
D5
R1
10
V1 = 0
V2 = -100
TD1 = 0.1 m
TC1 = 400 n
TD2 = 0.1004 m
TC2 = 0.85 m
V2
C1
1000 u
I1
1
IC = 13.5
0
ISO7637-2 Pulse 1 Pspice Model
Figure 10. Pspice simulation result
2 200 W
Probe Cursor
Diode dissipation power
A1 = 103.192 u,
A2 = 222.340 u,
dif = -119.149 u,
100 W
118.574
1.3069
117.267
0W
1 120V
SEL>>
100 W
V (I 1: +, D3 : C)* I ( V3: +)
2 2. 0A
Diode reverse voltage
80V
Diode reverse current
1. 0A
40V
0V
> >
0A
0s
1
0. 1ms
V (I 1: +, D3 : C)
2
0. 2ms
I ( V3)
0. 3ms
0. 4ms
0. 5ms
0. 6ms
0. 7ms
0. 8ms
0. 9ms
1. 0ms
Time
The blue curve in Figure 10 is the power dissipated in the diode avalanche. It is a triangular
shape curve with a peak power at 118 W during 120 µs. This waveform is equivalent to a
59 W square shape pulse of 120 µs duration.
In order to evaluate if the diode is able to dissipate this energy in the avalanche, two
elements are relevant:
●
PARM(1 µs, Tj = 25° C) is the repetitive peak avalanche power
●
PARM(Tp)/ PARM(1 µs, Tj = 25 °C) curve Figure 11.
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Choosing the appropriate Schottky diode
AN3361
In the example, we have selected the STPS5H100BY where:
PARM(1 µs, Tj = 25 °C) = 7200 W.
The derating curve Figure 11 shows the equivalent avalanche power the STPS5H100BY is
able to dissipate is 0.035 · PARM (1 µs, Tj = 25 °C) = 252 W
Therefore in this example the STPS5H100BY meets the ISO 7637-2 requirements and
ensures a good reverse battery protection.
Figure 11. Normalized avalanche power derating versus pulse duration for
STPS5H100BY
1
PARM(tp)
PARM(1µs)
0.1
0.035
0.01
tp(µs)
0.001
0.01
0.1
1
10
1000
100
120 µs
Note:
The derating curve for STPS5H100BY can be found as Figure 3 in the datasheet for this
device.
Table 6 indicates which Schottky diode can withstand Pulse 1 of ISO 7637-2 standard.
Table 6.
Compliance of Schottky diodes with ISO 7637-2 Pulse 1
Pulse 1 surge voltage (V)
Vs = -100 V
STPS2H100UY
Yes
STPS5H100BY
Yes
STPS8H100GY
Yes
Table 6 shows that only a few Schottky diodes can handle this constraint.
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3
Conclusion
Conclusion
Protecting automotive electronic modules from polarity inversion due to poor battery
handling and load-dump surge during battery disconnection while the alternator is still
charging usually involves the use of diodes, especially Schottky diodes rather than bipolar
ones because of their better performance in direct conduction. The choice must consider
the worst-case surge conditions of ISO 7637-2 which are pulses 1 and 5b.
Usually Schottky diodes with a breakdown voltage of 150 V are preferred for this application.
This article shows that a breakdown voltage of 100 V may be selected to withstand
avalanche mode during the negative pulse 1 test (starting from a 2 A Schottky type). This
results in the saving of power during direct conduction.
Note:
ST parts numbers listed in this application note were given as examples and are not an
exhaustive list. Please contact your sales or marketing representative for more automotive
grade rectifier devices.
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Revision history
4
AN3361
Revision history
Table 7.
14/15
Document revision history
Date
Revision
09-Sep-2011
1
Changes
Initial release.
Doc ID 018589 Rev 1
AN3361
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