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Load dump protection: Old vs. new ISO standards | EDN
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Soo Man (Sweetman) Kim, Vishay -February 07, 2014
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For many years, the load dump surge condition test for automobiles was defined in the
ISO-7637-2 standard and was used by major automotive component manufacturers worldwide. In
2010, this standard was replaced by a new automotive load dump test: ISO-16750-2. This article
describes the changes to the load dump test condition and defines the maximum surge
suppressing capability on an example load dump TVS under those conditions.
New vs. old ISO standard
The significant difference between the old and new load dump tests is that the new test requires
10 pulses in 10 minutes with a one minute interval per pulse, as shown in Table 1. The old load
dump test (ISO-7637-2, 2004) specified only a single pulse.
Table 1 Pulse requirements for ISO-16750-2
Parameter
Type of system
Minimum
UA = 12 V
UA = 24 V
Us (V)
79 to 101
151 to 202
Ri (Ohm)
0.5 to 4
1 to 8
td (ms)
40
100
tr (ms)
10 / +0 / -5
to 400
to 350
test requirements
10 pulses
at intervals of 1
min.
10 / +0 / -5
The test condition of 10 pulses in 10 minutes shows the reliability of the load dump protection
device, which assists the circuit designer in choosing the correct device based on surge capability
in high-temperature environments.
Load dump protection for 12 V systems using an AEC-Q101-qualified TVS series
Figure 1 shows the load dump protection device clamping in the specified voltage range, which is
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different from the standard waveform. The reason for this difference is the active voltage range of
the clamping device in the load dump pulse, as shown in Figure 1. The device in Figure 2 is
clamping at 101V Us, 1.75Ω Ri, and a 400ms pulse width.
Figure 1 Input waveform of an ISO-16750-2 pulse: 5A, 79V U , 400ms pulse width condition
s
Figure 2 Clamped waveform of the SM5S24A at an ISO-16750-2 pulse: 5A, 101V U , Ri =
s
1.75Ω, 400ms pulse input condition
Protecting electronic devices from load dump by using a TVS diode
Now let's examine how to protect devices by clamping the voltage below the maximum input
voltage for the voltage regulator or other electronic components in the circuit, without halting or
powering down the system. The protection device will not operate until the line voltage reaches
24V for 1 to 10 minutes or longer in withstand test conditions.
The clamping voltage of the load dump TVS device rises as the junction temperature changes
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during continuous clamping operation. Figures 3 and 4 show the first and last clamping waves for
10 pulses using TVS devices at Us = 79V, Ri = 1.0Ω, and 400ms pulse width at 1 minute intervals
between each pulse.
Figure 3 First clamped waveform for SM5S24A
Figure 4 Last clamped waveform for SM5S24A
Using the new test condition, 10 continuous pulses affected the surge suppressing capability of
the load dump TVS, as shown in the following three graphs:
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Figure 5 Capabilities of Vishay's SN5S24A under the new load dump test conditions. Suffix "M"
lines are multiple pulses using the new test condition standards (ISO-16750-2). Suffix "S" lines
are single pulses as defined by the old test condition standards (ISO-7637-2).
These values are based on normal room temperature with the recommended pad size as listed in
the datasheet, but the actual capability varies by PCB type, pad size, and temperature conditions.
Appendix
Easy reference for load dump TVS vs. peak current (Amps)
10 pulses in 10
minutes (ms)
Single pulse (ms)
40
ms
100 220
ms ms
400
ms
40
ms
100
ms
220
ms
400
ms
SM5S24A 73
57
46
34
58
52
38
29
SM6S24A 95
65
52
39
65
60
44
35
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SM8S24A 115
80
67
55
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78
69
60
50
Another way to select the correct load dump protection device is to refer to the maximum
clamping current capability of the TVS and estimate the clamping current of the circuit based on:
Circuit condition
Ri = 2Ω
Peak voltage of alternator output in load dump = 100V
Target clamping voltage = 35V
Pulse width = 200ms
Pulse numbers = 10 pulses in 10 minutes
SM5S24A has 38A clamping capability in 10 pulses condition and its peak clamping current is
32.5A ((100V - 35V) / 2Ω).
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