Example of application
Schottky diode choice for reverse battery protection is clearly defined by the electronic module normal operating current on one side (If in Figure 1), and the need of withstanding the ISO7637-2 pulses.
Figure 1: Typical schematic of a powered automotive module using a Schottky diode as reverse battery protection.
Let’s consider now the STPS5H100 Schottky diode
. This device has the following characteristics listed in Table 1.
Table 1: STPS5H100 characteristics
Regarding the Pulse 5b surge test (Figure 8), which is the most positive energetic surge in the case of an auto-protected alternator, having a surge voltage of 36V and a series resistor of 0.5Ω and a pulse duration of 300 ms—we can evaluate the maximum current that will cross the STPS5H100 during the surge.
Figure 9: Pulse 5b surge test schematic
A measurement on test bench according to Figure 9 yields the following curves:
Figure 10: Current and voltage at the transient suppressor side
The graph above shows the current though the reverse battery protection and voltage across the transient suppressor. What is remarkable is the current pulse duration. To make sure this current surge is compatible with the STPS5H100, it is necessary to compare with the IFSM
(“surge non repetitive forward current”) given in Table 1 that says the STPS5H100 is able to support 75 ARMS
max for a 10 ms sine wave signal.
This 17.2A, 70 ms exponential surge is equivalent to a 17.2A, 154 ms sine waveform surge (refer to AN316 application note
). The sine wave variation is tied to the law i²t = constant. Then a surge of 19A, 154 ms is equivalent in terms of energy to a sine wave of 67A, 10 ms. Thus we can compare this result with the IFSM
specification of 75A, 10 ms. Therefore we can see the STPS5H100 will have no problem to pass Pulse 5b test as described.
Now if we consider Pulse 1 as shown in Figure 11, below, things are different because the Schottky diode is conducting in reverse mode.
Figure 11: Example of an application with Pulse 1
For instance this 100V VRRM
diode will be activated in reverse because the voltage at its terminations will be -113.5V (Vsurge
due to the charge of the capacitor).
A Pspice simulation shows the power involved in the STPS5H100 as given in Figure 13 (second below) according to the schematic of Figure 12 (immediately below).
Figure 12: Pspice model of the Pulse 1 surge test
Figure 13: Pspice simulation result
What we see in Figure 13 is that the peak power is a 118W triangular shape that lasts roughly 120 µs. This triangular waveform is equivalent to a 59W square shape pulse of 120 µs duration.
Now to make sure this is compatible with the STPS5H100 characteristics we have to see what is given in Figure 14 (below).
Figure 14: Normalized avalanche power derating versus pulse duration (Figure 3 of STPS5H100 datasheet)
This derating curve shows that the equivalent avalanche power the STPS5H100 is able to dissipate is 0.035 * PARM
= 252W. Therefore in this example the STPS5H100 is compatible with ISO7637-2 and ensures a good reverse battery protection.
Electronic modules connected to automotive power rails may be affected by polarity inversion due to poor battery handling. In order to make the electronics safe, module manufacturers often add some reverse battery protection such as Schottky diodes rather than bipolar types because of their good performance in direct conduction. They tend to chose a “high” breakdown voltage (150V) Schottky to pass the ISO7637-2 Pulse 1 and Pulse 3a tests which impose a -100V and -150V surge test, respectively. As a result, direct conduction performance is not optimized because higher the breakdown voltage, higher the forward voltages drop.
In spite of this, it is possible to use a lower breakdown voltage Schottky diode, such as 100V, for example. This will bring some gain in direct conduction losses thanks to its lower voltage drop and ability to work in avalanche mode during ISO7637-2 negative surge tests, as noted above.
Helene Gouin is a strategic marketing engineer in power system applications and Philippe Merceron is an automotive application and system engineer at STMicroelectronics, Tours, France.
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