Automotive electronics modules require reverse battery protection to avoid the risk of damage from poor battery operation. Schottky diodes are the device of choice for this application because of their low forward voltage drop performance.
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Although Schottky diodes are well suited to meet the above requirements, they must support ISO7637-2 pulses, so products with high breakdown voltages are usually selected for testing with negative pulse 1 and pulse 3a - but this does not help to get the most Good forward voltage drop performance because the inherent compromise performance of Schottky diodes follows the rule that the higher the breakdown voltage, the greater the forward voltage drop.
However, it is possible to reconcile these two states. In fact, Schottky diodes have the ability to consume some power in the reverse state - this involves the PARM parameters (repetitive peak avalanche power). For example, a Schottky diode with a breakdown voltage of 100V supports the negative pulse 1 and the pulse 3a specified in the ISO 7637-2 standard on the one hand and a very good forward direction due to the very low voltage drop on the other hand. Work performance.
This article discusses how to choose the best Schottky diode in automotive applications to retain both low forward voltage drop performance and ISO7637-2 pulse testing.
Status quo of technology development
The ISO16750 standard considers that some changes may occur in the car's power rail. Since the reverse battery connection caused by incorrect maintenance is described as a large risk item, the electronic module supplier knows that certain measures should be taken to deal with this problem. So they added a battery reverse protection device to protect their electronic modules.
In general, the reverse battery protection solution consists of adding a series diode that prevents reverse current flow when the battery is connected in reverse.
Figure 1: Typical schematic of an active automotive module with a Schottky diode for reverse battery protection.
One of the drawbacks of this solution is that there is a certain voltage drop across the diode, which consumes a certain amount of power. For this reason, Schottky diodes have become the preferred product because of its lower forward voltage drop than conventional bipolar diodes.
Harsh environment application
Automotive electronic modules must pass several positive and negative pulse tests as specified in ISO 7637-2.
Some of the most stringent pulses are:
Pulse 1: "Transient pulse due to power supply disconnected from inductive load"
Figure 2: ISO7637-2 pulse 1.
Pulse 2a: "Transient pulse due to sudden interruption of current in the device in parallel with the device under test (DUT) due to harness inductance"
Figure 3: ISO7637-2 pulse 2a.
Pulse 2b: "Instantaneous pulse generated by a DC motor used as a generator after the ignition circuit is switched off"
Figure 4: ISO7637-2 pulse 2b.
Pulse 3a: "Transient pulse (negative pulse) caused by switching process"
Figure 5: ISO7637-2 pulse 3a.
Pulse 3b: "Transient pulse generated by switching process" (positive pulse)
Figure 6: ISO7637-2 pulse 3b.
Pulse 4: "The voltage drop due to the energization of the starter motor of the internal combustion engine".
Figure 7: ISO7637-2 pulse 4.
Pulse 5b: "The alternator is generating a charging current, and the load-dump transient pulse occurs when the discharged battery is disconnected, provided that the alternator has an automatic protection function"
Figure 8: ISO7637-2 pulse 5b "clamped load dump".
In fact, the most stringent positive pulse is pulse 5b, the peak voltage is typically +36V, the duration is 300ms, and the series resistance is 0.5Ω. The most stringent negative pulse is pulse 1, which has a peak value of -100V, a duration of 2ms, and an instantaneous peak current of 10A.
The indicator for pulse 3a is -150V, 50Ω series resistance and 100ns duration, and its energy is much smaller than pulse 1 - meaning that if the Schottky diode specification meets pulse 1 requirements, then pulse 3a has no problem at all.
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