Automotive electronic modules require reverse battery protection to avoid the risk of destruction following the poor handling of the battery. Schottky diodes are preferred in this application because of their low forward voltage drop performance.
Although they are well suited to fulfill this requirement, they have to support the ISO7637-2 pulses, thus they are quite often chosen with a high breakdown voltage to pass the negative Pulse 1 and Pulse 3a tests—which does not help in getting the best forward performances because of the Schottky intrinsic trade-off obeys the rule: Higher the breakdown voltage, higher the forward voltage drop.
However there is a possibility to conciliate both conditions. Indeed, Schottky diodes have the ability to dissipate some power in the reverse condition—which deals with PARM parameter (repetitive peak avalanche power). For instance a 100V breakdown voltage Schottky diode may support on one hand the negative Pulse 1 and Pulse 3a of the ISO7637-2 standard and on the other hand offers a very good performance in forward due to very low voltage drop.
This article explains how to choose the best Schottky diode in automotive applications in order to preserve the low forward voltage drop performance on one side and the ability to pass the ISO7637-2 pulses.
State of the art
ISO16750 standard recognizes that automotive power rails may be subjected to some variations. Reverse battery connection due to poor maintenance is described as a big risk and electronic module suppliers know that some care shall be taken to handle this problem. Thus they add a battery reverse protection device to make their module survive.
Most of the time the reverse battery protection solution consists in adding a diode in series that prevents negative current to flow as the battery connection is reversed (Figure 1).
Figure 1: Typical schematic of a powered automotive module using a Schottky diode as reverse battery protection.
One of the drawbacks of this solution is that some voltage drop occurs through the diode and therefore some power dissipation. For this reason Schottky diode is preferred as its forward voltage drop is less than a conventional bipolar diode.A rugged environment
Automotive electronic modules have to survive to several ISO7637-2 positive and negative pulses.
Some of the most critical are:Pulse 1:
“transients due to supply disconnection from inductive loads"
Figure 2: ISO7637-2 Pulse 1
“transients due to the sudden interruption of current through in a device connected in parallel with the DUT due to the inductance of the wiring harness”
Figure 3: ISO7637-2 Pulse 2aPulse 2b:
“transients from a DC motor acting as a generator after the ignition is switched off”
Figure 4: ISO7637-2 Pulse 2bPulse 3a:
“transients which occur as a result of the switching processes” (negative pulses)
Figure 5: ISO7637-2 Pulse 3aPulse 3b:
“transients which occur as a result of the switching processes” (positive pulses)
Figure 6: ISO7637-2 Pulse 3b
“voltage reduction caused by energizing the starter-motor of internal combustion engines”
Figure 7: ISO7637-2 Pulse 4Pulse 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”
Figure 8: ISO7637-2 Pulse 5b "clamped load-dump"
As a matter of fact most severe positive pulse is Pulse 5b which is commonly +36V with a duration of 300 ms and a series resistor of 0.5O.
The most severe negative pulse is Pulse 1. It can reach -100V for a duration of 2 ms and a peak current of 10A in short.
Pulse 3a is specified -150V but with 50O series resistor and 100 ns duration which is far much less energy than is the Pulse 1 case—which means if the Schottky diode specification is compliant with Pulse 1, Pulse 3a will be no problem at all.