ST AN3361 Application note

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	AN3361

1 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

+

I F

VF

Battery reverse protection

protectionTransient

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	Origin	Pulse	12V system
			polarity	Vpeak	tp
	N° 1	Supply disconnection from inductive loads	Negative	-100 V	2 ms
		The sudden interruption of current through a
	N° 2a	device connected in parallel with the device	Positive	+50 V	50 µs
		under test (DUT) due to the inductance of the
		wiring harness
	N° 2b	DC motor acting as a generator after the	Positive	10 V	2 s
		ignition is switched off
	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	Negative	-7 V	40 ms
		starter-motor of internal combustion engines
		Load-dump transient occurring in the event
	N° 5b	of a discharged battery being disconnected	Positive	87 V	Application
		while the alternator is generating charging			dependant
		current, case with auto-protected alternator

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

U	td
	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
U	t2
t3
0.1xUS	t
	US
0.9xUS
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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AN3361	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
U	t4	t5
		t
		US
	t1
		0.1xUS
		US
		0.9xUS
		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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