Power components

Select Schottky diodes based on avalanche performance

February 06, 2012 | Helene Goulin and Philippe Merceron | 222904062

Helene Goulin and Philippe Merceron, STMicroelectronics examines how to select the best Schottky diode in automotive applications to preserve the low forward voltage drop performance on one side and the ability to pass the ISO7637-2 pulses.

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

Pulse 2a: “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 2a

Pulse 2b: “transients from a DC motor acting as a generator after the ignition is switched off”

Figure 4: ISO7637-2 Pulse 2b

Pulse 3a: “transients which occur as a result of the switching processes” (negative pulses)

Figure 5: ISO7637-2 Pulse 3a

Pulse 3b: “transients which occur as a result of the switching processes” (positive pulses)

Figure 6: ISO7637-2 Pulse 3b

Pulse 4: “voltage reduction caused by energizing the starter-motor of internal combustion engines”

Figure 7: ISO7637-2 Pulse 4

Pulse 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 0.5Ω.

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 50Ω 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.

How to choose the appropriate Schottky diode

Example of application
Schottky diode choice for reverse battery protection is clearly defined by the electronic module normal operating current on one side (I_f 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 I_FSM (“surge non repetitive forward current”) given in Table 1 that says the STPS5H100 is able to support 75 A_RMS 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 I_FSM 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 V_RRM diode will be activated in reverse because the voltage at its terminations will be -113.5V (V_surge + V_bat 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 * P_ARM = 252W. Therefore in this example the STPS5H100 is compatible with ISO7637-2 and ensures a good reverse battery protection.

Conclusion
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.

0 comments (add yours?)

Please login to post your comment - click here