Application Note, V1.0, March 2006
Repetitive Short Circuit
Operation
Lifetime Estimations for Robust ECU Design
Automotive Power
N e v e r
s t o p
t h i n k i n g .
Repetitive Short Circuit Operation
Table of Contents
Table of Contents
Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1
1.1
1.2
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Background and Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2
2.1
2.1.1
2.1.2
2.1.3
2.1.4
2.2
2.2.1
2.2.2
2.2.3
2.3
2.3.1
2.3.2
2.3.3
2.4
2.4.1
2.4.2
2.4.3
2.4.4
Short Circuit Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrical Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Primary Impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Secondary Impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Test Coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Device-specific Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Automatic Restart Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Latching Shutdown Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Status Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Simulated Retry Methodology of Testing Repetitive Short Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cold Repetitive Short Circuit - Short Pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cold Repetitive Short Circuit - Long Pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hot Repetitive Short Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Determining the Worst-case Short Circuit Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Does the Device Have a Latching Shutdown or Automatic Restart Behavior? . . . . . . . . . . . . . . . . .
Does the Device Have a Status Feedback Signal? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Does the Status Signal Have a Delay? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Worst-case Short Circuit Decision Tree . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
3.1
3.1.1
3.1.2
3.2
3.2.1
3.2.2
3.2.3
3.3
Test Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Test Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sample Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Test Termination Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Simulated Retry Methodology of Testing Repetitive Short Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cold Repetitive Short Circuit - Short Pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cold Repetitive Short Circuit - Long Pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hot Repetitive Short Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Statistical Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
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Lifetime Estimations for Robust ECU Design
1
Introduction
1.1
Abstract
Repetitive Short Circuit
Operation
This application note provides an overview to the testing methodolgy and resulting data regarding repetitive short
circuit lifetimes of smart power switches. This data is necessary to define appropriate retry strategies for ECUs
(Electronic Control Units). The first section of this application note will describe Infineon’s approach of defining a
standard for short circuit conditions. The second section covers the practical realization of these conditions in a
test setup and explains how the resulting data is analyzed. The actual results for tested products are published as
addenda to this document.
1.2
Background and Motivation
One of the most important features of automotive smart power switches is to provide embedded protection
functions. As described in datasheets, these integrated protection functions are designed to prevent damage to
the integrated circuit under fault conditions. However, these fault conditions are often outside of the normal
operating conditions of smart power switches and are, in many cases, even outside the maximum ratings. The
stress from operation outside normal operating conditions can have a significant impact on the overall reliability of
a smart power device. Therefore, it is important to determine the effect of such fault conditions on the lifetime of
the device.
For the majority of smart power switches, an overload or short circuit condition will be indicated by the integrated
diagnostic feedback via the status pin, the sense pin or the SPI interface. This status feedback signal can then be
used to alert the microcontroller about any fault conditions. There exist some devices, however, such as the
HITFET and mini-ProFET families, which do not have such a status feedback signal.
With this status feedback signal, the µC can react and switch off the affected channel. However, it may not be
desirable to permanently switch off the smart power switch after a fault condition is reported by the status pin.
Because the objective of the ECU design is to maintain the maximum possible functionality under all
circumstances, it is therefore common practice to turn the shorted switch back on even after the short has been
recognized by the µC. This is done in order to regain the application functionality, e.g. in case of an intermittent
short circuit or a repair/removal of the short.
As a typical example, the µC can react to the status signal after a fault filter time, then switch off the output for
some seconds and then retry. This sequence can be repeated multiple times before the switch is completely
deactivated or until an external event occurs, such as a reset done after an inspection. The specific way the retry
sequence is implemented in the ECU firmware is called the retry strategy. This strategy and assumptions on car
usage like ignition- or service-intervals are determining factors for device lifetime results.
Ideally, the purpose of this retry strategy would be to limit the stress on the device to minimize the potential effect
on its lifetime. Put another way, this means that the maximum number of self-protection and retry events a smart
power switch can undergo during its lifetime in an application must be limited. This maximum number cannot be
determined by simulation, but rather requires testing and analysis of recorded experimental data. Furthermore,
due to a large number of potentially different combinations of application parameters, testing must take into
account all possible worst-case conditions that could occur in an application. This application note describes
Infineon’s methodology of modelling, testing and analysis of the effect of potential worst-case short circuit
conditions on the lifetime of a smart power switch.
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Repetitive Short Circuit Operation
Short Circuit Conditions
2
Short Circuit Conditions
In the event of a short circuit, a smart power switch is exposed to an amount of stress dependent on application
specific parameters such as:
•
•
•
•
•
•
battery voltage Vbat
primary impedance Rsupply and Lsupply (impedance between battery and smart power switch)
secondary impedance R2 and L2 (impedance between the smart power switch and short circuit)
control pattern at the input (controller diagnosis and retry strategy)
ambient temperature Tamb
cooling conditions, e.g. PCB layout
With so many possible combinations of parameters, it is possible for many different short circuit conditions to
appear in applications. On the other hand, it is necessary to define standard test conditions in order to standardize
and compare data for different devices and also to reduce the effort of testing. After discussions with numerous
major automotive electronics companies and car manufacturers, Infineon has drafted a proposal for such a
standard. This proposal has been presented and is under discussion in the Automotive Electronics Council [“A
Novel Test Standard for Short Circuit Reliability Characterization of Smart Power Devices”, W. Kanert, M.
Glavanovics, W. Troeger, AEC Workshop, Detroit, April 2005].
Whenever possible, parameters are fixed to constant values. If a certain range of values is required by the
application, the worst-case value is always selected for the test to ensure a worst-case estimation of device lifetime
in the event of a short circuit. This worst-case operating point is determined by a theoretical analysis of the
implemented protection mechanisms and then further verified by lab characterization prior to repetitive short circuit
testing.
2.1
Electrical Parameters
The electrical parameters of influence to the short circuit life-time are shown in Figure 1 and Figure 2 and Table 1.
primary
impedance
Rsupply
Lsupply
Vbat
secondary
impedance
V bb
OUT
DUT
RECU
L harness
R harness
GND
Impedance depending on
diameter and length of harness
Standard model for power net
Figure 1
Short circuit test conditions for smart power high-side switch
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Repetitive Short Circuit Operation
Short Circuit Conditions
Lharness
Rharness
RECU
Vbat
OUT
DUT
GND
Figure 2
Short circuit test conditions for smart power low-side switch
Table 1
Standard Test Conditions
Parameter
Symbol
Limit Values
Min.
Typ.
Max.
Unit
Conditions
Battery Voltage
Vbat
–
14
–
V
Internal impedance is modelled
by Rsupply and Lsupply
Battery Voltage
(high-current ProFET)
Vbat
9
–
14
V
Internal impedance is modelled
by Rsupply and Lsupply
–
10
–
mΩ
Fixed value for all DUT
–
5
–
µH
–
20
–
mΩ
–
16
–
mΩ/m For Ishort < 100 A
Harness Series Resistance R’harness- –
(thick wire harness)
thick
6
–
mΩ/m For Ishort > 100 A
Harness Series Inductance L’harness –
1
–
µH/m
Fixed value for all DUT
Cable Length
–
5
m
Fixed range for all DUT
Rsupply
Supply Series Inductance Lsupply
ECU Series Resistance
RECU
Harness Series Resistance R’harness
Supply Series Resistance
(normal wire harness)
2.1.1
l
0
Supply Voltage
For most products (e.g. standard ProFETs), 14 V is chosen as the standard test condition. These devices typically
rely on current limitation and overtemperature shutdown as their protection mechanisms.
The high-current ProFET family integrates an additional protection mechanism: the VDS shutdown. The VDS
shutdown deactivates the output as soon as the drain-source voltage exceeds a certain threshold. Because this
shutdown occurs relatively quickly, it reduces the stress on the device. There exist scenarios, however, where the
VDS shutdown may not become activated. For example, a combination of low battery voltage, wire harness
impedance and resistance in the short circuit can result in a drain-source voltage lower than the shutdown
threshold. In such cases, the VDS shutdown may not be triggered, thus requiring the device to rely on its other
protection mechanisms such as current limitation and overtemperature shutdown. But because these protection
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Repetitive Short Circuit Operation
Short Circuit Conditions
mechanisms shut down the device more slowly than the VDS shutdown, they result in more stress to the smart
power switch. Therefore, in order to properly test the worst-case condtion where the VDS shutdown is not active,
high current ProFETs may be tested at a battery voltage between 9 V and 14 V.
2.1.2
Primary Impedance
To better define standard conditions, fixed values were chosen for Rsupply and Lsupply. The values are given in
Table 1. The values follow the standard automotive LISN (Line Impedance Stabilization Network, also known as
AN, Artificial Network) which is described e.g. in the ISO 7637 and CISPR 25 standards.
2.1.3
Secondary Impedance
The secondary impedance is modelled as shown in Figure 3. RECU reflects the contribution of the ECU to the
secondary impedance, i.e. connector and PCB-trace resistance. ECU-contribution to the inductance is neglected.
L harness
OUT
RECU
Rharness
DUT
R2 = R ECU+ l*R’ harness
L 2 = l*L’ harness
Figure 3
Secondary impedance model
Rharness and Lharness reflect the contribution of the wiring harness. They are defined by constant values (resistance
per length R’harness and inductance per length L’harness) multiplied by the length l of the harness between the ECU
and the location of the short. This length is assumed to be between 0 and 5 m. The values for RECU, R’harness and
L’harness depend on the smart switch under investigation. Switches designed for low and medium currents utilize
normal connectors, PCB-traces and wire harnesses with resistance R’harness. Switches designed for higher
currents are often used with lower resistance connectors, wider PCB-traces and bigger diameter wires with
resistance R’harness-thick. The specific inductance L’harness, is assumed to be 1 µH/m and independent of the cable
gauge. Table 1 lists all standard and DUT dependent parameters.
2.1.4
Test Coverage
An important objective of the testing is to cover all possible short circuit scenarios that can occur in an automotive
electronics application. In practice, short circuits can occur either at the ECU, load or anywhere along the length
of the wire harness. A graphical depiction of this range of possibilities is shown in Figure 4. Point P1, with
resistance RECU and very little inductance (< 1 µH), reflects a short circuit directly at the connector of the ECU. The
reason there is almost no inductance at this point is because the ECU itself contributes very little inductance. Point
P3, with resistance RLmax-P3 and inductance Lmax reflects a short circuit at the load, where RLmax-P3 is the sum of
the resistances contributed by the ECU (RECU) and by the full length of the wire harness (l*R’harness) and Lmax is
the ontribution to inductance by the full length of the wire harness (l*L’harness). By association, a line that connects
these two points reflects any possible short circuit along the length of the wire harness. And because the two end
points reflect the limits of possible short circuit scenarios, testing at these two points ensures that the entire range
of harness lengths is covered.
If the short circuit itself possesses some quantity of resistance, then the total resistance of the system will be
increased. Such possible short circuits are reflected in the shaded area of Figure 4. But because the resistance
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Repetitive Short Circuit Operation
Short Circuit Conditions
has been increased, the severity of the short circuit is accordingly decreased. Therefore, covering the line
connecting P1 and P3 is sufficient to cover the entire shaded area. And because testing at P1 and P3 ensures
that all possible short circuits along the line are covered, testing at P1 and P3 also ensures that the entire shaded
area is covered.
A special testing consideration must be made for high-current ProFETs. Because this family of devices is designed
to drive higher load currents and could experience a short circuit current (Ishort) that may exceed 100 A, highcurrent ProFETs are generally connected to their loads with a thicker wire harness. This leads to a reduction in
the contribution to resistance by the wire harness (R’harness-thick). A thicker wire harness, however, generally leads
to no change in its contribution to inductance. Thus, high-current ProFETs are tested at P2 rather than P3, as
shown in Figure 4.
Table 2 summarizes the respective parameters of each test point and its corresponding short circuit description.
l = l max
L2
L max
P2
P3
R 2 > R ECU+ l*R’ harness
l=0
l = 0 ... 5 m
L2 = l*L’harness
P1
RECU
R2
RLmax
Figure 4
Secondary impedance range (the gray area indicates the impedances that can occur in
practice)
Table 2
Definition of Impedances for Short Circuit Characterization
Parameter
Symbol
Terminal Short Circuit (P1)
Load Short Circuit (P3)
Load Short Circuit (P2)
2.2
RECU
LECU
RLmax-P3
Lmax
RLmax-P2
Lmax
Limit Values
Unit
Description
–
mΩ
Short circuit at module
–
1
µH
–
100
–
mΩ
–
5
–
µH
–
50
–
mΩ
–
5
–
µH
Min.
Typ.
Max.
–
20
–
Short circuit at load, Ishort < 100 A
Short circuit at load, Ishort > 100 A
Device-specific Considerations
For smart power switches, there are two main reactions of smart power switches in the event of a short circuit
condition: automatic restart or latching shutdown.
2.2.1
Automatic Restart Behavior
Devices with an automatic restart behavior turn off the output only after the device has reached its
overtemperature detection threshold, then proceeds to “toggle” according to the temperature hysteresis of its
thermal shutdown. This thermal toggling continues until the input signal commands the channel to turn off. As a
result, if an automatic restart device is left permanently in ON-state in a short circuit condition, the device will
continue its “toggling” behavior.
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Repetitive Short Circuit Operation
Short Circuit Conditions
2.2.2
Latching Shutdown Behavior
Unlike devices with automatic restart behavior, latching devices shut down immediately once the fault condition is
detected. The device remains latched off until the microcontroller initiates a reset either by either switching of the
input signal or by clearing the fault flag of the device.
2.2.3
Status Feedback
A device with status feedback provides either a dedicated status signal or a serial interface (e.g. SPI) that reports
if there is a fault condition detected by the smart power switch. After being notified of the fault condition, the
microcontroller can be programmed to either immediately shut down the affected device or wait for a fault filter
time has elapsed before proceeding with device shutdown.
2.3
Simulated Retry Methodology of Testing Repetitive Short Circuit
In order to reflect the different stress conditions caused by different control patterns and retry strategies of the
smart switch, three different scenarios are tested.
2.3.1
Cold Repetitive Short Circuit - Short Pulse
This condition reflects the usage of the device when controlled by a microcontroller that monitors the status output
and immediately switches off the smart switch as soon as an error is reported by the status feedback signal.
Subsequent retries must then be triggered by the microcontroller. As the pause between two subsequent retries
can be large compared to the thermal time constant of the smart switch, the device may have enough time to cool
down completely. This means that the range of temperatures ∆T (difference between the ambient temperature and
the overtemperature detection threshold when it reports an error) experienced by the silicon switch under such
conditions can be rather high during each short circuit cycle. As this ∆T is a critical stress parameter for the
semiconductor device, this means that a repetitive short circuit - short pulse represents one worst-case stress per
cycle condition for the smart power device.
Because this ∆T is largest at the lowest ambient temperatures, the worst-case condition for the Cold Repetitive
Short Circuit - Short Pulse test occurs at -40 °C, generally the lowest temperature limit for automotive applications.
However, for the purpose of understanding the influence of ambient temperature on the device lifetime, an
additional test can be performed at 25 °C.
In the test setup, this control method is reproduced by turning on the device into a short circuit and switching it off
with the control input immediately after the device’s internal short circuit shutdown has been triggered. After a
given OFF-time that allows the DUT to cool down to the ambient temperature, the next cycle is performed.
2.3.2
Cold Repetitive Short Circuit - Long Pulse
Just as in the Cold Repetitive Short Circuit - Short Pulse scenario, a device in this short circuit condition
experiences a temperature increase ∆T from the ambient temperature to the overtemperature detection threshold.
But rather than immediately switching off the smart switch, a fixed fault filter time (300 milliseconds) expires after
an error is reported before the device is switched off. During the fixed fault filter time period, a device with
automatic restart behavior would engage in thermal toggling. The ∆T during thermal toggling is much lower than
during the start of the short circuit event because the device is not allowed to completely cool down to the ambient
temperature between toggle cycles. However, because toggling causes a high number of cycles in a short amount
of time, the effect of this behaviour must also be determined.
As mentioned, during this test, devices with automatic restart behavior undergo thermal toggling according to the
temperature hysteresis of its thermal shutdown. Taking this into consideration, the ambient temperature has only
a minor influence on the ∆T during this period. However, because the ∆T at the beginning of the short circuit event
is identical to the Cold Repetitive Short Circuit - Short Pulse test, the worst-case ∆T also occurs at the lowest
ambient temperatures. Therefore, an ambient temperature of -40 °C must also be applied in the Cold Repetitive
Short Circuit - Long Pulse test.
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Repetitive Short Circuit Operation
Short Circuit Conditions
In the test setup, this control method is reproduced by turning on the device into a short circuit and, after the status
feedback signal indicates an error and the fixed fault filter time elapses, the device is shut off. After a given OFFtime that sufficiently allows the DUT to cool down to the ambient temperature, the next cycle is performed.
In some devices, there can be a long delay between the onset of the short circuit event and the reporting of the
error by the status feedback signal. If this delay is longer than the fixed fault filter time, then the fault filter time
plays an insignificant role in the total amount of time that elapses after the short circuit event. Under these
circumstances, the Repetitive Short Circuit - Long Pulse test is sufficient.
Smart Switches with automatic restart behavior
For smart switches with an automatic restart behavior, this condition is modelled by keeping the device on with a
short circuit at the output. The automatic restart behavior will lead to a continuous sequence of shutdown and
restart toggling by the output with a restart frequency predominantly determined by characteristics of the DUT
itself. The toggling occurs until the fault filter time elapses and the input signal instructs the device to switch off.
Smart Switches with latch-off behavior
For smart switches with a latching shutdown behavior, the DUT does not automatically restart after a short circuit
shutdown. Instead, it remains in OFF-state until it is reactivated by an external signal. Because such devices do
not engage in "thermal toggling", the device behaves identically in both the Short and Long Pulse tests. Therefore,
the Repetitive Short Circuit - Short Pulse test already represents the worst-case test condition for this class of
devices.
2.3.3
Hot Repetitive Short Circuit
This test condition reflects the situation if the microcontroller keeps the device in ON-state under a short circuit
condition and, in the event of an overtemperature fault, allows the device to toggle for an extended period due to
its own internal automatic restart behavior. The ∆T under such a condition is much lower than for the Cold
Repetitive Short Circuit conditions because the device is not allowed to completely cool down to the ambient
temperature. However, the internal toggling behavior causes a high number of cycles in a short amount of time.
The effect of such a condition must also be determined.
In hot repetitive short conditions, the device toggles according to the temperature hysteresis of its thermal
shutdown. The ambient temperature has only a minor influence on ∆T and the average temperature of the device
during a short circuit event. Based on this reasoning, Hot Repetitive Short Circuit testing at an ambient
temperature of +25 °C is sufficient.
Certain smart power switches, in the event of a short circuit event, enter a latched shutdown state. Because they
do not automatically restart after a thermal hysteresis, Hot Repetitive Short Circuit testing is not relevant for this
category of devices.
2.4
Determining the Worst-case Short Circuit Condition
Due to the variety of implemented reactions to short circuit events, the true worst-case condition can vary for each
individual smart power switch. To streamline the testing process and derive the most useful results, it is necessary
to determine which short circuit testing condition is the worst-case for each unique device.
2.4.1
Does the Device Have a Latching Shutdown or Automatic Restart Behavior?
Both the cold repetitive short circuit - long pulse and hot repetitive short circuit tests are not relevant for devices
with a latching shutdown behavior. Because the device proceeds into latching shutdown once a short circuit event
is detected, the device does not require an OFF command from the input signal before turning itself off. As a result,
the presence of a fixed fault filter time in the Cold Repetitive Short Circuit - Long Pulse test does not contribute
any additional stress for devices in latched shutdown. For the same reason, the hot repetitive short circuit test is
also irrelevant, as the device ignores the ON command from the microcontroller after it enters its latched shutdown
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Repetitive Short Circuit Operation
Short Circuit Conditions
state. Therefore, the only relevant test for devices with a latching shutdown behavior is Cold Repetitive Short
Circuit - Short Pulse.
For devices that have an automatic restart behavior, further information is necessary to determine worst-case
condition and the proper method of testing.
2.4.2
Does the Device Have a Status Feedback Signal?
Though many smart power switches have a dedicated status feeback signal, some devices lack this feature. For
such devices, a cold repetitive short circuit - short pulse test is not relevant, as, without any instantaneous
feedback signal, there may be no way for the microcontroller to immediately ascertain the presence of an error
condition. In the event the application includes an external diagnosis circuit that checks for and reports a short
circuit condition, the Cold Repetitive Short Circuit - Long Pulse test would be valid. If no external diagnosis circuit
is present, due to the lack of any feedback signal, it is assumed that the microcontroller will take no responsive
action to any fault condition. In this case, the Hot Repetitive Short Circuit test is the most relevant.
For devices that have both automatic restart behavior and a status feedback signal, further information is
necessary to determine the proper course of testing.
2.4.3
Does the Status Signal Have a Delay?
For many smart power switches, the delay between when the device first detects the short circuit and when the
fault condition is reported by the status feedback signal is very short (typically 100µs). For such devices, the Cold
Repetitive Short Circuit - Long Pulse test presents the worst-case scenario. As another reference point to ascertain
the effect of the fixed fault filter time on the device, a Cold Repetitive Short Circuit - Short Pulse test can also be
performed.
However, there exist some devices that have have a significant delay before the status signal indicates a fault
condition. To streamline the testing process of such devices, for all devices with status signal delays longer than
1.5 seconds, the Cold Repetitive Short Circuit - Long Test is the only relevant test.
2.4.4
Worst-case Short Circuit Decision Tree
The preceeding arguments for which test is the most relevant for differing devices is summarized in the decision
tree shown in Figure 5.
Application Note
10
V1.0, 2006-03-20
Repetitive Short Circuit Operation
Short Circuit Conditions
Latching
Latching
or Autorestart
Behavior?
Autorestart
Cold Repetitive
(Short Pulse)
T amb = -40˚C / 25˚C
Status Signal
Feedback Available?
No
Yes
Hot Repetitive
T amb = 25˚C
Delayed Status
Feedback Signal?
(>1.5s after event)
No
Cold Repetitive
(Long Pulse)
T amb = -40˚C
Yes
Cold Repetitive
(Long Pulse)
T amb = -40˚C
And
Cold Repetitive
(Short Pulse)
T amb = -40˚C / 25˚C
Figure 5
Decision tree used to determine the worst-case condition to test smart power switches
Application Note
11
V1.0, 2006-03-20
Repetitive Short Circuit Operation
Test Method
3
Test Method
To properly test devices according to the specifications outlined in the previous section, a test setup was
constructed to fulfill the following requirements:
•
•
•
Fixed primary impedance values
Variable secondary impedance values
Variable ambient temperature
Because large amounts of data are required to provide sufficient statistical significance for lifetime calculations,
the test setup must also have the following capabilities:
•
•
Parallel test of multiple (up to 512) devices
Real-time detection of non-conformance of each individual device to properly record the exact number of
cycles to non-conformance (CTNC)
Lastly, the data must be analyzed to determine its statistical significance. This analysis can then be used by
system designers to ensure ECU reliability.
3.1
Test Hardware
Devices are assembled on PCBs with a construction and layout to replicate application conditions, such as the
example shown in Figure 6. The individual PCBs are inserted into sockets on a main test board, which also carries
other external components. These components include power resistors and air coils, used to model the primary
and secondary impedances. An example test board is shown in Figure 6. Test boards are then loaded into a
temperature chamber and controlled with an automated test setup that is able to monitor up to 64 DUTs
simultaneously. The short circuit condition is performed with an electronically-controlled short circuit switch
capable of fast slew rates. Real-time computer-controlled monitoring for both open and short circuit conditions at
the output is implemented by measuring the current flowing through each device.
Figure 6
Images of test hardware used for short circuit testing. On the left, an example PCB using a
layout that replicates application conditions. On the right, an example test board, complete
with air coils to model primary and secondary impedances.
3.1.1
Sample Size
Samples for repetitive short circuit testing are drawn from three independent lots. At least 10 DUTs per lot per test
are used. This sample size is large enough to guarantee the statistical validity of the data.
Application Note
12
V1.0, 2006-03-20
Repetitive Short Circuit Operation
Test Method
3.1.2
Test Termination Criteria
Non-conformance is defined as the state of a device that is incapable of turning off (electrical short) or turning on
(open circuit). All tests are conducted until either more than 50% of the DUTs have reached non-conformance or
a certain maximum number of cycles has been tested (typically 1000 hours or 100,000 cycles).
If the test is terminated before at least 50% of all DUTs have reached non-conformance, all remaining functional
devices are then subjected to testing by the production test program. This testing determines if the remaining units
conform to the device specification limits. Devices that are outside of the specification limits are counted as nonconforming devices for statistical analysis.
3.2
Test Procedures
Throughout testing, devices under test are exposed to a fixed short circuit condition at their outputs. The
parameters of the short circuit are kept constant during testing.
3.2.1
Cold Repetitive Short Circuit - Short Pulse
This test is designed to stress devices with integrated protection circuitry that are externally controlled with a
microcontroller which attempts to restart the smart power switch in the event of a fault condition.
Figure 7 shows the input and resulting output signals from such a test. At time point A, the DUT is turned on into
a short circuit and, at time point C, switched off at the control input after a fixed ON-time. The ON-time (point B to
C) is chosen long enough to guarantee that the integrated short circuit protection of the device will shut down the
output and is chosen so that a few thermal toggling events (for devices with automatic restart behavior) may also
be included. This ON-time is typically 10 milliseconds.
The input is turned off after the chosen ON-time has elapsed. After a fixed OFF-time (typically 10 seconds), this
procedure is repeated, as shown at time point D. The OFF-time is chosen to be long enough to allow the DUT to
cool down to the ambient temperature of the temperature chamber. Both ON- and OFF-times are experimentally
determined for each device beforehand. One turn-on event plus the subsequent off-time is counted as one cycle.
During each cycle, the DUT is checked for open or short at the output. If either an open or short is detected, the
number of cycles the DUT has undergone is recorded as the cycles-to-non-conformance (CTNC) for this sample.
IN
Iout
Status
A
B
C
D
Figure 7
Timing diagram of Cold Repetitive Short Circuit - Short Pulse
3.2.2
Cold Repetitive Short Circuit - Long Pulse
Unlike the Cold Repetitive Short Circuit - Short Pulse test, the Cold Repetitive Short Circuit - Long Pulse test waits
until a fault filter time period has elapsed before switching off the input signal. During this time, the integrated
overtemperature protection function initiates thermal toggling of the device. Figure 8 shows the input and output
signals from such a test on a device with auto-restart behavior. Time point A and B show the first turn-on and turnoff of the output, respectively. Time point B is also when the Status signal of the device makes a high-to-low
transition, indicating to the microcontroller that a fault event has occured. The period between time point B and C
Application Note
13
V1.0, 2006-03-20
Repetitive Short Circuit Operation
Test Method
is the fixed fault filter time, during which the device is in thermal toggling. At time point C, the input signal is
switched off. After the same OFF-time as used in the Cold Repetitive Short Circuit - Short Pulse test (typically 10
seconds), the input signal is again turned on at time point D and the procedure is repeated. One turn-on event plus
the subsequent off-time is counted as one cycle. Just like in the Cold Repetitive Short Circuit - Short Pulse test, a
check for open or short at the output is performed during each cycle and, if a malfunction is detected, the CTNC
for the sample is recorded.
A fixed fault filter time of 300ms was chosen because it reflects an application-relevant maximum waiting time
before a microprocessor in an automotive application shuts off a device that reports an error. Although a system
designer can choose a shorter fault filter time for an actual application, because longer times cause more thermal
toggling and, consequently, more stress for the device, this is the worst-case condition.
As discussed in Section 2.4.1, because latched shutdown devices do not engage in thermal toggling, they are
excluded from Cold Repetitive Short Circuit - Long Pulse testing.
IN
Iout
Status
A
B
C
D
Figure 8
Timing diagram of Cold Repetitive Short Circuit - Long Pulse
3.2.3
Hot Repetitive Short Circuit
Unlike the Cold Repetitive Short Circuit tests, the Hot Repetitive Short Circuit test does not toggle the input signal.
Rather, the device remains constantly ON in a short circuit condition, leading to thermal toggling of the DUT by the
device’s integrated overtemperature protection function. Figure 9 shows the input and output signals from such a
test. Time point A and B show the first turn-on and turn-off of the output, respectively. Time point C and D show
the turn-on and turn-off events after the DUT has settled into a state of constant repetitive shutdown and restart
cycling. Each turn-off event of the DUT is counted as one cycle and the cycle period is recorded and included in
the test report. This repetitive cycling continues for the entire duration of the testing and the output current is
always monitored. The device is continuously checked for open and short at the output. As soon as nonconformance is recognized, the number of cycles the DUT has undergone is recorded as the cycles-to-nonconformance (CTNC).
Application Note
14
V1.0, 2006-03-20
Repetitive Short Circuit Operation
Test Method
IN
Iout
A
B
C D
Figure 9
Timing diagram of Hot Repetitive Short Circuit for devices with Automatic Restart
3.3
Statistical Analysis
The degredation mechanisms that limit the repetitive short circuit lifetime are generally described in literature by
a log-normal distribution. Therefore, the CTNC gathered from the test setup will likewise also fit a log-normal
distribution. An example of a plot containing actual test data fitted to a log-normal distribution is shown in
Figure 10. From this plot, it is possible to ascertain the mean CTNC for a given non-conformance rate, such as
100 or 1000 parts per million (ppm). This can be achieved by following the best-fit line of the gathered data to the
desired ppm level. An example document that summarizes the results from Figure 10 can be found in Figure 11.
Cumulative
Failures
99%
BTS740S2 Cold Repetitive Short Circuit Test Result
95%
84%
(+sigma)
50%
(mean)
-40°C P1
16%
(-sigma)
-40°C P3
5%
+25°C P1
1,0%
+25°C P3
1000ppm
100ppm
10ppm
1ppm
1e3
Figure 10
2
3
5
2
3
1e4
Cycles to Non-Conformance (CTNC)
5
1e5
2
Plot showing log-normal distribution of test data from BTS 740S2. The four data series show
test results from cold repetitive short circuit - short pulse at -40°C and +25°C and at two
different secondary impedances, referred to as “P1” and “P3”. P1 represents a short at the
ECU (20 mΩ, 0 µH), while P3 represents a short after 5m cable (100 mΩ, 5 µH). Note that ppm
rates can be ascertained by extrapolating the best-fit line to the desired ppm level, as
indicated by the arrows.
Application Note
15
V1.0, 2006-03-20
Repetitive Short Circuit Operation
Test Method
BTS 740S2 Repetitive Short Circuit Test Result
preliminary V0.5
1
Repetitive Short Circuit Test Result
1.1
Background and Additional Information
This document contains the result of Infineon’s standard test for repetitive short circuit
robustness. The device under test is the BTS 740S2.
The BTS 740S2 is an automatic restart device with status feedback. Therefore, the Cold
Repetitive Short Circuit - Short Pulse and Cold Repetitive Short Circuit - Long Pulse tests
are the most relevant for this device.
For details on test setup, test procedure and statistical data analysis, please refer to the
Application Note “Repetitive Short Circuit Operation”, March 2006.
1.2
Test Results
BTS 740S2
Vbat = 14 V, RECU = 20 m:, Lmax = 5 PH, RLmax = 100 m:
Refer to Figure 1 and Figure 2 for definition of test condition parameters
Test Procedure
Sample
size
Tamb
ppm-level
Cycles to NonConformance
Cold Repetitive Short Short Pulse
30
25°C
100
3,4 x 104
1000
4,0 x 104
100
5.5 x 103
1000
7,9 x 103
30
-40°C
Cold Repetitive Short Long Pulse
–
-40°C
100
N/A
1000
N/A
Hot Repetitive Short
60
25°C
100
1,4 x 108
1000
1,6 x 108
Average toggle period during hot repetitive short test is 20 ms (DUT on test-PCB with
6 cm2 cooling area with forced convection, the toggling period may be different in the
application)
Repetitive Short Circuit Test Result
Figure 11
1
preliminary V0.5 2006-03-20
First page of an example document that summarizes results from repetitive short circuit
testing of the BTS 740S2. Note that the worst-case values out of the P1 and P3 results shown
in Figure 10 are used. “Figure 1” and “Figure 2” of the example document correspond to
Figure 1 and Figure 4 of this application note.
Application Note
16
V1.0, 2006-03-20
Repetitive Short Circuit Operation
Conclusion
4
Conclusion
As a service to our customers, Infineon Technologies provides an analysis of the effect of repetitive short circuit
on the lifetime of smart power switches under the test conditions described in this document. The intention is to
enable customers to create reliable ECU designs. The data is generated with a novel test method developed
specifically for this purpose, summarized by the points below:
•
•
•
•
•
•
•
A physical model of a short circuit is recreated based on conditions present in typical automotive applications.
If a certain range of values is required, the worst-case value is always selected.
In order to reflect the different stress conditions caused by different control patterns and retry strategies of the
smart switch, three different testing scenarios are tested.
The worst-case testing scenario is chosen based on the reaction of the smart switch in the event of a short
circuit, summarized in Figure 5.
At least 30 devices from three independent lots are subjected to each testing scenario.
Devices are tested until they reach non-conformance (electrical short or open circuit). Devices outside of the
specification limits are also considered non-conforming. The cycles-to-non-conformance (CTNC) of each
tested device is recorded.
The CTNC data is fitted to a log-normal distribution, such as the one shown in Figure 10. From this plot, it is
possible to ascertain the mean CTNC for a given non-conformance rate.
The non-conformance rates of 100 and 1000 parts per million (ppm) are published for customers in the form
of the document shown in Figure 11.
With test results and statistical analysis provided by this test method, the customer can now define his/her software
retry strategy to ensure a required level of ECU robustness in their respective application.
Visit us at: www.infineon.com/power
Application Note
17
V1.0, 2006-03-20
Template: ap_a4_tmplt.fm / 1 / 2005-04-01
Edition 2006-03-20
Published by Infineon Technologies AG,
St.-Martin-Strasse 53,
81669 München, Germany
© Infineon Technologies AG 2006.
All Rights Reserved.
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