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Texas Instruments Designing the front-end DC/DC conversion stage to withstand automotive transient Application notes
Analog Applications Journal
Automotive
Designing the front-end DC/DC conversion
stage to withstand automotive transients
By Vijay Choudhary
Systems and Application Engineer, Power Product Solutions
Introduction
Introduction to automotive transients
With rapidly expanding electronic content in the latest
generation of cars, there is an ever increasing need for
power conversion from the car’s battery rail. The 12-V
battery rail is subject to a variety of transients. This presents a unique challenge in terms of the power architecture
for off-battery systems.
This article introduces the types of transients that occur
in automotive battery rails, the causes of those transients,
and the standards and specifications defining the test
conditions for those transients. Different power architectures are covered for power-conversion and protection
circuits to ride out the transients and minimize power
interruption to the loads. Included are the advantages and
trade-offs associated with buck-boost, boost, and pre-boost
approaches for surviving cold-cranks and load dumps. Also
presented are different approaches for reverse-polarity
protection, which includes a comparison of smart diodes
to alternate methods. This information can equip the
designer with a deeper understanding of automotive transients and the approaches to tackle these transients when
designing the power conversion stage.
A variety of factors are responsible for the battery-rail
transients in automotive systems . The purpose of the
front-end power stage is to insulate the sensitive electrical
and electronic loads from these wide variations and to
power the loads with a conditioned voltage rail. Because of
a large number of different vehicles, and the varied conditions of operation, it may be difficult for the designer to
foresee every potential transient that will occur on the
battery rail to a module. This means that a variety of
testing standards must be used to determine the requirements for power conditioning.
To address this concern, many original equipment
manufacturers (OEMs) and organizations describe the
immunity tests and the standardized test conditions for
off-battery loads. A number of these tests are summarized
in ISO 16750-2 and ISO 7637-2 standards.[1, 2] However,
many of the extreme transients are taken care of using the
transient protection shown in Figure 1. Subsets of these
stresses that are often tackled in the power-stage design,
Figure 1. Front-end power conditioning circuit
Automotive Front-End Power Stage
Battery:
12 or 24 V
Reverse
Polarity
Protection
Transient
Protection
Texas Instruments
EMI
Filter
DC/DC
Converter
3 to 42 V
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Analog Applications Journal
Automotive
Table 1. Electrical stresses and their origins[1-3]
in addition to their physical origins, are summarized in
Figure 2 and Table 1. The ISO standards and a few
OEM-specific documents describing these tests are referenced in Table 1.
Test
Starting
profile
Simulates the disturbances ISO 16750-2 (sec 4.6.3),
during and after cranking. FMC1278 CI 230-231
Noise
Residual voltage ripple due
to rectified sinusoid from a ISO 16750-2 (sec 4.4)
generator.
Superimposed
Superimposed pulses
AC
FMC1278 CI 210, 220,
simulate sudden highGMW3172,
current loads switching on
BMW E-06
the battery rail.
Jump Start
Nominal
Reference Document
Load dump
Figure 2. Stresses on automotive battery rail
Load
Dump
What it Simulates
Battery disconnection with
alternator running with the ISO 16750-2 (sec 4.6.4),
other load remaining on
FMC1278 CI 222
the alternator rails.
Crank
Reverse
Polarity
Noise
Designing the power conversion stage
Reversed
voltage
Reversed battery connection when using an
auxiliary starting source.
ISO 16750-2 (sec 4.7)
Jump start
DC voltage overstress due
to a generator failure or
jump start using a 24-V
battery.
ISO 16750-2 (sec 4.3),
FMC1278 CI270
The DC/DC conversion stage must be able to withstand
voltages of up to ~42 V (for 12 V battery) during load
dumps and must be able to supply power to the load
during cold-crank, which can be lower than 4 V (Figure 3).
The DC/DC converter that needs to regulate the output
voltage within this range must be able to step down under
high-rail conditions and step up under low-rail condition.
Additionally, the designer must design the reverse-polarity
protection circuit to prevent or limit the damage in case of
an accidental reverse-polarity connection.
Figure 3. Voltage requirements for automotive
power-conversion stage
42 V
Load Dump
24 V
Overvoltage and Jump Start
Battery Voltages
16 V
Input to
DC/DC
converter
Normal Operating Range
8V
Warm and Cold Crank
4V
0V
Reversed Polarity Connection
Blocked by
reverse-polarity
protection
circuit
–14 V
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Automotive
Boost + buck power stage
jump-start conditions. The following buck stage must be
rated for the full load-dump voltage, which is usually
around 42 V in most practical designs. This results in the
size and cost of two stages that are both rated for wideinput voltage and full-load currents.
An additional cost of having two stages is the inherent
double conversion in this architecture where both stages
incur switching as well as conduction losses. This double
conversion happens all the time, even when the battery
voltage is within operating range and only step-down
conversion would have been otherwise sufficient. To avoid
this extra power loss due to the always-on boost stage in
Figure 4a, a smarter approach is shown in Figure 4b that
uses an on-demand boost stage. The on-demand boost is
normally in a bypass-mode as shown by the red dashed
line in Figure 4b, and only starts switching when the
battery voltage falls below a pre-determined value based
on the drop-out characteristic of the following buck stage.
Since the boost converter is off most of the time, this
Figure 4 includes advantages and limitations for a few
approaches to implement off-battery DC/DC conversion.
One approach is to use a boost converter as the first DC/DC
stage to create a higher voltage rail (Figure 4a). This is
followed by a second DC/DC stage, which is a wide-VIN
buck converter. The boost action facilitates disruption-free
operation when the battery-rail voltage drops too low, for
example during cranking. The buck stage then steps down
the voltage to the appropriate level. An important advantage of this approach is that the boost-input inductor
current has relatively small ripple and it provides significant reduction in the ripple current going back to the
battery rail. This reduces the attenuation required in the
electromagnetic interference (EMI) filter, which means
the size and cost of the EMI filter are lower.
A limitation of the boost front stage is that while it
levels the dips in the battery rail voltage, it has no capability to limit the spikes, for example, during a load-dump or
Figure 4. Approaches to off-battery DC/DC conversion
Prevents dips but
not excursions. All
components 42 V
VBAT
Boost
Converter
Reverse Polarity
Protection
3 V to 42 V
16/24 V
16 V to 42 V
LM5022/
LM5122
Smart Diode
LM74610
Audio Power
(for example)
Wide-V IN
2-MHz Buck
8 V,
5 V,
3.3 V
LM5140
(a) Always-on boost + buck
Prevents dips but
not excursions. All
components 42 V
“On demand” boost.
Needs extra circuit
for bypass
V BAT
Reverse Polarity
Protection
3 V to 42 V
Boost
Converter
8 V to 42 V
LM5022/
LM5122
Smart Diode
LM74610
Wide-V IN
2-MHz Buck
5 V,
3.3 V
LM5140
(b) On-demand boost + buck
V BAT
Reverse Polarity
Protection
3 V to 42 V
Smart Diode
LM74610
Buck-Boost
Converter
Stable Rail
5 V/8 V/12 V/16 V
LM5175
Buck
5 V,
3.3 V,
1.8 V
Lower voltage
(<20 V)
(c) Buck-boost DC/DC stage
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Automotive
saves the switching losses in the boost stage. The boost
converter must respond quickly enough to prevent the
load input voltage from dropping too low. Additional
circuitry may be needed to sense the battery drop and
switch over from bypass to boost-on mode.
Since the on-demand boost is only expected to switch
when battery voltage drops, this architecture is suitable
only for relatively lower-voltage rails, such as 5 V, 3.3 V, in
other words, well below the normal range of battery voltage.
leg (buck or boost) is switching in a cycle, it avoids the
higher losses associated with a two-stage conversion.
Unlike a boost pre-regulator, which only lifts the output
voltage for low VIN but cannot clamp the output voltage
below VIN, the buck-boost provides immunity against both
dips and excursions in input voltage. For automotive applications with the output voltage above the nominal battery
range (≥16 V), the buck-boost converter offers low ripple
at the input and provides overload and short-circuit protection, as well as inrush current limiting. A buck-boost power
stage also eliminates the need for bulky low-frequency
passive filters otherwise required to suppress the superimposed alternative voltage that happens on the 12-V battery
rail due to rectification of alternator AC output.
For regulated outputs below the nominal battery voltage
(5 V, 3.3 V), the buck-boost topology provides a singlestage solution with higher efficiency than the pre-boost +
buck architecture. However, the size advantage of a singleinductor buck-boost is somewhat tempered by the fact
that it typically requires a larger EMI filter.
For automotive systems, the buck-boost converter of
Figure 5 is an ideal pre-regulator. This converter combines
the benefits of a boost-converter front stage, such as lowinput ripple (for 16- to 24-V output range, Figure 4c) and
cranking protection. This converter also includes loaddump protection (VIN excursions) and overcurrent/shortcircuit protection typically associated with a buck
converter. Additionally, it offers true input-output disconnection when in shutdown mode.
Buck-boost power stage
Buck-boost converters facilitate single-stage conversion to
handle the wide-range battery voltage (Figure 3) on the
input and provide a regulated rail at the output. A number
of different topologies are used for buck-boost conversion.[4]
The example in Figure 4c shows the LM5175 four-switch
buck-boost converter because of its higher efficiency and
power-handling capabilities.
A wide-VIN four-switch DC/DC converter can both step
up and step down the input voltage and is able to regulate
the output, even when the input voltage is equal to the
output voltage. The simplified diagram and switching
waveforms are shown in Figure 5. When the input voltage
is higher than the target output, it operates in buck-mode
with the output stage in the pass-through mode. When the
input voltage is lower than the target output, it operates in
boost mode with the input stage is in the pass-through
mode. When VIN is close to VOUT, it interleaves buck and
boost cycles to maintain smooth operation. Since only one
Figure 5. Wide-VIN four-switch buck-boost converter
V OUT
V IN
QH1
SW1
V IN
SW2
V OUT
SW1
SW2
V IN
V OUT
SW2
V OUT
SW1
V IN
Buck
QH2
SW1
SW2
QL1
QL2
LM5175
Texas Instruments
Buck- Boost
Boost
Only one switching event per
switching cycle
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Automotive
Reverse-polarity protection
Figure 6. Reverse-polarity protection methods
A reverse-polarity protection circuit is needed in the front
end to protect the components connected to battery rail
from negative voltage, which can result from improper
connection of an external power supply to start the
vehicle. Many approaches are taken in automotive systems
to prevent reverse-current damage, ranging from fuses,
Schottky diodes, p-channel field-effect transistors (PFETs),
and n-channel FETS (NFETs) as shown in Figure 6.
For lower current applications, a simple Schottky diode
can be used for reverse-polarity protection. PFETs can
handle higher current, but the driver circuit usually
requires a pull-down resistor and a zener clamp that dissipates power. Furthermore, PFETs have inferior RDS(on)
characteristics compared to NFETs and usually are more
expensive. Smart-diode controllers combine the best
performance of an n-channel MOSFET with the simplicity
of a diode connection.
Schottky Diode
Load
–
• Simplest but higher loss
• Limited to low currents
(a) Schottky diode
PFET
+
Accidental
Reverse
Polarity
Connection
Conclusions
The front-end power-conversion stage for automotive offbattery applications must deal with a wide voltage variations on the input-voltage or battery rail. The tests to
simulate these variations are covered in automotive standards and OEM-specific documents. Examples of the
stress tests that are required in the power-stage design are
reverse-polarity connection, cold and warm cranks during
engine start/re-start, load dump, and superimposed AC
within the nominal battery voltage range. The positive
voltage transients and operating-voltage variations on the
battery rail necessitate the use of DC/DC converters with
a wide-input voltage rating to regulate or pre-regulate the
bus. Depending on the load and sub-systems being
powered, designers can design the power stage using a
pre-boost, pre-boost and a buck, or a single-stage buckboost converter. A four-switch, buck-boost converter
provides the best combination of versatility, small size, and
high efficiency. There are many approaches to reversepolarity protection but smart diodes provide the best
performance and a simple design.
.
Load
–
• PFET has poor specs and higher cost
• Needs drive resistors and zener clamp
(b) PFET switch
Smart Diode
Anode
Accidental
Reverse
Polarity
Connection
Cathode
+
Load
LM74610
–
• NFET provides lower RDS(on) and lower loss
(c) Smart diode
References
5. Vijay Choudhary and Mathew Jacob, “Smart Diode
and 4-Switch Buck-Boost Provide Ultra High
Efficiency, Compact Solution for 12-V Automotive
Battery Rail,” PCIM Europe 2016, 10 – 12 May 2016,
Vol. 1, pp 2019.
6. Matthew Jacob, “Reverse-polarity protection
­comparison: diode vs. PFET vs. a smart diode
­solution,” Texas Instruments, Behind the Wheel blog,
December 21, 2015.
1. ISO 16750-2: Road vehicles—Environmental conditions
and testing for electrical and electronic equipment.
2. ISO 7637-2: Road vehicles – Electrical disturbances
from conduction and coupling Part 2: Electrical
­transient conduction along supply lines only.
3. FMC1278: Electromagnetic Compatibility
Specification for Electrical/Electronic Components
and Subsystems, Ford Motor Company (FMC1278),
July 2015.
4. Vijay Choudhary, Timothy Hegarty and David Pace,
“Under the hood of a non-inverting buck-boost converter,” TI Power Supply Design Seminar 2016.
Texas Instruments
+
Accidental
Reverse
Polarity
Connection
Related Web sites
Product information:
LM5140-Q1
LM5122
LM5022
LM5175
LM74610-Q1
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Analog Applications Journal
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