By Nitin Kalje, Senior Scientist, Maxim Integrated Products,
Sunnyvale, Calif., and Greg Dygert, Strategic Applications
Engineer, Maxim Integrated Products, Brighton, Mich.
If protected against load dumps and other
transients, switching regulators with relatively
low voltage ratings can be configured to operate
efficiently at high switching frequencies.
T
Selecting the proper switching frequency is critical because a switching converter poses its own set of problems. For
example, electromagnetic radiation caused by the switching
converter can interfere with other electronics. The AM radio
receiver is sensitive to the interference between 530 kHz and
1700 kHz. The switching converter’s fundamental switching
frequency and third harmonics can contribute significant interference to other electronics around the power supply. The
even harmonics cancel each other out, and the odd harmonics higher than fifth order typically have little energy, which
is usually easy to filter out. Selecting a switching frequency
greater than 1800 kHz eliminates fundamental and other
harmonic interference from the AM frequency band.
However, a high switching frequency increases the power
loss, partly offsetting the advantage of using a switching
converter. The switching losses increase with higher input
voltages, as they are proportional to the square of the operating voltage. The automotive environment typically demands
high-voltage processes (40 V or higher) for power controller
ICs to withstand overvoltage transients such as load dump.
High-voltage processes use relatively larger geometries and
he increasing sophistication of electronic systems
in automobiles presents unique challenges and
opportunities for power system designers. Most
automotive modules need low voltages like 3.3 V
and 5 V. The voltage conversion from battery to
such lower voltages using linear regulators means a significant power dissipation. High power dissipation makes the
thermal management difficult and expensive. The higher
power requirement of faster processors and ASICs have
steered the power-conversion method from simple, lowcost, inefficient linear regulators to the more complex but
higher-efficiency switching converters.
The size of a switching converter depends on the switching frequency. The passive components like power inductors and capacitors become smaller with higher switching
frequency. These high-efficiency converters reduce power
dissipation by eliminating bulky and expensive heatsinks.
The entire power supply can shrink significantly when using
switching converters. These advantages make the switching
converter an obvious choice for power management of body
electronics, infotainment and engine-control modules.
Power Electronics Technology September 2006
14
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Fig. 1. In this conventional voltage-limiting protection circuit, the p-channel MOSFET blocks voltages higher than the breakdown voltage of Z2.
Pulse type
VP
Pulse 1
Pulse 2a
Pulse 3a
Pulse 3b
Pulse 5a
ISO 7637-1
OEM #1
OEM #2
OEM #3
OEM #4
OEM #5
OEM #6
OEM #7
-75 V to -100 V
-100 V
-100 V
-100 V
-150 V
-100 V
-100 V
-80 V
TD
2 ms
2 ms
2 ms
2 ms
2 ms
5 ms
50 µs
140 ms
RS
10 
10 
10 
10 
10 
25 
10 
5
VP
50.5 V to 63.5 V
163.5 V
+50 V
+100 V
+75 V
+200 V
110 V
TD
50 µs
50 µs
50 µs
50 µs
50 µs
2 ms
5.7 µs
RS
2
4
4
10 
2
10 
0.24 
VP
-98.5 V to 136.5 V
-300 V
-150 V
-150 V
-112 V
-150 V
-260 V
TD
100 ns
50 µs
100 ns
100 ns
100 ns
-150 V
-260 V
34 
RS
50 
4
50 
50 
50 
50 
VP
88.5 V to 113.5 V
+100 V
+100 V
+100 V
+75 V
+100 V
TD
100 ns
100 ns
100 ns
100 ns
100 ns
100 ns
RS
50 
50 
50 
50 
50 
50 
VP
78.5 V to 100.5 V
73.5 V
32 V
113.5 V
82.5 V
80 V
TD
40 ms to 400 ms
150 ms
400 ms
400 ms
250 ms
120 ms
RS
0.5  to 4 
0.5 
0.5 
0.5 
0.5 
2.5 
Table. Examples of typical OEM conducted immunity requirements.
and protect electronic circuitry from excessive voltage conditions that are conducted via electrical connections to the
automotive electrical system, in particular those connections to the main voltage supply. The ability to withstand
conducted disturbances is generally known as conducted
immunity (CI).
Automotive manufacturers and standards organizations
specify various test methods to evaluate the CI of electronic
components and systems. While each automotive OEM tends
to have specific requirements, the ISO7637 standard provides
the basis for many of these. The following is not meant to
be a comprehensive description of all CI requirements, but
rather a brief summary of the typical OV conditions relevant
to automotive applications.
higher gate thicknesses. The resulting longer channel lengths
lead to longer propagation delays. Obviously, high-voltage
processes are inherently slow and could be very inefficient
as the transition losses increase due to longer rise and fall
times of the switch.
Certain fabrication processes at Maxim are suited for
extremely high-speed converters at moderate voltage levels.
A recent example is the MAX5073, a dual-output 2.2-MHz
buck or boost converter that can tolerate up to a 23-V input.
An effective switching frequency of 4.4 MHz is achieved by
using ripple phase operation. The switching converters are
supposed to be immune to the interference present on the
power source. As far as automotive applications are concerned, high-voltage controllers are not an absolute necessity when designing these switching converters. This article
describes the most common automotive power disturbances
and ways to protect the low-voltage electronics from them.
Steady-State OV Conditions
Certain OV conditions are of long enough duration to be
considered steady-state from an electronic circuit standpoint.
For example, any OV condition that persists comparatively
longer than the thermal time constant of an electronic de-
Power Line Overvoltage Stress Conditions
Overvoltage (OV) protection devices are used to isolate
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15
Power Electronics Technology September 2006
AUTOMOTIVE CONVERTER
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Fig. 2. The MAX6398
X6398 includes an internal charge pump to drive an external n-channel MOSFET gate above VBAT to overcome the device’s gatesource threshold voltage.
vice can be considered steady-state. In these situations, the
continuous power dissipation and resulting temperature rise
are of primary concern. Conditions that can be considered
steady-state include a failed alternator regulator, a doublebattery jump-start and reverse battery connections. The
following is a brief description of these conditions.
The output of the alternator is regulated with respect to
speed, load and temperature by regulating the magnitude of
the current in the field winding. This function is typically
provided by an electronic circuit (that is, voltage regulator)
that pulse width modulates the field winding to achieve a
constant, regulated output voltage from the alternator. The
output set point of the voltage regulator is typically about
13.5 V. It is possible for the alternator voltage regulator to
fail in such a way as to provide full field current irrespective
of load or output-voltage conditions. When this happens,
voltage in excess of the typical 13.5 V may be applied to the
entire system, with the actual voltage level being dependent
on vehicle speed, loading and other conditions. The typical
OEM failed-regulator test requirement is on the order of
18 V for 1 hour. Most systems are required to operate under
these conditions, although certain comfort or convenience
functions are allowed to deviate.
Power Electronics Technology September 2006
Another OV condition that is effectively steady-state is the
double-battery jump-start. This typically occurs when a tow
truck or service station uses a 24-V system to jump-start a
disabled vehicle or charge a dead battery. The typical OEM
double-battery test requirement is on the order of 24 V for
2 minutes. Certain engine management and safety-related
systems are required to operate under these conditions.
It is possible for a steady-state reverse potential to be
applied to the vehicle electrical system during manufacturing or service. In general, most systems are required to
survive but not operate under this condition. The typical
requirement is –14 V for 1 minute. This can be a challenging
requirement for systems with high current or low-voltage
drop input requirements.
Transient OV Conditions
The majority of transient OV conditions in an automobile are due to switching inductive loads. Examples of such
loads include the starter motor, fuel pump, window motors,
relay coils, solenoids, ignition components and distributed
circuit inductances. Whenever current is interrupted in these
inductive loads, an OV pulse will typically be produced.
Due to the amplitudes and durations involved, filters, metal
16
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AUTOMOTIVE CONVERTER
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Fig. 3. A pulse 5 (80 V, 120 ms, Toyota OEM #5) CI waveform, as shown in the table, is applied at the battery-connected input of a protector
(the MAX6398). The output of the protector, and outputs 1 and 2 of the dual buck converter are monitored as follows: CH1 = VBAT, CH2 = VPROT,
CH3 = Output 1, CH4 = Output 2.
oxide varistors (MOVs) or transient voltage suppressors
are required for suppressing these types of OV transients. A
description of pulses based on the ISO 7637 standard, shown
in the table, is as follows:
Pulse
1 is aad
negative-going
repetitive
pulseAMranging
HVPSI
Meter
7-06 7/21/06
10:14
Pagefrom
1
–80 V to –150 V in amplitude with a duration of 1 ms to
140 ms. The source impedance is typically on the order
of 5  to 25 . Pulse 2 is a positive-going repetitive pulse
ranging from 75 V to 150 V with a typical duration of 50 µs.
The source impedance is typically 2  to 10 . Pulse 3a is a
17mm_ad 9/14/06 10:50 AM Page 1
series of negative pulses that are on the order of –150 V and
100 ns. Pulse 3b is a series of positive pulses on the order
HVPSI’s digital
high voltage meter
(supplied with a 4’ 22
AAWG, 40kV, UL 3239
high voltage cable with
male connector and alligator
clip termination) is ideal
� Maximum Input
for performing laboratory
Voltage: 40,000 VDC
measurements, troubleshooting
high voltage circuits and
� Input Impedance:
monitoring very high impedance
10,000 M Ω
circuitry supplies for intensifiers,
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Accuracy: 1% ±1 count
electronic antistatic devices,
� Input Voltage: 120 VAC
ionizers, etc.
or 240 VAC, 50/60 hz
HIGH VOLTAGE POWER SOLUTIONS, INC.
P Box 700968 � Dallas, TX 75370
P.O.
(972) 248-7691 � (972) 381-9988 FAX
e-mail: sales@hvpsi.com � www.hvpsi.com
Power Electronics Technology September 2006
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AUTOMOTIVE CONVERTER
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of 100 V and 100 ns. The impedance of the signal source is
typically 50 .
Pulse 5, also known as a load dump, is a condition that
occurs when an alternator is supplying high current to a
discharged battery and the battery is suddenly disconnected.
Since the alternator is a magnetic device, the sudden reduction in stator current induces a high voltage at the alternator
output to maintain the energy of the system. The duration of
this transient is based on the electrical time constant of the
alternator field circuit and regulator response time.
Due to conditions described earlier, the battery voltage
cannot be fed directly to the low-voltage, high-performance
switching converters. Transient voltage suppressors like
MOVs and bypass capacitors, followed by traditional input
voltage limiters are used. These circuits are simple and built
around the p-channel MOSFET (Fig. 1). The p-channel
MOSFET must be rated at 50 V or 100 V, depending on the
voltage transients expected at the input voltage (VBAT). The
12-V Zener diode Z1 limits the gate-to-source voltage of
the MOSFET below the VGSMAX. The MOSFET operates in
saturation when VBAT is below the breakdown voltage of the
Zener Z2. During the input voltage transient, the MOSFET
blocks the voltages higher than the Z2 breakdown voltages.
The disadvantage is an expensive p-channel MOSFET and
too many components around it.
Another approach is to use the npn transistor with collector connected to the “plus” terminal of the battery and the
emitter to the downstream electronics. A clamping device
(VZ) then clamps the npn base voltage, which regulates
the emitter voltage at VBE below the VZ. It’s a lower-cost
but inefficient (PLOSS = IIN  VBE) solution. The drop also
increases minimum operating battery voltage, which is
especially critical through cold crank. The third possible
solution is using an n-channel MOSFET. N-channel
MOSFETs are widely available, cheaper and may be used as
a blocking element. However, the gate drive is more complicated because it needs to be higher than the source voltage.
The MAX6398 includes an internal charge pump to drive
an external n-channel MOSFET (Fig. 2).
Fig. 2 shows the implementation of an n-channel
MOSFET switch as a blocking device. The MOSFET can
be completely turned off as soon as VBAT increases above
the set limit during the load dump. The MOSFET remains
off as long as the VBAT remains above the set voltage. The
MAX6398 controls the n-channel MOSFET to protect the
high-performance power supply from the automotive OV
events, such as double-battery jump-starting and load
dumping. The MAX5073, a 2-MHz, two-output compact
buck converter is connected downstream.
As depicted in Fig. 3, the MAX6398 effectively blocks
automotive load-dump pulses and regulates the voltage seen
by low-voltage, high-performance electronics. The strategy
of using a combination of protector and low-voltage, highfrequency power electronics saves space and cost compared
to the high-voltage solutions operating at significantly lower
frequencies.
PETech
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