Silicon for new trends in automotive powertrain electronics
By Johan Janssens, Marcos Laraia and Bart De Cock
1. Introduction
Today’s cars are evolving towards high energy efficiency at minimum environmental
impact. While in the long term non-petroleum based powertrains seem to be the most
promising answer, in the meantime the car industry is introducing further improvements
building on available technology. A major trend is hybridization where a lot of growth is
visible in micro hybrids (including stop-start) and mild hybrids. These “modest hybrid”
solutions may seem to be already old-fashioned – yet a lot of electronic and mechanical
development is still ongoing for these applications.
This article will first examine some good examples of continuous improvements around
the Lundell electrical machine, better known as the alternator. Thanks to better
electronic control, its efficiency is increased, more energy can be recuperated and the
impact of frequent engine starts is smoothened. A second section will highlight the
addition of more sensors that will help further reduce petroleum-dependency of the
classic combustion engine. The last paragraph explains how existing inductive sensor
technology can improve brake pedals to help cars save even more energy.
2. Starter-Alternator
In a starter-alternator system, the passive rectification diodes are replaced by high
current switches. These switches are responsible for driving the starter-alternator as
motor (starter mode) and performing synchronous rectification on the generated stator
currents inside the alternator (alternator mode).
Synchronous rectification dramatically improves the energy efficiency of the alternator
by bypassing the (body) diode with a highly conductive channel, reducing the forward
drop below 150mV.
−more−
prepredrivers
drivers
Fig 1 : Synchronous rectification in an alternator
A main functional challenge in this application is to ensure a very fast switch off when
the sign of the stator current is reversing; any delay in the switch off would result in
unwanted battery discharge, in very much the same way as reverse recovery of a
regular diode would do. To this end, the predriver IC contains high slew-rate drivers
operating inside an autonomous gate control loop, designed to make the best possible
compromise between ohmic loss during rectification and transition loss when the current
changes sign. The integration of these predrivers in an IC is quite complex.
Firstly, it requires the coexistence of many different voltage domains on a common
silicon substrate, while at the same time ensuring reliable communication between
these domains.
Secondly, the driver IC of a starter alternator is placed in about the worst place possible,
being subject to a wide range of transients like battery reversal, load dump, negative
ground shifts, very large dV/dts on the stator phases (in the order of 100 Volts per
microsecond), electromagnetic disturbances, etc. Still, using differential techniques and
meticulous management of the parasitic (bipolar) effects in the silicon substrate, it is
possible to build this type of IC in a cost effective bulk technology as opposed to Silicon
on Insulator (SOI) technology.
Bootstrap loader
LSSUP
HV SUPPLY
LSS
HSS
SUP
D
Synchr.
rectifier
G
Predriver
Auto - Shutdown
S
LSS
Fig 2 : Robust Predriver to control a High Cgs MOSFET in a starter-alternator
Apart from the traditional lead-acid batteries, we see that more and more types of
energy storage elements are being taken up in the power network around start/stop
systems, such as Li-ion cells, supercapacitors, etc. In these systems, the safety aspect
becomes as important as the core functionality. As a result, we see the ISO26262
safety norm popping up more and more, occasionally leading to a significant portion of
the silicon dedicated to monitoring the application, checking the health of the IC and its
companion ICs,… and ensuring a safe state in case it is needed.
Lastly, the combination of intelligent circuits with high power components in the
immediate vicinity means that the junction temperatures of the control circuits are rising
considerably; it is not uncommon that operational junction temperatures above 175°C
need to be considered in the application. Moreover, during the qualification phase of the
component, temperatures up to 200°C may be used to further accelerate the
degradation processes in order to keep the duration of the life test within a reasonable
2000 hours. By using a silicon process with extended temperature profile and taking this
constraint into account during the design phase, this challenge can be effectively dealt
with.
3. Sensing in the Internal Combustion Engine
Sensors play a key role in allowing modern internal combustion engines to reach
unprecedented efficiency levels while emissions are minimized. For instance, the mass
airflow (MAF) sensor weighs the amount of air getting into the combustion chamber, so
the right quantity of fuel is precisely injected. And at the other end of the engine, oxygen
and NOx sensors directly measure the composition of the exhaust gas and forward the
information back to the Engine Control Unit (ECU).
The invasion of pressure sensors virtually everywhere is a trend that has accompanied
the evolution of the internal combustion engine and its quest for increased control. It
started with the manifold absolute pressure (MAP) sensor, which can be used
alternatively to the MAF sensor. As fuel injection technology advanced, gasoline direct
injection (GDI) and diesel direct injection (DDI) pressure sensors were needed to allow
measure of the fuel pressure injected via a common rail fuel line directly into the
combustion chamber of each cylinder. The latter sometimes also requires diesel
particulate filters (DPF) to reduce soot, and the DPF needs a pressure sensor to help
maintain appropriate operating conditions. Even outside the engine, tire pressure
monitoring systems (TPMS) make sure tires are properly inflated to provide not only
better safety, but also higher fuel efficiency due to lower rolling resistance.
The final frontier for pressure sensors is the combustion chamber itself. In order to
achieve ultimate combustion control, one of the necessary conditions is to know
precisely the pressure inside all cylinders at all times. Some kinds of clean diesel
engines are already running with the help of in-cylinder pressure sensors. Those same
sensors are also a key enabler of new engines being researched, an example being the
homogeneous charge compression ignition (HCCI), which aims to combine the low
emissions of the gasoline engine along with the efficiency of the diesel engine.
All these advancements present new technical challenges for which increasingly
sophisticated integrated electronics are required. For one, better control requires tighter
accuracy and tolerances of 0.5% are now commonplace. At the same time the
operating temperature range keeps being extended, as the pressure sensing function
moves closer to the heart of the engine. That puts additional contraints on the sensing
element and on the electronics needed to compensate for its non-ideal characteristics.
The block diagram of a new-generation pressure sensor IC is shown in Figure 3. Highprecision performance starts with a low-noise analog front end, followed by a highresolution sigma-delta A/D converter. Sophisticated digital signal processing provides
for nonlinear temperature compensation of both offset and sensitivity of the sensing
element. And the usual 5V analog output is being gradually replaced by standard digital
outputs such as SENT and PSI5. This approach reduces total quantization error by
eliminating need of the output D/A in the sensor and the A/D in the ECU side.
Each individual sensor is calibrated in production and the compensation coefficients are
stored in the internal EEPROM.
VCC
OVERVOLTAGE
REVERSE
BATTERY
VBDR
NCV7190
5V to 3.3V
LINEAR
REGULATOR
POR
GAIN NL
REGISTER
INM
X
G1
G2
X
INP
LPF
Main
ALU
S-D
A/D
+
X
+/-
S
-
S-D
D/A
LPF
OUT
D/A
X
COARSE OFF
REGISTER
REG
FBACK NL
REGISTER
OFFSET TC
REGISTER
SENT
Output
TX
GAIN TC
REGISTER
EEPROM
OFFSET TC (a,b,c)
COEFFICIENTS
ALU
A/D
LATCH
GAIN TC (a,b,c)
COEFFICIENTS
(feeds data
to registers)
SERIAL COMM INTERFACE
TEMP
INT TEMP
SENSE
G
chopping
chopping
SENSOR
T SENSE
OFFSET NL
REGISTER
MUX
VSS
RC OSC
CK GEN
TESTABILITY &
DIAGNOSTICS
Fig 3 : Block diagram of a next-generation precision signal conditioning interface IC for
pressure sensors
4. Inductive Position Sensor Interface for Brake Pedals
A typical brake pedal has only a switch that helps to determine when the brake light
should be turned on. With the addition of brake energy regeneration (regen) functions,
new brake pedal position sensors are needed. In essence the standard friction brake
system is upgraded with a control system that measures the exact displacement of the
brake pedal. When the brake pedal is only slightly depressed, the friction brake system
will not be activated yet. During this “regen band”, the energy regeneration system will
measure the brake pedal displacement and determine how much kinetic energy of the
moving car needs to be transferred towards a temporary energy storage. This energy
storage can take on many forms; OEMs may prefer a pneumatic or hydraulic reservoir,
a 48V battery, a supercapacitor, or even a flywheel. Mild hybrids will convert the stored
energy back into propulsion power for a limited time during the next acceleration phase,
whereas micro hybrids only use electrical regeneration to power the boardnet over
extended periods.
In order to measure the exact position of the brake pedal during the “regen band”,
similar technologies as for accelerator pedals can be used. Figure 4 shows a block
diagram of a contactless sensor solution for such an application.
Fig 4 : Block diagram of an inductive position sensor application
The custom inductive sensor interfaces developed by ON Semiconductor make use of
advanced front-end filters in combination with intelligent processing. The on-chip
drivers stimulate the sensor through at least one excitation inductor. The coupled
output inductor(s) of the sensor will generate signals that contain information on the
relative position of the excitation inductor(s) vs. the output inductor(s). The variation in
relative position of the inductors depends largely on the chosen sensor design, it is
typically the result of linear or rotational motions. The integrated circuit then translates
the electrical input and output signals of the sensor into digital position information.
This extracted position is then communicated to the microcontroller through an interface
depending on customer requirements or preferences. Proprietary mixed-signal
solutions can be selected to support sensor interface output formats that range from
ratiometric Voltage, Sin-Cos Voltage, PWM, SENT or PSI5.
Semiconductor suppliers in this sensor field should possess - besides the required
technical skills - also the right attitude towards ISO26262. Many of the pedal
applications in a car are directly related to safety and this needs to be addressed by
means of an appropriate ISO26262 understanding, methodology and toolset. Inductive
sensors can be composed in a redundant configuration sharing same structures for
some functions and providing independent data outputs, achieving ASIL-D on the
module level. The emerging regen applications combined with the new applicable
safety standards are driving the industry towards the development of new integrated
circuits for interfacing with inductive sensors.
5. Conclusion
Developers and suppliers of electronic components are making the difference on the
road towards future powertrains. Though the micro and mild hybrids offer a relatively
modest fuel economy improvement, they are cost-effective. It is exactly this robust
automotive step-by-step approach that will make the majority of cars evolve steadily
towards new technologies while at the same time building the fundaments of nextgeneration powertrains.
-Ends•
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About ON Semiconductor
ON Semiconductor (Nasdaq: ONNN) is driving energy efficient innovations, empowering customers to
reduce global energy use. The company offers a comprehensive portfolio of energy efficient power and
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April 2014
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