Article AC10 BLDC-Motor-Control

BLDC Motor
Control in
Automotive
Environment
Rainer Boehringer
Abstract
Automotive environments that approach the operational
limits of semiconductor devices are a challenge for system
designers. Under-the-hood applications require a wide
supply voltage range and have high maximum junction
temperatures. Designers must integrate increasing
functionality within their electronic units, hence the ICs need
to provide higher device integration levels. The environment
is also subject to stronger EMC radiation levels. Actuators
close to the turbocharger are typical examples for such hightemperature applications. These actuators serve to adjust
the flaps of exhaust gas recirculation systems, the so-called
waste gate. Further examples are coolant or oil pumps
operating at 125°C and more.
Remove Belt-Driven Actuators
With limited engineering resources and more stringent CO2
emission requirements, designers need to consider all the
power appliances within a car. It is no longer sufficient to
optimize just the engine. Continuously operating loads waste
a lot of energy. Loads driven by the engine's belt are using
power, even if not needed. It would be preferable to operate
the water pump or the cooling fan, for example, according to
21
actual requirements. Driving uphill the fan and water pump
must dissipate plenty of heat. When driving downhill the
motor is running in fuel cut-off and minimal heat is generated.
Unlike belt-driven devices, you can control electronic
actuators according to real demand while considering all
relevant parameters.
Unlike DC motors, BLDC (brushless DC) motors allow
precise control over a wide dynamic range of revolution
speed. BLDC motors help to efficiently and flexibly control
loads according to the power actually needed. This is why
electronically-commutated actuators should be your first
choice for automotive applications such as power steering,
HVAC (heating, ventilation and air conditioning) fans, power
windows, and all kind of pumps.
Automotive Requirements
High Integration Level
A typical BLDC motor control application comprises various
functions. There is the microcontroller (MCU), high-current
external MOSFETs, a pre-driver to switch those external
MOSFETs, the power supply plus a voltage regulator for
the digital supply of the ECU (engine control unit), and a
communication interface to the car (see figure 1).
© 2013 / www.atmel.com
Kommunikation
Communication
Schnittstelle
Interface
Mikrocontroller
Microcontroller
Versorgung
Supply
Integrierter
Integrated
Charge Pump
Gatetreiber
Gate Driver
PMWSteuerung
Control
PWM
via
mittels
Power
PowerStage
Stage Controller
Controller
Input
Logic
Input Logic
Emergency
Shut Down
E
Diagnose
Diagnosis
LINUART
LIN
UART
LIN Transceiver
Transceiver
LIN
System
System
Management
Management
5V
5V Regulator
Regulator
WD
WD Timer
Timer
B6 Bridge
obere
Externe Mosfets
3 High-side
3 obere
Gate Drivers
Treiber
Gate
BLDC
Motor
33
Low-side
untere
Gate
Gate Drivers
Treiber
untere
Externe Mosfets
Position Detection
Figure 1. BLDC System Architecture
IC manufacturers integrate as many functions as possible to
ease your design effort. A higher integration level also saves
space. If the chip has an integrated LIN (local interconnect
network) physical layer function, it does not need a discrete
LIN transceiver. If you reduce the size of the electronic park
brake control, you might have room to add ESP (Electronic
Stability Control) functions on your ECU.
A watchdog timer is mandatory in automotive safety
applications. For failsafe reasons, it needs to be on a different
die than the MCU. Since the watchdog timer consists of
digital logic and a counter, Atmel® integrated this function
onto the MOSFET gate-driver chip to save cost and space.
is still running. The inductance of the alternator windings
creates high-energy pulses with voltage peaks up to 120V.
This voltage is limited by a central load-dump protection unit.
The protected load dump output voltage depends on the
individual OEM requirements (typical example 36V).
Low operating voltages also challenge electric motor
controller systems. The most critical low-voltage condition
occurs during car start. Activating the ignition key or starting
the engine after the start/stop function can drop the battery
voltage as low as 4.5V. This is called crank pulse (figure 2).
The ECU must function properly during this crank pulse. You
can achieve this with electrolytic capacitors that you size
according to the lowest voltage and longest time expected
for the crank pulse.
Automotive Supply Voltage Range
A wide supply voltage range is a key criterion for applications
within an automotive environment. Both high as well as
low battery voltages are a challenge for the ECU. It needs
to withstand a high operating voltage during operation
conditions such as jump-starting and load dump.
Starting an engine with an external starter battery is called
jump start. The worst-case jump start is off a 24V truck
battery with 12 instead of 6 lead acid cells. This creates a
maximum voltage requirement of 28V. Load dump occurs
when a mechanic disconnects the battery while the engine
Automotive Compilation Vol. 10
U
Crank Pulse Cycle
6.5V
4.5V
t
Figure 2. Typical Crank Pulse Waveform
22
External MOSFETs
There are both N-channel and P-channel high-current
MOSFET switches. For the same die size, an N-channel
MOSFET will have half the on-resistance (RDSon) compared to
a P-channel device. Since die size is the fundamental factor
of the part’s cost, N-channel MOSFETs are the preferred
solution in most cases.
The control voltage that begins to turn on a MOSFET is
called the gate threshold VGth. This voltage drops at high
temperatures. In a hot engine compartment, logic-level
MOSFETs may not switch off completely, whereas non-logiclevel MOSFETs guarantee safe and proper switch-off.
To create this gate drive voltage, chip designers use an
integrated charge pump (figure 4). In addition, the charge
pump helps to stabilize the drive of the external low-side
MOSFETs. Non-logic-level MOSFETs require a gate voltage
of 8V. If you derive the low-side gate drive directly from the
battery you cannot maintain 8V during a crank pulse event.
A 2-stage charge pump solves this issue. The charge pump
output voltage is transferred by the VG regulator to the lowside gate circuitry (see figure 3).
Integrated Charge Pump vs. Bootstrap
To turn on a high-side MOSFET, you need to raise the gate
voltage above the supply voltage the MOSFET is switching
(see figure 3).
The ATA6843/44 charge pump is similar to a Dickson charge
pump with its 2-stage architecture (see figure 4). You can
generate the output voltage of a 2-stage charge pump to
a maximum value two times higher than the input supply
voltage. The 2-stage configuration enables a reliable gate
supply voltage range for the external MOSFETs. The MOSFET
gates are protected from load dump and the gate drive
voltage is maintained during a crank pulse event.
Closing the high-side switch increases voltage on motor
phase A to the level of the battery supply voltage VSupply. This
means the voltage on the source pin of the MOSFET is at
VSupply. The gate threshold voltage, VGth, is always relative to
the FET source pin. Hence, the gate voltage VGSH needs to
reach a level of at least VSupply plus VGth.
Competing products often use bootstrap gate drive
techniques. Bootstrap circuits will double the power supply
voltage. But bootstrapping will not maintain gate drive during
a low-voltage crank pulse condition. Bootstrap circuits need
an oscillating motor drive output to work. If the motor output
is fully on or fully off the bootstrap circuit cannot keep its
Gate Drive
Charge
Pump
U
VSupply
VGSH
VGSL
H1
S1
High-side Switch
VGSH
VG
Regulator
L1
VSupply
Motor
Phase A
Low-side Switch
VPHASE
VGSH
VGSL
t
Figure 3. Gate Drive
23
© 2013 / www.atmel.com
TDS 3034B
7 May 2013
15:33:09
Charge Pump Voltage
Supply Voltage
GND level
VSupply
CPOUT
CCP1
T1
CCP2
CCPOUT
T2
GND
Figure 4. ATA6844 Charge Pump Wafeforms
storage capacitor charged. Only a free-running charge pump
is able to provide a stable output voltage above battery
supply no matter what the motor duty cycle is.
Engineers often believe that a charge pump is a complicated
device and difficult to design into their application. Atmel
developed the ATA6843/44's integrated charge pump
to drive six N-channel MOSFETs. The chip only requires
three external ceramic capacitors. The on-chip charge
pump guarantees to easily create a reliable BLDC gate
drive system. There is no additional effort for comparators,
chopping, or switching. You don’t have to agonize over
complex design issues. The Atmel engineers considered
EMC (electromagnetic compatibly) radiation when they
developed the ATA6843/44's internal push/pull stages. They
included sufficient cross-conduction times to keep emissions
low so you can meet strict automotive regulations.
Automotive Compilation Vol. 10
Reverse-Voltage Protection
The integrated charge pump allows you to implement a
reverse-voltage protection circuit (figure 5). This requires
a single external N-channel MOSFET wired in the reverse
direction. At power-on the N-channel MOSFET conducts
via its intrinsic body diode. This starts the integrated charge
pump. Since the motor is not operating, the supply current
is low. The intrinsic body diode can power the chip without
overheating. As soon as the charge pump voltage exceeds
the protection MOSFETs' gate threshold, the MOSFET is
driven into active mode and conducts through its low on
resistance. The charge pump can now also provide the gate
drive voltage to the motor MOSFETs.
An NPN transistor plus a diode in series protects against fast
negative voltages. When the battery input goes negative
relative to chassis common, it turns on the NPN transistor.
The transistor then clamps the MOSFET gate and source
together.
24
+
CCPOUT
CCP2
PBAT
CPHI2
CPHI1
CPLO1
CCP1
CPOUT
CVINT
VG
VINT
VBAT
VMODE
CVINT
CPLO2
Battery
CVCC
VCC
VCC
Regulator
VG
Regulator
Charge Pump
/RESET
WD
VINT
Regulator
/IH1-3
DG1
DG2
WD
Timer
WDEN
GND
SCREF
EN
LIN
LINGND
LIN
RWD
ADC
High-side
Driver 2
H2
High-side
Driver 1
H1
S1
S3
CC
Timer
RCC
Low-side
Driver 1
L1
Low-side
Driver 2
L2
Low-side
Driver 3
L3
PGND
RX
DAC
Driver
Control
Atmel ATA6843/44
DG3
H3
S2
Supervisor:
Short Circuit
Overtemperature
Undervoltage
/COAST
TX
Oscillator
VBG
CC
SLEEP
Control
Logic
RWD
Microcontroller
IL1-3
High-side
Driver 3
CCC
VCC
U
V
W
LIN
KL 15
Figure 5. ATA6843/44 Application Schematic
High-Temperature Operation
The AEC-Q100 standard defines ambient temperature
ranges for automotive applications. Grade 1 covers ambient
temperatures of -40°C to 125°C, grade 0 is suitable
for under-the-hood applications up to 150°C ambient
temperature.
Atmel manufactures motor driver ICs on its own BCD-on-SOI
(bipolar/CMOS/DMOS on silicon-on-insulator) technology.
This enables the IC to operate at junction temperatures
up to 200°C and ambient temperatures up to 150°C. SOI
technology offers specific design advantages. There is
significant cross-talk reduction between power and digital
circuits on the same die, as well as easy integration of highquality power devices, and immunity to radiation. Compared
to devices manufactured with existing bulk technology,
devices manufactured with SOI wafers achieve a higher
level of integration and processing speed at reduced power
consumption. The isolation of devices through an oxide layer
25
eliminates problems of parasitic capacitance and latch up,
thus minimizing internal coupling. This helps to simplify the
design and reduce risks.
Switching the MOSFETs
MOS gate threshold voltages typically decrease at higher
junction temperatures. This disturbs the IC-internal timing
and may prevent the MOS switches from turning off. Atmel
adjusts the gate threshold voltage of the high-temperature
BCD-on-SOI process to meet these high-temperature gate
threshold requirements.
Likewise, the decreasing gate threshold voltage affects the
external MOSFETs. The higher the temperature the lower
the MOSFET gate threshold voltage. One solution is to
apply non-logic level MOSFETs with higher gate threshold
voltages instead of P-channel MOSFETs. At the same die size,
N-channel MOSFETs have only half the RDSon resistance. The
reduction of thermal dissipation is an important benefit, in
particular for applications in hot environments.
© 2013 / www.atmel.com
Support Tools: The Development Kit
Diagnostics and Monitoring
In automotive applications monitoring and diagnostic
functions are mandatory. Voltage failures, thermal overload,
or overcurrent are events that require immediate action, for
example, instant stop or motor driver unlock, independent
of any microcontroller operation. In addition, ECUs must
provide feedback on malfunctions to the host controller, and
record them in central failure protocols to enable appropriate
countermeasures.
One emergency shut-down function is the coast feature.
The ATA6844's COAST pin enables the motor to rotate in
coasting mode by activating a single input pin. In case of
emergency, all six output gate drivers immediately switch
to off mode, the external MOSFETs are deactivated, and the
motor will coast to a stop.
The ATA6844-DK development kit enables designers to take
first steps in high-temperature BLDC motor control. It consists
of two connected boards plus a standard BLDC motor. The
+
CCPOUT
PBAT
CPHI1
VG
CPOUT
CCP2
CPLO1
CCP1
CPHI2
CVG
VINT
VBAT
CVINT
CPLO2
Battery
Figure 6. Development Kit ATA6844-DK
CVCC
VMODE
VCC
SCREF
DG1
DG2
3.3/5V VCC
Regulator
13V
Regulator
Supervisor:
Short Circuit
Overtemperature
Undervoltage
Charge Pump
VINT 5V
Regulator
VBG
Oscillator
DG3
High-side
Driver 3
H3
High-side
Driver 2
H2
High-side
Driver 1
H1
S3
Logic Control
WD
Atmel ATA6843/44
/IH1-3
IL1-3
EN1
WD
Timer
LIN
CC
Timer
RWD
L1
Low-side
Driver 2
L2
Low-side
Driver 3
L3
CC
WDEN
RWD
GND
LINGND
EN2
LIN
LIN
TX
Low-side
Driver 1
RCC
Back-EMF
Conditioning
PGND
/RESET
RX
S1
S2
COAST
CCC
Power Board
Controlller Board
ATmega32M1
UART
ATmega32U2
Figure 7. Evaluation Kit Block Diagram
Automotive Compilation Vol. 10
26
power board handles all BLDC functions except the MCU
microcontroller unit. Six discrete N-channel FETs are arranged
in a BLDC bridge architecture. An Atmel SBC ATA6844
handles the basic electronic control unit functions, a lowdropout regulator, LIN transceiver, and window watchdog.
The controller board features the Atmel ATmega32M1
8-bit AVR® MCU dedicated to BLDC motor control. An
ATmega32U2 microcontroller is on the board for debugging.
Power Board
The power board comprises all the BLDC motor control
functions:
• Six N-channel MOSFETs arranged as a B6 bridge supply
the motor current. The output terminals U, V, and W
attach to the motor connector to operate the included
BLDC motor.
• For emergency purposes, you can adjust the shortcircuit shutdown current with potentiometer SCREF.
• For EMC purposes, you can modify the serial resistors to
achieve gate voltage shaping to adjust the slew rates of
the discrete MOSFETs.
Controller Board
While an actual automotive application will have the BLDC
microcontroller placed close to the gate driver chip, this kit
has the MCU on a separate board to increase flexibility. All
MCU signals required to drive the power board are available
on the interface connector. This approach enables the
customer to use any motor control MCU by simply connecting
the relevant control signals to the interface connector. All
Atmel MCU evaluation boards, e.g. STK®600, can be used.
The controller board provides three debugging methods. The
standard debug interface is a UART interface. The Tx and Rx
connections are accessible via jumper connectors. Since the
ATA6844 has a LIN transceiver, diagnostics can also be done
via the LIN interface. Thirdly, the on-board ATmega32U2
enables RS232 interfacing. The MCU's output USB interface
can be directly connected to a PC and controlled by a hyper
terminal application.
For further information please refer to the ATA6844-DK
application note at
http://www.atmel.com/tools/ATA6844-DK.aspx.
• 82mOhm shunt for motor current measurement can be
adjusted for various motor current loads
• Charge pump for external gate voltage supply
• 3 capacitors for complete charge pump function
• Test pin CPOUT allows access to the charge pump
output voltage
• The charge pump output voltage is also used to
implement reverse battery protection. Typical supply
voltage is 12V. A seventh MOSFET the same size of the
B6 bridge MOSFETs is controlled by the charge pump
output. The reverse voltage protection control circuit
ensures fast switch off during any negative supply
voltage spikes.
Motor position feedback is a key feature of BLDC applications.
The ATA6844-DK offers both Hall sensor feedback and
B-EMF (back-electromotive force) feedback. The option can
be set via jumpers.
For Hall sensor feedback the jumpers directly connect the
motor Hall sensor output signals to the microcontroller.
The microcontroller uses these digital Hall signal outputs
for commutation. A resistor and capacitor network provide
for B-EMF feedback position detection. For this mode the
jumpers connect 3 motor control signals and their dedicated
neutral point signals to the microcontroller interface.
12V Supply, GND
Reverse Voltage Protection
N-Channel MOSFET
B6 Bridge
N-Channel MOSFETs
B asis Board
Motor
Connector
Reverse Voltage
Protection
Control
Shunt resistor
Gate Control
Charge Pump
capacitors
Short Circuit
Threshold
adjustment
Controller Board
Position
Feedback:
Back-EMF Signal
Conditioning
Controller Board
Interface
Connector
Motor controller
ATmega32M1
Control / Diagnosis
Interface
ATmega 32U2
Operating
Status
Jumpers to select
MCU communication
channel
- LIN
- UART
- USB
Figure 8. Application Board Top View, Functional Blocks
27
© 2013 / www.atmel.com
Summary
The Atmel ATA6843/44 integrated gate driver enables to
overcome the challenges of present day automotive BLDC
designs. It lets you design such applications utilizing fewer
external components. These motor driver devices feature
a high maximum junction temperature to meet the strict
automotive grade 0 requirements for under-the-hoodapplications. The IC has an integrated 2-stage charge pump
ensuring that designers can easily create a reliable BLDC
gate drive system without any additional design effort. The
development kit allows engineers to quickly get familiar with
high-temperature BLDC motor control.
For further information on AVR motor control designs, see
http://www.atmel.com/products/AVR/mc/?family_id=607.
Automotive Compilation Vol. 10
28
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