Texas Instruments | Brushless-DC Made Simple – Sensored Motor Control (Rev. A) | Application notes | Texas Instruments Brushless-DC Made Simple – Sensored Motor Control (Rev. A) Application notes

Texas Instruments Brushless-DC Made Simple – Sensored Motor Control (Rev. A) Application notes
Brushless-DC Made Simple – Sensored Motor Control
Matt Hein, Analog Motor Drive
Introduction
Brushless-DC (BLDC) motors remain an important and
fast-growing motor type with many performance
benefits over brushed-DC (BDC) and stepper motors.
BLDC motors are more efficient, higher power, higher
torque, quieter, longer lifetime, and higher speed
compared to their BDC counterparts. Today, we see
many different products that specifically advertise
themselves as having “brushless” technology. Being
able to keep up with the market trends is important in
order to design a product that is relevant and
successful on the market. Some examples of these
products are cordless power & garden tools, cordless
vacuums, drones & remote-control toys, fans & air
purifiers, and automated window blinds.
However, the difficulty of implementing a BLDC motor
in a customer system remains a large barrier to entry
for many product design teams, especially when
comparing the complexity of BLDC control versus a
BDC motor. This even applies to systems using
sensored trapezoidal control which will be the main
topic of this document.
Brushed-DC Systems
If we look at a typical medium-power brushed-DC
motor system, we have four external MOSFETs and
an associated H-bridge gate driver (e.g. DRV8701). A
low-dropout regulator (LDO) generates 3.3V from the
main motor supply for the microcontroller (MCU) and
Hall-effect sensor (the LDO may be integrated into the
gate driver). The system MCU takes inputs (buttons,
commands, etc) and controls the motor using two
output, a direction signal (clockwise or
counterclockwise spinning) and a PWM signal (0% to
100% duty cycle at a fixed frequency). The Hall-effect
sensor is used for motor speed feedback to the MCU.
24 V
3.3 V
LDO
The control loop used in such a system can be either a
speed control loop or a position/servo control loop.
The Hall-effect sensor output frequency is directly
proportional to the motor speed and is used to close
the speed control loop. If we apply an
integrator/accumulator to the Hall-effect sensor signal,
we can determine the motor position and perform a
position control loop.
MCU
Target
Speed
+
+
Error
Gate Driver, FETs, BDC
Controller
PWM
Process
Motor
Speed
±
Speed Feedback
H
Figure 2. Brushed-DC motor speed control loop
Traditional Brushless-DC Systems
Looking at a similar BLDC system, we can understand
why engineers would be concerned about the
complexity of implementing a brushless-DC motor in
the same way. The traditional solution for BLDC
control involves three ½ H-bridge gate drivers and six
external MOSFETs. These gate drivers feature a 6x
PWM interface, such that each FET requires a
controlling signal (six input signals in total). The
brushless-DC motor requires electrical commutation,
meaning that it is the system’s responsibility to keep
the motor spinning by energizing the phases in the
correct sequence. For a sensored control scheme,
three Hall-effect sensors are integrated in the motor to
give position feedback. In some motors, the Hall-effect
sensors are replaced by Hall elements, which have
analog outputs and require additional comparators to
implement proper feedback. As seen in Figure 3, the
MCU requirements increase dramatically in terms of
inputs and output required compared to a brushed-DC
motor scheme.
3.3 V
Direction
Inputs
MCU
PWM
Gate Driver
(e.g. DRV8701)
External
MOSFETs
(4x)
H
BDC
Current Limit
Speed
Feedback
Figure 1. Brushed-DC motor system
SLVA980A – April 2018 – Revised August 2018
Submit Documentation Feedback
Brushless-DC Made Simple – Sensored Motor Control Matt Hein, Analog Motor Drive
Copyright © 2018, Texas Instruments Incorporated
1
www.ti.com
This allows for a system with reduced complexity and
simple control. The motor control requirements from
the MCU are identical between the brushed-DC Motor
system example given earlier and this simplified
brushless-DC motor example.
24 V
3.3 V
LDO
3.3 V
H
PWM1H
PWM1L
Inputs
PWM2H
MCU
PWM2L
Gate Driver
(3x ½-H)
MCU
External
MOSFETs
(6x)
H
Target
Speed
+
PWM3H
Error
+
Gate Driver, FETs, BLDC
Controller
PWM
Motor
Speed
Process
±
PWM3L
H
Speed Feedback
H
Current Limit
Figure 5. Simplified brushless-DC motor speed
control loop
Speed
Feedback
Figure 3. Traditional brushless-DC motor system
Simplified Brushless-DC Systems
In order to “simplify” the brushless-DC system, let’s
examine DRV8306, a three-phase Smart Gate Driver.
This device integrates a six-step (trapezoidal)
commutation table in order for the DRV8306 to control
the brushless-DC motor commutation. This offloads
processing requirements from the MCU, as well as
reduces the number of GPIOs required on the MCU.
The integration of the commutation table allows the
DRV8306 device to implement a BLDC design as
simple as a brushed-DC motor system: a direction and
PWM command. The DRV8306 also integrates Hall
element comparators, which allow it to be used with
either Hall-effect sensors or Hall elements without
additional comparators or circuits. All three Hall signals
are combined in the DRV8306 to send one speed
feedback signal to the MCU.
Conclusion
Design complexity is cited as a common reason for not
implementing brushless-DC motors in products, and
while this is true for the traditional brushless-DC
system, devices like DRV8306 allow brushless-DC
motor control to be simplified so that the control
complexity is similar to a brushed-DC system.
Device Recommendation
The DRV8306 is not the only device that can execute
trapezoidal control of a sensored motor, other devices
may be selected based on the necessary system
requirement; however the DRV8306 can support Hall
element inputs while other devices only support Halleffect sensors without external comparators (in 1x
PWM mode).
Table 1. Alternative Device Recommendations
Device
Optimized Parameters
Performance
Trade-Off
DRV8304
40-V abs max voltage
150mA / 300 mA (Source / Sink)
3x Current Sense Amplifiers
No Hall element
comparators
DRV8320
65-V abs max voltage
1 A / 2 A (Source / Sink)
No Hall element
comparators
No current sense
amplifiers
DRV8350
100-V abs max voltage
1 A / 2 A (Source / Sink)
No Hall element
comparators
No current sense
amplifiers
DRV8323
65-V abs max voltage
1 A / 2 A (Source / Sink)
3x Current Sense Amplifiers
No Hall element
comparators
DRV8353
100-V abs max voltage
1 A / 2 A (Source / Sink)
3x Current Sense Amplifiers
No Hall element
comparators
24 V
3.3 V
LDO
3.3 V
H
Inputs
MCU
External
MOSFETs
(6x)
Direction
PWM
Gate Driver
(DRV8306)
H
H
Current
Limit
Speed
Feedback
XNOR
Figure 4. Simplified brushless-DC motor system
We see in Figure 5 that the control loop is similar to
that of a brushed-DC motor once the motor
commutation is handled by the gate driver.
2
Table 2. Adjacent Tech Notes
SLVA960
Reduce Motor Drive BOM and PCB Area with TI
Smart Gate Drive
SLVA939
Field Oriented Control (FOC) Made Easy for
Brushless DC (BLDC) Motors Using TI Smart Gate
Drivers
Brushless-DC Made Simple – Sensored Motor Control Matt Hein, Analog Motor Drive
Copyright © 2018, Texas Instruments Incorporated
SLVA980A – April 2018 – Revised August 2018
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