STMicroelectronics AN414 Variable speed drives have become widespread in the home appliance field Datasheet

AN414
Application note
Controlling a brushed DC motor
with an ST6265C or ST6260C MCU
Introduction
Variable speed drives have become widespread in the home appliance field. The most
classical way to achieve it is to use an AC universal motor with TRIAC and a specific circuit
which controls the TRIAC firing angle. We chose rather to use a permanent magnet DC
motor driven by a chopper. The chopper is driven by a high frequency PWM signal.
Controlling the PWM duty cycle is equivalent to controlling the motor voltage, which in turn
adjusts directly the motor speed.
Why did we make such a choice?
■
The main reason is the need of a speed sensor and a closed loop regulation for the
universal motor. For the permanent magnet DC motor, the speed is almost proportional
to the motor supply voltage. Therefore it is sufficient to control the motor voltage (through
duty cycle adjustment) if a 10% speed accuracy is sufficient. This saves the expensive
speed sensor.
■
The DC motor with chopper has higher efficiency, as there are no 50 Hz copper and iron
losses, due to lower current ripple.
■
The DC operation allows to reduce the acoustic noise, especially at 100 Hz.
Figure 1.
Block diagram
DC Motor
M
ST6265C
IGBT
MS19688V1
Table 1 lists the microcontroller products concerned by this application note.
.
Table 1.
February 2013
Applicable products
Type
Part number
Microcontroller
ST6265C, ST6260C
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AN414
Figure 2.
AC universal motor versus permanent magnet DC motor
Imotor
Imotor
ΔΙ
ΔΙ
t
t
N
L
M
M
MS19689V1
The DC motor has a lower current ripple and therefore higher efficiency.
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Principle
Principle
The drive is supplied from the rectified mains voltage, and consists of a chopper driven by a
PWM signal generated by the ST6265C microcontroller. The motor voltage control is
achieved by measuring the rectified mains voltage with the analog-to-digital converter
present on the ST6265C MCU and adjusting the PWM signal duty cycle accordingly: the
voltage seen by the motor is the product of the supply voltage by the PWM duty cycle. So
the motor speed can be stable even if the mains voltage ripple is high, as the MCU corrects
the duty cycle according to this ripple.
This avoids the use of a speed measurement device and a speed regulation loop. If this
voltage measurement / duty adjustment process is carried out often enough, it also results
in reducing motor current ripple, and therefore copper and motor iron losses, thus improving
motor efficiency. The ripple period being 10 milliseconds (for 50 Hz mains), the later process
must occur at least every millisecond to get a large ripple reduction. In our example, the
voltage compensation loop lasts 380 micro-seconds.
This drive also implements a power limitation: the motor current is measured by the
analog-to-digital converter of the ST6265C MCU, and is used, together with the DC mains
voltage, to compute the electrical power taken up by the motor. If it exceeds the maximum
programmed power, the PWM signal duty cycle is reduced in real time by the ST6265C to
stay below this maximum. This limitation requires only a few discrete components used to
measure the current (2 resistors, a diode and a capacitor make up a peak current detector),
it is otherwise fully software. Our application features a 300 watts maximum power, but this
value can be easily adjusted by modifying a data table in the program, therefore allowing a
very short design modification time if the drive must be used with various motors.
2
Why use a microcontroller?
The classical implementation of such a high frequency chopper usually involves an
application-specific circuit, and some analog circuitry to implement the additional features
such as voltage compensation, power limitation or other.
At first look, a microcontroller should reduce the overall cost of the electronics, as a single
chip would ideally replace several chips. The real world is not so simple: the ST6265C
outputs the PWM signal with a 5 volts level, so a buffer is needed to drive the IGBT with 15
volts, which takes 2 or 3 signal transistors. The MCU must be supplied with a voltage
between 3 and 6 volts, and the 15 volts must be generated, so we need a small auxiliary
supply to extract 5 and 15 volts from the mains. In addition, the MCU cannot generally
replace the fast transistor protection needed for the IGBT, as its response time is in the
range of 10 micro-seconds, while less than 1 micro-second is required.
In conclusion, the MCU approach comes out at the same price, or slightly more expensive
than the analog one, at least if we only account for components costs. We must, however,
consider the overall system cost, not only the components cost of one function.
Three factors at least should be considered:
1.
In many cases, the appliance driven by the motor includes some kind of user interface:
buttons, potentiometers, LED displays, even LCD displays. If the analog approach is
used, more circuitry will be needed to interface them with the motor drive. With the
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Why use a microcontroller?
AN414
ST6265C MCU approach, the MCU itself can sense the buttons, potentiometers, drive
LEDS...
In some existing appliances, the analog approach is used in conjunction with a
microcontroller, the latter being dedicated to the user interface. This is obviously a
waste, as one ST62 MCU could take care of both tasks. If we consider the case of LCD
displays, which are becoming more and more popular, a dedicated LCD driver circuit is
generally used to interface them to a micro.
We have implemented a software LCD drive function in the ST6265C MCU, using its
free output pins; the ST6265C MCU can at the same time run the motor, drive the LCD
and sense the user speed demand (in our case, two buttons "faster" and "slower"). (Cf.
ref. [4] page 12).
4/14
2.
Time-to-market and design time must be considered: the ST6265C MCU software
contains all the characteristics of the application; it is much faster, if the application
must be modified, to modify software than hardware especially when OTP parts are
used for volume production.
We have implemented a software motor power limitation: the motor characteristics are
known by the software in the form of a look up table. If the application must
accommodate a different motor type, it is very quick to modify the data table in the
software. Most modifications of the application can be achieved by simple software
modifications.
If the ST6265C MCU replaces an ASIC, the time and money saved when it is
necessary to modify the initial design goes without saying. The ST62 MCU can also be
used to customize the application: to limit the stock inventory, several applications, for
example a low-end and high-end vacuum cleaner can use the same electronics board,
with the same MCU, but the MCUs have different softwares, or the software is
programmed into the ST62T65Cxx order codes (OTP version of ST6265C) at the end
of the board assembly line.
3.
The MCU lets you do things that are not possible otherwise. Simple examples are:
a)
Execute pre-programmed sequences of motor speeds, diagnostic faults and alert
the user (with temperature sensors, speed sensors, ... or sometimes without them,
by deducing motor speed from current, or temperature from cumulated current ...).
b)
Store in non-volatile memory (EEPROM) user settings which must be saved when
power is turned off.
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Application description
Application description
Figure 3.
Permanent magnet DC motor control with ST6265C MCU and IGBT
M
Supply
15 V
IGBT
5V
0V
Buffer
Vdd
Ud
PWM
MCU
Vss
User
interface
Ic
Peak current
detectoe
MS19690V1
The power part of the hardware is a bridge rectifier, an input filter capacitor, and the
chopper. This converter drives the permanent magnet DC motor. The chopper switch is an
IGBT.
The control part of the hardware is made of the ST6265C MCU, the user interface and the
power interface. The latter is made of a low-power supply (5 volts for the MCU and 15 volts
for the IGBT gate), a level shifter to interface MCU and IGBT, a peak current detector and a
resistor bridge to measure the loop DC voltage, Ud.
The complete schematic with component values is shown in the Permanent magnet motor
drive.
3.1
What microcontroller to use?
The MCU generates a 5 volts PWM signal which drives the IGBT and permits to control the
average voltage across the motor, and consequently the motor speed through duty cycle
adjustment. On the MCU are also implemented software functions such as a line voltage
compensation and a motor power limitation described later.
In order to implement these functions, the MCU should include the following special
features:
●
PWM timer: it must be autoreload to leave the CPU free for other tasks, and fast to
obtain high switching frequencies.
●
Analog-to-digital converter, necessary to measure the DC voltage and the motor
current.
●
Good noise immunity to withstand the chopper and brush sparks proximity.
●
A watchdog for safe operation
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Application description
AN414
We used the ST6260C / ST6265C: it is a low-end 8-bit MCU with 20 pins (or 28 pins
respectively), depending upon the I/O request of the user interface, with an internal
analog-to-digital converter and a fast PWM generator.
Figure 4.
ST6260C / ST6265C Block diagram
EEPROM
128
RAM
128
PWM
A/D
TIMER
CONVERTER
TIMER
8 BIT DATA BUS
WATCHDOG
SPI
TIMER
ROM
4K
8 BIT CPU
PORT
A
PORT
B
PORT
C
20 or 28 pins package
MS19692V1
3.2
PWM control
The PWM feature is required to control the average voltage on the motor and therefore its
speed. The autoreload timer of the ST6265C is used to achieve the PWM control (See
Figure 5: PWM timer operation (Operation of the Autoreload PWM timer: The CPU controls
the period with the Reload register, and the duty cycle with the Compare register.)). At each
end of count, it reloads itself without CPU operation. The CPU is then available for other
tasks after it has programmed the frequency and duty cycle of the PWM signal.
●
6/14
The duty cycle can be adjusted by software from 0 to 100%. The maximum frequency
is: 31.25 kHz with a resolution of 256 steps (0.4% resolution). For our application, we
chose a frequency of 8 kHz.
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AN414
Application description
Figure 5.
PWM timer operation (Operation of the Autoreload PWM timer: The CPU
controls the period with the Reload register, and the duty cycle with the
Compare register.)
SWITCHING PERIOD
TIMER
255
COMPARE
REGISTER
t
000
DUTY CYCLE
PWM OUTPUT
MS19693V1
3.3
Voltage compensation
Using such feature on a Permanent magnet DC motor, the speed is kept almost constant by
keeping the motor voltage constant even if the rectified mains voltage is strongly swinging
across the filter capacitor. So a closed loop speed control is not necessary and the speed
sensor is saved.
A compensation feature is implemented in order to:
●
Obtain a motor speed insensitive to mains voltage variations (static performance),
●
Reduce acoustic motor noise and dissipation by reducing the motor current ripple
(dynamic performance).
The average motor voltage Vmot must be kept constant. The duty cycle d is modulated as a
function of the direct voltage Ud around a reference point given by:
●
User request duty cycle (d0),
●
Nominal direct voltage (Udnom).
Vmot = d x Ud
Vmot = constant = d0 x Udnom
To achieve this function, Ud is measured using the A/D converter of the ST6265C and
quantized on 32 levels, and d0 is quantized on 16 levels. A duty cycle correction is picked up
from a look-up table versus Ud and d0. Then the correction is added to d0 and the sum d is
loaded in the PWM timer register.
The table could also contain values chosen specifically for the application.
In order to reduce motor current ripple, the control loop must be fast, so as to minimize the
time between voltage reading and duty cycle updating.
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Application description
AN414
Our compensation algorithm takes 380 µs, so in one Ud ripple cycle (10 ms for a 50 Hz line),
the correction is made approximately 26 times.
Figure 6 shows results obtained for the static performance of the compensation: the motor
voltage is measured while the DC input voltage varies from 100 to 400 volts. It is shown that
the motor voltage remains within a 5 to 15% range of the voltage demand. This
compensation is good enough for most home appliance applications.
The motor voltage and therefore, motor speed, remains constant while the DC line voltage
ud varies.
Figure 6.
Voltage compensation static performances
V motor (V)
250
200
150
100
Δ Vmotor < 10%
50
0
100
200
300
400
Ud (V)
500
MS19694V
Figure 7 shows the dynamic performance of the compensation: the 100 Hz ripple in motor
current is considerably reduced when using the compensation (lower trace), compared to
the ripple without compensation (upper trace).
The top trace shows the motor current without compensation.
The measurement is made with a loaded motor:
Imotor = 2.5 amps (average) / Vmotor = 105 volts. The mains voltage is 230 volts.
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Application description
Figure 7.
Voltage Compensation (Dynamic performance: motor current ripple
reduction (1 A/Div, 2 Ms/Div)
ΔI = 2 Amp
0 Amps --
ΔI = 1 Amp
2 ms/dv ; 1 A/dv
0 Amps -Imotor av = 2.5 Amps ; Vmotor = 105 V
Uac = 230 V ; C = 100 μF
3.4
MS19695V1
Motor power limitation
The purpose of the power limitation is to keep the motor within its safe operating area in
case of excessive loading (avoid overheating and hard brush switching). The motor current
is evaluated every 3 milliseconds by measuring the peak chopped current in the IGBT
emitter.
If we call d the duty cycle of the PWM signal, Ud the motor DC supply voltage and Imot the
average current in the motor, the electrical power fed to the motor is:
P = Vmot x Imot = d x Ud x Imot
If we want to limit P to a maximum value Pmax (300 W in our example), we must keep the
duty cycle below a maximum duty cycle "d_max":
d £ d_max with d_max = Pmax / (Ud x Imot)
The program repetitively measures Imot and Ud using the A/D converter of the ST6265C.
Then it extracts d_max from a table addressed by Ud and Imot. It then compares d to d_max
and decreases d if it is larger than d_max.
The power limitation presents "smooth" transitions between "limited" condition and
"non-limited" condition, in order to avoid oscillations or jerks.
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Application description
AN414
Figure 8 shows the measured performance of the power limitation. To modify the power limit
target, the look-up table can easily be modified. When the motor load changes, the motor
current varies. The ST6265C MCU measures this current and limits the motor voltage if
necessary.
Figure 8.
Motor power limitation performance
Power (W)
Vmotor (V)
250
500
200
400
Power
150
300
100
200
50
100
Vmotor
0
0
0
1
2
3
Imotor (A)
3.5
4
5
6
MS19695V1
Program highlights
The total program takes 400 bytes of memory space, plus 1024 bytes for the tables. One
timer remains free, and 2500 bytes remain available on the ST6260C / ST6265C for
additional software or enlargement of the look-up tables.
It is possible, for example, to drive an LCD display by software, to execute pre-programmed
sequences of motor speeds, current limitations, etc. The EEPROM also remains available,
for example, to store user adjustments which must be saved when the power is off.
The line voltage compensation algorithm must be executed at high speed, as already
explained, while the power limitation algorithm can be executed less often. Therefore, the
complete program executes seven line compensations for each power limitation. The line
voltage is compensated every 380 micro-seconds while the power is sensed and limited
only every 3 milliseconds.
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Conclusion
Conclusion
Microcontrollers are often limited to controlling and monitoring the user interfaces. This
paper describes a digital solution based on a general purpose MCU with on-chip PWM
generator, the ST6265C. Besides the user interface such as an LCD drive, the MCU
controls directly a DC motor with few external components and sophisticated control. A
simple instruction set and the use of look-up tables allow for a short time-to-market, while
avoiding the investment into an ASIC circuit. The demonstrated concept is applicable to
home appliances (food processor, washing machine, refrigerator, HVAC ...) and to industrial
applications where high speed accuracy is not required (pumps, fans ...).
5
References
[1] - Microcontroller and TRIAC on 120/230 v mains
A.N 392/A - Ph. RABIER and L. PERIER (SGS-THOMSON Microelectronics)
[2] - Design with microcontrollers in noisy environment
A.N 435 - L. PERIER (SGS-THOMSON Microelectronics)
[3] - Versatile and cost-effective induction motor drive with digital three phase generation
A.N 424 - B. MAURICE, JM. BOURGEOIS, B. SABY (SGS-THOMSON
Microelectronics)
[4] - Direct Software LCD Drive with ST621x and ST626x
A.N 594 - T. CASTAGNET, J. NICOLAI, N. MICHEL (SGS-THOMSON
Microelectronics)
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P1
R17
12K/2W
3A
F1
0.47/2W
R11
T3
R10
0.47/3W
BYT11-600
D10
100nF
C8
D5
STTA806DI
R3
R15
8.2K
220
STGP10N50
C5
100nF
D1
1N4148
2N2222A
T2
R2
270K
T1
BC337
C3
100uF
R4
4.7K
22K
R5
D3
13V
47K
R6
R1
6.8K
R19
15K
R20
2.2K
1K
R16
R9
10K
T4
BC327
R7
1N4148 C6100
1nF
D2
27pF
C10
C9
27pF X1
8MHz
15
16
17
18
19
20
21
22
23
24
25
26
27
28
ST6265
PA3
PA2 14
PA4
PA1 13
PA5
VSS 12
PA6
VCC 11
PA0 10
PA7
XTAL PB7/TIM20 9
EXTAL PB6/TIM21 8
NRES U1
PB5 7
CKOUT
PB4 6
PB3 5
PC4
PB2 4
PC3
TST/VPP 3
PC2
PB1 2
PC1/TIM1
PC0
PB0 1
C7
47n
SW2
SW1
C4
200uF
D4
5.1V
R12
D7
LED
3.9K
R13
D8
LED
3.9K
Figure 9.
100u380V
C1
P4
Appendix A
R18
12K/2W
D6
4X3A 600V
P2
P3
Permanent magnet motor drive
AN414
Permanent magnet motor drive
The permanent magnet motor drive schematic is shown in Figure 9.
Complete schematic of the permanent magnet motor drive
MS19697V1
AN414
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Revision history
Revision history
Table 2.
Document revision history
Date
Revision
Changes
Nov-1993
1
Initial release.
05-Feb-2013
2
Modified title and part numbers to include ST6265C and ST6260C.
ST62T65 MCU replaced with the ST62T65Cxx order codes.
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