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Texas Instruments LVDS to Improve EMC in Motor Drives Application notes
LVDS to Improve EMC in Motor Drives
With growing demand for higher switching frequencies
and more compact packaging in motor drive systems,
the risk of noisy signals and Electro Magnetic
Interference (EMI) increases dramatically. Noise and
EMI management have become a necessity for
acceptable and robust functioning of motor drive
systems and also for meeting the external agency
Electro Magnetic Compatibility (EMC) standards.
There are many sources of noise generation in AC
Drives (Variable Frequency Drives), Servo Motor
Drives, Computer Network Connected (CNC) and
Robotic multi-axis motor drive systems. The noise that
is generated can find multiple paths to propagate as
the motor sense and control signals move from one
stage to another in the motor drive system. If the noise
is high enough the power stage of the motor drive
system has the risk of getting into false switching
states impacting the speed, torque and position of the
motor. While the traditional noise and EMI
management techniques help with noise reduction,
signal integrity and noise reduction are still a
challenge.
We still have room to improve the signal integrity
further by employing Low Voltage Differential Signaling
(LVDS) for signal transfer between the different boards
instead of using single ended CMOS/TTL techniques.
Motor Drive System
If we were to simplify the system architecture of a
typical motor drive system, it consists of an AC voltage
input stage, which is usually a 3 phase input power
supply. This is followed by the rectifier stage that
converts the AC voltage into DC power rails (DC+ and
DC-). This is followed by the inverter stage. DC
capacitors provide both DC filtering to the rectifier and
switching currents to the Integrated Gate Bipolar
Transistor (IGBT) gate drivers in the inverter stage.
The isolated gate driver drives the IGBT and
implements isolation of the power stage. The power
stage is also called the hot side because it handles
high voltages. Figure 1 shows the different stages of a
motor drive system.
The motor drive is a closed loop feedback system. An
encoder located close to the motor along with voltage
and current sense circuits provides both position and
speed feedback to the control stage. The processor or
the FPGA in the control unit receives these feedback
signals and provides the input to the Pulse Width
Modulator (PWM). PWM drives the isolated IGBT gate
drivers.
Rectifier
IGBT
M
P um p
DC+
AC Input
Power Supply
Encoder
DCPWM unit
PWM Control
Data
V & I Sense
(e.x.Del-Sig
Modulator)
SD-Dx
User Interface
Speed, Torque
& Position
Feedback
Processor
e.x. C2000
Power Module
Control Module
Cold Side
Hot side
Figure 1. Typical Motor Drive System Block
Diagram
There could be an additional communications unit (not
shown in Figure 1) with real-time processing capability
based on the system architecture. The I/O block which
is accessible to humans is the user interface. It could
be connected to a Programmable Logic Controller
(PLC) or Human Machine Interface (HMI) through
standard communication interfaces. The control unit,
communications unit, and the I/O unit are also referred
to the cold-side of the motor drive system.
It is important that the noise does not get transferred
between the different motor drive stages. The standard
interfaces used between these boards/modules vary
from directly sending single ended TTL/CMOS signals
through wires, using SPI interface format, or using
differential interfaces like RS-422, RS-485. The next
section analyzes the advantages of using differential
signals and how LVDS can help with noise reduction.
Single Ended Signals Vs Differential LVDS Signals
LVDS is an industry standard differential signaling
interface used for both low and high-speed
transmission of digital data over copper. It is a physical
layer used for point to point transmission of data, clock
and control signals.
The main advantages of using LVDS are:
•
•
•
•
•
Noise Reduction
Low EMI Emission
Low Power
Supports High Data Rates
Common Mode Noise Immunity
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interface. An LVDS TX/RX can be added between the
control board and the communication board (not
shown here), between the encoder and the control
board, and between the control and I/O user interface
board.
Rectifier
DC+
M
LVDS
TX/RX
AC Input
Power Supply
Encoder
DCPWM unit
LVDS interface also generates low EMI compared to
single-ended transmission lines where electric field
lines are free to radiate away from the conductor.
Some of these fields can travel as Transverse Electro
Magnetic (TEM) waves which escape the system and
cause EMI problems to adjacent circuits. As
complementary current runs in the differential pair,
both lines will generate magnetic fields but in the
opposite direction. In turn, the magnetic field partially
cancels each other. Only the stray fringing fields are
allowed to escape to the far field. Therefore they have
much less field energy available to propagate as TEM
waves and affect the adjacent systems.
LVDS standard also consumes about 10 times lower
power compared to single ended interfaces and other
differential signaling interface standards like RS-422,
RS-485 and Ethernet. This is because LVDS only has
350 millivolt voltage swing. LVDS works well in both
low and high speeds from few Kbps up to 1350 Mbps.
The next section explores where we can use LVDS in
the motor drive system and what it buys us.
LVDS Implementation 1: Board to Board
Communication
LVDS Transmitters/Drivers and Receivers help with
the noise reduction between PCB’s both at low and
high data rates and also short and long distances.
Figure 2 shows the different implementations of LVDS
drivers and receivers.
IGBT
P um p
Sending single ended CMOS/TLL signals through a
standard cable allows for external noise to impact the
signal integrity. LVDS uses balanced differential but
equal and opposite signals. When external noise is
present in the environment, both wires will receive
nearly equal amount of noise. Because the receiver
only considers the voltage potential between the two
wires, the external noise will be canceled out and
therefore helps with noise reduction. This property
enables LVDS to have a very high Signal-to-Noise
Ratio (SNR), and is one of the reasons why LVDS
technology is so robust.
SD-Dx
LVDS
TX/RX
PWM Control
Data
LVDS
TX/RX
LVDS
TX/RX
V & I Sense
(e.x.Del-Sig
Modulator)
User Interface
LVDS
TX/RX
LVDS
TX/RX
Speed, Torque
& Position
Feedback
Processor
e.x. C2000
Power Module
LVDS
TX/RX
Control Module
Cold Side
Hot side
Figure 2. LVDS Implementation 1: Board to Board
Communication
LVDS Implementation 2: Between ADC and
Processor
Using LVDS TX/RX between the voltage and current
sense ADC in the power board and the processor in
the control board helps with noise reduction. Instead of
using a standard single ended cable to tranfer the
signals, using a LVDS TX/RX to send the data and
clock signals between the ADC and the processor
reduces the noise in the output of the ADC. Figure 3
(A) shows the noise in the output voltage of the deltasigma modulator at a data rate of 20 Mbps and 20
MHz clock sent along a 20-cm ribbon cable. Figure 3
(B) shows a ~600-mV improvement in noise in the
output of delta-sigma modulator AMC1305 after using
low cost LVDS TX/RX. This significant noise reduction
helps with the overall signal integrity.
A single ended ribbon cable or a cat 5 cable is used
usually to transfer signals physically between the
multiple boards in the motor drive system. Adding an
LVDS Driver (TX) and Reciever (RX) between the
different boards converts the signals to the differential
format thus offering the benefits of a differential
Figure 3. AMC1305 ADC Output Voltage Waveform
with and without LVDS
2
LVDS to Improve EMC in Motor Drives
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Control Module
Processor
( e.x C2000
TSMC320F28x7x)
Power Module
Delta- Sigma Modulator
w/ LVDS output
(e.x. AMC1305L)
DS90LV012A
SD-Dx
DOUT
RX
SD-Cx
PWMx
CLKIN
TX
DS90LV011A
Figure 4. LVDS Implementation Between ADC with
LVDS Output and Processor
Figure 4 shows the implementation of an LVDS
receiver between AMC1305L and C2000 MCU.
Figure 5 shows the implementation of an LVDS
transmitter and receiver between the ADC and a
C2000 MCU. Two single channel driver
(DS90LV011A) and receiver (DS90LV012A) pairs are
used to transmit data and clock signals between the
boards.
signals over a LVDS driver and receiver helps with the
noise reduction. To showcase this advantage of
sending SPI signals over LVDS TI has released the TI
Design TIDA-060017. Please refer to TIDA-060017
(TIDUED8) to download the detailed implementation of
SPI over LVDS and checkout our demo board.
Figure 6 shows a reconstructed sine wave postprocessed by the controller after receiving a noisy
single-ended SPI signal versus through LVDS Line
Driver and Reciever.
SPI Over LVDS Time Domain Analysis
SPI Over Single-ended Time Domain Analysis
Control Module
Delta- Sigma
Modulator
(e.x AMC1305M)
Processor
(e.x. C2000)
DS90LV012A
SD-Dx
SD-Cx
PWMx
RX
TX
DS90LV011A
DS90LV011A
TX
DOUT
CLKIN
RX
DS90LV012A
Figure 5. LVDS Implementation Between ADC and
Processor
LVDS Implementation 3: SPI over LVDS
SPI ports are still popular for communication on the
same PCB or between two PCB’s in a noisy motor
drive environment. SPI is a single ended signal
transmission protocol which is more susceptible to
noise. SPI communication involves using four lanes of
data/clk communication lines which generates EMI that
can affect the other adjacent circuitry. Sending SPI
Figure 6. SPI Over LVDS vs SPI Over Single Ended
As noise reduction and signal integrity become more
and more critical in motor drive applications, using an
external LVDS TX/RX offers noise reduction and
several other advantages at a very low cost.
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