Texas Instruments | Space saving design techniques for multi-channel high-voltage DI modules (Rev. B) | Application notes | Texas Instruments Space saving design techniques for multi-channel high-voltage DI modules (Rev. B) Application notes

Texas Instruments Space saving design techniques for multi-channel high-voltage DI modules (Rev. B) Application notes
Space Saving Design Techniques for Multi-Channel HighVoltage Digital Input Modules
Charles Lin, Field Application Engineer
Introduction
Digital input modules are frequently used in
Programmable Logic Controllers (PLCs), Computer
Numerical Control (CNC) and Railway Signaling Data
Loggers, to detect 24 V or higher voltage Digital Inputs
(DI) from sensor transmitters. In some larger PLCs
and Data Logger systems, more than 1000 digital
inputs detection circuits need to be integrated in one
board. Traditional Optocoupler solutions[1] result in
high board temperatures (heating up to 120℃ with
ambient temp at 58℃), which can lead to reliability
concerns. The implementation of high wattage
resistors and cooling fans to counter these
temperatures add unneeded cost.
Industrial digital input modules usually use screw
terminals to connect field sensors over long cables.
This setup can pick up voltage and current noise from
a variety of disturbances in harsh environments, such
as surge, electrostatic discharge (ESD), and electrical
fast transients (EFT). Surge can be caused by events
such as lighting strikes. ESD can happen when a
component or connector comes in contact with a
human operator. EFT can be caused by cables near
high-voltage and high-frequency signals.
Multi-channel digital input boards are a growing trend
that will continue well in to the future. The efficient
cost, compact size and low power dissipation
encourage the grown of the trend. In this paper,
techniques will be introduced to optimize the space
and thermal rise in the multi-channel HV digital input
board design. Other functionality and cost benefits are
also presented.
Methods to reduce board space
IN
5 V or 3.3 V or 2.5 V
Backplane supply
ISO1212
RTHR
SENSE1
CIN
VCC
VCC1
RSENSE
IN1
OUT1
FGND1
Host
Controller
SENSE2
±24 V
RSENSE
OUT2
IN2
FGND2
GND1
GND
COM
Current for positive polarity
Current for negative polarity
(A)
RTHR
VF
ISO1212
CIN
±24 V
SENSE1
RSENSE
VCC1
IN1
OUT1
FGND1
OUT2
SENSE2
IN2
FGND2
GND1
(B)
Figure 1. Two solutions for bi-directional signal
detection (a) ISO1212 with sourcing and sinking
input (b) ISO1212 with Bridge rectifier
Use MUX and Decoder to reduce the total number
of Microcontroller I/O Pins
New Solutions for Digital Input Modules
Texas Instrument’s ISO1212 device provides a simple
and low-power solution with an accurate current limit
to enable a more compact and high-density digital
input board design. The current limit feature results
lower power dissipation, and lower board temperature
rise, compared to traditional Optocoupler based
solutions. These benefits are shown in this TI Tech
Note, How To Simplify Isolated 24-V PLC Digital Input
Module Designs. Two solutions are shown here to
detect bi-directional signal in the real PLC or CNC
system. Figure 1(a) shows the use of two channel
digital input receivers with one COM terminal. Channel
1 and channel 2 are active when the digital input is
sinking or sourcing respectively. But the state of the
SLLA420B – July 2018 – Revised October 2018
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signal has to be considered by the state of OUT1 and
OUT2. It will consume more microcontroller I/O pins
and increase logic requirements. Figure 1(b) uses the
rectifier followed by ISO1212, it can be used to detect
two bi-directional signals for each device. For ‘N’
channels digital input board design, the latter can save
more PCB space and BOM cost for reducing usage of
‘N’ ISO1212s. The board can be more compact in
America and Europe, where only unidirectional signals
must be detected.
For an N channel digital inputs board design in a large
system, it is impractical to use N Microcontroller I/Os
to detect these signals. The ISO1212 provides an
output-enable pin on the controller side (EN) to solve
this. Setting the EN pin to 0 causes the output buffer to
be in the high-impedance state. This feature can
multiplex the ISO1212 on the controller side with
external multiplexer and decoder.
Assuming that a 2 × N × M channel digital inputs
board needs to be constructed; a block diagram of a
digital input board system is shown in Figure 2. A
module refers to the circuit chosen to detect bidirectional signals. The system is been divided into M
groups, and each group has the same output OUT1 to
Space Saving Design Techniques for Multi-Channel High-Voltage Digital
Input Modules Charles Lin, Field Application Engineer
Copyright © 2018, Texas Instruments Incorporated
1
www.ti.com
OUT2N, which will save (M-1) × N lines to a Micro. A
Multiplexer and Decoder can be used to reduce more
Microcontroller I/Os when detecting the state of OUT1
and OUT2N. For example, a 16:1 mux and 3-line to 8line decoder can be used to detect 128 digital input
signals while only using 8 I/O pins from the
microcontroller. The usage of MUX, decoder and
enable function of ISO1212 can be used to reduce the
number of routed traces, which will save board space
and reduce microcontroller I/O pin count.
N
MUX
Connector to MCU
Decoder
ISO
ISO
ISO
ISO
ISO
ISO
ISO
ISO
ISO
ISO
ISO
ISO
Connector
1
Connector
2
Connector
3
Connector
4
Connector
5
Connector
6
M
TOP
(a)
GroupM
Module 1
Module 1
MUX
Group1
N
OUT1 to
OUT2N
Module 2
Module 2
OUT1
to
OUT2N
IN
to
IN2MN
OUT1
to
OUT2N
Module N-1
Module N
Module N
MCU
EN1 to
ENM
Decoder
Address
Decoder
Module N-1
EN1
MUX
Connector
5
Signal 2
Connector
4
ISO
ISO
ISO
ISO
ISO
ISO
ISO
ISO
ISO
ISO
ISO
ISO
Connector to
MCU
IN1
to
IN2N
Connector
6
Signal 1
Address
and data
M
Connector
1
Connector
2
Connector
3
ENM
TOP
(b)
Figure 2. Block diagram of a DI board system
Legend:
Field-side routing
Field-side routing
Layout suggestions to make the board compact
Field-side routing
Field-side routing
Use Symmetric Layout to save space
Figure 3. Two floor design solution for multichannels digital input board design
(a) Using one side to place connectors (b) Using
two sides to place connectors.
Both the bottom and top layers should be used to
place the ISO1212 to realize a more compact board.
The external spacing between the two sides that are
being isolated must be maintained to pass safety
standards. This distance is referred to as "Clearance".
The IEC 60950-1 is the primary standard describing
the clearance requirement[4]. The shortest terminal-toterminal distance through air of ISO1212 is 3.7 mm.
For some lower isolation voltage case, such as PLC
system, the minimum clearance should be no less
than 3 mm. Figure 3 shows the two different layout
designs. Figure 3(a) shows the solution using one side
to place the connector, which will make the length of
the board too long. The minimum clearance of the red
and dark teal zone must be larger than 3.7 mm.
Figure 3(b) shows the solution using two sides of the
board to place the connector, the shape of this design
will be square. It contains two distinct advantages: one
is that this design will be more common in most
application, the other is that the design saves
clearance between the field side and MCU side signal
in the red and dark teal zone. One thing that must be
carefully considered is that the signal 1 in the
Figure 3(b) must be the MCU side signal. If field side
signals are routed here, it will increase 3 mm
clearance to the MCU connector.
2
Reduce decoupling capacitor in the VCC1 side
For most cases, a 100 nF decoupling capacitor is
required on VCC1 side. A decoupling capacitor is used
to filter voltage noise and spikes that pass through DC
signal. In some low speed multi-channels digital input
signal board design (switching frequency < 1 kHz),
some decoupling capacitors can be removed. For the
signal 2 in the Figure 3(b), 2 × M pieces of ISO1212
are place in the near zone. Only M pieces of ISO1212
are required for these 2 M devices to filter and make
the supply stable. M pieces of capacitor can be placed
in the middle of this zone to make two side ISO1212
use the same capacitor, which can save the space of
M times each capacitor size.
Use more internal layers when routing the signal
To make the board more compact, a multilayer board
can be used to route the signals. Increase the use of
internal layers to route the most signals, less signal
routing in the top/bottom layers will save routing
space. The internal spacing must be designed to meet
some system level standard, such as IEC61010-1,
which will mandate a surge or impulse test for different
isolation voltages. FR4 has a dielectric strength of 20
kV/mm. So a 1 mm minimum clearance between the
field side signal and MCU signal must be left to meet
Space Saving Design Techniques for Multi-Channel High-Voltage Digital
Input Modules Charles Lin, Field Application Engineer
Copyright © 2018, Texas Instruments Incorporated
SLLA420B – July 2018 – Revised October 2018
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www.ti.com
Connector
5
ISO
ISO
2
VCC1
FGND1
EN
OUT1
4
OUT2
5
Isolation
3
IN1
ILIM
GND1
14
SUB1
13
SUB2
12
SENSE2
7
NC
IN2
ILIM
NC
GND1
15
Isolation
6
8
16
11
10
FGND2
9
Figure 4. Top view of ISO1212 DBQ Package
Decoder
SENSE1
1
ISO
ISO
ISO
M
ISO
Utilize the top and bottom layers to place the ISO1212
to increase the channel density. Figure 4 shows the
top view of ISO1212. to achieve good board density, a
specific layout of top and bottom layers should be
made. Let the MCU signal side of the ISO1212 in the
top layer have the same direction as the ISO1212 in
the bottom layer. For example, Figure 5 shows the
floor design of the multi-channel board. Assume that N
pieces of ISO1212 are controlled by the EN1 signal in
the top layer, and N pieces of ISO1212 are controlled
by the EN2 signal in the bottom layer. For the output
signals that have been multiplexed, some effort can be
made to let the output signal of ISO1212 in the top
layer (controlled by EN1) have the same position as
the output signal of ISO1212 in the bottom layer
(controlled by EN2). Then a via can be used to
connect the same nets. The benefit of this symmetric
routing is half of the output signal routing space can be
saved by using vias.
ISO
Connector
4
Connector to
MCU
Use symmetric layout when routing the signal
N
Connector
6
MUX
the surge voltage compliance. From this point, if more
internal layers are used to route the signal, only 1 mm
clearance must be maintained, rather than 3 mm in the
top/bottom layers, thus the board will be more
compact.
ISO
Connector
1
ISO
ISO
Connector
2
ISO
EN1 and EN2
ISO
Connector
3
TOP
Figure 5. Floor plan design of multi-channels DI
board
Conclusion
The ISO1212 devices offer a simple, low-power
solution with an accurate current limit. The ISO1212
devices enable a more compact and high-density I/O
module. The schematic and layout suggestions are
introduced in this paper to make the digital input board
more compact and meet the isolation compliance.
Using different methods in different situations will
reduce more space in the board design.
Related Documentation
1. Texas Instruments, How To Simplify Isolated 24-V
PLC Digital Input Module Designs TI TechNote
2. Texas Instruments, How to use isolation to improve
ESD, EFT and surge immunity in industrial systems
analog applications journal
3. David Lohbeck, Understanding isolator standards
and certification to meet safety requirements,
National Instruments, EDN, January 2016.
4. Texas Instruments, ISO121x Isolated 24-V to 60-V
Digital Input Receivers for Digital Input Modules
data sheet
5. Texas Instruments, Low-Emission Designs With
ISOW7841 Integrated Signal and Power Isolator
application report
Table 1. Alternative Device Recommendations
SLLA420B – July 2018 – Revised October 2018
Submit Documentation Feedback
Device
Optimized Parameters
Performance Trade-Off
SN65HVS880
8-channel digital input
serializer
Non-isolated, 3.6mA current
limit, Needs field side supply
SN65HVS885
8-channel digital input
serializer
Non-isolated, 3.6mA current
limit, Needs isolated DC-DC
Space Saving Design Techniques for Multi-Channel High-Voltage Digital
Input Modules Charles Lin, Field Application Engineer
Copyright © 2018, Texas Instruments Incorporated
3
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