Texas Instruments | How to Isolate Signal and Power in Isolated CAN Systems | Application notes | Texas Instruments How to Isolate Signal and Power in Isolated CAN Systems Application notes

Texas Instruments How to Isolate Signal and Power in Isolated CAN Systems Application notes
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How to Isolate Signal and Power in Isolated CAN Systems
Neel Seshan, Product Marketing Engineer, Isolation, Interface Group
Introduction:
With the increase in the usage of signal isolation in
many industrial and automotive applications, the need
for isolated power has also increased. The benefits of
isolation are lost if the power supplies on either side of
the isolation barrier are simply shorted. At the same
time, if the isolated power sub-systems are not
designed carefully, it affects the overall system
performance like temperature rise due to poor power
transfer efficiency, data corruption due to emissions,
and so on. To simplify the design process of isolated
CAN sub-systems, this document provides various
options (discrete and integrated) to isolate CAN
signals and power.
Discrete implementation of isolated power:
Figure 1 shows an example of a discrete isolated
signal and power solution for an isolated CAN
subsystem. The isolated CAN functionality is achieved
using a digital isolator ISO7721, which provides one
channel in each direction for transmit (TXD) and
receive (RXD) signals, and TCAN1042H, a fault
protected CAN transceiver with flexible data-rate.
The SN6501 device is a push-pull transformer driver
that can be used in conjunction with an external
transformer, rectifier diodes and an LDO to generate
the isolated power supply. Depending on the
regulation needed, the LDO can be optional. For
transformer driver currents up to 350 mA on the
primary side of the transformer, the SN6501 would
suffice. If the driver current needs to be larger, the
SN6505A or SN6505B with 1A drive current may be
more suitable.
By replacing the discrete isolator and CAN from
Figure 1 with an isolated CAN device allows for a
simplified signal chain as depicted in Figure 2. The
isolated power solution is the same as the previous
discrete implementation. The isolated CAN ISO1042
device in this design reduces the device count by one,
improves the loop time and provides high levels of
protection. This solution has all of the benefits of the
previous discrete solution but with no additional
drawbacks.
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If there are multiple isolated CAN devices in the
system powered from the same isolated power supply,
this architecture makes the signal chain section
compact as output of the LDO can be used to power
all the isolated CAN devices. TIDA-01255 is a detailed
reference design for the discrete implementation, that
is applicable to automotive and industrial CAN
systems.
• Advantages of this solution:
– Higher power transfer efficiency (~ 70-80%) due
to use of external transformer.
– Emissions can be minimized by lower switching
frequencies of the transformer drivers.
– Flexibility of using any CAN transceiver or
digital isolator or single chip isolated CAN.
– Isolated power supply has ability to provide
current for multiple isolated CAN transceivers.
GND
3.3V
D2
EN SN6501 VCC
SN6505A/B
1
3.3V
Vdd
2
TXD
DGND
1
7
6
3
1
5
VCC2 8
VCC1
3
INA 7
OUTA
ISO7721
OUTB 6
INB
4
GND1
RXD
MCU
8
3
2
2
D1
CLK
4
GND2 5
1
STB
TXD
IN
OUT
TPS76350
EN
GND
NC
5
4
8
2 VCC
7
CANH
TCAN1042
6
3
CANL
GND
Optional Bus
protection
function
NC 5
4 RXD
Galvanic
Isolation Barrier
Digital
Ground
ISO
Ground
Figure 1. Discrete implementation of Isolated CAN
sub-system
GND
3.3V
D2
EN SN6501 VCC
SN6505A/B
3.3V
1
Vdd
2
TXD
MCU
RXD
DGND
Digital
Ground
8
1
3
2
7
6
3
1
5
2
D1
CLK
4
VCC2
VCC1
TXD
CANH
ISO1042
GND
NC
5
4
8
7
6
3
RXD
CANL
4
GND1
GND2 5
Galvanic
Isolation Barrier
IN
OUT
TPS76350
EN
Optional Bus
protection function
ISO
Ground
Figure 2. Discrete implementation of Isolated CAN
sub-system with Isolated CAN transceiver
How to Isolate Signal and Power in Isolated CAN Systems Neel Seshan, Product
Copyright © 2018, Texas Instruments Incorporated
Marketing Engineer, Isolation, Interface Group
1
www.ti.com
•
Drawbacks of this solution:
– Larger board space due to discrete
components.
– Transformers are bulky and require more
height, which restricts closer stacking of boards.
– Every isolation component needs to be certified
individually.
Isolated CAN with integrated isolated power:
The same functionality of isolated CAN and power can
be achieved with a two chip solution: a single device
that integrates all the isolation functionality and a CAN
transceiver. Figure 3 shows this implementation with
ISOW7821, a 2 channel isolated data and power
device along with the CAN transceiver TCAN1042H.
The integration of the transformer inside the
ISOW7821 chip saves space not only in the X and Y
dimension, but also in the Z (height) dimension.
Additionally if the ISOW7821 device is placed on one
side of the board and the CAN device is placed on the
other side, it helps in further optimizing board space.
For a more detailed design, refer the isolated CAN
module with integrated power reference design TIDA00893.
3.3V
Vdd
1
VISO 16
OUTA 14
ISOW7821
13
4 OUTB
INB
3
TXD
MCU
RXD
DGND
Digital
Ground
VCC1
1
INA
2
8 GND1
GND2
9
TXD
STB
8
7
VCC
CANH
TCAN1042 6
3
CANL
GND
4 RXD
NC 5
stitching capacitors and good board layout.
More details on reducing emissions can be
found in this application note.
– Current capability of the isolation device
determines the number of CAN transceivers
that can be driven.
Special case
There could be instances when 3.3 V or 5 V is not
readily available to power up the primary side in either
the discrete or the integrated isolated power
implementations. For ex. PLC applications may have a
24 V supply while automotive applications may have a
12 V battery. In these cases a suitable LDO can be
added before the transformer driver SN650x as shown
in Figure 4 to step down the voltage to the 5 V levels
needed by the transformer driver. The transformer is
driven by the high voltage power supply and the turns
ratio is chosen to provide 5 V on the secondary side to
power the isolated CAN. Since the transformer center
tap is connected to higher voltage than the SN650x
ratings, high voltage FETs are used to protect the
outputs D1, D2 of the transformer drivers. Care must
be taken to size the FETs to keep the Rds_on as low
as possible to reduce loss of efficiency.
12V
5V
LDO
12:5
GND
Optional Bus
protection
function
5V
Galvanic
Isolation Barrier
D2
EN SN6501 VCC
SN6505A/B
D1
CLK
12V
4
8
1
3
2
7
6
3
1
5
2
IN
OUT
TPS76350
EN
GND
NC
5
TO CAN
SUPPLY
4
ISO
Ground
Figure 4. High primary side voltage
Figure 3. Integrated implementation of Isolated
CAN sub-system
Conclusion
•
•
2
Advantages of this solution:
– Compact solution, including lower height of the
solution.
– Ease of certification and design due to only one
isolated component.
– Reduces the burden of finding a suitably rated
transformer.
– Flexibility of using any CAN transceiver.
Drawbacks of this solution:
– Power transfer efficiency is limited to 50% due
to the on-chip transformers.
– Since the transformer is integrated, the
emissions are generally higher than discrete
implementation due to higher switching
frequencies. These can be addressed with
Choosing the right components for designing isolated
CAN systems is critical. The selection of the discrete
or integrated solutions depends on the trade-offs
between size, ease of design versus efficiency,
emissions. Discrete implementation of isolated power
supplies provides higher efficiency of power transfer,
lower emissions while the integrated solution for
isolated power provides a compact solution for space
critical applications.
Table 1. Alternative Device Recommendations
Device
Optimized Parameters
Performance Trade-Off
ISOW7841
4 Channel Digital Isolator
With Integrated Power
4 channels for additional
isolated signals
ISO1050
Isolated 5 V CAN
Transceiver
Basic isolation, 1 Mbps
How to Isolate Signal and Power in Isolated CAN Systems Neel Seshan, Product
Marketing Engineer, Isolation, Interface Group
Copyright © 2018, Texas Instruments Incorporated
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