EMI Prevention of CAN-Bus-Based Communication in … · EMI Prevention of CAN-Bus-Based Communication in Battery Management Systems Chin-Long Wey, Chung-Hsien Hsu*, Kun-Chun …

International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:05 6

131805-7070-IJECS-IJENS © October 2013 IJENS I J E N S

EMI Prevention of CAN-Bus-Based Communication

in Battery Management Systems

Chin-Long Wey, Chung-Hsien Hsu*, Kun-Chun Chang*, Ping-Chang Jui*, and Muh-Tian Shiue*

Department of Electrical Engineering, National Chiao Tung University, Hsinchu, Taiwan

*Department of Electrical Engineering, National Central University, Jhongli, Taiwan

Abstract— Controller Area Network (CAN) bus has been

popularly employed in most of vehicles and it will be heavily

involved to the electric vehicle (EV) applications. A vehicle is a

noisy system, with electromagnetic interference (EMI) over a wide

range of frequencies. EMI generated by the inductive loads is the

major cause of performance degradation of CAN bus

communications. In order to enhance and protect the reliability

and robustness of the CAN bus communication and to maintain

safe operation of the battery stack, galvanic isolator is required

for the data transmission. This study compares two different

galvanic isolators, Optocoupler and Digital Isolator, and three

different isolation schemes are evaluated by their protection

capability for various EMI affected signals. The best isolation

scheme has been recommended for EMI prevention of the CAN-

bus-based BMS.

Index Term-- Controller Area Network (CAN), Battery

Management System (BMS), Electromagnetic Interference (EMI),

Galvanic Isolators, Electrical Vehicle.

I. INTRODUCTION

Battery Management System (BMS) is simply battery

monitoring which keeps checking on the key operational

parameters during charging and discharging such as voltages,

currents, and temperatures (internal and ambient). The BMS

normally provides inputs to protection devices which generate

alarms or disconnect the battery from the load or charger when

any of the parameters become out of limits. The major

objectives of BMS are [1,2]: (1) to protect the cells or the

battery from damage; (2) to prolong the life of the battery; and

(3) to maintain the battery in a state in which it can fulfill the

functional requirements of the application for which it was

specified. Thus, the BMS may incorporate one or more of the

following functions: cell protection, charge control, demand

management, state of charge (SOC) determination, state of

health (SOH) determination, cell balancing, communication,

and etc.

The BMS communicates with other system devices or with

external equipment via a data link for performance monitoring,

data logging, diagnostics, or system parameters setting. The

choice of the communication protocol is determined by the

application. The System Management Bus (SMBus) [3] has

been commonly used for the BMSs applied for notebook PC.

Recently, Controller Area Network (CAN) bus has been

popularly employed in most of vehicles and it has been heavily

involved to the Electric Vehicle (EV) application. The CAN bus

is very robust with error detection and fault tolerance, but it

carries significant communications overhead and high materials

cost. While an interface from the battery system to the main

vehicle CAN bus may be desirable, I2C communications can be

advantageous within the battery pack. The BMS system with

CAN bus allows the vehicle system to detect whether the

battery is overloaded through the CAN bus nodes when the

vehicle starts or overloads, and then temporarily disables non-

safety-critical devices, such as air condition, entertainment

systems, and etc., to reduce the battery load to prolong the life

of battery.

The battery stack voltage can be as high as 400 V in many

EV’s, this high voltage is needed to deliver enough power to the

motor. A vehicle is a noisy system, with electromagnetic

interference (EMI) over a wide range of frequencies. EMI

generated by the inductive loads has been found to be the

major cause of performance degradation of CAN bus

communications [4-6].

The ultimate goal of this study is to enhance and protect the

reliability and robustness of the CAN bus communication and

to maintain safe operation of the battery stack for data

transmission from high voltage battery to low voltage

electronics elsewhere in the vehicle. The use of galvanic

isolators [7] can achieve this goal. Therefore, this study will

compare two different galvanic isolators, Optocoupler [8] and

Digital Isolator [9], and evaluate the protection capability of

three different isolation schemes for various EMI affected

signals. Results of this study will demonstrate that the best

isolation scheme is recommended for EMI prevention of the

CAN-bus-based BMS.

In the next section, the CAN-bus-based BMS architectures are briefly reviewed. The effect of EMI is also briefly discussed. Section III compares two different galvanic isolators and evaluates three different isolation schemes using these isolators. A concluding remark is given in Section IV.



II. BACKGROUND

This section reviews the architecture of CAN-bus-based BMS and EMI effects to CAN bus.

2.1 CAN-bus-based BMS

In order to improve the conventional rechargeable batteries by

indicating the residual power therein, a smart battery was

jointly developed with the Automotive Research & Test Center

[10], Taiwan, under the collaborative research project “Smart

Battery Management System with Distributed Battery Units”

[11] sponsored by the Ministry of Economic Affairs (MOEA),

Taiwan. The battery implements a residual power monitoring

device, a temperature sensor and other sensors to detect the

variation of the charge-discharge cycle taking place in the

smart battery, so as to prevent the smart battery from

overcharge or over-discharge.

Fig.1(a) shows the developed BMS under the project. The

SOC of each cell in the BMS can be monitored by a BIM

(Battery Interconnect Manager), as shown in Fig. 1(b), where

each BIM is instructed by the BWM (Battery Module

Manager), as shown in Fig. 1(c), to communicate with its next

neighbor BIM through a communication bus. Once

overcharging or over-discharging of a cell occurs, the BIM

reports to its supervised BWM and self-purge to maintain the

safety of the system. The BIM configuration provides very

easy interface with its neighbors and offer the salient feature of

plug-and-play.

Due the insufficient bandwidth of the SMBus and high

battery stacked voltage for vehicle applications, the CAN bus

was employed in our BMS development. The CAN bus is very

robust with error detection and fault tolerance, but it carries

significant communications overhead and high materials cost.

While an interface from the battery system to the main vehicle

CAN bus may be desirable, I2C communications can be

advantageous within the battery pack.

(a)

(b)

(c)

Fig. 1 Developed BMS: (a) Block Diagram; (b) Battery Interconnect Manager (BIM); and (c) Battery Module Manager (BMM).

The CAN bus includes three major components: Host

Controller (HC), CAN-controller (CC), and CAN-Transceiver

(or call bus driver, BD). Fig 2 shows a block diagram of a

CAN-controller, SJA-1000, a product of Philips. The controller

converts the data from the HC, a process chip or PC, to CAN

bus protocol data, and sends to the CAN transceiver, and then to

the CAN bus.

Fig. 2 CAN Bus.

On the other hand, the transceiver transmits the data from the CAN bus to the CAN controller to convert the data and then send to the host controller. The developed BMS [11] takes a simple 8-bit MCU, such as 8051, as the host controller, SJA-1000 as the CAN controller, and PCA82C250 as the CAN-transceiver to form a simple CAN node. Fig. 3 shows a simple BMS, where the battery monitoring data is processed by 8051 and then display at the monitor through a RS-232.

Fig. 3 CAN Bus and CAN Nodes [11].

The TI BQ-29312 AFE protection IC integrates an I

2C

compatible interface to extract battery parameters such as cell voltages and control output status. It provides safety protection



for overcharge, overload, short-circuit, and overvoltage conditions in conjunction with the battery management host. It allows user to program the parameters such as current protection thresholds and delays for overload and short-circuit during charge and discharge. The 8051 maintains an accurate record of available capacity and other critical parameters of the battery pack and reports the information to the system host controller over the RS232.

2.2 EMI Effects on CAN BUS

Fig. 4(a) shows that the electromagnetic interference [4], where the EM field of the power line produces the electric field E to inference the signal on the communication bus. The magnitude of Iinference depends upon the distance between both the power line and the communication bus, and significantly affects the signal-noise ratio (SNR).

Fig. 4(b) shows the waveform of data frames at the bit rate

of 1 Mbps. The amplitude of data frames is 1.8 V. One data

frame lasts from 0.08 ms to 0.019 ms, while the data filed lasts

from 0.1 ms to 0.164 ms. Fig. 4(c) shows the waveform of

EMI, where the voltage peak is 4 V. The length of EMI is

approximately 0.65 ms. Fig. 4(d) shows the waveform of EMI.

On average, every data frame has 6 error bits affected by EMI

[4]. These errors can be detected by CAN, and wrong data

frames can be retransmitted.

(a) (b)

(c)

(d)

Fig. 5 CAN Bus [4]: (a) EMI Principle; (b) Data Frame; (c) EMI

Effect; and (d) Data Frame with EMI.

2.3 Error Effects of CAN BUS

A Data Frame is composed of seven different bit fields:

Start of Frame, Arbitration Filed, Control Field, Data Field,

CRC Field, ACK Field, and End of Frame. A Data Field can be

of length zero. There are five different error types which are not

mutual exclusive [12]: Bit Error; Stuff Error; CRC Error; Form

Error; and Acknowledgment Error.

Every CAN bus controller takes two error counters, REC

(Receiver Error Counter) and TEC (Transmit Error Counter)

and three states, Error Active, Error Passive, and Bus Off, to

distinguish between temporary and permanent failures. The bus

off state is a state in which the unit cannot participate in

communication on the bus. When in this state, the unit is

disabled from all transmit/receive operations.

III. ISOLATORS AND PROTECTABILITY

In order to enhance and protect the reliability robustness of

CAN bus communication and to maintain safe operation of the

battery stack, signal isolator is needed for the data transmission

from high voltage battery to low voltage electronics elsewhere

in the vehicle. This section evaluates the degree of protection

from the isolators. We first introduce the possible isolation

schemes for protection and then discuss the degree of

protection.

3.1 Galvanic Isolators

To maintain safety voltage at the interface and to prevent

transients from being transmitted from the sources, galvanic

isolation is required. Two isolators are considered in the BMS

implementation [13]: Optocouplers [8] and Digital Isolator

[4,9], as shown in Fig. 6.

(a)

(b)

Fig. 6 Galvanic Isolators: (a) Optodecupler [8]; and

(b) Digital Isolator [9].

The optocouplers rely on light emitting diodes to convert the

electrical signals to light signals and on photo detects to

convert the light signals back to electrical signals. On the other

hand, the digital isolator is magnetic coupler based on chip-

scale transformers, as compared with the photodiodes used in

optocoupler. The digital isolator offers higher speed, up to 150

Mbps, and lower power consumption, about 1/10 of the



potocoupler, lower cost, about 40% lower per channel. No

external discrete components are needed. The digital isolator is

more stable operation with temperature than optocouplers.

3.2 Isolation Schemes

Fig. 7(a) shows the typical implementation of a CAN node,

where two CAN nodes are connected to the CAN bus. Each

node is comprised of a modular controller, CAN controller

(CC), and a CAN transceiver (or bus driver, BD). Typically, an

isolator is added between the CAN controller and the CAN

transceiver, as illustrated in Fig. 7(b), referred to as Type B

isolation scheme. The isolator protects both host controller and

CAN controller to enhance the safety of the CAN node. Fig.

7(c) shows an alternative architecture by placing the isolator in

between CAN transceiver and the CAN bus, referred to as

Type C isolation scheme. As such, the isolator can avoid some

invalid or incorrect data to be transmitted to the bus to reduce

the bus loading. Recently, TI has integrated both isolator and

CAN transceiver as the component, TI ISO 1050, as shown in

Fig. 7(c), referred to as Type A isolation scheme.

(a)

(b) (c) (d)

Fig. 7 CAN Node with Isolator: (a) Two CAN Nodes;

(b) Type B Isolation Scheme; (c) Type C Isolation Scheme;

and (d) Type A Isolation Scheme.

In practice, however, when an unexpected signal, such as

static, high current, or high voltage, occurs at either pin

CAN_L or CAN_H of the CAN bus, it may damage the CAN

transceiver and further may affects the CAN_H (CAN_L) of

some other CAN nodes. The occurrence may cause the entire

vehicle system to be unable to properly retrieval the critical

data and execute the safety-critical operation, such as braking.

Thus, we propose to protect both CAN_L and CAN_H signal

line and move the isolator down to the bus connection, as

shown in Fig. 7(c).

For Type A isolation shown in Fig. 7(d), the isolator is

embedded inside of the CAN transceiver IC. Even though the

TI ISO 1050 has excellent EMI protection to the transceiver

IC, the signals of the differential pair on the bus, as shown in

Fig. 8, are still not fully protected. More specifically, the

isolator well protects the circuitry on its left-hand side.

However, when the bus is affected by EMI, the circuitry on its

right-hand side is not protected. The un-protected circuitry may

cause the wrong data to be transmitted or received.

Fig. 8 Function Diagram of TI ISO 1050.

For Type B isolation, when the bus is attacked by EMI

signal, the CAN Transceiver will receive the higher voltage,

higher current, and impulse noise signals. If the signals are

sufficiently large, the un-protected CAN transceiver directly

receives the bus signals which may include the EMI signals and

then transmit to the CAN controller which is well protected by

the isolator. The CAN controller may misjudge the received

signal as a correct signal and thus causing a node error. The

typical errors include re-transmission error, delay transmission

error, and bit error. These errors may cause the node to stop

data transmission. On the other hand, if the EMI effects are not

too severe, the CAN bus error detection mechanism may be

carried out to set the modes of re-transmission, stop-

transmission, or Bus off. However, when re-transmission mode

was issued, the correct data can still be re-transmitted even if

the CAN transceiver is not well protected.

For Type C isolation, the isolators are placed on the front-

end so that it well protects all the components as long as the

EMI signals are within the tolerance of the isolators. The

isolator can directly receive, filter out, and isolate the bus signal

with EMI. On the other hand, if the EMI signals are sufficiently

large, the bus signals will still be affected. However, the CAN

bus error detection mechanism may also be carried out to set the

modes of re-transmission, stop-transmission, or Bus off. It

should be mentioned that delay transmission error and bit error

are generally not occurred in this type of isolation scheme as

long as the EMI does not directly attack the frame during the

transmission.

In summary, for sufficiently small EMI signals, Type A

isolation scheme is generally the best solution. It is followed by

Types B and C which are better than no isolation. For



sufficiently large EMI signals or the EMI signals are negative

Vp-p, however, only Type C can survive.

3.3 Experimental Results

A complete CAN data frame includes ID frame, Control

frame, CRC frame, and END frame, as illustrated in Fig. 9(a).

In these experiments, EMI signals with various amplitude and

offset voltages are applied, as shown in Fig. 9(b).

(a)

(b)

Fig. 9 (a) Frames in CAN; and (b) EMI signals Applied to CAN Bus.

If the amplitude and offset voltage of the EMI signals do not

completely destroy the CAN data frame, the CAN bus error

detection mechanism may be carried out to set the modes of re-

transmission, stop-transmission, or Bus off. The re-

transmission may be issued, as illustrated in Fig. 10(a)-10(c),

while stop-transmission or bus off is issued, as shown in Fig.

10(d). Note that the upper signal waveforms (in yellow) in Fig.

10(a)-10(d) represent the received signals by the controller,

while the lower signal waveforms (in light blue) are the bus

signals.

(a)

(b)

(c)

(d)

Fig. 10 EMI affected signals to CAN Bus.

A. Type A Isolation Scheme

The TI ISO1050 is used as the isolator for Type A isolation

scheme. Fig. 11(a) is the CAN communication of two nodes

with Type A isolation scheme. In this experiment, the data

frame is partially affected by EMI, i.e., the front part is affected

by EMI. Since the transmitted data “D6” is affected, the

receiver will not be able to receive the ACK signal. Since the

controller is well protected by the isolator, the affected data will

then be transmitted.

(a) Type A



(b)

Fig. 11 Type A Isolation Scheme: (a) Experiment Set-up;

and (b) Measurement Results.

B. Type B Isolation Scheme

In this experiment, the optocoupler is used as the isolator to

protect the CAN nodes. The optocouplers rely on light emitting

diodes to convert the electrical signals to light signals and on

photo detects to convert the light signals back to electrical

signals.

Type B isolation scheme in Fig. 7(b) is implemented as

shown in Fig. 12(a). Fig. 12(b) and 12(c) illustrate the probed

signal waveforms, where Channels 1 and 2 respectively probes

the protected signals received by the CAN controller in Node

#2 and the bus signals. With the amplitude of 10 Vp-p EMI

signals (in light blue) in Fig. 12(b), the bus signal is affected.

After being demodulated by the CAN bus driver (without

protection), the bus signals (in yellow) is destroyed causing

form error, CRC error, CRC error, ACK error, stuff error, and

bit error. The CAN bus error detection mechanism may be

carried out to set the modes of re-transmission or stop-

transmission. In this case, some bytes may be demodulated

correctly which is better than no protection in Type A. In Fig.

12(c), with an amplitude of 2 Vp-p EMI signal, the affected data

still can be demodulated correctly by the bus driver, but the

ACK signal still cannot be received. As such, the re-

transmission mode may be issued.

(a)

(b)

(c)

Fig. 10 Type B Isolation Scheme.

C. Type C Isolation Scheme

This experiment also takes the optocoupler as the isolator to

protect the CAN nodes. Type C isolation scheme in Fig. 7(c) is

implemented as shown in Fig. 13(a). Fig. 13(b) shows the

probed signal waveforms, where Channels 1 and 2 respectively

probes the bus signals and the protected signals received by the

CAN controller in Node #2. With the amplitude of 10 Vp-p EMI

signals (in light blue), the bus signal is affected. After being

demodulated by the CAN bus driver (without protection), the

affected signal can be correctly demodulated, but it still cannot

send the ACK signal. Thus, a re-submission mode is issued.

(a)



(b)

Fig. 11 Type C Isolation Scheme.

IV. CONCLUSION

The CAN bus is very robust with error detection and fault

tolerance. While an interface from the battery system to the

main vehicle CAN bus may be desirable, I2C communications

can be advantageous within the battery pack. The proposed

CAN-bus-based BMS allows the vehicle system to detect

whether the battery is overloaded through the CAN bus nodes

and to temporarily disable the non-safety-critical devices, if the

battery is overloaded, so that the battery can be protected to

prolong the life of battery.

To maintain safety voltage at the interface and to prevent

transients from being transmitted from the sources, the isolators

are added at the connection of the CAN node to the bus so that

the reliability and robustness of the CAN bus can be enhanced

and the safe operation of the battery stack can be properly

maintained.

This paper demonstrates various isolation schemes and

evaluate their protection capability to the CAN bus with the

EMI effects. Results show that Type C has the best protection

capability, even though Type A may be better than Type C

when the sufficiently small EMI signals are affected. However,

with a slight modification of the optocoupler, the circuit will

have the same protection capability as Type A for small EMI

signals.

For highly reliable battery management system, reliability

and safety are very important elements for electric vehicle

applications. Type C has been adapted to the battery

management system which is currently being developed.

ACKNOWLEDGMENT

This work was supported in part by the Taiwan National

Science Council under the project No. 102-2220-E-009-052 and

the project No. 101-3113-P-110-004.

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http://www.mpoweruk.com/bms.htm

http://www.mpoweruk.com/communications.htm%23smbus

http://www.electronics-manufacturers.com/info/optoelectronics/optocoupler-optical-coupler.html

http://www.electronics-manufacturers.com/info/optoelectronics/optocoupler-optical-coupler.html

EMI Prevention of CAN-Bus-Based Communication in … · EMI Prevention of CAN-Bus-Based Communication in Battery Management Systems Chin-Long Wey, Chung-Hsien Hsu*, Kun-Chun …

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