EMC design scheme of CAN bus

One . CAN Interface EMC Design Overview

Controller Area Network Referred to as CAN, It is mostly used in automobile and industrial control , For data transmission control . In the process of application, the communication cable is easy to be coupled with external interference, which will have a certain impact on signal transmission , The interference inside the board may also form external radiation through the cable .

This scheme starts from EMC On the principle of , Schematic diagram of the interface 、PCB、 The structure and cable shall be designed for interference suppression and anti sensitivity , Solve from the design level EMC problem .

This scheme can pass the automotive electronic standard CISPR 25-2008 And ISO7637 Series standard , Or you can pass the following test items ：

Serial number	Interface test items	Test level	Performance criterion
1	Radiation emission (RE)	CISPR25 LV5
2	Conducted emission （CE）	CISPR25 LV5
3	RF magnetic field immunity (RI)	400~4000MHz  150V/m Dwell time  2s	Performance criterion A
4	High current injection BCI	1~400MHz 150mA 150mm  Dwell time   2s	Performance criterion A
5	Conducted immunity 7637-2 （24V System ）	wave form 1：Us=-450V 10000 pulses wave form 2a：Us=+50V 10000 pulses wave form 2b：Us=+20V 20 pulses wave form 2a：Us=-150V 1h wave form 3b：Us=+150V 1h wave form 5b：Us=-+30V Td=250ms	Performance criterion C Performance criterion A Performance criterion C Performance criterion A Performance criterion A Performance criterion A
6	Electrostatic immunity ESD	Contact ：+-8KV air ：+-15KV	Performance criterion A

 

Two .  Schematic design scheme

 

1.  CAN Interface anti-static design

 

chart 1 CAN Interface anti-static design

circuit EMC Design description ：

 

（1）  Key points of circuit filter design ：

L1 For common mode inductance , It is used to filter the common mode interference on the differential line , The impedance selection range is 120Ω/100MHz ~2200Ω/100MHz, Typical value selection 600Ω/100MHz;

C1、C2 Is the filter capacitance on the signal line , Provide a low impedance return path for interference , The selection range of capacitance value is 22PF~1000pF, Typical value selection 100pF;

C3 It is the jumper capacitance between interface ground and digital ground , The typical value is 1000pF, The withstand voltage shall meet the requirements of 2KV above ,C3 The capacitance value can be adjusted according to the test situation ;

（2）  Key points of circuit protection design ：

D1、D2 It is a transient suppression diode , Typical selection requires reverse shutdown voltage 3.5V above ; because TVS Just for electrostatic protection ,TVS The power is not required .TVS The junction capacitance of the transistor has a certain influence on the signal transmission ,CAN It is recommended that the junction capacitance be less than 100pF Of TVS tube .

 

Interface circuit design remarks ：

If the equipment is metal enclosure , At the same time, the board can be divided into interfaces independently , Then the metal shell is directly electrically connected with the interface ground , And the single board and the interface pass through 1000pF Capacitors are connected ; If the equipment is non-metallic shell , So the interface PGND And veneer ground GND Direct electrical connection .

 

3、 ... and . PCB design scheme

 

1.  CAN Interface division design

chart 2 CAN Interface division design

 

  Program features ：

（1）    In order to suppress the high-frequency noise of the internal board, the radiation is transmitted to the outside through the interface , In order to enhance the anti-interference ability of the single board to external interference . stay CAN Protective and filter isolation devices shall be added at the interface , It is bounded by the position and size of the isolation device , Divide the interface ground ;

（2）    The capacitance can be selectively increased in the isolation band as the connection between the two ground , The recommended value of capacitance is 1000pF; Signal line series common mode inductance filtering , And the common mode inductance is required to be placed in the isolation band ; In order to prevent strong external interference, it is coupled into the interior through the port PCB, Cause performance degradation of internal devices , Add protective devices on the signal line near the port TVS tube , The specific layout is shown in the figure .

Scheme analysis ：

（1）    When there are circuits with poor compatibility or incompatibility between the interface and the board , The interface needs to be connected with the board “ Division of land ” Handle , That is, according to different port voltages 、 Set the ground wire according to the signal level and transmission rate .“ Division of land ”, It can prevent the superposition of return signals of incompatible circuits , Prevent impedance coupling of common ground wire ;

（2）   CAN The interface signal transmission rate is high , Inside PCB The high-frequency noise of the board is easily transmitted and radiated from the common ground through the interface , Therefore, the common ground is divided and connected through capacitance , It can block the propagation path of common mode interference .

 

2.  CAN Interface circuit layout

chart 3 CAN Interface filtering and protection circuit layout

 

Program features ：

（1） Protective devices and filter devices shall be placed close to the interface and shall be placed compactly and neatly , Protective devices on signal lines TVS The tube and filter capacitor shall be connected to the interface ground ; Place the device according to the signal flow direction , When routing, try to avoid routing twists and turns ;

（2） Common mode inductance and jumper capacitance shall be placed in the isolation band .

 

Scheme analysis ：

（1） The interface and interface filter protection circuit shall not be wired and high-speed or sensitive devices shall not be placed around ;

（2） The projection layer below the isolation belt shall be hollowed out , No routing .

 

Four ． Structural design scheme

 

1. CAN Signal connector pin definition

chart 4 CAN Signal connector

 

Connector signal pin definition

（1）  If it is a single group CAN The signal , Then select two adjacent pins as the differential signal ;

（2）  If multiple groups CAN The signal is in the same connector , Then a ground pin should be defined between each group of signals to isolate each other . As shown in the figure above ：

 

5、 ... and .  Cable design scheme

 

1. CAN Signal cable assembly

chart 5 CAN Signal cable assembly

 

Cable design ：

（1） CAN The signal cable adopts the shielding method of mesh braided shielding layer , And the braiding density of the mesh braid shall not be less than 90%;

（2）  When assembling lines inside , The differential cable adopts twisted pair transmission , The twisted pair pitch is generally 3 times ; The wiring method is shown in the above figure ：

（3）  Magnetic rings need to be added at both ends of the cable , The inner diameter of the magnetic ring should be closely combined with the outer diameter of the cable , Try to choose thick and long magnetic rings .

Wiring design ：

（1） CAN The signal cable shall be far away from other strong interference sources , Such as power module ;

（2）  It is better to route the cable separately or keep it with other analog and power cables 250px The distance above , Do not mix and bind with other cables .

EDP The software is introduced

EMC design platform (EDP) , According to the most professional EMC Expert program knowledge base , Quickly output the guidance that meets the requirements of product design EMC Solution .

Main function modules ：

 

EDP Software workflow