CAN bus interface protection circuit design guide

The CAN bus has a wide range of applications and the application environment is quite complex. Some interferences such as static electricity and surges are easily coupled to the bus and directly act on the CAN bus interface. In order to meet some high-level EMC requirements, it is necessary to add additional peripheral protection circuits.

Why protection circuit is needed

General CAN transceiver chips have low ESD and surge protection levels. For example, the SM1500 isolated CAN transceiver has an isolation withstand voltage of 3500VDC. In the case of bare metal, the CAN interface ESD can reach 6kV, but it cannot meet common surge test requirements. Industrial products have higher EMC level requirements for communication interfaces. Many applications require IEC61000-4-2 electrostatic discharge level 4, IEC61000-4-5 surge immunity level 4, etc. In this case, necessary protection must be added. circuit to meet the requirements.

Interface protection and working principle

1. Recommended circuit

Figure 1 shows the recommended protection circuit for the CAN interface. Reasonable protection can greatly improve the anti-interference ability of the interface. The bus interface protection is divided into three levels. The first level realizes large energy discharge, the second level implements current limitation, and the third level implements voltage clamping. Circuits at all levels perform their own duties and work together to achieve the best protection effect.

Figure 1 Recommended protection circuit for CAN interface

2. Working principle - differential mode circuit

As shown in Figure 2, when a differential mode interference voltage is applied to pins 1 and 2 of the interface, TVS1 responds fastest and is turned on first, and the voltage between the chip bus pins CANH and CANL is clamped. The R2 and R3 resistors limit the current flowing through TVS1 to prevent it from being damaged by overpower. GDT responds the slowest and turns on last, dissipating most of the energy and limiting the residual voltage to a low level.

Figure 2 Schematic diagram of differential mode leakage circuit

3. Working principle - common mode loop

As shown in Figure 3, in order to ensure good protection effect, the communication reference ground CANG should be grounded at a single point after networking. When a common-mode interference voltage is applied to pins 1 and 2 of the interface, TVS1 responds fastest and is turned on first, and the voltage between the chip bus pin and CANG is clamped. The R2 and R3 resistors limit the current flowing through TVS1 to prevent it from being damaged by overpower. GDT responds the slowest and turns on last, dissipating most of the energy and limiting the residual voltage to a low level.

Figure 3 Common mode leakage circuit diagram

Application Notes

1. The protection circuit must be reliably grounded

Common-mode interference requires the earth (or protective ground) as a discharge circuit, and the protection circuit must be reliably grounded. Otherwise, the common-mode protection part has no return path, and the protection circuit fails, which may cause damage to the front-end chip or circuit, as shown in Figure 4.

Figure 4 Ungrounded common-mode current path

2. Reduce the introduced capacitance as much as possible 

The CAN bus has extremely high requirements on bus capacitance, and the equivalent capacitance of the protection circuit itself should be reduced as much as possible. Designed according to the circuit structure recommended in Figure 1, the total differential capacitance of the protection circuit can be controlled at around 10pF, which while providing sufficient protection, basically avoids the impact on CAN bus communication.

Design example

Design a protection circuit that meets IEC61000-4-5 Class4

According to the standard, Class4 is suitable for applications where communication interconnection lines are deployed outdoors.

This level of surge test has an open circuit voltage of 4kV and a short circuit current of up to 100A. Since CAN cables are symmetrical communication lines, only line-to-ground (common mode) testing is required.

● Gas discharge tube: The flow rate can be 500A, the opening voltage is 90V, and the package is 1206. If there is enough space, you can choose a device with a larger flow rate to achieve better protection.

● Current-limiting resistor: The resistor should not be too large, otherwise the signal amplitude will be too low. Generally choose within 10 euros, such as 4.7 euros. If the TVS tube conduction voltage is 12V, the peak current flowing through the resistor is approximately (90-12)/4.7=16.5A. You should choose high-current resistors that can pass a peak current of 16.5A, such as winding resistors, PTC, etc. Do not choose ordinary metal film or carbon film resistors!

● TVS tube: The conduction voltage should be higher than the signal amplitude and lower than the maximum DC withstand voltage of the pin, such as 12V. The peak current should be greater than 2*16.5=33A, such as P6KE12CA.

● Diode: The reverse withstand voltage is greater than the maximum conduction voltage of TVS, and the peak current is greater than 16.5A, such as 1N4007.

Products Recommended

Zhiyuan Electronics provides a complete CAN interface product solution, which can save customers from the design troubles of CAN interface and protection circuits. The SM1500 isolated CAN receiver chip, combined with the SP00S12 surge protection module, can easily meet the IEC61000-4-5 test requirements and provide customers with a compact and highly reliable CAN bus interface.

Figure 5 Typical connection between SM1500 and SP00S12