Although the CAN bus has strong anti-interference capabilities, it is still susceptible to electrostatic discharge and surge interference in practical applications. How can we improve the surge protection capability of the CAN bus in a network? It's actually not difficult; these devices will give you peace of mind.
First, let's understand some typical transient disturbances.
Table 1 Comparison of several transient disturbances
As shown in the table, surges have the highest energy and the largest overcurrent, and therefore pose the greatest risk. The following three types of surge protection devices are introduced.
1. TVS
Figure 1. Characteristic diagram of bidirectional TVS diode
Reverse cutoff voltage VRWM: The highest voltage at which the TVS does not conduct;
Clamping voltage VC: The voltage across a diode when it is conducting and a certain current flows through it; it increases with increasing current.
Reverse current IR: Reverse leakage current under VRWM voltage;
Breakdown voltage VBR: The voltage at which the TVS diode conducts when a specified test current IT (usually 1mA) is applied; it indicates the voltage at which the TVS diode is turned on.
Peak current (IPP): The maximum peak current that a TVS diode is allowed to pass through in a 10/1000μs or 8/20μs waveform. Exceeding this current may cause permanent damage. Due to power limitations, diodes with higher breakdown voltages allow a smaller peak current.
CJ: The junction capacitance of a TVS diode is much larger than that of an ESD device, and the unidirectional junction capacitance is larger than that of a bidirectional junction capacitance. The junction capacitance affects the response time of the TVS diode and limits the bus bandwidth when used in a communication bus.
Selection considerations: VC, IPP, CJ
2. Gas discharge tube
DC breakdown voltage VDC: The voltage at which the discharge tube breaks down when a voltage with a rising slope of 100V/s is applied. This is the nominal voltage value of the discharge tube, and this parameter varies considerably.
Pulse breakdown voltage (VSI): The voltage at which the discharge tube breaks down when a voltage with a rising slope of 1 kV/μs is applied. Impulse discharge current (ID): Divided into 8/20 μs and 10/1000 μs impulse discharge currents.
Selection considerations: VDC, ID
Datasheets for devices such as TVS diodes and gas discharge diodes typically provide test parameters for two current waveforms: 8/20μs and 10/1000μs. The main difference between the two is the duration and peak current. The 8/20μs peak current is in the kA range, while the 10/1000μs peak current is in the A range. The durations are shown in Figures 2 and 3, respectively.
Figure 28/20μs current waveform
Figure 310/1000μs current waveform
3. PTC resistor
In surge protection circuits, the PTC resistor acts as a current limiter and voltage divider, preventing overcurrent damage to the TVS diode and also boosting the voltage to enable the gas discharge tube to conduct. The characteristics of the PTC are shown in Figure 4 below.
Figure 4 PTC resistance characteristics
Maximum operating voltage VMAX: The maximum voltage that a PTC resistor can continuously withstand under the highest permissible temperature;
Holding current Ihold: The current that keeps the PTC resistance stable at its operating point value;
Trigger current (Itrip): The minimum current that can cause a step increase in the resistance value;
Rated zero-power resistance Rn: The initial resistance of the PTC at room temperature;
Switching temperature TC: The temperature at which the resistance increases in a step, at which point the resistance is twice the minimum resistance.
Models to consider: Rn, Itrip, Ihold.
4. Integrated surge protection solution
When building a system using custom components, it's not only necessary to consider the layout of components on the board, but also to select appropriate voltage ratings for the components. Even slight design flaws can prevent the system from meeting expected standards. Furthermore, human intervention during the production phase can affect consistency, making the process time-consuming and stressful. Zhiyuan Electronics has launched a comprehensive integrated surge protection solution.
CTM Series + Bus Protector
This bus protector integrates all surge protection devices into one unit, suitable for various signal transmission systems. It suppresses harmful signals such as lightning strikes, surges, and overvoltages, protecting equipment signal ports. This product is particularly suitable for surge protection in communication fields such as CAN and RS-485. When paired with ZLG's CTM series isolation modules, it can comprehensively enhance the surge protection capability of the bus. A recommended peripheral application diagram is shown in Figure 5 below.
Figure 5 Surge Recommended External Circuit Diagram
High protection level isolation module
The CTM1051(A)HP series is the latest high-protection-level isolated CAN transceiver from Zhiyuan Electronics. It conforms to the international ISO11898-2 standard, offering electrostatic discharge protection up to ±8kV for contact, ±15kV for air discharge, and ±4kV for surge protection. This isolated CAN solution is shown in Figure 6 and is suitable for various harsh industrial environments. It is easy to use, plug and play, as shown in Figure 7.
Figure 6 shows the EMC performance of the CTM1051(A)HP.
Figure 7 Application Principle Diagram
