Influence of static electricity on feedback circuit of core board and protective measures

When winter comes, the human body can release static electricity of up to tens of thousands of volts. At the end of the year, the project is also in the final stage. Don't let the motherboard burn out by directly touching the live motherboard with static electricity!

Figure 1 Human hands and circuit boards can not be too close?

It is a common knowledge of electronics engineers that human hands cannot directly touch or touch a circuit board. Why?

First, the harm of static electricity

In different environments, the electrostatic voltage carried by the human body ranges from a few hundred volts to several tens of thousands of volts. Contact with electronic components (conductors) can cause electrostatic discharge, which can damage the device and reduce reliability. In severe cases, electrostatic discharge can cause breakdown of the device and cause the product to be directly scrapped.

In addition, the electrostatic discharge process simultaneously radiates radio waves at a certain frequency, interfering with the peripheral microprocessor, causing the application to run disorderly, which seriously affects the normal operation of the device.

Figure 2 pay attention to static electricity

Therefore, in daily production and work, it is necessary to standardize the operation (including the correct wearing of the electrostatic ring); try to avoid direct contact with electronic components, especially in the charged state.

Second, the impact of static electricity on the feedback circuit of the core board

Take the M3352 industrial grade core board (Cortex-A8 core) as an example (the core board passed the electromagnetic compatibility industry level 4 test, including static electricity, surge, pulse group, conducted disturbance, etc.), in the power-on state, the engineer Touching the yellow circle position shown in Figure 3 can induce a system restart. What is the reason for this?

Figure 3 M3352 industrial grade core board

The yellow circled area (R80, C116, R79, R78) of Figure 3 is the negative feedback and compensation network (FB part) of the system main power supply 3.3V DC-DC, as shown in Figure 4.

Figure 4 system power circuit

The human touch (the introduction of interference by the human equivalent resistance) changes the characteristics of the feedback loop (including the offset of the pole), causing the feedback loop to oscillate, resulting in unstable DC-DC output voltage.

The output voltage of the DC-DC is unstable. The light system causes the system to restart (DC-DC automatically reduces the output voltage). In severe cases, the main chip of the board (the DC-DC automatically increases the output voltage) is burned out. .

Figure 5: The motherboard is live with no touch

Third, how to prevent static electricity

1, anti-static cordless wrist strap

Figure 6 anti-static cordless wrist strap

According to the "corona discharge" effect and the principle of tip discharge, when the accumulated charge exceeds a certain value, the potential difference is discharged to the space, thereby achieving the purpose of eliminating static electricity. Features: Convenient and reliable, mobile positions are allowed to wear. Static dissipative time: less than 0.5s.

2, anti-static wrist strap

Figure 7 anti-static wrist strap

The human skin is in direct contact with the conductive material on the wrist strap. When the wrist strap is grounded, the static electricity generated by the movement of the human body is quickly released through the grounding system. Static discharge time: less than 0.1s.

3, anti-static PU coated gloves

Figure 8 Anti-static PU coated gloves

Anti-static PU gloves further prevent human static electricity from being transmitted to the board through hand contact. The distance between the conductive wires of the back of the hand is 10 mm.

Fourth, the core board against static electricity protection measures

Industrial-grade core board protection against static electricity Here, ZLG Zhiyuan Electronics' M3352 industrial-grade core board is taken as an example. Figure 9 shows the M3352 core board, and Figure 10 shows the M3352 evaluation board.

M3352 uses AM3352 of Cortex-A8 platform, clocked at 800MHz, dual CAN, dual network port, 6 serial port, working temperature -40 °C ~ +85 °C. The M3352 core board is equipped with an evaluation board for electrostatic protection from both hardware and software protection.

Figure 9 M3352 industrial grade core board

Figure 10 M3352 Evaluation Kit

Hardware Protection - Electromagnetic Compatibility Industry Level 4

Electrostatic discharge immunity: air discharge ± 15KV, contact discharge ± 8KV;

Electrical fast transient pulse group: interference frequency 5KHz, 100KHz, pulse group time 300 (1±20%) ms;

Lightning strike (surge) immunity: 2KV capacitive coupling coupled with gas discharge tube, 1 time / minute;

Conducted disturbance immunity: 3V capacitive coupling coupled with gas discharge tube, test frequency 150KHz ~ 80MHz.

Software Protection - Dual System Architecture Design

The operating system is usually stored in Flash. Frequent erasing of Flash is prone to bad blocks. Static interference of the product and accidental power failure may cause the operating system to be lost and the file system to be damaged. The M3352 core board is redundantly designed with a backup system outside the normal operating system. In the case of bad blocks in the normal storage system of the Flash, the backup system is automatically started to ensure that the product can still work stably.