ESD protection circuit design for pcb

Static electricity from the human body, the environment, and even inside electronic devices can cause various damages to precision semiconductor chips, such as:

Penetrating the thin insulating layer inside the components;
Destroying the gate of MOSFET and CMOS components;
Locked triggers in CMOS devices;

Short-circuiting reverse-biased PN junctions;
Short-circuiting forward-biased PN junctions;
Melt welding wires or aluminum wires inside active devices.
In order to eliminate the interference and damage of electrostatic discharge (ESD) to electronic equipment, a variety of technical means need to be taken to prevent it.
In the design of PCB boards, we can achieve PCB anti-ESD design through layering, proper layout and installation.
In the design process, most design modifications can be limited to adding or removing components through prediction. By adjusting the PCB layout and wiring, ESD can be well prevented. The following is a typical general ESD protection circuit:

CAN Bus protection data line and interface protection share personal ESD protection 9 measures
Recently, I have been doing ESD testing of electronic products. From the test results of different products, I found that this ESD is a very important test.
If the circuit board is not designed well, when static electricity is introduced, it will cause the product to freeze or even damage the components.
In the past, we only noticed that ESD would damage components, but we didn’t expect that electronic products should also be given enough attention.
ESD is what we often call electrostatic discharge. From what we have learned, we know that static electricity is a natural phenomenon, usually generated by contact, friction, induction between electrical appliances, etc. Its characteristics are long-term accumulation, high voltage (can generate several thousand volts or even tens of thousands of volts of static electricity), low power, small current and short action time.
For electronic products, if the ESD design is not well designed, it often causes electronic and electrical products to operate unstably or even damaged.
Two methods are usually used when doing ESD discharge testing: contact discharge and air discharge.
Contact discharge is to discharge directly to the device under test; air discharge is also called indirect discharge, which is generated by the coupling of strong magnetic fields to adjacent current loops.
The test voltages of these two tests are generally 2KV-8KV, and the requirements in the same region are different. Therefore, before designing, you must first understand the market for the product.
The above two situations are basic tests for electronic products that cannot work when the human body comes into contact with electronic products due to human body electrification or other reasons.
Humidity conditions vary around the world, but at the same time, in a region, if the air humidity is different, the static electricity generated is also different.
From the collected data, it can be seen that static electricity increases as the air humidity decreases, which indirectly explains why static electricity sparks are generated when taking off sweaters in the winter in the north.
Since static electricity is so harmful, how can we protect it? When we design static electricity protection, we usually take three steps:
Prevent external charges from flowing into the circuit board and causing damage;
Prevent external magnetic fields from damaging the circuit board;
Prevent the harm caused by static electricity.
In actual circuit design, we will use one or more of the following methods for static electricity protection:

1. Avalanche diode for static electricity protection
   This is also a method often used in design. The typical practice is to connect avalanche diodes in parallel to the ground on the key signal lines.
   This method uses the avalanche diode to respond quickly and has the ability to clamp stably, which can consume the accumulated high voltage in a short time and protect the circuit board.
2. Use high-voltage capacitors for circuit protection
   This practice usually places ceramic capacitors with a withstand voltage of at least 1.5KV at the I/O connector or key signal position, and the connecting wires are as short as possible to reduce the inductive reactance of the connecting wires. If a capacitor with a low withstand voltage is used, it will cause damage to the capacitor and lose its protective function.
3. Use ferrite beads for circuit protection
   Ferrite beads can effectively attenuate ESD current and suppress radiation. When faced with two problems, a ferrite bead would be a very good choice.
4. Spark gap method
   This method was seen in a material. The specific method is to use triangular copper foils with mutually aligned tips in the microstrip line layer composed of copper foils. One end of the triangular copper foil is connected to the signal line, and the other triangular copper foil is connected to the ground. When there is static electricity, tip discharge will occur and consume electrical energy.
5. Use LC filter method to protect the circuit
   The filter composed of LC can effectively reduce the high-frequency static electricity entering the circuit.
   The inductive reactance characteristics of the inductor can effectively suppress the high-frequency ESD from entering the circuit, and the capacitor diverts the high-frequency energy of ESD to the ground.
   At the same time, this type of filter can also smooth the signal edge and reduce the RF effect, and the performance has been further improved in terms of signal integrity.
6. Multilayer board for ESD protection
   When funds allow, choosing a multilayer board is also an effective means to prevent ESD.
   In a multilayer board, since there is a complete ground plane close to the trace, ESD can be coupled to the low-impedance plane more quickly, thereby protecting the key signal.
7. The method of leaving a protective belt around the circuit board
   This method usually draws traces without a soldering layer around the circuit board.
   If conditions permit, connect the trace to the housing, and pay attention to the fact that the trace cannot form a closed loop, so as not to form a loop antenna and introduce greater trouble.
8. Use CMOS devices or TTL devices with clamping diodes to protect the circuit board
   This method uses the principle of isolation to protect the circuit board. Since these devices are protected by clamping diodes, the complexity of the design is reduced in actual circuit design.
9. Use more decoupling capacitors
   These decoupling capacitors should have low ESL and ESR values. For low-frequency ESD, decoupling capacitors reduce the loop area. Due to the effect of ESL, the electrolyte effect is weakened, which can better filter out high-frequency energy.
   In short, although ESD is terrible and may even bring serious consequences, only by protecting the power supply and signal lines on the circuit can the current of ESD be effectively prevented from flowing into the PCB.