Shulou(Shulou.com)05/31 Report--

Most people do not understand the knowledge points of this article "what are the ESD protection methods?", so the editor summarizes the following content, detailed content, clear steps, and has a certain reference value. I hope you can gain something after reading this article. Let's take a look at this "what are the ESD protection methods?"

The theoretical study of electrostatic discharge (ESD) has been quite mature. In order to simulate and analyze electrostatic events, many electrostatic discharge models have been designed. The common electrostatic models are: human body model (HBM), live device model, field induction model, field enhancement model, machine model and capacitance coupling model and so on. The chip level is generally tested by HBM, while electronic products are tested by the discharge model of IEC 1000-4-2. In order to standardize the testing of ESD, in terms of industrial standards, the European Community's IEC 61000-4-2 has established a strict transient shock suppression standard; electronic products must meet this standard before they can be sold to the member states of the European Community.

Therefore, most manufacturers regard IEC 61000-4-2 as the de facto standard for ESD testing. China's national standard (GB/T 17626.2-1998) is equivalent to I EC 6 1000-4-2. Most of the electrostatic generators used in laboratories are divided into contact discharge and air discharge according to the standard of IEC 1000-4-2. The model of the electrostatic generator is shown in figure 1. The discharge head is divided into sharp head and round head according to contact discharge and air discharge.

The waveform of electrostatic discharge of IEC 61000-4-2 is shown in figure 2. It can be seen that the main current of electrostatic discharge is a rising edge around the 1nS. To eliminate this rising edge, the response time of the ESD protection device is less than this time. The energy of electrostatic discharge is mainly concentrated in dozens of MHz to 500MHz. In many cases, we can consider it from the spectrum, such as filtering the energy of the corresponding frequency band to achieve electrostatic protection.

IEC 61000-4-2 specifies several test levels. At present, mobile phone CTA tests are carried out at level 3, that is, contact discharge 6KV and air discharge 8KV. Many mobile phone manufacturers implement higher levels of electrostatic protection internally.

When the integrated circuit (IC) is subjected to electrostatic discharge (ESD), the resistance of the discharge circuit is usually very small, which can not limit the discharge current. For example, when the cable with static electricity is plugged into the circuit interface, the resistance of the discharge circuit is almost zero, resulting in an instantaneous discharge spike current as high as tens of amperes, which flows into the corresponding IC pin. Instantaneous high current will seriously damage the IC, and the local heat will even melt the wafer core. The damage caused by ESD to IC also includes the burning of the internal metal connection, the destruction of the passivation layer and the burnout of the transistor unit.

ESD can also cause IC's deadlock (LATCHUP). This effect is related to the activation of thyristor-like structural units in CMOS devices. High voltage can activate these structures to form large current channels, usually from VCC to ground. The deadlock current of serial interface devices can be as high as 1A. The deadlock current remains until the device is powered off. By then, however, the IC will usually have been burnt out due to overheating.

Circuit-level ESD protection methods

1. Parallel discharge devices

The commonly used discharge devices are TVS, Zener diode, varistor, gas discharge tube and so on. As shown in the picture

1.1.Zener diode (Zener Diodes, also known as Zener diode): the reverse breakdown characteristic of Zener diode can protect ESD sensitive devices. But Zener diodes usually have a capacitance of dozens of pF, which can cause signal distortion for high-speed signals such as 500MHz. The Zener diode also has a good absorption effect on the surge on the power supply.

Transient Voltage canceller TVS (Transient Voltage Suppressor): TVS is a solid-state diode designed to prevent ESD transient voltage from damaging sensitive semiconductor devices. Compared with the traditional Zener diodes, the area of the TVS diode Phammer N junction is larger. The improvement in this structure makes the TVS have a stronger ability to withstand high voltage and reduce the voltage cutoff rate, so it has a better effect for protecting the safety of the low voltage circuit of handheld devices.

The transient power and transient current performance of TVS diodes are proportional to the junction area. The junction of the diode has a large cross-sectional area and can deal with the high transient current caused by lightning and ESD. TVS also has junction capacitors, usually 0.3 pF to dozens of pF. TVS is unipolar and bipolar, so you should pay attention to it when using it. The TVS used on the mobile phone is about 0. 01 $, and the low-volume value is about 2-3 cents.

1.3.Multilayer metal oxide structure devices (MLV): generally known as varistors in mainland China. MLV can also effectively suppress the instantaneous high voltage shock. This kind of device has a nonlinear voltage-current (impedance performance) relationship, and the cut-off voltage can reach 2 ~ 3 times of the initial stop voltage. This characteristic is suitable for electrostatic or surge protection of lines and devices that are less sensitive to voltage, such as power circuits, key inputs, etc. The varistor for mobile phone is about 0.0015 $, which is about 1 / 6 of the price of TVS, but the protective effect is not as good as TVS, and the varistor has aging life.

2. Series impedance

Generally, the discharge current of ESD can be limited by series resistors or magnetic beads to achieve the purpose of anti-static. As shown in the picture. For example, the high input impedance port of the mobile phone can be protected by 1K ohmic resistors, such as ADC, input GPIO, keys and so on. Don't worry that 0402 of the resistance will be broken, practice has proved to be unbreakable. There is no detailed analysis here. Using resistors to protect against ESD hardly increases the cost. If you use magnetic beads, the price of magnetic beads is about $0.002, which is similar to that of varistors.

3. Add filter network

The energy spectrum of static electricity mentioned above can also achieve the purpose of electrostatic protection if the main energy is filtered out by a filter.

For low-frequency signals, such as GPIO input, ADC, audio input can use the capacitance of 1k+1000PF to do electrostatic protection, the cost can be ignored, the performance is not worse than the varistor, if you use the varistor of 1K+50PF (the compound protection measures described below), the effect is better, experience has proved that the protective effect is sometimes better than TVS.

For the microwave signal of RF antenna, if TVS tube, varistor and other capacitive devices are used for electrostatic protection, the RF signal will be attenuated, so the capacitance of TVS is required to be very low, which increases the cost of ESD measures. For the microwave signal, dozens of nH inductors can be connected to the ground to provide a discharge channel for static electricity, which has almost no effect on the microwave signal. 22nH inductors are often used for 900MHZ and 1800MHz mobile phones. This absorbs a lot of energy from the main energy spectrum of static electricity.

4. Compound protection

There is a device called EMI filter, which has a good ESD protection effect, as shown in the picture. EMI filter is also based on TVS transistors and varistors. The former works well, but it is very expensive, while the latter is cheap. The price of a 4-way EMI based on varistors is generally 0.02 $.

In practical application, one of the following resistors + a varistor can be used. It not only has the function of low-pass filter, but also has the function of varistor and the function of resistor series current limit. Is the best cost-effective protection, for high resistance signal can use 1K resistance + 50PF varistor; for headphones and other audio output signal can use 100ohm resistor + varistor; for TP signal series resistance can not be too large otherwise affect the linearity of TP, can use 10 ohms resistor. Although the resistance is low and the effect of the low-pass filter is gone, the current limiting effect is still very important.

5. Increase the absorption circuit

Copper leakage can be added to sensitive signal accessories to absorb static electricity. The reason is the same as the principle of lightning rod. Placing a tip discharge point (spark gap) on the signal line is also often used in shanzhai mobile phone design.

The above is about the content of this article on "what are the methods of ESD protection". I believe we all have a certain understanding. I hope the content shared by the editor will be helpful to you. If you want to know more about the relevant knowledge, please follow the industry information channel.

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.