The theoretical research of electrostatic discharge (ESD) has been quite mature. In order to simulate and analyze electrostatic events, many previous electrostatic discharge models have been designed.
Common electrostatic models include human body model (HBM), charged device model, field induction model, field enhancement model, machine model and capacitive coupling model. The chip level is generally tested with HBM, while the electronic product is tested with the discharge model of IEC 6 1000-4-2. In order to standardize the testing of ESD, in the respect of industry standards, the IEC 61000-4-2 of the European Community has established strict transient shock suppression standards; electronic products must meet this standard before they can be sold to the European Community. The various member states.
Therefore, most manufacturers regard IEC 61000-4-2 as the de facto standard for ESD testing. The national standard of our country (GB/T 17626.2-1998) is equivalent to I EC 6 1000-4-2. Most of the electrostatic generators used in the laboratory are classified into contact discharge and air discharge according to IEC 61000-4-2. The electrostatic generator model is shown in Figure 1. Discharge head according to the contact discharge and air discharge split tip and round head.
The waveform of the electrostatic discharge of IEC 61000-4-2 is shown in Fig. 2. It can be seen that the main current of electrostatic discharge is a rising edge with a rising edge of about 1nS. To eliminate this rising edge, it is required that the response time of the ESD protection device be less than this time. Electrostatic discharge energy is mainly concentrated in the tens of MHz to 500 MHz. In many cases, we can consider the spectrum, such as the filter to filter out the energy of the corresponding frequency band to achieve electrostatic protection. Its discharge spectrum is as follows, this picture is my own painting, can only qualitatively see, can not be quantitative.
IEC 61000-4-2 stipulates several test levels. At present, the mobile CTA test is performed at level 3, namely contact discharge 6KV and air discharge 8KV. Many mobile phone manufacturers perform higher static protection levels internally.
When the integrated circuit (IC) is subjected to electrostatic discharge (ESD), the resistance of the discharge loop is usually very small, and it is not possible to limit the discharge current. For example, when an electrostatically charged cable is plugged into a circuit interface, the resistance of the discharge loop is almost zero, causing instantaneous discharge spike currents of up to tens of amps to flow into the corresponding IC pin. A momentary high current can severely damage the IC, and the local heating heat can even melt the silicon die. ESD damage to ICs also includes internal metal connections being blown, passivation layers damaged, and transistor cells burned. ESD also causes the IC's deadlock (LATCHUP). This effect is related to the activation of thyristor-like structural units inside the CMOS device. A high voltage can activate these structures, forming a large current channel, typically from VCC to ground. Serial interface devices can have up to 1A of dead-lock current. The deadlock current will remain until the device is powered down. But then, ICs are usually already burned by overheating. Circuit-level ESD protection method 1, parallel discharge devices commonly used discharge devices TVS, Zener diodes, varistors, gas discharge tubes. Pictured
1.1 Zener Diodes (Zener Diodes): The use of Zener diodes for reverse breakdown protection of ESD sensitive devices. However, Zener diodes typically have a capacitance of tens of pF, which can cause signal distortion for high-speed signals (eg, 500 MHz). Zener diodes also have a good absorption of surges on the power supply.
1.2 Transient Voltage Suppressor (TVS): TVS is a solid-state diode specifically designed to prevent ESD transients from damaging sensitive semiconductor devices. Compared with traditional zener diodes, TVS diodes have a larger P/N junction area. This structural improvement allows TVS to have a higher voltage withstand capability while also reducing the voltage blocking rate, thus protecting the handheld device from low operation. The safety of the voltage loop is better. The TVS diode's transient power and transient current performance are directly proportional to the junction area. The diode's junction has a large cross-sectional area and can handle high transient currents caused by lightning and ESD. TVS also has junction capacitance, usually 0.3 pF to tens of pF. TVS is unipolar and bipolar and should be used with caution. The TVS used on mobile phones is about 0.01$, and the low-valued TVS is about 2-3 cents.
1.3. Multilayer Metal Oxide Structure Devices (MLV): The mainland is generally called a varistor. The MLV can also perform effective transient high-voltage impulse suppression. This type of device has a nonlinear voltage-current (impedance behavior) relationship with a cutoff voltage that is 2 to 3 times the initial shutdown voltage. This feature is suitable for electrostatic or surge protection of circuits and devices that are less sensitive to voltage, such as power circuits, key inputs, and so on. The varistor used in mobile phones is about 0.0015$, which is about 1/6th of the price of TVS. However, TVS is not as good as the protective effect, and the varistor has an aging life.
2, series impedance
Generally, ESD discharge current can be limited by series resistance or magnetic beads to achieve anti-static purposes. As shown. For example, the high input impedance port of a mobile phone can be protected by a series of 1K ohm resistors, such as ADCs, input GPIOs, and buttons. Don't worry about 0402's resistance being broken. Practice has proven to be unbreakable. Not analyzed in detail here. ESD protection with resistors adds little cost. If you use magnetic beads, the price of magnetic beads is about 0.002$, which is similar to varistors.
3, increase the filter network
As mentioned earlier, the electrostatic energy spectrum can be used to achieve the purpose of electrostatic protection if the filter filters out the main energy.
For low-frequency signals, such as GPIO input, ADC, audio input can use 1k+1000PF capacitor to do electrostatic protection, the cost is negligible, performance is not worse than varistor, if you use 1K+50PF varistor (combined protection Measure), the effect is better, experience has shown that this protective effect sometimes exceeds TVS.
For microwave signal of RF antenna, if TVS tube and pressure sensitive capacitive device are used for electrostatic protection, RF signal will be attenuated, so the capacitance of TVS is required to be very low, thus increasing the cost of ESD measures. For a microwave signal, an inductance of several tens of nH can be connected in parallel to the ground to provide a discharge channel for the static electricity, which has almost no influence on the microwave signal, and a 22nH inductor is often used for the 900MHZ and 1800MHz mobile phones. This can absorb much of the energy in the main energy spectrum of static electricity.
4, composite protection There is a device called EMI filter, he has a very good ESD protection effect, as shown. The EMI filter is also based on TVS tubes and based on varistors. The former is effective, but it is expensive. The latter is cheap, and the general 4-way varistor-based EMI price is 0.02$.
In practice, one of the following resistors + one varistor can be used. He has both a low-pass filter function and a varistor function, as well as a resistor-series current limiting function. Is the best cost-effective protection, for high impedance signal can be used 1K resistor +50 PF pressure sensitive; for audio output signals such as headphones can use 100 ohm resistor + varistor; for TP signal series resistance can not be too large or affect the linearity of TP You can use 10 ohm resistors. Although the resistance is small and the low-pass filter has no effect, the current limiting effect is still very important.
5, increase the absorption circuit can increase the leakage of copper in the sensitive signal accessories to absorb static electricity. The principle is the same as the lightning rod principle. Placing a sharp discharge point (a spark gap) on the signal line is also often used in the design of mobile phones.
With Yuchai Engine Diesel Generator
With Yuchai Engine Diesel Generator,Yuchai Diesel Generator,Yuchai Diesel Generator Set,Yuchai Generator
Shanghai Kosta Electric Co., Ltd. , https://www.ksdgenerator.com