The theoretical research on electrostatic discharge (ESD) has been quite mature. In order to simulate and analyze electrostatic events, predecessors have designed many electrostatic discharge models.
Common electrostatic models are: human body model (HBM), charged device model, field induction model, field enhancement model, machine model and capacitive coupling model, etc. The chip level is generally tested with HBM, while electronic products are tested with the discharge model of IEC 61000-4-2. In order to standardize ESD testing, in terms of industrial standards, the European Community’s IEC 61000-4-2 has established a strict transient impact suppression standard; electronic products must meet this standard before they can be sold to the European Community Member states.
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 the laboratory are divided into contact discharge and air discharge according to the standard of IEC 61000-4-2. The model of the electrostatic generator is shown in Figure 1. According to contact discharge and air discharge, the discharge head is divided into two types: pointed and round.
The ESD waveform of IEC 61000-4-2 is shown in Figure 2. It can be seen that the main current of ESD is a rising edge with a rising edge of about 1nS. To eliminate this rising edge, the response time of the ESD protection device must be shorter than this time. The energy of electrostatic discharge is mainly concentrated in tens of MHz to 500 MHz. In many cases, we can consider from the frequency spectrum, such as filters to filter out the energy of the corresponding frequency band to achieve electrostatic protection. The discharge frequency spectrum is as follows. This picture is drawn by myself. It can only be viewed qualitatively, not quantitatively.
IEC 61000-4-2 specifies several test levels. The current CTA test for mobile phones is performed at level 3, namely, contact discharge 6KV and air discharge 8KV. Many mobile phone manufacturers implement higher levels of electrostatic protection internally.
When an integrated circuit (IC) is subjected to electrostatic discharge (ESD), the resistance of the discharge circuit is usually very small and cannot limit the discharge current. For example, when a static-charged cable is inserted into the circuit interface, the resistance of the discharge circuit is almost zero, causing an instantaneous discharge spike current of up to tens of amperes to flow into the corresponding IC pin. The instantaneous high current will seriously damage the IC, and the local heating heat will even melt the silicon die. The damage of ESD to IC also includes internal metal connections being burned, passivation layer being damaged, and transistor cells being burned out.
ESD can also cause IC deadlock (LATCHUP). This effect is related to the activation of the thyristor-like structural unit inside the CMOS device. High voltage can activate these structures to form a high current channel, generally from VCC to ground. The deadlock current of serial interface devices can be as high as 1A. The deadlock current will remain until the device is powered off. But by then, the IC is usually burned out due to overheating.
Circuit-level ESD protection method
1. Parallel discharge devices
Commonly used discharge devices include TVS, Zener diode, varistor, gas discharge tube, etc. As shown
1.1 Zener Diodes (Zener Diodes, also known as Zener Diodes): Use the reverse breakdown characteristics of Zener diodes to protect ESD sensitive devices. However, the Zener diode usually has a capacitance of tens 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.
1.2 Transient Voltage Suppressor TVS (Transient Voltage Suppressor): TVS is a solid-state diode, specially used to prevent ESD transient voltage from destroying sensitive semiconductor devices. Compared with traditional Zener diodes, TVS diodes have a larger P/N junction area. This structural improvement makes TVS more capable of withstanding high voltages, and at the same time reduces the voltage cut-off rate, which is useful for protecting handheld devices. The safety of the voltage circuit has a better effect.
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 handle high transient currents caused by lightning and ESD. TVS also has junction capacitance, usually 0.3 pF to dozens of pF. TVS has unipolar and bipolar, please pay attention when using it. The TVS used on the mobile phone is about 0.01$, and the low-capacity value is about 2-3 cents.
1.3. Multilayer metal oxide structure device (MLV): China is generally called a varistor. MLV can also perform effective instantaneous high voltage surge suppression. This type of device has a non-linear voltage-current (impedance performance) relationship, and the cut-off voltage can reach 2 to 3 times the initial stop voltage. This feature is suitable for electrostatic or surge protection of circuits and devices that are not very sensitive to voltage, such as power circuits, key input terminals, etc. The varistor for mobile phones is about 0.0015$, which is about 1/6 of the price of TVS, but the protection effect is not as good as that of TVS, and the varistor has ageing life.
2. Series impedance
Generally, series resistors or magnetic beads can be used to limit the ESD discharge current to achieve the purpose of anti-static. As shown in the figure. For example, the high input impedance port of the mobile phone can be protected by a series of 1K ohm resistors, such as ADC, input GPIO, buttons, etc. Don't worry that the 0402 resistor will be broken, practice has proved that it cannot be broken. No detailed analysis here. Using resistors for ESD protection hardly increases the cost. If magnetic beads are used, the price of magnetic beads is about $0.002, which is similar to a varistor.
3. Add filter network
The energy spectrum of static electricity was mentioned earlier. If the main energy is filtered out by a filter, the purpose of static electricity protection can also be achieved.
For low-frequency signals, such as GPIO input, ADC, and audio input, 1k+1000PF capacitors can be used for electrostatic protection. The cost can be ignored, and the performance is not worse than varistor. If 1K+50PF varistor is used (composite protection described below) Measures), the effect is better. Experience has shown that this protection effect sometimes exceeds TVS.
For the microwave signal of the radio frequency antenna, if capacitive devices such as TVS tubes and pressure sensitive devices are used for electrostatic protection, the radio frequency signal will be attenuated. Therefore, the capacitance of the TVS is required to be very low, which increases the cost of ESD measures. For microwave signals, an inductance of tens of nH can be connected in parallel to the ground to provide a discharge channel for static electricity, which has almost no effect on the microwave signal. For mobile phones of 900MHZ and 1800MHz, an inductance of 22nH is often used. This can absorb a lot of energy in the main energy spectrum of static electricity.
4. Composite protection
There is a device called EMI filter, which has a good ESD protection effect, as shown in the figure. There are also TVS tube-based and varistor-based EMI filters. The former is effective but very expensive, and the latter is cheap. Generally, the price of 4-channel EMI based on varistor is 0.02$.
In practical applications, the following resistor + varistor can be used. It not only has the function of low-pass filter, but also the function of varistor, as well as the function of resistor series current limiting. It is the best cost-effective protection method. For high-impedance signals, 1K resistance + 50PF varistor can be used; for audio output signals such as headphones, 100 ohm resistance + varistor can be used; for TP signals, the series resistance should not be too large, otherwise it will affect the linearity of TP. , 10 ohm resistors can be used. Although the resistance is small and the effect of the low-pass filter is gone, the current limiting effect is still very important.
5. Increase the absorption circuit
You can add ground copper leakage to sensitive signal accessories to absorb static electricity. The principle is the same as that of a lightning rod. Placing a tip discharge point (spark gap) on the signal line is also often used in the design of copycat mobile phones.
