Circuit-level electrostatic protection design techniques and ESD protection methods

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Circuit-level electrostatic protection design techniques and ESD protection methods [Copy link]

The research on electrostatic discharge (ESD) theory is quite mature. In order to simulate and analyze electrostatic events, many electrostatic discharge models have been designed by predecessors. Common electrostatic models include: human body model (HBM), charged device model, field induction model, field enhancement model, machine model and capacitive coupling model. HBM is generally used for chip level testing, while electronic products are tested with the discharge model of IEC 6 1000-4-2. In order to unify the standards for ESD testing, in terms of industrial standards, the European Community's IEC 61000-4-2 has established strict transient impulse suppression standards; electronic products must meet this standard before they can be sold to various member states of the European Community. Therefore, most manufacturers regard IEC 61000-4-2 as the de facto standard for ESD testing. my country's national standard (GB/T 17626.2-1998) is equivalent to IEC 61000-4-2. Most laboratory electrostatic generators are divided into contact discharge and air discharge according to the IEC 61000-4-2 standard. The model of the electrostatic generator is shown in Figure 1. The discharge head is divided into two types: pointed head and round head according to contact discharge and air discharge. The waveform of the electrostatic discharge of IEC 61000-4-2 is shown in Figure 2. It can be seen that the main current of the electrostatic discharge is a rising edge of about 1nS. To eliminate this rising edge, the response time of the ESD protection device must be less than this time. The energy of electrostatic discharge is mainly concentrated in the range of tens of MHz to 500MHz. In many cases, we can consider it from the spectrum, such as filtering out the energy of the corresponding frequency band to achieve electrostatic protection. The discharge spectrum is as follows. This figure is drawn by myself and can only be viewed qualitatively, not quantitatively. IEC 61000-4-2 stipulates several test levels. Currently, the mobile phone CTA test is performed at level 3, that is, contact discharge 6KV and air discharge 8KV. Many mobile phone manufacturers implement higher electrostatic protection levels 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 cable with static electricity is plugged into a circuit interface, the resistance of the discharge loop is almost zero, causing an instantaneous discharge peak current of up to tens of amperes to flow into the corresponding IC pins. The instantaneous high current can seriously damage the IC, and the local heat can even melt the silicon die. ESD damage to ICs also includes internal metal connections being burned off, passivation layers being damaged, and transistor units being burned out. ESD can also cause IC latchup. This effect is related to the activation of thyristor-like structural units inside CMOS devices. High voltage can activate these structures to form a high current channel, generally from VCC to ground. The latchup current of serial interface devices can be as high as 1A. The latchup current will remain until the device is powered off. However, by then, the IC has usually been burned out due to overheating.

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Circuit-level ESD protection method 1. Parallel discharge devices Commonly used discharge devices include TVS, Zener diodes, varistors, gas discharge tubes, etc. As shown in the figure 1.1. Zener diodes (Zener Diodes, also known as voltage regulator diodes): The reverse breakdown characteristics of Zener diodes can be used to protect ESD sensitive devices. However, Zener diodes usually have a capacitance of tens of pF, which will cause signal distortion for high-speed signals (such as 500MHz). Zener diodes also have a good absorption effect on surges on the power supply. 1.2. Transient Voltage Suppressor TVS: TVS is a solid-state diode specifically used to prevent ESD transient voltages from destroying sensitive semiconductor devices. Compared with traditional Zener diodes, TVS diodes have a larger P/N junction area. This structural improvement enables TVS to have a stronger high-voltage tolerance and also reduces the voltage cutoff rate, so it has a better effect on protecting the safety of low-working voltage circuits of handheld devices. The transient power and transient current performance of TVS diodes are proportional to the area of the junction. The junction of this 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 tens of pF. TVS has unipolar and bipolar types, so be careful when using it. The TVS used in mobile phones is about $0.01, and the low-capacitance is about $2-3 cents. 1.3. Multilayer metal oxide structure device (MLV): generally called varistor in mainland China. MLV can also effectively suppress transient high-voltage impact. Such devices have a nonlinear 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 supply circuits, key input terminals, etc. The varistor used in 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 TVS, and the varistor has life aging.

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2. Series impedance Generally, the ESD discharge current can be limited by series resistors or magnetic beads 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 1K ohm resistor, such as ADC, input GPIO, buttons, etc. Don't worry that the 0402 resistor will be damaged. Practice has proved that it cannot be damaged. I won't analyze it in detail here. Using resistors for ESD protection almost does not increase the cost. If magnetic beads are used, the price of magnetic beads is about 0.002$, which is similar to that of varistors. 3. Add filtering network As mentioned above, the energy spectrum of static electricity can be achieved if the main energy is filtered out by a filter. 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 that of varistors. If 1K+50PF varistors are used (composite protection measures described below), the effect is better. Experience has shown that this protection effect sometimes exceeds that of TVS. For microwave signals of RF antennas, if TVS tubes, varistors and other capacitive devices are used for electrostatic protection, the RF signal will be attenuated. Therefore, the capacitance of TVS is required to be very low, which increases the cost of ESD measures. For microwave signals, an inductor 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 microwave signals. For 900MHZ and 1800MHz mobile phones, 22nH inductors are often used. This can absorb a lot of energy on the main energy spectrum of static electricity.

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4. Composite protection There is a device called EMI filter, which has a good ESD protection effect, as shown in the figure. EMI filter also has TVS tube-based and varistor-based ones. The former has a good effect but is very expensive, while the latter is cheap. Generally, the price of a 4-way EMI based on varistor is 0.02$. In actual applications, the following one resistor + one varistor can be used. It has both the function of a low-pass filter and the function of a varistor, as well as the function of resistor series current limiting. It is the most cost-effective protection method. For high-impedance signals, 1K resistor + 50PF varistor can be used; for audio output signals such as headphones, 100 ohm resistor + varistor can be used; for TP signals, the series resistor cannot be too large, otherwise it will affect the linearity of TP, and a 10 ohm resistor can be used. Although the resistance is small, the low-pass filter effect is no longer there, but the current limiting function is still very important. 5. Add absorption circuit You can add ground leakage copper near sensitive signals to absorb static electricity. The principle is the same as that of lightning rods. Placing a sharp discharge point (spark gap) on the signal line is also often used in the design of knockoff mobile phones.