Circuit-level electrostatic protection design skills and ESD protection methods

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 include: human body model (HBM), charged device model, field induction model, field enhancement model, machine model and capacitive coupling model, etc. HBM is generally used for testing at the chip level, while electronic products are tested using the discharge model of IEC 6 1000-4-2. In order to unify ESD testing, in terms of industrial standards, the European Community's IEC 61000-4-2 has established strict transient impact suppression standards; electronic products must meet this standard before they can be sold to the European Community. of each member state.

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 I EC 6 1000-4-2. Most electrostatic generators used in laboratories are divided into contact discharge and air discharge according to the standard of IEC 6 1000-4-2. 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 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 with 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 tens of MHz to 500MHz. In many cases, we can consider it from the spectrum, such as using a filter to filter out the energy in the corresponding frequency band to achieve electrostatic protection. The discharge spectrum is as follows. This picture was drawn by myself. It can only be viewed qualitatively, not quantitatively.


IEC 61000-4-2 stipulates several test levels. The current CTA test for mobile phones is level 3, which is 6KV contact discharge and 8KV air discharge. 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 loop is usually too small to limit the discharge current. For example, when a statically charged cable is plugged into a circuit interface, the resistance of the discharge circuit is almost zero, causing an instantaneous discharge peak current of up to tens of amps to flow into the corresponding IC pin. A large instantaneous current will seriously damage the IC, and the localized heat may even melt the silicon die. ESD damage to ICs also includes internal metal connections being burned out, passivation layers being damaged, and transistor units being burned out.

ESD can also cause IC deadlock (LATCHUP). This effect is related to the activation of thyristor-like structural units inside the CMOS device. High voltages activate these structures, forming a high-current channel, typically 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 down. But by then, the IC has usually burned out due to overheating.

Circuit Level ESD Protection Methods.

1. Parallel discharge devices

Commonly used discharge devices include TVS, Zener diodes, varistors, gas discharge tubes, etc. As shown in the pictureimage.png

1.1. Zener Diodes (Zener Diodes, also called Zener Diodes): The reverse breakdown characteristics of Zener diodes can be used to protect ESD sensitive devices. But Zener diodes typically have a capacitance of tens of pF, which for high-speed signals (e.g., 500MHz) can cause signal distortion. Zener diodes are also very good at absorbing surges 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 voltages from damaging sensitive semiconductor devices. Compared with traditional Zener diodes, TVS diodes have a larger P/N junction area. This structural improvement gives TVS a stronger high-voltage withstand capability and also reduces the voltage cut-off rate, making it ideal for protecting handheld devices at low operating costs. The safety of voltage circuit has better effect.

The transient power and transient current performance of TVS diodes are directly proportional to the junction area. The diode's junction has a large cross-sectional area to handle the high transient currents caused by lightning and ESD. TVS will also have junction capacitance, usually 0.3 pF to dozens of pF. TVS is available in unipolar and bipolar types, so be careful when using them. The TVS used on mobile phones is about 0.01$, and the low capacitance value is about 2-3 cents.

1.3. Multilayer metal oxide structure device (MLV): generally called varistor in mainland China. MLVs can also perform effective transient high-voltage surge suppression. Such devices have a nonlinear voltage-current (impedance behavior) relationship and a cut-off voltage that can reach 2 to 3 times the initial cut-off voltage. This feature is suitable for static electricity or surge protection of circuits and devices that are not 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. However, the protection effect is not as good as that of TVS, and the varistor has a lifespan and is subject to aging.

2. Series impedance

Generally, the ESD discharge current can be limited by connecting resistors or magnetic beads in series to achieve the purpose of anti-static. As shown in the picture. For example, the high input impedance ports of mobile phones can be protected by stringing 1K ohm resistors, 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. No detailed analysis here. Using resistors for ESD protection adds almost no cost. If magnetic beads are used, the price of magnetic beads is about 0.002$, which is similar to that of a varistor.


3. Add filter network

As mentioned earlier, the energy spectrum of static electricity can also be used to achieve the purpose of electrostatic protection if a filter is used to filter out the main energy.

For low-frequency signals, such as GPIO input, ADC, and audio input, 1k+1000PF capacitors can be used for electrostatic protection. The cost is negligible and the performance is no worse than a varistor. If a 1K+50PF varistor is used (composite protection discussed below) Measures), the effect is better, and experience has proved that the protective effect sometimes exceeds that of TVS.


For the microwave signal of the radio frequency antenna, if TVS tubes, pressure sensitive and other capacitive devices are used for electrostatic protection, the radio frequency signal will be attenuated, so the capacitance of the 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 will have almost no impact on the microwave signal. 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.


4. Composite protection

There is a device called EMI filter, which has very good ESD protection effect, as shown in the picture. There are also EMI filters based on TVS tubes and varistor-based ones. The former is effective but very expensive, while the latter is cheap. Generally, the price of 4-channel EMI based on varistor is 0.02$.


In practical applications, the following method of a resistor + a varistor can be used. It has the functions of a low-pass filter, a varistor, and a series resistor current limiting function. It is the most cost-effective protection method. For high-impedance signals, you can use 1K resistor + 50PF varistor; for audio output signals such as headphones, you can use 100 ohm resistor + varistor; for TP signals, the series resistance should not be too large otherwise it will affect the linearity of TP. , you can use a 10 ohm resistor. Although the resistance is small and the low-pass filter effect is gone, the current limiting effect is still very important.


5. Increase absorption loop

Ground leakage copper can be added to sensitive signal accessories to absorb static electricity. The principle is the same as that of a lightning rod. Placing tip discharge points (spark gaps) on signal lines is also often used in the design of copycat mobile phones.

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