Static Electricity Protection Measures in Circuits

qwqwqw2088

Static Electricity Protection Measures in Circuits [Copy link]

1. Parallel discharge devices.

Commonly used discharge devices include TVS, Zener diodes, varistors, gas discharge tubes, etc.

1.1 Zener diodes (also called 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 several tens of pF, which can 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 that is specifically designed to prevent ESD transient voltage from damaging sensitive semiconductor devices. Compared with traditional Zener diodes, TVS diodes have a larger P/N junction area. This structural improvement makes TVS

It has stronger high-voltage bearing capacity and also reduces the voltage cut-off rate, so it is more effective in protecting the safety of the low-working voltage circuit 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, so be careful when using it. The TVS used in mobile phones is about 0.01$, and 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 characteristic is suitable for static or surge protection of circuits and devices that are not very sensitive to voltage, such as power supply circuits, key input terminals, etc. The price of 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 TVS, and the varistor has life aging.

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, as 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 varistors.

3. Add filter network

The energy spectrum of static electricity was mentioned earlier. If a filter is used to filter out the main energy, 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 no worse than that of varistors. If 1K+50PF varistors are used (the composite protection measures described below), the effect will be better. Experience has shown that such protection 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 signals 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 several 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.

qwqwqw2088

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 EMI filters. The former has a good effect but is very expensive, while the latter is cheap. Generally, the price of a 4-way EMI filter 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 resistor series current limiting function. It is the most cost-effective protection method. For high-impedance signals, a 1K resistor + 50PF varistor can be used; for audio output signals such as headphones, a 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 and the low-pass filter effect is no longer there, the current limiting function is still very important.

5. Increase the 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 copycat mobile phones.

Where should the TVS tube or varistor be placed on the flip phone receiver?

Many people think that ESD protection devices should be placed close to ports during layout, and that the TVS or varistor on the receiver (receiver, rec) of a flip phone or slider phone should be placed on the upper board near rec. This is not the case. Please see the analysis below.

This is an equivalent circuit of a flip phone or a slider phone. The TVS is on the upper board, the main IC is on the main board, and they are connected in the middle by a relatively long FPC. The rec is a coil, which is not afraid of static electricity, but the IC is afraid of static electricity.

The inductors in the middle are the equivalent inductances of the ground network and signal line on the FPC. When static electricity hits rec, due to the clamping effect of TVS and the existence of equivalent capacitance, the voltage Ub on the REC signal line and the voltage Ug on the small board at the moment of electrostatic discharge can be regarded as equal. The static charge must be transferred from the ground network of the FPC to the ground of the main board. At this time, a voltage difference Ug-0=Ug will be generated between the ground of the main board and the ground of the small board. For the signal line, if it is in the high-ancestor state, even if the FPC has a larger equivalent inductance, it will not generate current because of high impedance. Ua and Ub can be regarded as consistent, so Ua=Ub=Ug, that is, a voltage difference will be generated between the chip end and the ground. This voltage difference is almost equal to the voltage difference between the small board ground and the main board ground at the moment of discharge. If this voltage difference is large, the IC will be damaged. The size of the voltage difference depends on the level of ESD discharge and the equivalent impedance and inductance on the FPC ground. If it is a non-high-ancestor signal, there will be a little current, and the chip will be damaged if the current is large. This can also be approximately regarded as Ua=Ub=Ug. Therefore, if the static electricity of TVS or varistor reaches rec when placed on the upper board, there will be problems.

If static electricity hits the ground of the small board, due to the clamping and equivalent capacitance of the TVS or varistor, Ub=Ug. Due to the equivalent inductance of the FPC ground, there will be a voltage difference between Ug and the motherboard ground. There is no current Ua=Ub=Ug on the signal line. Ua added to the chip port will damage the chip.

If static electricity hits the mainboard ground, part of the charge on the mainboard ground will be transferred to the small board ground, generating current. The current generates voltage on the equivalent inductance of the FPC ground. Similarly, the voltage between the mainboard ground and the small board ground will be directly added to both ends of the chip and burn the chip.

If TVS or varistor is added to the motherboard, as shown in the figure, due to the clamping effect of TVS or varistor, the voltage between Ua and ground will not be too high, which can protect the chip. Static electricity is placed on rec, and the current is conducted to the motherboard and absorbed by TVS. The charge hits the small board ground and the main board ground and has nothing to do with the signal line.

Where should the anti-static device be placed?

This is true for rec, and also for other signal line power supplies.

The above analysis is about connecting two boards with FPC. In fact, even if there is one board, there will be impedance on the ground. So the closer the ESD protection device is to the chip, the better. Especially for boards without a complete ground layer.

But there is a problem that if the chip is close to the port to the TVS, there will be a large discharge current and voltage, which will interfere with the adjacent signal line for protection. Since the mobile phone motherboard has a complete ground layer and the equivalent impedance on the ground is relatively small, it is recommended to place the electrostatic protection device at the interface of the motherboard, but not on the small board, keypad, etc.

alan000345

It is very detailed and has great reference value in circuit design and protection.