December 09, 2019

Overview of ESD protection circuit and its circuit design

ESD Electrostatic discharge brings fatal harm to your electronic products. It not only reduces the reliability of the product, but also increases the maintenance cost and does not meet the European Community's industry standard EN61000-4-2, which will affect the sales of the products in Europe, so the electronics Device manufacturers typically consider ESD protection circuits early in the circuit design. This article discusses several methods for ESD protection circuits.

1 ESD generation and harm

Electrostatic discharge ESD occurs when two objects collide or separate, that is, the static charge moves from one object to another. The static charge moves between two objects with different potentials, similar to a small lightning process discharge. The size and duration of discharge depend on various factors such as the type of object and the surrounding environment. When ESD has high enough energy, it will cause damage to the semiconductor device. Electrostatic discharge ESD may occur at any time, such as plugging or unplugging cables or human contact devices. The /O port or a charged object contacts the semiconductor device. The semiconductor device touches the ground and the electrostatic field and electromagnetic interference generate a sufficiently high voltage to cause the electrostatic discharge ESD.

ESD can basically be divided into three types, one is ESD caused by various machines, the other is ESD caused by furniture movement or equipment movement, and the third is ESD caused by human contact or equipment movement, all three kinds of ESD for semiconductor devices. Production and production of electronic products are very important. The use of electronic products is most susceptible to damage from the third type of ESD. Portable electronic products are particularly vulnerable to human contact with ESD. ESD generally damages the interface devices connected to it. In this case, the device after the ESD strike may not be damaged immediately, but the performance may cause the product to malfunction prematurely.

When the integrated circuit IC is subjected to ESD, the resistance of the discharge loop is usually very small, and the discharge current cannot be limited. For example, when the cable with static electricity is inserted into the circuit interface, the resistance of the discharge loop is almost zero, causing an instantaneous discharge spike of up to several tens of amps. Current, flowing into the corresponding IC pin, a large current will seriously damage the heat generated by the IC local heat or even melt the silicon die ESD. The damage to the IC generally includes the internal metal connection being blown, the passivation layer is destroyed, and the transistor unit is burnt. Bad.

ESD can also cause IC deadlock LATCHUP This effect is related to the thyristor-like structural unit inside the CMOS device being activated. High voltage can activate these structures to form a large current channel, typically from VCC to the ground serial interface device. The current can be as high as 1 amp. The lock current will remain until the device is powered off. However, the IC usually burns out due to overheating. At the time of ESD shock, there may be two problems that are not easily found. The average user and the IEC test organization use the traditional ring. Road feedback methods and insertion methods for testing usually do not detect these two problems.

1 Receiver cross crosstalk to the transmitter in the RS-232 interface circuit

The ESD protection structure in the RS-232 interface circuit of similar products may cause ESD or a certain ESD impulse voltage failure of a certain waveform to cause a path between the receiver input and the transmitter output after ESD strike, causing the receiver to transmit. If the RS-232 interface circuit is related to the circuit, the ESD is more likely to generate intermodulation after the ESD impact. The communication will fail and the transmitter will still output even if the device is powered off. The shutdown failure causes the other party's RS-232 to be in the receiving state.

2 RS-232 interface circuit generates reverse drive to power supply

The ESD protection structure in some RS-232 interface circuits may form a current path between the input terminal and the power supply Vcc after ESD strike. Figure 2: The reverse drive is applied to the power supply. If the power supply does not sink current, the power is usually There is a forward diode in the power output loop which will cause the power supply voltage Vcc to increase and thus damage

Bad RS-232 interface circuit and other circuits in the system burn out the circuit because the voltage at the input of the RS-232 interface circuit is between 5V and 25V, making Vcc more than 9V beyond the maximum range of the supply voltage.

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