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Application of TVS diode in ESD of portable equipment

ESD protection of portable equipment is very important, and TVS diode is a very effective protection device, which has its unique advantages compared with other devices. However, devices should be selected according to different protection objects in application, because different ports may be subject to different electrostatic shocks, and different devices require different degrees of protection. Attention should be paid to the corresponding parameter identification and different design of each manufacturer, and reasonable PCB layout should be carried out at the same time. This paper introduces how to use TVS diode in ESD protection of portable equipment.
Portable devices such as laptops, mobile phones, PDAs, MP3 players, etc. are vulnerable to electrostatic discharge (ESD) impact due to frequent contact with human body. If proper protective devices are not selected, the performance of the machine may be unstable or damaged. The worse situation is that the exact reason can not be found out, so that users mistakenly think it is the product quality problem and damage the reputation of the enterprise.
In general, anti-static protection is required for ports exposed to the outside of such equipment that may contact with human body, such as keyboard, power interface, data interface, I / O interface, etc. At present, the general ESD standard is IEC61000-4-2. The range of test voltage is 2KV ~ 15kV (air discharge), the maximum peak current is 20A / NS, and the duration of the whole pulse is not more than 60ns. The total energy generated by such a pulse is no more than a few hundred microjoules, but it is enough to damage sensitive components.
The IC devices used in portable devices are mostly high integration and small volume products. The precise processing technology makes the silicon oxide layer very thin, so it is easier to break down, and some of them will be damaged at about 20V. Traditional protection methods are no longer generally applicable, and some even cause interference to equipment performance.

Characteristics of TVS Diode
There are many ESD protection devices that can be used in portable devices. For example, designers can use discrete devices to build protection loops. However, due to the limitation of portable devices on space and avoidance of loop self induction, this method has gradually been replaced by more integrated devices. Multilayer metal oxide devices, ceramic capacitors and diodes can be effectively protected. Their characteristics and performances are different. The unique performance of TVs diodes in such applications has won more and more market.
One of the most remarkable characteristics of TVS diode is its rapid response, which makes the instantaneous pulse be effectively contained before it causes damage to the circuit or device. The other is that the cut-off voltage is relatively low, which is more suitable for the low-voltage circuit environment powered by battery. In addition, the improvement of TVS Diode design makes it have lower leakage current and junction capacitance, so it has better performance when dealing with the electrostatic impact of high-speed conduction circuit (Figure 1).

Advantages of TVS Diode
TVs and Zener Diodes: compared with the traditional zener diodes, TVs diodes have larger P / n junction area. The improvement of this structure makes TVs have stronger high-voltage withstand capacity, and also reduces the voltage cut-off rate, so it has a better effect on the safety of low working voltage circuit of handheld devices.
TVs and ceramic capacitors: many designers are willing to use surface mount ceramic capacitors for ESD protection, which is not only cheap but also simple to design, but also weak to withstand high voltage. The impact of 5kV will cause about 10% of the ceramic capacitor to fail. When 10kV, the damage rate reaches 60%, and TVs can withstand 15kV voltage. In the use of handheld devices, due to frequent contact with human body, each port must be able to withstand at least 8Kv contact impact (IEC61000-4-2 standard), so the use of TVs can effectively ensure the qualification rate of the final product.
TVs and MLV: multilayer metal oxide devices (MLV) can also effectively suppress the instantaneous high voltage impulse. Such devices have a nonlinear voltage current (impedance performance) relationship, and the cut-off voltage can reach 2-3 times of the initial stop voltage. This feature is suitable for the protection of lines and devices that are not very sensitive to voltage, such as power supply circuits. TVS diode has a better voltage cut-off factor (Figure 2) and a lower capacitance, which is very important for the high-frequency port of handheld device, because too high capacitance will affect data transmission, cause distortion or degradation. Various surface packages of TVs diodes are suitable for assembly line, and the chip structure is easy to integrate other functions, such as EMI and RFI filter protection, which can effectively reduce the cost of devices and optimize the overall design.
Another feature that can not be ignored is that the diode can be easily integrated with other devices on a chip. There are many devices that integrate EMI filtering, RFI protection and other functions with TVs, which not only reduces the number of devices used in the design and reduces the cost, but also avoids the associated self inductance easily induced when wiring on PCB.