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Portable devices such as notebook computers, mobile phones, PDAs, MP3 players, etc., are very susceptible to electrostatic discharge (ESD) due to frequent contact with the human body. If you do not choose the appropriate protection device, the performance of the machine may be unstable or damaged. The worse case is that the exact reason cannot be found, which makes the user mistakenly believe that it is a product quality problem and damages the corporate reputation.
Under normal circumstances, anti-static protection is required for ports that may be in contact with the human body when such devices are exposed to the outside, such as keyboards, power interfaces, data ports, I / O ports, and so on. Now the more general ESD standard is IEC61000-4-2, using human body electrostatic mode, the test voltage range is 2kV ~ 15kV (air discharge), the peak current is up to 20A / ns, and the entire pulse duration does not exceed 60ns. The energy generated under such a pulse does not exceed a few hundred microjoules in total, but it is sufficient to damage sensitive components.
Most of the IC devices used in portable equipment are high-integration and small-volume products. The precise processing technology makes the silicon crystal oxide layer very thin, so it is easier to break down, and some will be damaged around 20V. Traditional protection methods are no longer universally applicable, and some may even cause interference with the performance of the equipment.
Features of TVS diodes
There are many ESD protection devices that can be used in portable devices. For example, designers can use discrete devices to build protection circuits. However, due to the limitation of space in portable devices and the avoidance of circuit self-inductance, this method has gradually been replaced by more integrated devices. Multilayer metal oxide devices, ceramic capacitors and diodes can all be effectively protected. Their characteristics and performance are different. The unique performance of TVS diodes in such applications has won an increasingly large market.
The most notable feature of TVS diodes is that they respond quickly, so that instantaneous pulses are effectively contained before they cause damage to lines or devices. Second, the cut-off voltage is relatively low, which is more suitable for battery-powered low-voltage loop environments. In addition, the improvement of TVS diode design makes it have lower leakage current and junction capacitance, so it has more ideal performance when dealing with the electrostatic impact of high-rate conduction loop (Figure 1).
Advantages of TVS diodes
TVS and Zener diodes: Compared with traditional Zener diodes, TVS diodes have a larger P / N junction area. This structural improvement allows TVS to have a stronger high-voltage withstand capability, while also reducing the voltage cut-off rate. It has a better effect for protecting the safety of the low working voltage circuit of the handheld device.
TVS and ceramic capacitors: Many designers are willing to use surface-mounted ceramic capacitors for ESD protection, which is not only cheap and easy to design, but the resistance of such devices to high voltage is relatively weak. The impact of 5kV will cause about 10% of the ceramic capacitors to fail. By 10kV, the damage rate will reach 60%, and TVS can withstand 15kV. In the process of using the handheld device, due to frequent contact with the human body, each port must be able to withstand at least 8kV contact shock (IEC61000-4-2 standard). It can be seen that the use of TVS can effectively guarantee the pass rate of the final product.
TVS and MLV: Multi-layer metal oxide structure devices (MLV) can also effectively suppress instantaneous high-voltage shocks. Such devices have a nonlinear voltage-current (impedance performance) relationship, and the cut-off voltage can reach 2 to 3 of the initial stop voltage. This feature is suitable for the protection of circuits and devices that are less sensitive to voltage, such as power circuits. The TVS diode has a better voltage cut-off factor (Figure 2), and also has a lower capacitance, which is very important for high-frequency ports of handheld devices, because too high capacitance will affect data transmission, causing distortion or degradation. . Various surface packages of TVS diodes are suitable for assembly of assembly lines, and the chip structure facilitates the integration of other functions, such as EMI and RFI filter protection, which can effectively reduce device costs and optimize the overall design.
Another feature that cannot be ignored is that the diode can be easily integrated with other devices on one chip. There are many devices that integrate EMI filtering and RFI protection with TVS, which not only reduces the number of devices used in the design and reduces costs It also avoids the associated self-inductance that is easily induced when wiring on the PCB.
TVS related parameters
The best way to deal with the damage caused by transient pulses is to divert the transient current from sensitive devices. The TVS diode is connected in parallel with the protected circuit on the circuit board. When the instantaneous voltage exceeds the normal operating voltage of the circuit, the TVS diode avalanches, providing an ultra-low resistance path to the instantaneous current. The result is that the instantaneous current is diverted through the diode , Avoid the protected device, and keep the protected circuit cut-off voltage until the voltage returns to the normal value. When the transient pulse ends, the TVS diode automatically returns to a high-impedance state, and the entire loop enters normal voltage. After many devices are subjected to multiple shocks, their parameters and performance will be degraded, and as long as they work within the specified range, the diode will not be damaged or degraded.
It can be seen from the above process that when choosing a TVS diode, you must pay attention to the selection of the following parameters (Figure 3):
1. In order to meet the IEC61000-4-2 international standard, TVS diodes must be able to handle a minimum of 8kV (contact) and 15kV (air) ESD shocks. Some semiconductor manufacturers use higher shock resistance standards in their products. For some portable equipment applications with special requirements, designers can select devices as needed.
2. Vwm This is the voltage that the diode can withstand in a normal state. This voltage should be greater than or equal to the normal working voltage of the protected circuit, otherwise the diode will continue to cut off the loop voltage; but it needs to be as close as possible to the normal working voltage of the protected loop. It will not make the entire circuit face the threat of overvoltage before TVS works.
3. Vc is the voltage provided by the diode in the off state, that is, the voltage that passes through the TVS during the ESD shock state. It cannot be greater than the tolerable limit voltage of the protected circuit, otherwise the device faces the risk of damage.
4. Pppm rated pulse power This is based on the maximum cut-off voltage and the peak pulse current at this time. For handheld devices, generally 500W TVS is sufficient.
5. The higher the capacitance of the data / signal frequency loop, the greater the interference of the diode capacitance on the circuit, which forms noise or attenuates the signal strength. Therefore, the capacitance range of the selected device needs to be determined according to the characteristics of the loop. For high-frequency loops, the capacitor should be selected as small as possible (such as LCTVS, low-capacity TVS, and the capacitance is not more than 3pF), and the loop capacitance that does not require high capacitance can be selected to be higher than 40pF.
The following introduces the TVS requirements for ESD protection of several common handheld devices:
Audio input / output: The signal rate of the audio loop is relatively low, and the requirements on the device capacitance are not too high. Around 100pF is acceptable. In some designs, the headset and microphone are combined, and some are discrete lines. In the former case, a single TVS can be selected. In the latter case, if the two circuits are adjacent, you can use a multi-channel TVS array. One device can complete the protection of two circuits.
Buttons / switches: The data rate of these loops is very low, there is no special requirement for the capacitance of the device, and ordinary TVS arrays can be used.
Data video / USB2.0: The data rate is up to 480Mbps, and some video data rates reach more than 1G, so choose low-capacitance LCTVS, which is usually a low-capacitance diode and TVS diode in series to reduce the capacitance of the entire line (can be low At 3pF), to meet the requirements of high-speed loop.
SIM card / antenna: there is a device designed for this type of port that integrates ESD (TVS) / EMI / RFI protection on one chip, which fully reflects the infinite integration scheme of chip devices.
Power supply / charging port: Since it is a DC circuit, high-capacity devices can be used. This port may be impacted by high energy. You can choose devices that integrate TVS and overcurrent protection.
When selecting a device for different purposes, it is necessary to avoid making the device work near its design parameter limit. It is also necessary to select a device with a fast enough response and a high enough sensitivity according to the characteristics of the protected circuit and the characteristics that may withstand ESD shocks. It is very important to effectively play the role of protecting the device, and the device integrating other functions should also be considered first.
Many semiconductor manufacturers provide a variety of different TVS diode packaging forms, especially micro-packages such as SOT23 and SC-70, and flip chips of the same size as the chip, which only occupy about 4.8mm2 on the board, but Can protect multiple lines at the same time. Many recent new products are more suitable for the requirements of high integration and miniaturization of portable equipment. Integrating EMI / RFI / ESD protection in one device can not only effectively reduce the space, but also greatly reduce the cost, reduce the cost of device procurement and processing The cost is the first choice for designers for ports that need these protection functions at the same time.
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