ESD protection is of great significance for electronic devices with high density, miniaturization and complex functions. This paper discusses the influence of parameters such as minimum breakdown voltage and breakdown current, maximum reverse leakage current and rated reverse turn-off voltage on the circuit and the selection criteria when TVS diode is used to prevent ESD, and introduces some typical applications for video line protection, USB protection and RJ-45 interface in portable consumer electronic devices, set-top boxes and personal computers.

With the rapid development of mobile products, printers, PCs, DVDs, STBs and other products, consumers are demanding more and more advanced performance. Semiconductor components are becoming more and more miniaturized, high-density and complex in function. Especially, applications that require strict motherboard area, such as fashion consumer electronics and portable products, are vulnerable to electrostatic discharge. Some complex semiconductor functional circuits using deep submicron process and very fine linewidth wiring are more sensitive to the impact of circuit transient process, which will lead to the intensification of the above problems.

ESD protection principle

There are several technologies for circuit protection components. When selecting circuit protection components, designers can only provide wrong safety concepts if they choose inappropriate protection components. The selection of circuit protection elements shall be determined according to the wiring conditions to be protected, the available circuit board space and the electrical characteristics of the protected circuit. In addition, it is necessary to know the characteristics of the protection elements. One of the important factors to be considered is the clamping voltage of the device. The so-called clamp voltage is the voltage across the transient voltage canceller (TVS) in the ESD device. It is the strain voltage of the protected IC.

Because the oxide layer in IC circuits manufactured by advanced technology is relatively thin, the gate oxide layer is more vulnerable to damage. This means that a higher clamping voltage will generate a higher strain voltage on the protected IC device and increase the probability of failure.

Many protection elements are designed to absorb a large amount of energy, and they also have high clamping voltage due to element structure or design reasons. Because the clamping voltage of rheostat is too high, they can not provide effective ESD protection. In addition, due to the high capacitance of rheostat, they can not provide protection for high-speed data lines. TVS diode is produced to solve this problem. It has become a key technology to protect portable electronic devices.

TVS diode is a solid-state component specially designed to absorb ESD energy and protect the system from ESD damage. If applied properly, the TVS diode will limit the voltage across the protected device to be just above the rated operating voltage, but far below the damage threshold voltage.

TVs related parameters

The best way to deal with the damage caused by the instantaneous pulse is to divert the instantaneous current from the sensitive device. The TVS diode is connected in parallel with the protected circuit on the circuit board. When the instantaneous voltage exceeds the normal working voltage of the circuit, the TVS diode will avalanche and provide an ultra-low resistance path to the instantaneous current. As a result, the instantaneous current is led away through the diode to avoid the protected device, and the protected circuit will maintain the cut-off voltage until the voltage returns to the normal value. When the instantaneous pulse ends, the TVS diode will automatically return to the high resistance state, and the whole circuit will enter the normal voltage. The parameters and performance of many devices will degrade after many times of impact. As long as they work within the limited range, the diodes will not be damaged or degraded.

  

From the above process, it can be seen that when selecting TVS diode, attention must be paid to the selection of the following parameters:

1. minimum breakdown voltage VBR and breakdown current IR. VBR is the minimum breakdown voltage of TVs. At 25 ℃, TVs will not avalanche below this voltage. When the TVS flows through the specified 1mA current (IR), the voltage applied to the two poles of the TVS is its minimum breakdown voltage VBR. According to the dispersion degree of TVs VBR and standard value, VBR can be divided into 5% and 10%. For 5% VBR, vwm=0.85vbr; For 10% VBR, vwm=0.81vbr. In order to meet the IEC61000-4-2 international standard, TVs diodes must meet the test standard that can handle the fourth level of IEC61000-4-2 electrostatic protection. The minimum 8Kv (contact) and 15kV (air) ESD impact. Leimao electronics can meet the contact and air 30KV test on the TVS devices of automotive electronics. Therefore, for some applications with special requirements, designers can select devices as required.

2. maximum reverse leakage current ID and rated reverse turn-off voltage VWM. VWM this is the voltage that the diode can bear in the normal state. This voltage should be greater than or equal to the normal working voltage of the protected circuit, otherwise the diode will cut off the circuit voltage continuously; However, it needs to be close to the normal working voltage of the protected circuit as much as possible, so that the whole circuit will not face the threat of overvoltage before TVs works. When the rated reverse turn-off voltage VWM is applied between the two poles of TVs, it is in the reverse turn-off state, and the current flowing through it shall be less than or equal to its maximum reverse leakage current ID.

3. maximum clamping voltage VC and maximum peak pulse current IPP. When the pulse peak current IPP with a duration of 20ms flows through TVs, the maximum peak voltage at both ends is VC. VC and IPP reflect the surge suppression capability of TVs. The ratio of VC to VBR is called clamping factor, which is generally between 1.2 and 1.4. VC is the voltage provided by the diode in the cut-off state, that is, the voltage passing through the TVs in the ESD impact state. It cannot be greater than the withstand limit voltage of the protected circuit, otherwise the device will face the risk of damage.

4. PPPM rated pulse power, which is based on the maximum cut-off voltage and the peak pulse current at this time. For handheld devices, 500W TVs is generally enough. The maximum peak pulse power consumption PM is the maximum peak pulse power consumption that TVS can withstand. Under a given maximum clamping voltage, the greater the power consumption PM, the greater the surge current tolerance. Under a given power consumption PM, the lower the clamping voltage VC, the greater the surge current tolerance. In addition, the peak pulse power consumption is also related to the pulse waveform, duration and ambient temperature. Moreover, the transient pulse that TVS can withstand is not repeated, and the pulse repetition frequency (ratio of duration to intermittent time) specified by the device is 0.01%. If there are repetitive pulses in the circuit, the accumulation of pulse power should be considered, which may damage the TVs.

5. capacitance C. The capacitance C is determined by the cross section of TVS Avalanche junction and is measured at a specific 1MHz frequency. The magnitude of C is directly proportional to the current bearing capacity of TVs. Too large C will cause signal attenuation. Therefore, C is an important parameter for selecting TVs for data interface circuit. The higher the data / signal frequency of the circuit, the greater the interference of the diode capacitance to the circuit, resulting in noise or attenuation of signal strength. Therefore, the capacitance range of the selected device needs to be determined according to the characteristics of the circuit. Generally, the capacitance of high-frequency circuit shall be as small as possible (such as LCTVs and Low Capacitance TVS, the capacitance shall not be greater than 3pf), while the capacitance of circuit with low capacitance requirements can be higher than 40pf.

ESD application

1. application of bottom connector

Bottom connector design is widely used in mobile consumer products. At present, mobile phones, PDAs, DSC (digital cameras) and MP3 and other portable products are mainly used in the market.

As it is a DC circuit, ordinary capacitor devices can be selected. This port may be impacted by high energy, and integrated TVs or discrete devices can be selected.

  

2.rj-45 (10/100/100 20m Ethernet network)

RJ-45 interface is widely used in network connected interface equipment. The typical application is 10/100/1000m Ethernet network.

  

  

3. video line protection

At present, the common video output ports are D-sub, DVI (28 lines), SCART (19 lines) and d-terminal (mainly Japanese products in use). The video data line has a high data transmission rate, up to 480mbps. Some video data transmission rates reach more than 1G. Therefore, Low Capacitance TVS should be selected. It usually connects a low capacitance diode in series with the TVS diode to reduce the capacitance of the whole line (which can be less than 3pf) and meet the requirements of high-speed circuit. Ulc0524p of Shanghai leimao Electronics Co., Ltd. has good low capacitance characteristics and can provide 4-channel ESD protection, while its successor srda05-6 can provide up to 6-channel ESD protection. Srda05-x series products are applicable to the protection of all high-speed communication lines.

  

4. SIM card data line protection

SIM card data line protection has always been the focus of various companies’ products, and the device specially designed for such ports that integrates ESD (TVS) /emi/rfi protection in one chip fully reflects the infinite integration scheme of chip devices.

  

When selecting devices for different purposes, it is necessary to avoid making the devices work near the limit of their design parameters. Devices with fast response speed and high sensitivity should also be selected according to the characteristics of the protected circuit and the characteristics that may withstand ESD impact. This is very important for effectively playing the role of the protector. In addition, devices with other functions should also be considered first.

Leimao also provides a variety of TVS diode packages, especially micro packages such as SOT23 and SC-70, and flip chip 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 meet the requirements of high integration and miniaturization of portable devices. Integrating emi/rfi/esd protection into one device can not only effectively reduce the space, but also greatly reduce the cost, device procurement cost and processing cost. For ports that need these protection functions at the same time, it is the first choice for designers.

For the emerging fast charging USB, the surge protection of VBUS is particularly important. For fast charging USB, there is a corresponding ulc1654n to meet the protection requirements.

5. USB protection

  

  

  

  

6. audio / speaker data line protection

In terms of audio data line protection, due to the low signal rate of the audio circuit, the requirements for device capacitance are not too high, and the common capacitance indicators are acceptable. Some mobile phones are designed to combine headphones and microphones, and some are discrete lines. In the former case, single channel TVs can be selected. In the latter case, if the two circuits are adjacent, multi-channel TVS Array can be selected. Only one device can complete the protection of the two circuits.

  

7. keys / switches

For key and switch circuits, the data rate of these circuits is very low, and there is no special requirement for the capacitance of the device. Ordinary TVs arrays can be used. The above devices are used for ESD protection on mobile phones. In fact, they are 5V anti-static, which can meet the needs of static electricity.

  

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