Performance and Application of TVS Transient Interference Suppressor

Performance and Application of TVS Transient Interference Suppressor

TVS is the abbreviation of TRANSIENT VOLTAGE SUPPRESSORS

Chinese abbreviation: transient voltage suppression diode, transient voltage suppression diode, transient suppression surge absorber, transient interference suppressor

Transient interference

Transient interference refers to instantaneous interference signals such as surge voltage, ringing voltage, spark discharge, etc. that appear on the AC power grid. Its characteristics are that the action time is extremely short, but the voltage amplitude is high and the transient energy is large. Transient interference will cause fluctuations in the power supply voltage of the control system; when the transient voltage is superimposed on the input voltage of the control system, making the input voltage of the control system exceed the limit voltage of the internal components of the system, it will damage the internal equipment of the control system, so suppression measures must be adopted.

Silicon TVS Diodes

硅瞬变吸收二极管的工作有点象普通的稳压管，是箝位型的干扰吸收器件；其应用是与被保护设备并联使用。 硅瞬变电压吸收二极管具有极快的响应时间(亚纳秒级)和相当高的浪涌吸收能力，及极多的电压档次。可用于保护设备或电路免受静电、电感性负载切换时产生的瞬变电压，以及感应雷所产生的过电压。
TVS管有单方向(单个二极管)和双方向(两个背对背连接的二极管)两种，它们的主要参数是击穿电压、漏电流和电容。使用中TVS管的击穿电压要比被保护电路工作电压高10％左右，以防止因线路工作电压接近TVS击穿电压，使TVS漏电流影响电路正常工作；也避免因环境温度变化导致TVS管击穿电压落入线路正常工作电压的范围。

TVS tubes have a variety of packaging forms, such as axial lead products can be used on power feeders; dual in-line and surface mount are suitable for protecting logic circuits, I/O buses and data buses on printed boards.

Characteristics of TVS

TVS的电路符号和普通的稳压管相同。其电压-电流特性曲线如图1所示。其正向特性与普通二极管相同，反向特性为典型的PN结雪崩器件。图2是TVS的电流-时间和电压-时间曲线。在浪涌电压的作用下，TVS两极间的电压由额定反向关断电压VWM上升到击穿电压VBR，而被击穿。随着击穿电流的出现，流过TVS的电流将达到峰值脉冲电流IPP，同时在其两端的电压被箝位到预定的最大箝位电压VC以下。其后，随着脉冲电流按指数衰减，TVS两极间的电压也不断下降，最后恢复到初态，这就是TVS抑制可能出现的浪涌脉冲功率，保护电子元器件的过程。当TVS两极受到反向高能量冲击时，它能以10~12s级的速度，将其两极间的阻抗由高变低，吸收高达数千瓦的浪涌功率，使两极间的电位箝位于预定值，有效地保护电子设备中的元器件免受浪涌脉冲的损害。TVS具有响应时间快、瞬态功率大、漏电流低、击穿电压偏差小、箝位电压容易控制、体积小等优点，目前已广泛应用于家用电器、电子仪表、通讯设备、电源、计算机系统等各个领域。

The main parameters of TVS

Maximum reverse leakage current ID and rated reverse shutdown voltage VWM. VWM is the maximum continuous working DC or pulse voltage of TVS. When this reverse voltage is applied between the two poles of TVS, it is in reverse shutdown state, and the current flowing through it should be less than or equal to its maximum reverse leakage current ID. Minimum breakdown voltage VBR and breakdown current IR. VBR is the minimum breakdown voltage of TVS. At 25°C, TVS will not avalanche below this voltage. When the specified 1mA current (IR) flows through TVS, the voltage applied to the two poles of TVS is its minimum breakdown voltage VBR. According to the degree of discreteness 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.

最大箝位电压VC和最大峰值脉冲电流IPP。当持续时间为20mS的脉冲峰值电流IPP流过TVS时，在其两端出现的最大峰值电压为VC。VC、IPP反映了TVS的浪涌抑制能力。VC与VBR之比称为箝位因子，一般在1.2~1.4之间。

Capacitance C. Capacitance C is determined by the cross section of the TVS avalanche junction and is measured at a specific frequency of 1MHz. The size of C is proportional to the current carrying capacity of the TVS. If C is too large, the signal will be attenuated. Therefore, C is an important parameter for selecting TVS in data interface circuits.

最大峰值脉冲功耗PM。PM是TVS能承受的最大峰值脉冲功率耗散值。在给定的最大箝位电压下，功耗PM越大，其浪涌电流的承受能力越大；在给定的功耗PM下，箝位电压VC越低，其浪涌电流的承受能力越大。另外，峰值脉冲功耗还与脉冲波形、持续时间和环境温度有关。而且，TVS所能承受的瞬态脉冲是不重复的，器件规定的脉冲重复频率(持续时间与间歇时间之比)为0.01％。如果电路内出现重复性脉冲，应考虑脉冲功率的累积，有可能损坏TVS。

箝位时间TC。TC是从零到最小击穿电压VBR的时间。对单极性TVS小于1×10-12s；对双极性TVS小于10×10-12s。

Classification of TVS

TVS devices can be divided into unipolar and bipolar types according to polarity; general-purpose and special-purpose types according to use; and axial lead diodes, dual in-line TVS arrays, surface mount and high-power modules according to packaging and internal structure. The peak power of axial lead products can reach 400W, 500W, 600W, 1500W and 5000W. Among them, high-power products are mainly used in power feeders, and low-power products are mainly used in high-density installation occasions. For high-density installation occasions, dual in-line and surface mount packaging can also be selected.

Selection of TVS

Determine the maximum DC or continuous working voltage of the protected circuit, the rated standard voltage of the circuit and the maximum withstand voltage.
The rated reverse shutdown voltage VWM of the TVS should be greater than or equal to the maximum working voltage of the protected circuit. If the selected VWM is too low, the device may enter avalanche or the reverse leakage current may be too large to affect the normal operation of the circuit.

TVS的最大反向箝位电压VC应小于被保护电路的损坏电压。

在规定的脉冲持续时间内，TVS的最大峰值脉冲功率PM必须大于被保护电路可能出现的峰值脉冲功率。在确定了最大箝位电压后，其峰值脉冲电流应大于瞬态浪涌电流。一般TVS的最大峰值脉冲功率是以10/1000ms的非重复脉冲给出的，而实际的脉冲宽度是由脉冲源决定的，当脉冲宽度不同时其峰值功率也不同。如某600WTVS，对1000ms脉宽最大吸收功率为600W，但是对50ms脉宽吸收功率就可达到2100W，而对10ms的脉宽最大吸收功率就只有200W了。而且吸收功率还和脉冲波形有关：如果是半个正弦波形式的脉冲，吸收功率就要减到75％，若是方波形式的脉冲，吸收功率就要减到66％。

平均稳态功率的匹配对于需要承受有规律的、短暂的脉冲群冲击的TVS，如应用在继电器、功率开关或电机控制等场合，有必要引入平均稳态功率的概念。举例说明，在一功率开关电路中会产生120Hz，宽度为4ms，峰值电流为25A的脉冲群。选用的TVS可以将单个脉冲的电压箝位到11.2V。此中平均稳态功率的计算为：脉冲时间间隔等于频率的倒数1/120=0.0083s，峰值吸收功率是箝位电压与脉冲电流的乘积11.2V×25A=280W，平均功率则为峰值功率与脉冲宽度对脉冲间隔比值的乘积，即　280×(0.000004S/0.0083S)=0.134W。也就是说，选用的TVS平均稳态功率必须大于0.134W。

For the protection of data interface circuits, attention must also be paid to selecting TVS devices with appropriate capacitance C.

Select the polarity and packaging structure of TVS according to the application. It is more reasonable to use bipolar TVS for AC circuits; it is more advantageous to use TVS array for multi-line protection.

Temperature considerations: Transient voltage suppressors can work between -55℃ and +150℃. If the TVS needs to work at a variable temperature, its reverse leakage current ID increases with increasing temperature; power consumption decreases with increasing TVS junction temperature, from +25℃ to +175℃, it decreases linearly by about 50%; the breakdown voltage VBR increases with increasing temperature by a certain coefficient. Therefore, it is necessary to consult the relevant product information and consider the impact of temperature changes on its characteristics.

Matters needing attention in the use of TVS tubes

The relationship between the power absorption (peak value) of transient voltage and the pulse width of transient voltage. The manual only gives the power absorption (peak value) under a specific pulse width, but the pulse width in the actual circuit is unpredictable, so an estimate must be made in advance. For wide pulses, the rated value should be used.

To protect small current loads, you can consciously add a current limiting resistor in the circuit. As long as the resistance of the current limiting resistor is appropriate, it will not affect the normal operation of the circuit, but the current limiting resistor will greatly reduce the current generated by the interference. This makes it possible to use a TVS tube with a smaller peak power to protect the small current load circuit.

When suppressing repetitive transient voltages, it is particularly important to note whether the steady-state average power of the TVS tube is within a safe range.

Derating

As a semiconductor device, the TVS tube should pay attention to the derating problem when the ambient temperature rises. In particular, pay attention to the length of the TVS tube lead and its relative distance from the protected circuit. When there is no TVS tube with a suitable voltage, it is allowed to use multiple TVS tubes in series. The maximum current of the series tube is determined by the one with the smallest current absorption capacity among the tubes used. The peak absorption power is equal to the product of this current and the sum of the series tube voltage.

The junction capacitance of the TVS tube is a key factor affecting its use in high-speed lines. In this case, a TVS tube is generally connected back-to-back with a fast recovery diode. Since the fast recovery diode has a smaller junction capacitance, the equivalent capacitance of the two in series is also smaller, which can meet the requirements of high-frequency use.

Conclusion: Various electronic systems and communication networks are often subject to external electromagnetic interference, which mainly comes from transient processes of power lines, lightning, and cosmic radio. These interferences can cause system operation errors or even hardware damage. For these problems, comprehensive preventive protection measures should be taken. Designing and selecting appropriate silicon transient absorption diodes is a good solution to transient interference. However, as the electronics industry explores more electronic products that improve efficiency, increase functions, and continuously increase integration, designing and providing complete circuit protection solutions will form another revolution in circuit protection technology.