TVS and its application in circuit design

瞬态电压抑制器是一种二极管形式的高效能保护器件，具有极快的响应时间和相当高的浪涌吸收能力。当TVS的两端受到反向瞬态过压脉冲时，能以极高的速度把两端间的高阻抗变为低阻抗，以吸收瞬间大电流，并将电压箝制在预定数值，从而有效保护 电路 中的元器件免受损坏。本文讲述了TVS器件的主要特性参数和选用注意事项，同时给出了TVS在 电路 设计中的应用方法。

1 Characteristics and main parameters of TVS devices

1.1 TVS device characteristics

在规定的反向应用条件下，TVS对受保护的线路呈高阻抗状态。当瞬间电压超过其击穿电压时，TVS就会提供一个低阻抗的路径，并通过大电流方式使流向被保护元器件的瞬间电流分流到TVS二极管，同时将受保护元器件两端的电压限制在TVS的箝制电压。当过压条件消失后，TVS又恢复到高阻抗状态。与陶瓷 电容 相比，TVS可以承受15 kV的电压，但陶瓷 电容 对高压的承受能力比较弱。5 kV的冲击就会造成约10％陶瓷电容失效，而到10 kV时，其损坏率将高达到60％。

1.2 Main parameters of TVS devices

(1) Minimum breakdown voltage VBR

When the specified current flows through the TVS, the voltage across the TVS is called the minimum breakdown voltage. In this area, the TVS presents a low impedance path. At 25°C, below this voltage, the TVS will not undergo avalanche breakdown.

(2) Rated reverse shutdown voltage VWM

VWM is the voltage that TVS can withstand in normal state, and this voltage should be greater than or equal to the normal working voltage of the protected circuit. However, it needs to be as close as possible to the normal working voltage of the protected circuit, so that the entire circuit will not face the threat of overvoltage before the TVS works. According to the discreteness of TVS VBR and the standard value, VBR can be divided into 5% and 10%. For 5% VBR, VWM=0.85VBR; and for 10% VBR, VWM=0.81VBR.

(3) Maximum peak pulse current IPP

Ipp is the maximum pulse peak current allowed to pass through the device under specified pulse conditions when the TVS is working in the reverse state.

(4)箝位电压Vc

当脉冲峰值电流Ipp流过TVS时，其两端出现的最大电压值称为箝位电压Vc。Vc和Ipp反映了TVS的浪涌抑制能力。通常把Vc与VBR之比称为箝位因子(系数)，其值一般在1.2～1.4之间。实际使用时，应使Vc不大于被保护电路的最大允许安全电压，否则被保护器件将面临被损坏的可能。

(5) Maximum peak pulse power consumption PM

PM通常是最大峰值脉冲电流Ipp与箝位电压Vc的乘积，也就是最大峰值脉冲功耗。它是TVS能承受的最大峰值脉冲功耗值。在给定的最大钳位电压下，功耗PM越大，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、脉冲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可重复施加的。

(6) Capacitance C

The capacitance of TVS is determined by the cross-sectional area of ​​its silicon wafer and the bias voltage, which is measured at a specific frequency of 1 MHz. The size of C is proportional to the current carrying capacity of TVS. If C is too large, the signal will be attenuated. Therefore, capacitance C is an important parameter for selecting TVS in data interface circuits.

(7) Leakage current IR

IR is the leakage current of the TVS tube when the maximum reverse working voltage is applied to the TVS. When TVS is used in high impedance circuits, this leakage current IR is an important parameter.

2. Considerations for selecting TVS

When selecting TVS, the following main factors should generally be considered based on the specific situation of the circuit:

首先，由于双向TVS可以在正反两个方向吸收瞬时大脉冲功率，并把电压箝制到预定水平。因此，若电路有可能承受来自两个方向的浪涌电压冲击时，应当选用双向TVS。双向TVS一般适用于交流电路，单向TVS一般用于直流电路。另外，箝位电压Vc不大于被保护电路的最大允许安全电压。

Secondly, the maximum peak pulse power consumption PM must be greater than the maximum transient surge power in the circuit. However, in actual applications, surges may occur repeatedly. In this case, even if the energy of a single pulse is much smaller than the pulse energy that the TVS device can withstand, if it is repeatedly applied, the accumulation of these single pulse energies may exceed the pulse energy that the TVS device can withstand in some cases. Therefore, when designing the circuit, it is necessary to carefully consider this point and select a suitable TVS device so that the accumulation of repeated pulse energy within the specified interval does not exceed the pulse energy rating of the TVS device.

Third, in actual application, the maximum reverse working voltage must be correctly selected. The general principle is to select the maximum reverse working voltage of the TVS tube at 1.4 times the AC voltage. The DC voltage is 1.1 to 1.2 times the maximum reverse working voltage of the TVS tube.

3 Typical applications of TVS in circuit design

In actual application circuits, the best way to deal with the damage of transient pulses to devices is to divert the transient current away from sensitive devices. To achieve this goal, TVS is connected in parallel with the protected circuit on the circuit board. In this way, when the transient voltage exceeds the normal operating voltage of the circuit, TNS will undergo avalanche breakdown, thereby providing an ultra-low impedance path for the transient current. As a result, the transient current is diverted through TVS, thereby avoiding the protected device and keeping the protected circuit at the cut-off voltage until the voltage returns to normal. After this, when the transient pulse ends, the TVS diode automatically returns to the high-resistance state, and the entire circuit enters the normal voltage state. The following are several typical examples of TVS in circuit applications.

3.1 Application of TVS in AC Circuits

图1所示是一个双向TVS在交流电路中的应用电路。应用TVS可以有效地抑制电网带来的过载脉冲，从而起到保护整流桥及负载中所有元器件的作用。图1中的TVS箝位电压应不大于电路的最大允许电压。

3.2 Use TVS to protect DC voltage regulator Power supply

Figure 2 shows a DC regulated power supply. Adding a TVS at its regulated output can protect the equipment using the power supply and absorb the peak voltage between the collector and emitter of the transistor in the circuit, thereby protecting the transistor. It is recommended to add a TVS tube at the output of each regulated source, which can greatly improve the reliability of the entire machine.

3.3 Protecting transistor circuits with TVS

Various transient voltages can cause the EB junction or CE junction of the transistor to break down and be damaged. In particular, when the collector of the transistor has an inductive load (coil, transformer, motor), a high voltage reverse potential is usually generated, which may damage the transistor. In practical applications, it is recommended to use TVS as a protection device. Figure 3 shows four circuit examples of TVS protecting transistors.

3.4 Integrated Circuit Protection

As the integration of modern ICs is getting higher and higher, and their withstand voltage is getting lower and lower, they are easily damaged by transient voltage shocks, so protection measures must be taken. Usually, there are protection networks at the input and output ends of CMOS circuits. For the sake of reliability, various protection networks are also added to the external interfaces of each complete machine. Figure 4 shows the relevant circuit measures for using TVS to protect TTL and CMOS devices.

3.5 Using TVS to protect integrated op amps

Integrated operational amplifiers are very sensitive to external electrical stress. Therefore, when using the operational amplifier, if there is an operational error or abnormal working conditions, excessive voltage or current, especially surge and electrostatic pulse, will often occur, which can easily damage or fail the operational amplifier. Figure 5 shows a protection circuit that uses TVS to prevent overvoltage damage at the differential input of the operational amplifier.

4 Conclusion

This article mainly describes the characteristics and main parameters of TVS devices, as well as the precautions when selecting devices. At the same time, it gives the typical application circuits of the devices in circuit design, so as to deepen the circuit designers' understanding of TVS and provide a basis for designing high-reliability application circuits. Although the current application of TVS in China is in the promotion stage, we have reason to believe that as more and more designers continue to deepen their understanding of TVS and TVS shows excellent performance, TVS devices will eventually be widely used.