TVS Parameters and ESD Suppression Principles.

During winter, we often hear a "pop" sound when touching electronic devices, which is electrostatic discharge (ESD). Novices may complain that this is due to poor product design, but in fact, the reliability of the design depends on whether the device is abnormal after the "pop." If the device works normally, the design is reliable. If the device is abnormal, such as black screen, flashing, abnormal noise or even shutdown, it means that the design is unreliable.

We often hear two abbreviations, ESD and TVS, and some students are easily confused, but they are actually two completely different concepts. ESD stands for Electro-Static Discharge, which describes an objective phenomenon. TVS stands for Transient Voltage Suppressor, which is a diode-like protective device and refers to a device. We use TVS to suppress the impact of ESD on electronic circuits to ensure that the circuit system can work normally.

Figure 1-32 TVS protection circuit

As shown in Figure 1-32, TVS is connected in parallel with the protected device. Under normal conditions, TVS is in a high impedance state for signals. When the instantaneous voltage is large enough, TVS provides a low impedance path to release the large voltage interference, as shown in Figure 1-32. ESD generates a very high transient voltage, which lasts for a very short time, up to one billionth of a second, as shown in Figure 1-33. If such a high voltage is left unattended, it will cause serious damage to the circuit system. However, if TVS is added, its impact will be greatly reduced.

Figure 1-33 ESD Transient High Voltage.

We generally use TVS to absorb the energy of ESD to protect the normal operation of the circuit system. The symbol of TVS is shown in Figure 1-34. There are generally two types of TVS: bidirectional and unidirectional. TVS is a kind of diode that uses the reverse application of a diode and can be compared with a voltage regulator diode. By using the avalanche breakdown principle of the PN junction of the diode, high-voltage interference can be avoided from entering the low-voltage circuit behind it.

Figure 1-34 TVS Symbol.

Using TVS to suppress static electricity is a common solution. The VI characteristic curve of a unidirectional TVS is very close to the VI characteristic curve of a diode. We use its working area in the third quadrant, as shown in Figure 1-35. The IV characteristic curve of a bidirectional TVS is symmetric to the working area curve of a unidirectional TVS. We need to understand the various parameters of TVS, which will help us choose the appropriate TVS in practical circuits to effectively suppress ESD.

Figure 1-35 TVS Current-Voltage Characteristics Curve.

We need to choose the right type of TVS to effectively suppress the instantaneous large pulse caused by ESD while not affecting the normal operation of the circuit signal or power supply. Taking a unidirectional TVS tube as an example, we can interpret the main parameters of TVS. The IV characteristic curve of TVS is shown in Figure 1-36. TVS uses the working characteristics in reverse to clamp the voltage, similar to the diode's reverse connection and parallel connection in the circuit. When it works in the third quadrant, we introduce the main parameters in the third quadrant.

Figure 1-36 Unidirectional TVS Current-Voltage Characteristics Curve.

VRWM: Peak Reverse Working Voltage, also known as breakdown voltage. At this working voltage, we can see that the reverse leakage current IR is very small, and the TVS has very little current flowing through it, resulting in very low power consumption. It is important to note that VRWM should be greater than the working voltage of the device. For example, if a signal is 3.3V, the TVS with a parallel connection should have a VRWM greater than 3.3V, otherwise the TVS may suppress the 3.3V signal as interference or unnecessarily increase power consumption.

IR: Reverse Leakage Current @ VRWM, leakage current, the current that TVS works at VRWM, which is a leakage current with a small current value, approximately equal to 0 between 0V and VRWM voltage.

VBR: Breakdown Voltage @ IT, the voltage at which TVS breaks down. Under a certain test current IT, the voltage at both ends of TVS when it conducts in reverse direction. At this time, TVS is in a low impedance state and begins to conduct high pulse to the ground circuit. Avalanche breakdown starts at this time.

VC: Clamping Voltage @ IPP, the clamping voltage, under the action of peak current IPP, the voltage at both ends of TVS is called the clamping voltage, which is a very important parameter. Under the action of peak current IPP, most of the energy flows through TVS to the ground, and the voltage is clamped, thereby protecting the circuit behind TVS.

VC must be smaller than the maximum voltage that the subsequent circuit can withstand because if VC is higher than the device's withstand voltage, the peak pulse is still clamped at VC, and VC is still higher than the device's withstand voltage, causing the device to be damaged.

Using TVS to alleviate static electricity is a common method, but in actual engineering, it is often not as simple as imagined. For example, the antenna in a mobile phone often has static electricity problems. Generally speaking, it is best to place TVS close to the position where static electricity is introduced into the circuit, that is, to place TVS at the source end where static electricity is introduced. However, this often affects the antenna or RF performance, and in this case, it is necessary to consider placing TVS in other positions. If TVS is placed too far away, static electricity may affect the circuit through conduction or secondary discharge, which requires testing the specific effects of TVS in different layout positions. In addition, small capacitors or inductors can also be used to optimize static electricity problems.