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How to understand the TVS in EMC protective devices? in view of this problem, this article introduces the corresponding analysis and solution in detail, hoping to help more partners who want to solve this problem to find a more simple and feasible method.

The transient interference of voltage and current is the main cause of damage to electronic circuits and equipment, which often brings inestimable losses to people. These disturbances usually come from the start and stop operation of power equipment, the instability of AC power grid, lightning interference and electrostatic discharge, etc., transient interference is almost everywhere, all the time, making people feel difficult to prevent. Fortunately, with the emergence of a highly efficient circuit protection device, TVS, the transient interference is suppressed effectively.

TVS (TRANSIENT VOLTAGE SUPPRESSOR) or transient voltage suppression diode is a new product developed on the basis of voltage regulator technology. Its circuit symbol is the same as ordinary voltage regulator diode and its shape is the same as ordinary diode. When both ends of TVS tube are subjected to instantaneous high energy shock, it can suddenly reduce its impedance at a very high speed (up to 1mm 10-12 seconds) and absorb a large current at the same time. The voltage between the two ends is clamped at a predetermined value, thus ensuring that the subsequent circuit elements are protected from damage caused by transient high energy shocks.

Characteristics and parameters of TVS

Fig. 1 TVS characteristic curve

Characteristics of 1.TVS

If you use a grapher to observe the characteristics of TVS, you can get the waveform shown on the left in figure 1. If we look at this curve alone, there is no difference in the breakdown characteristics between the TVS tube and the ordinary voltage regulator, which is a typical PN junction avalanche device.

However, this curve reflects only one part of the characteristics of TVS, and the characteristic curve shown in the figure on the right must be supplemented to reflect all the characteristics of TVS. These are the current and voltage waveforms of TVS transistors under high current shock observed on a double-trace oscilloscope.

Curve 1 in the figure is the current waveform in the TVS transistor, which indicates that the current flowing through the TVS transistor suddenly rises from the 1mA to the peak value, and then decreases exponentially. The cause of this current shock may be lightning strike, overvoltage and so on. Curve 2 is the waveform of the voltage at both ends of the TVS tube, which indicates that when the current in the TVS rises suddenly, the voltage at both ends of the TVS also increases, but the maximum value only rises to the VC value, which is slightly larger than the breakdown voltage VBR, thus protecting the following circuit components.

Parameters of TVS

a. Breakdown voltage (VBR): the impedance of TVS decreases suddenly at this time and is in the state of avalanche breakdown.

b. Test current (IT): the breakdown voltage of TVS is measured by VBR at this current. In general, IT takes 1MA.

c. Reverse modification voltage (VRWM): the maximum rated DC operating voltage of the TVS. When the voltage at both ends of the TVS continues to rise, the TVS will be in a high resistance state.

d. Maximum reverse leakage current (IR): the maximum current through the TVS measured at the operating voltage.

e. Maximum peak pulse current (IPP): the maximum surge current allowed by the TVS, which reflects the surge suppression capability of the TVS.

f. Maximum clamp voltage (VC): when the TVS transistor withstands transient high energy shock, there is a large current in the tube, the peak value is IPP, and the terminal voltage no longer rises from VRWM to VC, thus realizing the protection effect. After the surge, the IPP attenuates exponentially with time. When the attenuation reaches a certain value, the voltage at both ends of the TVS begins to decrease from VC and returns to its original state. The ratio of maximum clamping voltage VC to breakdown voltage VBR is called clamping factor Cf, which is expressed as Cf= VC / VBR, and the general clamping factor is only 1.2 to 1.4.

g. Peak pulse power (PP): according to the different TVS of peak pulse power, PP is divided into four types, which are 500W, 600W, 1500W and 5000W. Maximum peak pulse power: maximum peak pulse power is PN= VC ·IPP. Obviously, the greater the maximum peak pulse power, the greater the peak pulse current IPP that the TVS can withstand; on the other hand, after the rated peak pulse power PP is determined, the peak pulse current IPP that the TVS can withstand increases with the decrease of the maximum clamping voltage VC. The maximum allowable pulse power of TVS is not only related to peak pulse current and clamp voltage, but also related to pulse waveform, pulse duration and ambient temperature.

The instantaneous pulse peak value of TVS can reach several hundred amperes, and the clamping response time is only 1: 10-12 seconds, and the allowable forward surge current of TVS can reach 50-200amperes under the condition of 25 ℃ and 120 seconds. Generally speaking, the instantaneous pulses that TVS can withstand are non-repetitive pulses. In practical application, repetitive pulses may appear in the circuit.

The TVS device specifies that the pulse repetition rate ratio (the ratio of pulse duration to intermission time) is 0.01%. If this condition is not met, the accumulation of pulse power may burn out the TVS. Circuit designers should pay attention to this. The work of TVS is reliable, even if it withstands the impact of high energy of non-repetitive large pulses for a long time, there will be no "aging" problem. The experimental results show that after 10000 pulses, the maximum allowable pulse power of TVS is still more than 80% of the original value.

TVS is mainly used for fast overvoltage protection of circuit components. It can "absorb" surge signals with power up to thousands of watts. TVS has many advantages, such as small size, high power, fast response, no noise, low price and so on. It has a wide range of applications, such as: household appliances; electronic instruments; instruments; precision equipment; computer system; communication equipment; RS232, 485 and CAN communication ports; ISDN protection; Igamo port; IC circuit protection; audio and video input; AC and DC power supply; motor, relay noise suppression and other fields. It can effectively protect against overvoltage shock caused by artificial operation errors such as lightning and load switch. here are several typical examples of TVS in circuit applications.

The selection method of TVS

1. Determine the DC voltage or continuous operating voltage of the circuit to be protected. If it is alternating current, the maximum value should be calculated, that is, the effective value * 1.414.

The reverse modification voltage of the 2.TVS is the operating voltage (VRWM)-the VRWM of the selected TVS is equal to or greater than the operating voltage specified in step 1 above. This ensures that the current absorbed by TVS under normal working conditions is negligible. If the voltage specified in step 1 is higher than the VRWM of TVS, TVS will absorb a large amount of leakage current and be in the avalanche breakdown state, thus affecting the operation of the circuit.

3. Maximum peak pulse power: determine the interference pulse condition of the circuit, and determine the TVS peak pulse power which can effectively suppress the interference according to the waveform and pulse duration of the interference pulse.

4. The maximum clamping voltage (VC) of the selected TVS should be lower than the maximum withstanding voltage allowed by the protected circuit.

5. Unipolar or bipolar-there is a common misconception that two-way TVS is used to suppress reverse surge pulses, but this is not the case. Two-way TVS is used for alternating current or from positive and negative two-way pulses. TVS is also sometimes used to reduce capacitance. If the circuit has only a forward level signal, then a unidirectional TVS is sufficient. The operation mode of TVS is as follows: in forward surge, TVS is in the state of reverse avalanche breakdown; in reverse surge, TVS turns on and absorbs surge energy like a forward bias diode. This is not the case in low capacitance circuits. Bi-directional TVS should be selected to protect the low capacitance devices in the circuit from reverse surge.

6. If you know the more accurate surge current IPP, then you can use VC to determine its power, if you can not determine the approximate range of power, generally speaking, it is better to choose a larger power.

The answer to the question about how to understand the TVS in EMC protective devices is shared here. I hope the above content can be of some help to you. If you still have a lot of doubts to be solved, you can follow the industry information channel to learn more about it.

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