Practical information | An article on EMC protection components - TVS

Transient interference of voltage and current is the main cause of damage to electronic circuits and equipment, often causing immeasurable losses to people. These interferences usually come from the start-stop operation of power equipment, the instability of the AC power grid, lightning interference and electrostatic discharge, etc. Transient interference is almost everywhere and at all times, making people feel hard to guard against. Fortunately, the emergence of a high-performance circuit protection device TVS has effectively suppressed transient interference.

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 that of ordinary voltage regulator diode, and its appearance is no different from that of ordinary diode. When the two ends of the TVS tube are subjected to a momentary high-energy impact, it can suddenly reduce its impedance at an extremely high speed (up to 1*10-12 seconds) and absorb a large current at the same time, clamping the voltage between its two ends to a predetermined value, thereby ensuring that the subsequent circuit components are protected from damage by transient high-energy impact.

Figure 1 TVS characteristic curve

1. Characteristics of TVS

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

However, this curve only reflects a part of the TVS characteristics. The characteristic curve shown in the right figure must be supplemented to reflect the full characteristics of the TVS. This is the current and voltage waveform observed on a dual-trace oscilloscope when the TVS tube is subjected to a large current shock.

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

TVS parameters

Figure 2 TVS characteristics and parameters

A. Breakdown voltage (VBR): At this point, the impedance of the TVS drops suddenly and it is in an avalanche breakdown state.

B. Test current (IT): The breakdown voltage VBR of TVS is measured at this current. Generally, IT is 1MA.

C. Reverse displacement voltage (VRWM): The maximum rated DC working voltage of TVS. When the voltage across the TVS continues to rise, the TVS will be in a high-resistance state.

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

E. Maximum peak pulse current (IPP): The maximum surge current allowed to flow through the TVS, which reflects the surge suppression capability of the TVS.

F. Maximum clamping voltage (VC): When the TVS tube is subjected to transient high-energy impact, a large current flows through the tube, with a peak value of IPP. The terminal voltage rises from the VRWM value to the VC value and then stops rising, thus achieving a protective effect. After the surge, the IPP decays exponentially over time. When it decays to a certain value, the voltage across the TVS starts to drop from VC and returns to its original state. The ratio of the maximum clamping voltage VC to the breakdown voltage VBR is called the clamping factor Cf, expressed as Cf = VC / VBR. Generally, the clamping factor is only 1.2 to 1.4.

G. Peak pulse power (PP): PP is divided into four types of TVS according to the peak pulse power, including 500W, 600W, 1500W and 5000W. Maximum peak pulse power: The 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 as the maximum clamping voltage VC decreases. In addition to the peak pulse current and clamping voltage, the maximum allowable pulse power of TVS is also related to the pulse waveform, pulse duration and ambient temperature.

The peak value of the instantaneous pulse that TVS can withstand can reach hundreds of amperes, and its clamping response time is only 1*10-12 seconds; the forward surge current allowed by TVS can also reach 50-200 amperes under the conditions of 25℃ and 1/120 seconds. Generally speaking, the instantaneous pulse that TVS can withstand is a non-repetitive pulse. In actual applications, repetitive pulses may appear in the circuit.

TVS devices stipulate that the pulse repetition rate ratio (ratio of pulse duration and intermittent 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. TVS works reliably, and will not have "aging" problems even if it is subjected to high-energy impact of non-repetitive large pulses for a long time. Tests have shown that after TVS works safely for 10,000 pulses, its maximum allowable pulse power 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 several kilowatts. TVS has many advantages such as small size, high power, fast response, no noise, low price, etc. It is widely used, such as: household appliances; electronic instruments; instruments; precision equipment; computer systems; communication equipment; RS232, 485 and CAN communication ports; ISDN protection; I/O ports; IC circuit protection; audio and video input; AC and DC power supplies; motor and relay noise suppression, etc. It can effectively protect against overvoltage shocks caused by human operation errors such as lightning and load switches. The following are several typical examples of TVS in circuit applications.

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

2. TVS reverse displacement voltage, i.e. working voltage (VRWM) - select TVS VRWM 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 TVS VRWM, TVS will absorb a large amount of leakage current and be in avalanche breakdown state, thus affecting the operation of the circuit.

3. Maximum peak pulse power: Determine the interference pulse situation of the circuit, and determine the TVS peak pulse power that can effectively suppress the interference based on 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 withstand voltage allowed by the protected circuit.

5. Unipolar or Bipolar - There is often a misunderstanding that bidirectional TVS is used to suppress reverse surge pulses, but this is not the case. Bidirectional TVS is used for AC or positive and negative bidirectional pulses. TVS is sometimes used to reduce capacitance. If the circuit only has positive level signals, then a unidirectional TVS is sufficient. The operation of TVS is as follows: during a forward surge, the TVS is in a reverse avalanche breakdown state; during a reverse surge, the TVS conducts and absorbs the surge energy like a forward biased diode. This is not the case in low-capacitance circuits. Bidirectional TVS should be used to protect low-capacitance devices in the circuit from reverse surge damage.

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

Source: Compiled from the Internet. If there are any copyright issues, please contact us in time to delete it.

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