Transient Voltage Suppressor (TVS) is a classification of Diodes. It is a high-efficiency circuit protection device developed on the basis of Zener diodes technology. Its circuit symbol and shape are the same as ordinary Zener diodes. Different from ordinary diodes, the main function of TVS diodes is to suppress transient interference and effectively protect circuit equipment. Transient interference of voltage and current is the main cause of damage to electronic circuits and equipment. These disturbances usually come from electrostatic discharge effects, instability of the AC power grid, lightning strikes, and noise from switching power supplies.
The Different Between TVS Diodes and Zener Diodes
Compared with the Zener voltage regulator, both can limit the voltage at both ends within a certain range, and the long-term operation withstanding current value is almost the same, which is related to the volume power consumption. Generally, the smaller the package size, the smaller the power. However, the working principle is not the same. For the Zener voltage regulator, when the reverse voltage reaches and exceeds the stable voltage, the reverse current is suddenly increased to keep the voltage at both ends constant.
Package of TVS Diodes
Although both TVS and Zener diodes can be used for voltage regulation, TVS is mainly used for transient voltage protection. It is not as accurate as Zener voltage regulators in terms of voltage accuracy. In terms of current capacity, the TVS diodes can reach several hundred amps, while the surge current of the Zener diodes is very small.
A TVS diode may be either unidirectional or bidirectional. according to their polarity. A unidirectional device is generally suitable for DC circuits, and A bidirectional TVS is generally suitable for AC circuits.
How TVS Diodes Work
When a large-scale transient interference voltage or pulse current occurs in the circuit due to various interferences, it can suddenly reduce its impedance at a very high speed, and quickly turn into a reverse conduction state, and the clamping voltage at both ends is a predetermined value. Since the clamping voltage is lower than the maximum withstand voltage value of other devices in the circuit at this time, it protects the circuit and ensures that the following circuit components are not damaged by the impact of transient high energy.
The action time of transient interference is very short, but the voltage amplitude is very high, and the transient energy is large, which is very destructive to electronic equipment. The instantaneous pulse power that TVS can withstand can reach up to kilowatts. In addition, it has the advantages of fast response, low leakage current, small breakdown voltage deviation, easy control of clamping voltage, and no damage limit. Therefore, it is widely used in computer systems, communications, etc. Equipment, AC/DC power supply, automobiles, household appliances, instrumentation and other fields.
Application of TVS diode in circuit
In the actual application circuit, the best way to deal with transient pulse damage to the device is to divert the transient current away from the sensitive device.
Usually, the TVS diode is connected in parallel with the protected circuit on the circuit board. When the instantaneous voltage exceeds the normal operating voltage, the TVS diode will have an avalanche breakdown, providing an ultra-low impedance path for the instantaneous current. The transient current is diverted through the TVS, avoiding the protected device and keeping the protected circuit at the cut-off voltage until the voltage returns to normal.
Many components will have their parameters and performance degraded after multiple shocks, but as long as they operate within a limited range, the diode will not be damaged or degraded.
In the AC circuit, the application of bidirectional TVS can effectively suppress the overload pulse brought by the power grid, and protect the rectifier bridge and all components in the load. Note that the TVS clamp voltage should not be greater than the maximum allowable voltage of the circuit.
Bidirectional TVS Diodes in AC Circuits
For DC regulated power supply, adding TVS to the regulated output end can not only protect the equipment of the power supply, but also absorb the peak voltage between the collector and the emitter of the transistor in the circuit, and protect the transistor. It is recommended to add a TVS diodes to the output of each voltage regulator, which can greatly improve the reliability of the whole machine.
TVS Protects DC Regulated Power Supply
Various transient voltages can break down the EB or CE junction of the transistor and damage it, especially when the collector of the transistor has an inductive (coil, transformer, motor) load, a high-voltage back EMF 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 protection transistors.
Four Circuit Examples of TVS Protection Transistors
Integrated op amps are very sensitive to external electrical stress. Therefore, in the process of using the op amp, if the operation error or abnormal working conditions are adopted, there will often be excessive voltage or current, especially surge and electrostatic pulse, which will easily damage the op amp or cause damage to the op amp. invalid. Figure 4 shows a protection circuit that uses TVS to prevent over-voltage damage at the differential mode input end of the op amp.
TVS Prevents Overvoltage Damage at the Differential Mode Input of the OP AMP
How to choose TVS Diodes?
1. Check the parameters of the circuit to be protected, such as the maximum DC or continuous operating voltage, the rated standard voltage of the circuit, and the "high-end" tolerance;
2. The rated transient power of the TVS diodes is greater than the maximum transient surge power that may appear in the circuit;
3. The rated reverse working voltage of TVS should be greater than or equal to the maximum working voltage of the protected circuit. Otherwise, the device may enter an avalanche or the normal operation of the circuit may be affected by excessive reverse leakage current;
4. The maximum clamping voltage VC of TVS should be less than the damage voltage of the protected circuit;
5. After determining the maximum clamping voltage of the TVS diodes, its peak pulse current should be greater than the transient surge current;
6. For the circuit protection of the data interface, it is also necessary to pay attention to the selection of TVS diodes with appropriate capacitance. (For example, when the signal frequency or transmission rate is high, TVS diodes of low capacitance series should be selected)
7. Select the polarity and package structure of the TVS according to the application. (such as AC circuit should use bipolar TVS)
8. If the TVS is required to operate at varying temperatures, the temperature must also be considered. The transient voltage suppressor can work between -55°C - +150°C. Since its reverse leakage current Id increases with temperature, the voltage Vbr will increase by a certain factor. Conversely, the power is then reduced, approximately linearly by 50% from +25°C to +175°C.
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