The circuit symbol of a Zener diode (also called a Zener diode) is: This diode is a semiconductor device with high resistance until the critical reverse breakdown voltage. At this critical breakdown point, the reverse resistance decreases To a very small value, the current increases while the voltage remains constant in this low-resistance area. Zener diodes are classified according to breakdown voltage. Because of this characteristic, Zener diode is mainly used as a voltage regulator or voltage regulator. Reference components are used. Its volt-ampere characteristic Zener diodes can be connected in series to be used at higher voltages, and more stable voltages can be obtained through series connection.

TVS diode (Transient Voltage Suppresser)

TVS diode is a commonly used new type of high-efficiency circuit protection device. It has extremely fast response time (sub-nanosecond level) and very high surge absorption capability. When both ends of it experience a momentary high-energy impact, the TVS diode can change the impedance value between the two ends from high impedance to low impedance at an extremely high speed to absorb a momentary large current, thus reducing the voltage across it. Clamped at a predetermined value to protect subsequent circuit components from transient high voltage spikes. Because of this, TVS diodes can be used to protect equipment or circuits from static electricity, transient voltages generated when switching inductive loads [4], and overvoltages generated by induced lightning.

TVS diodes, like Zener diodes, are also used in reverse. VR is called the maximum turning voltage, which is the critical voltage before reverse breakdown. VB is the breakdown voltage, and its corresponding reverse current IT generally takes a value of 1 mA. VC is the larger clamping voltage. When the peak current flowing in the TVS diode is the large current of IPP, the voltage at both ends of the tube will no longer rise. Therefore, TVS diodes can always limit the port voltage of the protected device or equipment to the effective area of ​​VB ~ VC. Different from the voltage regulator tube, the IPP value can reach hundreds of amperes, and the clamp response time is only 1×10-12s. The maximum allowable pulse power of TVS diodes is PM=VCIPP, and given a larger clamping voltage, the greater the power consumption PM, the greater its surge current tolerance.

The role of voltage regulator tube and TVS diode

Applications of voltage regulator tubes:

1. Surge protection circuit: The zener diode breaks down at an accurate voltage, which allows it to be used as a limiting or protective component. Because zener diodes of various voltages are available, it is particularly suitable for this application. Suitable. The Zener diode D in the picture is used as an overvoltage protection device. As long as the power supply voltage VS exceeds the Zener voltage value D of the diode, it will conduct, causing the relay J to attract the load RL and separate from the power supply.

2. Overvoltage protection circuit in the TV: EC is the main supply voltage of the TV. When the EC voltage is too high, D is turned on, transistor BG is turned on, and its collector potential will change from the original high level (5V) to Low level, the TV enters the standby protection state through the control of the standby control line.

3. Arc suppression circuit: If a suitable Zener diode is connected in parallel to the inductor coil (an ordinary diode can also be connected in the same principle), when the coil is cut off in the conductive state, the electromagnetic energy released will cause The high voltage is absorbed by the diode, so when the switch is turned off, the arc of the switch is eliminated. This application circuit is often used in industry, such as some larger power electromagnetic suction control circuits.

4. Series voltage stabilizing circuit: In this circuit, the base of the series voltage stabilizing tube BG is clamped at 13V by the voltage stabilizing diode D, then its emitter outputs a constant voltage of 12V. This circuit is used on many occasions. There are applications

TVS diode devices can be divided into unipolar and bipolar according to polarity; they can be divided into general-purpose and special-purpose types according to their uses; they can be divided into axial lead diodes and dual in-line TVS diode arrays according to packaging and internal structure. , SMD and high-power modules, etc. [1]. Axial lead products have peak powers of 400 W, 500 W, 600W, 1500W and 5000W. Among them, high-power products are mainly used in power feeders, while low-power products are mainly used in high-density installation situations. For high-density installation occasions, dual in-line and surface mount packaging forms can also be selected.

When selecting TVS diodes, the following main factors should be considered:

1. If the TVS diode is likely to withstand the impact of spike pulse voltages (surge voltage) from two directions, bipolar ones should be selected, otherwise unipolar ones can be selected.

2. The Vc value of the selected TVS diode should be lower than the higher voltage of the protected component. Vc is the voltage of the diode in the cut-off state, which is the voltage passing through the TVS diode in the ESD impact state. It cannot be greater than the tolerable limit voltage of the protected circuit, otherwise the device is in danger of being damaged.

3. The TVS diode should not be in a breakdown state under normal working conditions, and it is better to be below VR. The requirements of VR and VC should be comprehensively considered to select the appropriate TVS diode.

4. If you know the more accurate surge current IPP, you can use VCIpp to determine the power; if you cannot determine the approximate range of IPP, it is better to choose a TVS diode with higher power. PM is the maximum peak pulse power dissipation that the TVS diode can withstand. At a given larger clamping voltage, the greater the power consumption PM, the greater its surge current tolerance; at a given power consumption PM, the lower the clamping voltage VC, the greater its surge current tolerance. The bigger. In addition, the peak pulse power consumption is also related to the pulse waveform, duration and ambient temperature.

5. The transient pulses that TVS diodes can withstand are non-repetitive, and the pulse repetition frequency (ratio of duration to intermittent time) specified by the device is 0.01%. If repetitive pulses occur in the circuit, the accumulation of pulse power should be considered, otherwise the TVS diode may be damaged.

6. For the protection of small current loads, you can consciously add a current-limiting resistor to the line. As long as the resistance of the current-limiting resistor is appropriate, it will generally not affect the normal operation of the line, but the current generated by the interference will be affected by the current-limiting resistor. greatly reduced. However, it is possible to use TVS diodes with smaller peak power to protect small current load lines.

7. The capacitance C is determined by the avalanche junction cross-section of the TVS diode, which is measured at a specific frequency of 1 MHz. The size of C is proportional to the current carrying capacity of the TVS diode. If C is too large, the signal will attenuate. Therefore, C is an important parameter for selecting TVS diodes for data interface circuits. For loops with higher data/signal frequencies, the capacitance of the diode will interfere more with the circuit, causing more noise or attenuating the signal strength. Therefore, the capacitance range of the selected device needs to be determined based on the characteristics of the loop. Generally, the capacitance selected for high-frequency circuits should be as small as possible (such as LCTVS diodes and low-capacitance TVS diodes, the capacitance is no more than 3 pF). For circuits with low capacitance requirements, the capacitance of the capacitor can be selected to be higher than 40 pF.

8. In order to meet the IEC61000-4-2 international standard, TVS diodes must be able to handle smaller ESD impacts of 8 kV (contact) and 15 kV (air). Some semiconductor manufacturers use higher voltages in their products. impact resistance standards. For some portable device applications with special requirements, designers can select devices as needed.

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