1. Principle of Zener Diode and TVS Diode
1. Zener diode
Zener diode is a kind of use of PN junction in the reverse breakdown voltage basically remains unchanged, but the current can be changed within a certain range, the Zener diode has a high resistance state before the reverse breakdown voltage, beyond this critical point In a low resistance state. Zener diodes are widely used in various types of voltage regulator circuits, voltage reference components and other occasions. It can be used in series, so that a higher voltage value can be obtained.
Zener diodes are commonly used in circuits to indicate "ZD". The most important parameters of Zener diodes are: voltage regulation value, power dissipation, and maximum operating current. These three factors should be fully considered when designing the circuit, as shown in the figure below. The resistance-capacitance step-down circuit, the voltage output is 12V, and the 1N4742A voltage regulator diode is used. Its voltage regulation value is 12V and the power is 1W.
2. TVS diode
TVS tube is a transient suppression diode, a semiconductor device with surge absorption capability. Its response time is extremely fast, reaching the sub-nanosecond level. Therefore, once the circuit receives an instantaneous high-energy impact, it can remove this The energy is suppressed, and by absorbing the instantaneous large current and high voltage clamping from the circuit, the final result is to protect the subsequent equipment or circuit.
TVS diodes are divided into one-way and two-way. One-way is mostly used in DC circuits, and two-way is mostly used in AC circuits. It is connected in parallel with the protected circuit in the circuit. Once the instantaneous voltage exceeds the normal working voltage of the circuit, the TVS diode will have an avalanche effect. Give the instantaneous current an ultra-low resistance path, so that the protected device or equipment is prevented from being damaged. Therefore, this TVS diode is very suitable for ESD sensitive circuits and overvoltage circuits. As shown in the figure below, when the unidirectional TVS tube is connected to the circuit, the polarity must be opposite to the polarity of the power input, so that the circuit can work.
2. The difference between Zener diode and TVS diode
1. Symbol package
The circuit symbols of Zener diodes and TVS diodes are basically the same as Zener diodes, and the packaging is similar. Sometimes it is difficult to tell which is which in appearance;
2. Circuit connection
Zener diodes and TVS diodes are connected in the reverse direction of the circuit, that is, using its reverse characteristics, using the PN junction avalanche effect, there is a critical voltage before reverse breakdown, and both have a critical voltage in the reverse access circuit. The effect of voltage stabilization is different. Zener diodes are used to fix the input voltage at a certain value, while TVS diodes are mainly used to prevent transient high voltage from clamping the subsequent circuit;
3. Response time
Zener diodes generally don’t talk about response time, and more attention is paid to the voltage regulation value. However, TVS diodes are different. Its characteristics determine that it has different functions. TVS diodes have transient suppression and high-energy effects, so it is required to The higher energy is absorbed instantaneously in a short time, so this time is very short, reaching the nanosecond level;
4. Power size
The power of the Zener diode is generally not very large. Common Zener diodes are 1W, 2W, etc. Its power is determined by its allowable temperature rise, which is actually the product of the stable voltage and current. For example, for the Zener diode 1N4742A, it The maximum power is only 1W, and the voltage regulation value is 12V, so the current through it generally cannot exceed 80mA, otherwise the tube will heat up; for TVS diodes, it pays attention to instantaneous pulse power, which is very high and can reach Above kilowatts, but this time value remains small.
