Principle and application of zener diode

To understand the working principle of the zener diode, just understand the reverse characteristics of the diode. The basic characteristic of all crystal diodes is unidirectional conduction. That is to say, the positive pressure is on and the reverse pressure is not on. There is a condition here that the reverse pressure does not exceed the reverse pressure resistance value of the pipe. What is the result after exceeding the withstand voltage value? A simple answer is that the pipe is burnt. But that is not all the answers. The test found that as long as the reverse current value is limited (for example, a resistance is connected in series between the pipe and the power supply), the pipe will not burn down although it is broken down. It is also found that after the reverse breakdown of the tube, the current changes from large to small, and the voltage decreases only slightly, until it reaches a certain current value, the voltage drops sharply with the current drop. It is by taking advantage of this characteristic that people have created the zener diode. The key to use the zener diode is to design its current value.

The characteristic of the zener diode is that the voltage at both ends of the diode remains basically unchanged after breakdown. In this way, when the voltage regulator is connected to the circuit, if the voltage at each point in the circuit changes due to the fluctuation of the power supply voltage or other reasons, the voltage at both ends of the load will remain basically unchanged.

Working principle of zener diode and application circuit of zener diode

1、 Principle and characteristics of zener diode

Generally, the triode is on in the forward direction and off in the reverse direction; If the reverse voltage applied to the diode exceeds the bearing capacity of the diode, the diode will be broken down. However, there is a diode whose forward characteristic is the same as that of ordinary diodes, but its reverse characteristic is relatively special: when the reverse voltage is applied to a certain extent, the tube will be in a breakdown state, but it will not be damaged through a large current, and the repeatability of this phenomenon is very good; On the other hand, as long as the tube is in the breakdown state, although the electricity flowing through the tube changes greatly, the voltage at both ends of the tube changes very little to stabilize the voltage. This special diode is called a voltage regulator.

The models of voltage stabilizer tubes include 2CW, 2DW and other series, and their circuit symbols are shown in Figure 5-17.

The voltage stabilizing characteristic of the voltage stabilizing tube can be clearly shown by the volt-ampere characteristic curve shown in Figure 5-18.

The voltage stabilizing tube works by using the voltage stabilizing characteristic of reverse striking multiple areas. Therefore, the voltage stabilizing tube should be connected in the reverse direction in the circuit. The reverse breakdown voltage of the voltage stabilizer is called the stable voltage. The stable voltage of different types of voltage stabilizer is also different. The voltage stabilizer value of a certain type of voltage stabilizer is fixed in the specified range. For example, the stabilized voltage of 2CW11 is 3.2V to 4.5V, and the stabilized voltage of one pipe may be 3.5V, and that of the other pipe may be 4,2V.

In practical applications, if it is not possible to select a voltage stabilizing tube with the required voltage stabilizing value, you can select a voltage stabilizing tube with a lower voltage stabilizing value, and then connect one or several silicon diode "pillows" in series to increase the stable voltage to the required value. This is to use the characteristics of the forward voltage drop of the silicon diode to stabilize the voltage. Therefore, the diode must be positively connected in the circuit, which is different from the regulator.

The performance of voltage stabilizer can be expressed by its dynamic resistance r:

Obviously, for the same current variation Δ 1. Voltage variation at both ends of voltage stabilizer Δ The smaller the U, the smaller the dynamic resistance, and the better the performance of the regulator.

The dynamic resistance of the voltage stabilizer varies with the working current, and the greater the working current is. The smaller the dynamic resistance. Therefore, in order to achieve good voltage stabilization effect, the working current should be properly selected. If the working current is larger, the dynamic resistance can be reduced, but the allowable current (or dissipated power) of the tube cannot be exceeded. The working current and allowable current of various types of pipes can be found in the manual.

The stability performance of the stabilized voltage regulator is affected by temperature. When the temperature changes, its stable voltage will also change. The temperature coefficient of the stable voltage is often used to represent this performance. For example, the stable voltage Uw=12V of the 2CW19 stabilized voltage regulator, and the temperature coefficient is 0.095% ℃, which means that the stable voltage will increase 11.4 mV for every 1 ℃ rise in temperature. In order to improve the stability of the circuit, appropriate temperature compensation measures are often adopted. When the stability performance requirements are very high, it is necessary to use the stabilized voltage with temperature compensation, such as 2DW7A, 2DW7W, 2DW7C, etc.

II Circuit diagram of voltage regulator diode

The simple voltage stabilizing circuit composed of silicon voltage stabilizing tube is shown in Figure 5-l9 (a). Silicon voltage regulator DW and load Rfz are connected in parallel, R1 is current limiting resistance.

How is this circuit regulated?

If the grid voltage rises, the output voltage Usr of the rectifier circuit also rises, causing the load voltage Usc to rise. Since the voltage stabilizer DW is connected in parallel with the load Rfz, as long as there is a small increase in Usc, the current flowing through the voltage stabilizer will increase sharply, making I1 also increase, and the voltage drop on the current-limiting resistor R1 increase, thus offsetting the increase in Usr and keeping the load voltage Usc basically unchanged. On the contrary, if the grid voltage decreases, resulting in the decrease of Usr and the decrease of Usc, the current in the voltage stabilizer tube decreases sharply, resulting in the decrease of I1 and the decrease of voltage drop on R1, thus offsetting the decrease of Usr and keeping the load voltage Usc basically unchanged.

If Usr remains unchanged and the load current increases, the voltage drop on R1 increases, causing the load voltage Usc to drop. As long as Usc decreases a little, the current in the voltage stabilizer will decrease rapidly, so that the voltage drop on R1 will be reduced again, so that the voltage drop on R1 will remain basically unchanged, and the load voltage Usc will be stabilized.

To sum up, it can be seen that the voltage stabilizer tube plays the role of automatic current regulation, while the current-limiting resistor plays the role of voltage regulation. The smaller the dynamic resistance of the voltage stabilizer, the greater the current limiting resistance, and the better the stability of the output voltage.