The principle of Zener diode and precautions for use

Zener diode principle

Under normal circumstances, there is only a small current in a reverse-biased PN junction. This leakage current remains constant until the reverse voltage exceeds a certain value, after which the PN junction suddenly begins to conduct a large current. This sudden and significant reverse conduction is reverse breakdown. If there are no external measures to limit the current, it may cause damage to the device. Reverse breakdown usually sets the maximum operating voltage of a solid-state device. However, if proper precautions are taken to limit the current, the reverse breakdown junction can be used as a very stable reference voltage.

One mechanism that causes reverse breakdown is avalanche mulTIplicaTIon. Consider a reverse-biased PN junction. The depletion region widens as the bias increases, but it is not fast enough to prevent the electric field from strengthening. The strong electric field accelerates some carriers to pass through the depletion zone at very high speeds. When these carriers collide with atoms in the crystal, they hit loose valence electrons and create additional carriers. Because a carrier can generate thousands of additional carriers by impact, just like a snowball can generate an avalanche, so this process is called avalanche mulTIplicaTIon.

Another mechanism of reverse breakdown is tunneling. Tunneling is a quantum mechanism process that enables particles to move a short distance regardless of the presence of any obstacles. If the depletion region is thin enough, then carriers can jump through tunneling. Tunneling current mainly depends on the width of the depletion region and the voltage difference across the junction. The reverse breakdown caused by tunneling is called Zener breakdown.

The reverse breakdown voltage of the junction depends on the width of the depletion region. The wider the depletion region, the higher the breakdown voltage. As discussed earlier, the lighter the doping, the wider the depletion region and the higher the breakdown voltage. When the breakdown voltage is lower than 5 volts, the depletion region is too thin, mainly due to Zener breakdown. When the breakdown voltage is higher than 5 volts, it is mainly an avalanche breakdown. The designed PN diodes that mainly work in the reverse conduction state are called Zener diodes or avalanche diodes according to the dominant working mechanism. The breakdown voltage of a Zener diode is lower than 5 volts, and the breakdown voltage of an avalanche diode is higher than 5 volts. Usually engineers refer to them as Zener tubes regardless of how they work. Therefore, the 7V Zener, which is mainly operated by avalanche breakdown, may be confusing.

Actually, the breakdown voltage of the junction is not only related to its doping characteristics but also to its geometry. The above discussion analyzes a planar junction where two uniformly doped semiconductor regions intersect in a plane. Although some real knots approximate this ideal situation, most knots are curved. The curvature strengthens the electric field and reduces the breakdown voltage. The smaller the radius of curvature, the lower the breakdown voltage. This effect has a great influence on the breakdown voltage of the thin junction. Most Schottky diodes have an obvious fault at the edge of the metal-silicon interface. Electric field enhancement can greatly reduce the measured breakdown voltage of Schottky diodes, unless there are special measures to weaken the electric field at the edge of the Schottky barrier.

Precautions for the use of Zener diodes

1. Pay attention to the difference between general diode and Zener diode. Many general diodes, especially glass-encapsulated tubes, are similar in appearance and color to Zener diodes. If they are not carefully distinguished, they will be used incorrectly. The difference is: looking at the shape, many Zener diodes are cylindrical, shorter and thicker, while general diodes are more slender if they are cylindrical; look at the signs, the outer surface of the Zener diodes are marked with voltage stabilizer Value, such as 5V6, means the voltage regulation value is 5.6V; use a multimeter to measure, according to the unidirectional conductivity, use the X1K block to judge the positive and negative polarity of the tested diode first, then use the X10K block, and connect the black meter pen to the negative pole of the diode , The red test lead is connected to the anode of the diode, and the measured resistance is compared with the X1K gear. If the reverse resistance is very large, it is likely to be a normal diode. If the reverse resistance becomes very small, then It is a Zener diode.

2. Pay attention to the difference between the forward and reverse use of the Zener diode. When a Zener diode is used in forward conduction, it is basically the same as when a general diode is used in forward conduction. After the forward conduction, the voltage at both ends is basically unchanged, both about 0.7V. Theoretically speaking, the Zener diode can also be used as a Zener diode in the forward direction, but its voltage regulation value will be lower than 1V, and the voltage regulation performance is not good. Generally speaking, the forward conduction characteristic of the Zener diode is not used alone for stability. Instead, use the reverse breakdown characteristic to stabilize the voltage. The reverse breakdown voltage value is the regulated value. Sometimes two voltage regulator tubes are used in series, one using its forward characteristics and the other using its reverse characteristics, which can both stabilize voltage and temperature compensation to improve the effect of voltage stabilization.

3. Pay attention to the function of current-limiting resistor and the influence of resistance. In the voltage regulator diode voltage regulator circuit, a resistor R is generally connected in series, as shown in Figure 1 or 2. The resistor plays a role in limiting current and improving the effect of voltage stabilization in the circuit. If this resistance is not added, that is, when R=0, it is easy to burn out the voltage regulator tube, and the voltage regulator effect will be extremely poor. The larger the resistance of the current-limiting resistor, the better the voltage regulation performance of the circuit, but the voltage difference between the input and output will be too large, and the more power consumption will be.

4. Pay attention to the pressure difference between input and output. In normal use, the output voltage of the Zener diode voltage stabilizer circuit is equal to the voltage stabilized value at both ends of the Zener tube after reverse breakdown. If the voltage value input to the voltage stabilizer circuit is less than the voltage value of the Zener tube, the circuit will Loss of the voltage stabilization effect, only when it is greater than the relationship, the voltage stabilization effect, and the greater the pressure difference, the greater the resistance of the current-limiting resistor should be, otherwise it will damage the voltage regulator tube.

5. The voltage regulator tube can be used in series. After several voltage regulator tubes are connected in series, multiple different voltage regulator values ​​can be obtained, so it is more common to use them in series. The following example illustrates how to obtain the voltage regulation value after using two voltage regulator tubes in series. If the voltage of one zener tube is 5.6V and the other is 3.6V, and the voltage of the zener tube is 0.7V when the zener tube is turned on, there will be four different voltage values ​​after the series connection, as shown in the figure 1 shows.

6. Voltage regulator tubes are generally not used in parallel. After several zener tubes are connected in parallel, the voltage stabilization value will be determined by the lowest (including the voltage value after forward conduction). Take the above two voltage regulator tubes as an example to illustrate the calculation method of the voltage regulator value. There are four situations after the two are connected in parallel, and there are only two voltage stabilization values, as shown in Figure 2. Unless special circumstances, Zener diodes are not used in parallel.