The principle of Zener diode, what is its function?

For hardware designers , Zener tube should be one of the most commonly used devices in our project .

Zener diode , It is also called zener diode .

It plays a role in the circuit Steady voltage The role of . After the diode is reverse broken down , In a certain range of reverse current, the reverse voltage does not change with the reverse current .

The biggest difference from ordinary diodes is their main Working in reverse breakdown state Next .

However, what is reverse breakdown , What does reverse breakdown voltage mean ？ When understanding diodes , You must understand these nouns .

The term reverse breakdown , In fact, it is not only used in the study of diodes , In the future study courses of triode , We also come across this term .

The diode is forward conducting , Reverse cut-off . When a positive voltage is applied , Electrons can pass through a diode . However, when a reverse voltage is applied at both ends , Electrons cannot pass through diodes , At this time, the diode in the circuit is equivalent to an open circuit . But this produces “ Open circuit ” Depends on the reverse voltage applied across the diode . As long as the reverse voltage is large enough , When it reaches a certain value , The diode will be broken down , At this time, the reverse voltage of breakdown is the reverse breakdown voltage .

However , Zener diode is mainly used in reverse breakdown state , The current varies greatly , However, the characteristic that the voltage basically remains unchanged eventually plays a role in stabilizing voltage in the circuit , At the same time, the reverse breakdown of the voltage regulator is reversible . That is, after the loss of reverse voltage , The regulator continues to work normally . As long as it is under reverse voltage , The reverse current shall not exceed the maximum allowable range , The voltage stabilizing tube will not be damaged due to thermal breakdown .

Zener diode before and after reverse breakdown , according to R=U/I, The resistance will drop rapidly from a large value to a small value . Therefore, the voltage stabilized value of the zener diode is its breakdown voltage , Mainly with PN Junction doping concentration .

The peak voltage of the reverse voltage applied to the diode is specified according to the breakdown voltage of the diode URWM, It is generally half or two-thirds of the reverse breakdown voltage . Such as 2CZ52A The peak reverse operating voltage of the silicon diode is 25V, The reverse breakdown voltage is about 50V, Avoid excessive reverse voltage applied by diode when using .

The following figure shows the volt ampere characteristic curve of Zener diode
Key parameters of Zener diode

Steady voltage Uz
The voltage value when the voltage stabilizer works stably after reverse breakdown is called stable voltage .

Steady current Iz
The reverse current when the voltage stabilizer tube works stably after reverse breakdown is called stable current . The maximum reverse current allowed by the regulator is called the maximum stable current , When using a pressure stabilizing tube , The operating current shall not exceed , Generally, it is greater than 2 Times the output voltage ;

Dynamic resistance Rz
When the voltage regulator works in the curve of reverse breakdown , Voltage variation △u And current variation △i The ratio is called dynamic resistance , The more obvious the dynamic resistance is, the better the voltage stabilizing performance is ;

Rated power consumption Pz
Determined by the allowable temperature rise of the chip , Its rated value is stable voltage Uz And maximum allowable current Izm The product of the .

temperature coefficient α
The temperature change of the regulator tube will cause the stable voltage to change slightly , So the temperature changes 1℃ The relative change of the voltage at both ends of the tube is the temperature coefficient , The smaller the temperature coefficient, the better , It shows that the pressure stabilizing tube is little affected by temperature .

Since it is voltage stabilization , And the working state It belongs to the reverse breakdown state , When the zener diode is used , Its negative pole is connected to high potential , The positive pole is connected to the low potential .
The figure shows a simple voltage stabilizing circuit

When the grid voltage fluctuates
Load change

In a voltage stabilizing circuit , It is often combined with current limiting resistor R Use it together .

Operating conditions of Zener diode ：1 In reverse breakdown state 2 In reverse breakdown state Iz<Iz max.

About the voltage regulator in the circuit , We often see the use of multiple combinations , The following is a simple diagram , We should not only understand its function , Also know the final voltage value .

chart 1,6V And 8V The voltage stabilizing tubes are all broken down ,VCC What is the value of voltage stabilization for both .
chart 2. 6V The regulator is conducting in the positive direction ,8V Voltage regulator tube reverse breakdown voltage regulator ,VCC by 8V Voltage stabilization plus 6V The forward conduction voltage drop of the voltage stabilizing tube is 8.7V.
chart 3. 8V The regulator is conducting in the positive direction ,6V Voltage regulator tube reverse breakdown voltage regulator ,VCC by 6V Voltage stabilization plus 8V The forward conduction voltage drop of the voltage stabilizing tube is 6.7V.
chart 4,6V And 8V The voltage stabilizing tubes are conducting in the positive direction ,VCC Is the sum of forward conduction voltage drop, i.e 1.4V.

chart 1, Low stabilized voltage 6V The voltage stabilizing tube breaks down first ,8V The voltage regulator is in an open circuit state ,VCC by 6V.
chart 2 3 4 At least one regulator is conducting in the positive direction , therefore VCC by 0.7V.

Finally, the diode, especially the chip type , The distinction between positive and negative electrodes We also need to know , It is mainly divided into the following methods ：

1. Distinguish by means of a magnifying glass or microscope Color recognition band , Those with obvious marks such as color bands are diode negative electrodes , At the same time PCB On the pad , The negative pole is often marked with a vertical line .

2. Measure with a multimeter
   Turn the multimeter to diode position
   Touch the red and black probes to both ends of the diode , When the multimeter shows 0.3-0.6 Left and right values , At this time, the contact end of the black probe is the negative pole of the diode , The contact end of the red probe is the positive pole of the diode .