[electronic components] zener diode

One 、 Zener diode Zender Diode

utilize PN Junction reverse breakdown state , Its The current can vary over a wide range and The voltage is basically unchanged The phenomenon of , Made of a diode with voltage stabilizing effect .

working principle

The forward characteristic of volt ampere characteristic curve of Zener diode is similar to that of ordinary diode , But the difference is that when the reverse voltage is large to a certain extent . When the reverse voltage of diode is very large, it will be reverse breakdown , If thermal breakdown occurs, it will be damaged .
The reverse characteristic of Zener diode is when the reverse voltage is lower than the reverse breakdown voltage , The reverse resistance is very large , The reverse leakage current is very small .

however , When the reverse voltage approaches the critical value of the reverse voltage , The reverse current suddenly increases , be called breakdown , At this critical breakdown point , The reverse resistance suddenly drops to a very small value . Although the current varies over a wide range , and The voltage at both ends of the diode is basically stable near the breakdown voltage , Thus, the voltage stabilizing function of the diode is realized .

therefore , Regulators are often used as voltage regulators and voltage reference components , It is also classified according to its breakdown voltage ：3V ( Both ends 3V voltage , Opposite to the polarity of the regulator itself ),4V3,5V1,5V6,8V3,12V ( Less common ),13v ( Commonly used ),18V,27V,30V etc. .

Use of pressure stabilizing tube

The voltage regulator circuit is related to the three working areas divided by the volt ampere characteristic curve , These three working areas are similar to ordinary diodes , But there are many access methods ：

positive ：

1. dead zone ： The opening voltage does not reach Vf, Cut off status .
2. Forward bias region ： With the increase of forward bias , The forward current increases rapidly
   The voltage stabilizing pipe in this area is directly connected , Ordinary diodes use , But the forward conduction voltage of different types of Zener diodes Vf Different .

reverse ：

1. Reverse bias region ： When the reverse voltage does not reach Vz Before , Zener diode is not connected ; And when the reverse bias approaches Vz When the value of , The zener diode begins to turn on , produce Iz electric current . With the increase of reverse bias , The reverse current will change violently .
2. Breakdown region ： The working area of the zener diode
   The variation range of reverse bias voltage is very small , This voltage is the operating point of the zener diode 、 Voltage stabilizing value .

The voltage stabilizing tube is used in reverse

Zener diode needs parallel connection The connecting circuit works .

If the current from its negative end to its positive end is large enough （ Greater than 2mA, It can't be too big , Otherwise, short circuit , Current limiting resistor can be added ）, Stable voltage with little change is formed at both ends . As shown in the figure below , resistance R1 Current limiting resistance .

At work , If the voltage of the grid rises , That is, the circuit input voltage Vdc elevated , Will cause load voltage Vo The rise of . Regulator tube D1 Parallel with load ,Vdc A little increase will sharply increase the current flowing through the regulator , Make it flow through the current limiting resistance R1 The current on the 、 The voltage increases , So that Vo The voltage at both ends remains basically unchanged .

If at work Vdc Constant and the load current increases , Current limiting resistor R1 The pressure drop on the will increase , Cause load voltage Vo falling . But as long as Vo The voltage drops a little , The current of the voltage stabilizer will decrease rapidly, making R1 Pressure drop reduction , Maintain load voltage Vo The stability of the .

Be careful ：R1 Resistance must be present , Direct connection will cause the current on the zener diode to be too large and burned ;RL The value of load will also be limited （ If the load current is greater than the current limiting resistance current , According to Kirchhoff's current law , The current direction of Zener diode is wrong ）.

Two regulators are connected in series

• If two zener diodes are connected in reverse series , just 、 When the voltage in the opposite direction reaches the regulated value , The voltage is clamped （ That is, it can't rise any higher ）.
• Two diodes in reverse series can protect the circuit connected in parallel from overvoltage , When the circuit is over-voltage , The diode first breaks down and is short circuited .
  
  When u When the absolute value of is large , Positive or negative , There will be a diode in the positive bias state , The other diode is in the voltage stabilizing state of reverse bias , Thus, it plays the role of clamping voltage .
  
• Often in high-power amplifier circuit , Grid of power tube G And source S That is, the emission junction is a zener diode , This is done by limiting the voltage to G-S Play a protective role , prevent G-S The insulating layer between is punctured by too high voltage .

The example is as follows ：

Common parameters

Common parameters	english	Symbol	Company
Steady voltage	Nominal Zener Voltage	Vz @ Iz	V
Steady current	Zener Current	Iz	mA
Dissipated power	Power Dissipation	Pm	W

Note appended

Zener diode 、DCDC chip 、LDO Chips are similar , But it can't replace the power chip ：

1. Zener diode is not suitable for large load scenarios , The heating of high current scene is serious , It is usually suitable for dozens of milliampere load scenarios ;
2. Lack of feedback loop , There is a problem of unstable output . The jump of load and input power supply will make the output voltage fluctuate violently .

Two 、 Three terminal integrated voltage stabilizing circuit

seeing the name of a thing one thinks of its function , Three ends IC It refers to this kind of integrated circuit for voltage stabilization , Only three pins output , They are inputs 、 Ground terminal and output terminal . It looks like an ordinary triode ,TO-220 Standard package , Also have 9013 What it looks like TO-92 encapsulation （ Common three terminal voltage stabilizing integrated circuits have positive voltage output 78×× Series and negative voltage output 79×× series ）.

TO-220 Footprint ( Transistor Outline Package) It is a kind of high-power transistor 、 A kind of in-line packaging form often used by small and medium-sized integrated circuits .

This series integrates voltage stabilization IC In the model 78 or 79 The following numbers represent the output voltage of the three terminal integrated voltage stabilizing circuit , Such as 7806 Indicates that the output voltage is positive 6V,7909 Indicates that the output voltage is negative 9V.

The representative symbols in the circuit are as follows ：

• Need one that can output 1.5A Regulated power supply for above current ： Usually, several three terminal voltage stabilizing circuits are connected in parallel , The maximum output current is N individual 1.5A（ High current output , The size of the heat sink should be large enough , Otherwise, high temperature protection or thermal breakdown will be caused ）, However, attention should be paid to when applying ： The integrated voltage stabilizing circuit used in parallel shall be from the same manufacturer 、 Products of the same batch number , To ensure the consistency of parameters . In addition, a certain margin is left on the output current , In order to avoid the chain burning of other circuits caused by the failure of individual integrated voltage stabilizing circuits .
• Typical circuits ： The output is positive 5V DC voltage stabilized power supply circuit .IC Adopt integrated voltage regulator 7805,C1、C2 Filter capacitors at input and output respectively ,RL Is the load resistor .
  
  78** The limit input voltage of the series of voltage stabilizing integrated blocks is 35V, The minimum input voltage is higher than the output voltage 2V. You can usually use multiple 78 Series of regulators are connected in series .