Basic knowledge of diode

Basic concepts ：
The diode consists of a die 、 The tube shell and two electrodes form . The die is a PN junction , stay PN A lead wire is led out at both ends of the knot , Combined with plastic 、 Glass or metal material is used as the packaging shell , It forms a crystal diode , As shown in the figure below .P The electrode leading out of the region is called positive electrode or anode ,N The electrode leading out of the region is called negative electrode or cathode .

Volt ampere characteristics of diodes
The volt ampere characteristic of a diode refers to the relationship between the voltage applied to both ends of the diode and the current flowing through the diode , The curve used to qualitatively describe the relationship between the two is called volt ampere characteristic curve . The volt ampere characteristics of silicon diode are observed by transistor grapher, as shown in the figure below .

Forward characteristic
1、 When the applied forward voltage is small , The diode presents a high resistance , The forward current is almost zero , curve OA The section is called non-conductive zone or dead zone . Generally, the dead time voltage of silicon tube is about 0.5 v , The dead time voltage of germanium is about 0.2 v , This voltage value is also called threshold voltage or threshold voltage .
2、 When the applied forward voltage exceeds the dead band voltage ,PN The electric field in the junction is almost offset , The resistance of the diode is very small , The forward current begins to increase , Enter the forward conduction area , But at this time, the voltage is not proportional to the current, such as AB paragraph . The forward current increases rapidly with the increase of applied voltage , Such as BC The section characteristic curve is steep , The volt ampere relationship is approximately linear , In a fully conductive state .
3、 The forward voltage at both ends after the diode is turned on is called the forward voltage drop ( Or tube pressure drop ), And almost constant . The pressure drop of the silicon tube is about 0.7V, The pressure drop of germanium tube is about 0.3V.
Reverse characteristic
1、 When the diode is subjected to reverse voltage , Strengthened PN Internal electric field of junction , The diode presents a high resistance , There is only a small reverse current . Like a curve OD The segment is called the reverse cut-off zone , At this time, the current is called reverse saturation current . Practical application , The higher the reverse current, the greater the reverse resistance of the diode , The better the reverse cut-off performance . Generally, the reverse saturation current of silicon diode is below tens of microamps , Germanium diodes can reach hundreds of microamps , The high-power diode is slightly larger .
2、 When the reverse voltage increases to a certain value ( In the figure D spot ), The reverse current increases sharply , Enter the reverse breakdown region ,D The voltage corresponding to the point is called the reverse breakdown voltage . After the diode is broken down, excessive current will damage the tube , Therefore, in addition to the regulator , The reverse voltage of the diode shall not exceed the breakdown voltage .

Rectifier circuit ：

Unidirectional half wave rectifier circuit
The diode is like an automatic switch ,u2 When it is positive half cycle , Automatically connect the power supply to the load ,u2 When it is negative half cycle , Automatically cut off the power supply and load . therefore , As shown in the figure below , The direction of the load remains unchanged 、 Fluctuating DC voltage with varying magnitude uo As shown in the figure below . Because the circuit is only used in u2 There is output in the positive half cycle of , So it is called half wave rectifier circuit . If the polarity of the rectifier diode is reversed , Negative DC pulsating voltage can be obtained .
Full wave rectifier circuit
Rectifier principle ：
Set the voltage at the secondary side of the transformer as ：
1) When u2 When it is positive half cycle ,A The point potential is the highest ,V The point potential is the lowest , diode V1 and V3 Conduction ,V2 and V4 end , The path of current is A→V1→RL→V3→B.
2) When u2 When it is negative half cycle ,B The point potential is the highest ,A The point potential is the lowest , diode V2 and V4 Conduction ,V1 and V3 end , The path of current is B→V2→RL→V4→A.
so , stay u2 A cycle of change , load RL The current always flows from top to bottom , The waveform of voltage and current is a full wave pulsating DC voltage and current , As shown in the figure below .

Filter circuit ：
The rectifier circuit changes alternating current into pulsating direct current , But it contains a lot of communication ( It is called ripple voltage ). Therefore, it is necessary to filter out the AC component in the pulsating DC , This process is called filtering .
Capacitive filtering
The characteristics of capacitive filtering are ：
1、 The average value of the output voltage is related to the filter capacitance C And load resistance RL About the size of ,C Capacity or RL The greater the resistance is , The slower the discharge rate , The higher the output voltage , The better the filtering effect .
2、 When using large capacity filter capacitor , The instantaneous charging current of the power supply is particularly large . Capacitor filter has simple structure , Load DC voltage UL Higher , The ripple is also small , However, the output characteristics are poor , Therefore, it is suitable for high load voltage , When the load changes little .

Parameter selection ：

1. Output voltage ：UL=U2( Half wave ) UL=1.2*U2( Full wave or bridge )
2. Selection of capacitance ：C>=(0.03~0.05)/RL
3. Diode selection ：Urm=1.41*U24
   Inductive filtering :
   Characteristics of inductive filter ： Due to the action of self induced electromotive force, the conduction angle of diode is larger than that of capacitor filter circuit , The peak current flowing through the diode decreases , Good external characteristics , Strong load carrying capacity . Inductive filter circuit is mainly used in the occasions where the capacitive filter is not competent for high current load or the load changes frequently , It is rarely used in low-power electronic equipment .
   
   For DC component : XL=0 It's equivalent to a short circuit , The voltage drops mostly at RL On .
   For harmonic components : f The higher the ,XL The bigger it is , The voltage drops mostly at XL On . therefore , A relatively smooth DC voltage is obtained at the output .
   When ignoring the DC resistance of the inductive coil , The average output voltage is about ：UL=0.9U2
   RC – pai Type filtering
   When the current is small 、 In the case of low requirements , Resistance is often used instead of inductance L, constitute RC-pai Type a filter . It's low cost 、 Small volume , Good filtering effect . But because the resistance consumes power , Therefore, the power loss of the power supply is large , The efficiency of the power supply is reduced , It is generally suitable for occasions with small output current .
   
   Zener diode ：
   
   When the zener diode works in the reverse breakdown state , When the working current Iz stay Izmax and Izmin Between time , The voltage at both ends is approximately constant .