What is a rectifier diode, how does it works?

A rectifier diode is a semiconductor device that converts AC power into DC power. Usually, it contains a PN junction with anode and cathode terminals. Its structure shows in Figure 1. The carriers in the P region are holes, and the pages in the N region are electrons, forming a specific potential barrier between the P region and the N region. When a positive voltage is applied to the P region relative to the N region, the potential barrier is lowered. And storage carriers are generated near both sides of the likely border, which can pass a large current and have a low voltage drop (typically 0.7V), which is called forward conduction—pass status. Suppose the opposite voltage is applied to increase the potential barrier. In that case, it can withstand high reverse voltage and flow a small reverse current (called reverse leakage current), called reverse blocking state. The rectifier diode has obvious unidirectional conductivity.

What is a rectifier diode, how does it works?
What is a rectifier diode, how does it works?

The rectifier diode is a large-area power device with large junction capacitance and low operating frequency, generally in kilohertz. The reverse voltage is from 25 volts to 3000 volts. There are 2CP series with a current capacity below 1A and 2CZ series above 1A. Rectifier diodes can be made of materials such as semiconductor germanium or silicon. Silicon rectifier diodes have high breakdown voltage, small reverse leakage current, and good high-temperature performance. Usually, high-power rectifier diodes are made of high-purity single crystal silicon. This device has a large junction area and can pass a large current (up to thousands of amps), but the operating frequency is not high, generally below tens of kilohertz. Rectifier diodes are mainly used in various low-frequency rectifier circuits.

Selection of rectifier diodes

Rectifier diodes are generally planar silicon diodes used in various power rectifier circuits. When choosing a rectifier diode, the parameters such as its maximum rectifier current, maximum reverse working current, cut-off frequency, and reverse recovery time should be considered. The rectifier diodes used in ordinary series regulated power supply circuits do not have high requirements on the reverse recovery time of the cut-off frequency. The rectifier diodes with the maximum rectification current and the maximum reverse working current meet the requirements according to the needs of the circuit—for example, 1N series, 2CZ series, RLR series, etc.
The rectifier diodes used in the switching regulated power supply and the pulse rectifier circuit should have higher operating frequency and shorter reverse recovery time (RU series, EU series, V series, 1SR series, etc.) or Fast recovery diode.

Standard parameters of rectifier diodes

(1) Maximum average rectified current IF

refers to the maximum forward average current allowed to pass through the diode during long-term operation. This current is determined by the junction area of ​​the PN junction and heat dissipation conditions. When using, it should note that the average current through the diode cannot be greater than this value, and the heat dissipation conditions must be met. For example, the IF of a 1N4000 series diode is 1A.

(2) The highest reverse working voltage VR

refers to the maximum reverse voltage allowed to be applied across the diode. If it is more significant than this value, the reverse current (IR) will increase sharply, and the unidirectional conductivity of the diode will be destroyed, thereby causing reverse breakdown. Usually take half of the reverse breakdown voltage (VB) as (VR). For example, the VR of 1N4001 ($0.0084) is 50V, and the VR of 1N4007 ($0.0084) is 1OOOV

(3) Maximum reverse current IR

It is the reverse current that the diode is allowed to flow under the highest reverse working voltage. This parameter reflects the unidirectional conductivity of the diode. Therefore, the smaller the current value, the better the quality of the diode.

(4) Breakdown voltage VR

refers to the voltage value at the sharp inflection point of the reverse volt-ampere characteristic curve of the diode. When the reverse is a soft characteristic, it relates to the voltage value under the condition of a given reverse leakage current.

(5) The highest operating frequency FM

It is the diode’s highest operating frequency under normal conditions. It is mainly determined by the junction capacitance and diffusion capacitance of the PN junction. If the operating frequency exceeds FM, the unidirectional conductivity of the diode will not be well-reflected. For example, the form of a 1N4000 series diode is 3kHz.

(6) Reverse Recovery time

It is the reverse recovery time under the specified load, forward current, and maximum reverse transient voltage.

(7) Zero bias capacitor CO

refers to the sum of the capacity of the diffusion capacitor and the junction capacitor when the voltage across the diode is zero. It is worth noting that due to the limitations of the manufacturing process, even the same type of diodes have excellent dispersion of their parameters. The parameters given in the manual are often a range. If the test conditions change, the corresponding parameters will also change. For example, the IR of 1N5200 series silicon plastic encapsulated rectifier diodes measured at 25°C is less than 1OuA, while at 100°C, IR becomes less than 500uA.

Causes of damage to the rectifier tube

(1) The lightning protection and overvoltage protection measures are weak.

The rectifier is not equipped with lightning protection and overvoltage protection devices. Even if lightning protection and overvoltage protection devices are installed. Its operation is unreliable, and the rectifier tube is damaged due to lightning strikes or overvoltage.

(2) Bad operating conditions

For the generator set with the indirect transmission, the generator runs at high speed due to the incorrect calculation of the ratio of rotation speed or the balance of the diameters of the two belt pulleys not meeting the requirements of the percentage of rotation speed for a long time. The rectifier tube is also at a higher voltage for a long time. Under working conditions, the rectifier tube is accelerated to age and is damaged by the premature breakdown.

(3) Poor operation and management

The operators on duty are irresponsible in their work, do not understand the changes in external loads (especially between midnight and 6 am the next day), or when a load shedding failure occurs in the outside world. The operators fail to perform corresponding operations in time Treatment will cause overvoltage to break down and damage the rectifier tube.

(4) Equipment installation or manufacturing quality is not up to standard

Due to the long-term operation of the generator set in a significant vibration. The rectifier tube is also under the interference of the external force of this vibration. At the same time, due to the generator set’s high and low speed, the rectifier tube’s working voltage also fluctuates—the common ground changes, which significantly accelerates the aging and damage of the rectifier tube.

(5) The specifications and models of the rectifier tubes do not match

When replacing a new rectifier tube, the line whose working parameters do not meet the requirements is replaced by mistake or the wiring is wrong, resulting in breakdown and damage of the rectifier tube.

(6) The safety margin of the rectifier tube is too small

The overvoltage and overcurrent safety margins of the rectifier tube are too small. The rectifier tube can not withstand the attack of the overvoltage or overcurrent transient process peak in the generator excitation circuit and be damaged.

How to check the rectifier tube

First, remove all the rectifier diodes in the rectifier. And use the 100×R or 1000×R ohm gear of the multimeter to measure the two lead wires of the rectifier diode (the head and the tail are reversed once for each measurement). Suppose the resistance values ​​measured twice are very different, for example. In that case, the significant resistance value is as high as several tens of thousands Ω. In contrast, the small resistance value is only a few hundred Ω or even smaller, indicating that the diode is suitable (a diode with a soft breakdown). Except). If the resistance values ​​measured twice are almost the same, and the resistance value is minimal, it means that the diode has been damaged by the breakdown and cannot be used.

Technical characteristics of rectifier diodes

A semiconductor device converts alternating current electrical energy into direct current electrical power. Usually, it contains a PN junction with anode and cathode terminals. Its structure is shown in Figure 1. The carriers in the P region are holes, and the pages in the N region are electrons, forming a specific potential barrier between the P region and the N region. When a positive voltage is applied to the P region relative to the N region, the potential barrier is lowered. And storage carriers are generated near both sides of the likely border, which can pass a large current and have a low voltage drop (typically 0.7V), called forward conduction—pass status. Suppose the opposite voltage is applied to increase the potential barrier. In that case, it can withstand high reverse voltage and flow a small reverse current (called reverse leakage current), called reverse blocking state. The rectifier diode has obvious unidirectional conductivity, and its volt-ampere characteristics and circuit symbols are shown in Figure 2. Rectifier diodes can be made of materials such as semiconductor germanium or silicon. Silicon rectifier diodes have high breakdown voltage, small reverse leakage current, and good high-temperature performance. Usually, high-voltage and high-power rectifier diodes are made of high-purity single-crystal silicon (it is easy to reverse breakdown when doped more). This device has a large junction area and can pass a large current (up to thousands of amps), but the operating frequency is not high, generally below tens of kilohertz. Rectifier diodes are mainly used in various low-frequency half-wave rectifier circuits. To achieve full-wave rectification, they must be connected to form a rectifier bridge.
When selecting a rectifier diode, parameters such as its reverse peak voltage, maximum rectifier current, maximum reverse working current, cut-off frequency, and reverse recovery time should be considered.

Commonly used rectifier diodes

such as 1N4001, the colored end indicates the negative electrode.
Its characteristic parameters: Vrm≥50v, If=1A,

The AC and DC resistances of the rectifier diodes

Forward DC resistance Rf=Vf/If=(Vf/Is) exp(-qVf/mkT)
Forward AC resistance rf=dVf/dIf=mkT/qIf
Reverse DC resistance Rr=Vr/Ir=Vr/Is
Reverse AC resistance rr=dVr/dIr=(mkT/qIf) exp(qVr/mkT)

The rectification ratio of the rectifier diode

DC rectification ratio Rr/Rf=(Vr/Vf) exp(qVf/mkT)
AC rectification ratio rr/rf=(If/Is) exp(qVf/mkT)
Where Vf and If is the forward voltage and forward current, respectively, Vr and Ir are the reverse voltage and reverse current, respectively, Is is the reverse saturation current (ignoring the current generated by the recombination center), and m is the ideality factor (m= 1 to 2).
The DC rectification ratio and the AC rectification ratio have a great relationship, respectively, with the forward voltage and reverse voltage. And as long as the voltage is not too low, under a specific forward bias voltage, the AC resistance of the p-n junction will always be smaller than its corresponding DC resistance Rf. In addition, the bold DC resistance can be rewritten as rf=(Mkt/qIf) ln(If/Is)=rf ln(If/Is). It can see that when the forward bias voltage and the corresponding forward current are constant, the PN junction The DC resistance is much larger than the corresponding AC resistance (Rf is ln(Jf/Js) times larger than rf). The forward DC conductance of the PN junction is much smaller than the forward AC conductance. Of course, for a reverse-biased p-n junction, the reverse DC resistance is much smaller than the reverse AC resistance.

Leave a Comment