Tech

# Introduction to Rectifier Basics and Circuit Types

### Rectifier Diode Introduction

The rectifier diode is a semiconductor device that converts AC power into DC power. Usually it contains a PN junction with two terminals, a positive electrode and a negative electrode. The most important characteristic is unidirectional conductivity. In electronic circuits, with high breakdown voltage and small reverse leakage current, the high temperature performance is good.

##### Rectifier Diode Materials

Generally, rectifier can be made of materials such as semiconductor germanium or silicon. In addition, high-voltage and high-power rectifier diodes use the high-purity single crystal silicon. Because it is easy to reverse breakdown when there is more doping). This kind of device has a large junction area and can pass a large current (up to thousands of amperes). But the operating frequency is not high, generally below tens of KHz. Rectifier diodes are common in various low-frequency half-wave rectifier circuits. If require full-wave rectification, use several diodes to form a rectifier bridge.

### 1 Rectifier Diode Common Parameters

#### 1.1 Main Parameter of Rectifier Diode

Rectifier diodes use the unidirectional conductivity of the PN junction to convert alternating current into pulsating direct current. Rectifier diodes have a large leakage current, and most of them are smd package. The parameters of the rectifier diode include the maximum rectifier current. It refers to the maximum current value allowed by the rectifier diode for long-term operation. It is the main parameter of the rectifier diode and the main basis for the option of the rectifier diode.

#### 1.2 Other Important Parameters of Rectifier Diode

##### Voltage and Current

(1) Maximum average rectified current IF
It refers to the maximum forward average current allowed to pass through the diode. The current is determined by the PN junction area and the heat dissipation conditions. Pay attention, the average current passing through the diode cannot be greater than this value, and has heat dissipation.

(2) Maximum reverse working voltage VR
It refers to the acceptable maximum reverse voltage to be applied across the diode. If it is greater than this value, the reverse current (IR) will increase sharply, and the unidirectional conductivity of the diode will fade. Result in reverse breakdown. Usually take half of the reverse breakdown voltage VB as VR.

(3) Maximum reverse current IR
It is the acceptable reverse current to flow through the diode. And this will happen under the highest reverse working voltage. This parameter reflects the quality of the unidirectional conductivity of the diode. Therefore, the smaller the current value, the better the diode quality.

(4) Breakdown voltage VB
It refers to the voltage value at the sharp bend point. And it is in the reverse volt-ampere characteristic curve of the diode. When the reverse is a soft characteristic, it refers to the voltage value under a given reverse leakage current condition.

##### Other Points

(5) The highest operating frequency fm
It is the highest operating frequency of the diode 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.

(6) Reverse recovery time trr
It refers to the reverse recovery time under the specified load, forward current and maximum reverse transient voltage.

(7) Zero-bias capacitor CO
It refers to the sum of the capacitance of the diffusion capacitance and the junction capacitance. When the voltage across the diode is zero. Due to the limitation of the manufacturing process, even the same type of diode has a large dispersion of its parameters.
The parameters given in the manual are often within a range. If the test conditions change, the corresponding parameters will also change. For example, the IR of the 1N5200 series silicon plastic rectifier diode. It at 25°C is less than 10uA, and at 100°C IR becomes less than 500uA.

### 2 Rectifier Diodes Selection

#### 2.1 Required Conditions in Rectifier Diode Circuit

Rectifier diodes are generally planar silicon diodes, which are popular in various power rectifier circuits. When selecting a rectifier diode, the parameters such as its maximum rectifier current, maximum reverse working current, cut-off frequency and reverse recovery time should be mainly considered.

The rectifier diode used does not require high reverse recovery time of the cut-off frequency. It is in the ordinary series stabilized power supply circuit. The rectifier diode with the maximum rectified current and maximum reverse working current should meet the requirements of the circuit.

#### 2.2 Rectifier Diode in Series Connection

Talk about the rectifier circuit of the switching regulated power supply and the pulse rectifier circuit. it should be a rectifier diode with a higher operating frequency and shorter reverse recovery time. Such as RU series, EU series, V series, 1SR series, etc.) or select fast recovery diodes, or Schottky rectifier diode.

### 3 Rectifier Diode Common Failures

(1) Inadequate lightning protection and poor overvoltage protection.
The rectifier device is not equipped with lightning protection and overvoltage protection devices. Or insufficient routine maintenance of the equipment.

(2) Poor operating conditions.
In the indirect drive generator set, the calculation of the speed ratio is incorrect or the ratio of the diameters of the two belt pulleys does not meet the speed requirements. The generator will run at a high speed for a long time. So the rectifier is at a higher voltage for a long time. It accelerates the rectifier aging, and was damaged by premature breakdown.

(3) Poor operation management.
The load failure or diode breakdown doesn’t fixed in time.

(4) Poor equipment installation or manufacturing process.
The generator set has been operating under large vibration for a long time, which affects the rectifier tube operation. At the same time, because the generator set speed is unstable, the working voltage of the rectifier tube also fluctuates. It greatly accelerates the aging and damage of the rectifier tube.

(5) The specifications and models of the rectifier tube do not match.
When replacing a new rectifier tube, wrong replacement and wiring will cause the rectifier to breakdown and damage.

(6) The safety margin of the rectifier tube is too small.
The overvoltage and overcurrent safety margin of the rectifier tube is too small. So that the rectifier tube cannot withstand the overvoltage. Or the peak value of the overcurrent transient process that occurs in the generator excitation circuit.

### 4 Rectifier Diodes Detection

Here is a more general and simple method. Remove all the rectifier diodes in circuit.Use the 100×R or 1000×R ohm range of a multimeter to measure the two lead wires of the rectifier. If the resistance values measured twice are very different.

For example, the resistance value is as high as a few hundred kΩ to infinity. Or the resistance value is only a few hundred Ω or less. They indicate that the diode is good (except under special circumstances). Same resistance values and or very small, it means that the diode has been broken down. In addition, if both infinite, it means that the diode has been internally disconnected.

### 5 Rectifier Diode Replacement

#### 5.1 Rectifier Diode Replacing Rules

After the rectifier diode is bad, you should replace with the same model or another model with the same parameters.
Generally, rectifier diodes with high withstand voltage (reverse voltage) are suitable for rectifier diodes with low withstand voltage. While rectifier diodes with low withstand voltage cannot be replaced with high withstand voltage. A diode with a high rectification current value can replace with a diode with a low current value. While a diode with a low rectification current value cannot do it reversely.

#### 5.2 Commonly Used Rectifier Diode List

1N4001, 1N4002, 1N4003
1N4004, 1N4005, 1N4006
1N4007, 1N4148, 1N5391
1N5392, 1N5393, 1N5394
1N5395, 1N5396, 1N5397
1N5398, 1N5399, 1N5400
1N5401, 1N5402, 1N5403
1N5404, 1N5405, 1N5406
1N5407, 1N5408

### 6 Rectifier Diode Circuit Types

The power grid supplies users with alternating current, and various electrical devices require direct current. Rectification is the process of converting AC into DC. Utilizing the device with unidirectional conductivity, the current of alternating direction and magnitude can be converted into direct current. The following introduces three main rectifier circuits composed of crystal diodes.

#### 6.1 Half-Wave Rectifier Diode Circuit

Fig 1. Half-Wave Rectifier Diode Circuit

The figure shows the simplest rectifier circuit, including power transformer B, rectifier diode D and load resistor Rfz. The transformer transforms the voltage into the required alternating voltage e2, and then D transforms the AC into pulsating DC.

Fig 2. Half-Wave Rectifier Diode Wave

The transformer threshold voltage e2 is a sine wave voltage. Its direction and magnitude change with time, which shows in Figure (a). In the 0~K time, e2 is a positive half cycle. That is, the upper end of the transformer is positive and the lower end is negative. At this time, the diode is in forward conductive conduction, and e2  connect with the load resistor Rfz through it. Within π~2π, e2 is in negative half cycle. The lower end of the transformer secondary is positive, and the upper end is negative. At this time, D bears the reverse voltage and does not conduct, and there is no voltage on Rfz.

##### Half-wave Rectification Form

In the time of π~2π, the process of 0~π time repeats. And in the time of 3π~4π, the process of π~2π time… half-cycle through Rfz, get a single right direction voltage on Rfz (up positive and lower negative). It shows in Figure (b), achieving the purpose of rectification. But the load voltage Usc, and the load current also changes with time, so it is usually called pulsating DC.

Removing the first half week and leaving half a week is the half wave rectification. It is not difficult to note that the half-wave rectification is at the expense of consuming half of the AC in circuit. And the current utilization rate is very low. According to it, half-wave rectifier diode is common in high voltage and small current occasions. And is rare in general radio devices.

#### 6.2 Full-Wave Rectifier Diode Circuit

Fig  3. Full-Wave Rectifier Diode Circuit

Making some adjustments to the structure of the rectifier circuit, a full-wave rectifier circuit can be obtained. The figure above is the electrical schematic diagram of the full-wave rectifier circuit.

The full-wave rectifier circuit is a combination of two half-wave rectifier circuits. Require a tap needs in the middle of the secondary coil of the transformer. It divides the secondary coil into two symmetrical windings. So as to get two voltages e2a and e2b of equal size. But opposite polarity to form two energized circuits.

##### Waveform Diagram

The full-wave rectifier circuit can be illustrated by the waveform diagram. Between 0 and π, e2a is a positive voltage to Dl, D1 turns on. While a up positive and down negative voltage is obtained on Rfz. e2b is a reverse voltage to D2, and D2 is not conductive (see Figure(b) ). In the time of π-2π, e2b is a positive voltage to D2, D2 turns on. And the voltage obtained on Rfz is still up positive and down negative voltage. Therefore e2a is a reverse voltage to D1, and D1 is not conductive (see figure (c).

Fig  4. Full-Wave Rectifier Diode Circuit Wave

In this way, have the two rectifier elements D1 and D2 conduct electricity in turn. The result is that the load resistor Rfz has the same direction of current at the positive and negative half cycles. As shown in Figure(b). This is full-wave rectification. It not only uses the positive half-cycle, but also cleverly uses the negative half-cycle. Full-wave rectifier greatly improves the rectification efficiency.

Fig  5. Full-Wave Rectifier Diode Circuits

This circuit requires the transformer to have a secondary center tap. This makes the two ends symmetrical. And it brings a lot of trouble to the production. In addition, in this circuit, the maximum reverse voltage that each rectifier diode can withstand is twice the maximum value of the transformer secondary voltage. So diodes should withstand higher voltages.

#### 6.3 Bridge Rectifier Diode Circuit

Fig  6. Bridge Rectifier Diode Circuit

The bridge rectifier circuit is the most used rectification circuit. It has the advantages of a full-wave rectifier circuit. As long as two diode ports are connected to form a bridge structure. So its shortcomings are overcome to a certain extent.

The bridge rectifier circuit is as follows:

Fig  7. Bridge Rectifier Diode Circuit (a)

When e2 is a positive half cycle, D1, D3 and the direction voltage, D1, D3 turn on; D2, D4 are applied with reverse voltage, they turn off. E2, Dl, Rfz, and D3 are energized a loop in the circuit. On Rfz, get a positive and negative half-wave washing voltage.

When e2 is a negative half cycle, a positive voltage is applied to D2 and D4, and they are turn on. Apply reverse voltage to D1 and D3, they cut off. E2, D2Rfz. And D4 are energized a loop in the circuit. While the other half-wave rectified voltage on Rfz.

Fig  8. Bridge Rectifier Diode Circuit (b)

##### Full-wave Rectification Form

If repeated, a full-wave rectified voltage at Rfz is made. The waveform diagram is the same as the full-wave rectifier. It is not difficult to see from the figure. The reverse voltage of each diode is equal to the maximum value of the secondary voltage. It is half smaller than the full-wave cleaning circuit.

### 7 High-frequency Rectifier Diodes

#### 7.1 Rectifier Diode Types

The rectifier diode in the switching power supply is unique. It has the characteristics of low forward voltage reduction and fast recovery, and sufficient output power. Include the following three types of high-frequency diodes: fast recovery rectifier, ultra-fast recovery rectifier, and Schottky diode rectifier.

#### 7.2 Forward Voltage Drop

Fast recovery and the ultra- have moderate and high forward voltage drop. And the range is from 0.8 to 1.2V. These two types of rectifier diodes also have higher cut-off voltage parameters. Therefore, they are particularly suitable for use in low-power auxiliary power circuits with output voltages around 12V.

#### 7.3 Reverse Recovery Time

Compared with general rectifier diodes, the reverse recovery time difference is in the nanosecond level. Thus greatly improving the efficiency of the power supply.

According to experience, choose a fast recovery rectifier diode. Its reverse recovery time should be at least 1/3 of the rise time of the switching transistor. These two kinds of rectifier diodes also reduce the switching voltage spike. Because it will affect the ripple of the output DC voltage.

#### 7.4 Heat Sink

Whether they need a heat sink in switching power supplies. It depends on the maximum power of the circuit. Under normal circumstances, the allowable junction temperature is 175°C during manufacture. The manufacturer has a technical parameters. To calculate the maximum output operating current, voltage, and case temperature.

Even under the action of a large forward current, the forward voltage drop of Schottky rectifier diodes is very low, only about 0.4V. Moreover, as the junction temperature increases, its forward voltage drop decreases.

#### 7.5 Schottky Rectifier Diode

Schottky rectifier diodes are particularly suitable for low-voltage output circuits around 5V. Its reverse recovery time is negligible. Because this device is a semiconductor device with majority carrier. During the switching process of the device, there is no need to remove the stored charge of the minority carrier.

This kind of diodes have two major shortcomings. First, the reverse cut-off voltage tolerance is low, about 100V. Second, the reverse leakage current is large. It causes the device more susceptible to have heat breakdown than other types of rectifier devices. Of course,  adding a transient overvoltage protection circuit and appropriately controlling the junction temperature can over these shortcomings.

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