apogeeweb title
Home arrow Power Supplies

arrow left

arrow right

Aug 15 2018

Half Wave Rectifier Circuit Explanation: Working, Parameters and Application

Introduction

The half-wave rectifier circuit is a common circuit that uses the unidirectional conduction characteristics of diodes to perform rectification, and the rectification method that removes half a cycle and leaves half a cycle is called half-wave rectification. The function of it is to convert alternating current into direct current, that is, rectification.

Half Wave Rectifier Explanation

Catalog

Introduction

Catalog

I Working Principle and Circuit Diagram of Half-wave Rectifier

1.1 Introduction to Half-wave Rectifier Circuit

1.2 How a Half-wave Rectifier Circuit Works

1.3 Waveform

1.4 Features and Applications

II Parameters

2.1 The Reverse Peak Voltage (PIV) of the Bridge Rectifier Diode

2.2 Functions and Parameters of Power Diode

2.3 Ripple Coefficient and Rectification Filter

III Illustrate the Structure and Principle of Precision Half-wave Rectifier Circuit with Diagrams

IV Introduction to Single-phase Half-wave Rectifier Circuit

V Application of Half-wave Rectification

I Working Principle and Circuit Diagram of Half-wave Rectifier

1.1 Introduction to Half-wave Rectifier Circuit

The mains power (AC grid) needs to go through four processes of voltage transformation, rectification, filtering and stabilization to become stable DC power.

Using the unidirectional conductivity of the diode, the process of transforming the power frequency alternating current whose size and direction change with time into a unidirectional pulsating direct current is called rectification. After half-wave rectification, because half of the AC power is discarded, the output voltage is roughly half of the original voltage. For example, the input is 24V AC voltage. After half-wave rectification, the output DC voltage is about 12V.

Sometimes the transformer, rectifier circuit and filter circuit are collectively referred to as a rectifier.

Half-wave rectification: The secondary winding of the transformer is connected to the load, and a rectifier diode is connected in series, which is half-wave rectification. Utilizing the unidirectional conductivity of the diode, only half of the cycle has current flowing through the load, and the other half of the cycle is blocked by the diode without current. In this circuit, there is a direct current component flowing through the transformer, which reduces the efficiency of the transformer; the pulsating component of the rectified current is too large, and the requirements for the filter circuit are high. Only suitable for small current rectifier circuits.

Half-wave rectifier circuit diagram

Figure1. Half-wave Rectifier Circuit Diagram

1.2 How a Half-wave Rectifier Circuit Works

Let's see how the diode is rectified from the waveform diagram in the figure below.

Half-wave rectified waveform

Figure2. Half-wave Rectified Waveform

The transformer secondary voltage e2 is a sine wave voltage whose direction and magnitude change with time. Its waveform is shown in Figure 5-2(a). In the time between 0 and K, 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 turned on with the positive voltage, and e2 is added to the load resistance Rfz through it. Within π~2π, e2 is a 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. In the time of π~2π, repeat the process of 0~π; time, and in the time of 3π~4π, repeat the process of π~2π time. Repeating this way, the negative half cycle of the alternating current is "cut" off, and only the positive half cycle passes through Rfz, and a single rightward (upper positive and lower negative) voltage is obtained on Rfz, as shown in Figure (b). The purpose of rectification, however, the load voltage Usc and the size of the load current also change with time, so it is usually called pulsating DC.

This rectification method that removes the half cycle and the lower half of the diagram is called half-wave rectification. It is not difficult to see that the half-wave rectification theory is at the expense of "sacrificing" half of the AC in exchange for the rectification effect, and the current utilization rate is very low (calculation shows that the average value of the half-wave voltage obtained by the rectification in the entire cycle, that is, the load The DC voltage Usc =0.45e2) is therefore commonly used in high-voltage, low-current occasions, but rarely used in general radio devices.

1.3 Waveform

Half wave rectifier Circuit

Figure3. Half Wave Rectifier Circuit

In the negative half cycle of u2 (ωt =π~2π), diode D is cut off due to the application of reverse voltage, no current flows on RL, and the voltage on RL is uL=0. The rectified waveform can be drawn as shown in the figure.

It can be seen that due to the unidirectional conduction effect of the diode, the current flowing through the load resistance is a pulsating current, and the voltage is also a unidirectional pulsating voltage, and the average value of the voltage (output DC component) is

Half wave rectifier formula

The average current flowing through the load is

Formula of IL

 

The average current flowing through the diode D (ie forward current) is

Equation for ID

The highest reverse voltage applied across the diode is

Equation

1.4 Features and Applications

The half-wave rectifier circuit is simple, with few components, but the output voltage has a small DC component (only half a wave), large pulsation, and low rectification efficiency. It is only suitable for occasions with small output current, large allowable pulsation and low requirements.

II Parameters

2.1 The Reverse Peak Voltage (PIV) of the Bridge Rectifier Diode

(1) In the positive half cycle, the current starts from the upper end of Vs1 and flows back to the lower end of Vs1 (center tap) after D1 for load consumption. D1 bears a forward voltage of Vs1 (MAX). The DC output voltage is approximately equal to 0.45 times Vs1 (half-wave rectification).

(2) In the negative half cycle, the current starts from the lower end of Vs2 and flows back to the upper end of Vs2 (center tap) after D2 for load consumption. D2 bears a positive voltage of Vs2 (MAX). The DC output voltage is approximately equal to 0.45 times Vs2 (half-wave rectification).

(3) For a complete cycle of DC output Vs, the positive half cycle is provided by Vs1, and the negative half cycle is provided by Vs2. The two half-wave rectifiers overlap together to achieve the effect of full-wave rectification. The final DC output = 0.9 times Vs.

(4) Pay attention to the mark point on the end of the secondary symmetric winding with the same name. Assuming that the marked point is positive, then we look at Vs1 and Vs2, they are like two dry batteries in series, combined into a larger voltage power supply, this power supply is twice the voltage of a single dry battery. In the positive half cycle, the reverse voltage of D2 is Vs1+Vs2, and the reverse voltage of D1 during the negative half cycle is also Vs1+Vs2. Because the material of the diode will produce a voltage drop, the voltage applied across the diode after removing this voltage drop in the calculation is the diode reverse voltage value. So here PIV=VR(max)=2Vs(max)-Vr.

2.2 Functions and Parameters of Power Diode

The working principle of PN junction and power diode

The basic structure and working principle are the same as the diode in the information electronic circuit. It is composed of a larger area PN junction, leads at both ends and a package. From the appearance point of view, there are mainly two types of packages, bolt type and flat type.

Reverse breakdown of PN junction (two forms)

(1) Avalanche breakdown

(2) Zener breakdown

(3) Both may cause thermal breakdown

Capacitance effect of PN junction

(1) The amount of charge of the PN junction changes with the applied voltage, presenting a capacitance effect, which is called junction capacitance CJ, also called differential capacitance.

(2) The junction capacitance is divided into barrier capacitance CB and diffusion capacitance CD according to the difference of its generation mechanism and function.

(3) The capacitance affects the operating frequency of the PN junction, especially the high-speed switching state.

Forward average current IF (AV)

Rated current: The average value of the maximum power frequency sine half-wave current allowed to flow under the specified case temperature (referred to as case temperature, expressed by TC) and heat dissipation conditions

The average forward current is defined in accordance with the heating effect of the current, so the current rating should be selected according to the principle of equal effective value when using it, and a certain margin should be left.

Conversion relationship:

Equation

When used in high-frequency applications, the heat caused by switching loss can often not be ignored

When a power diode with a large reverse leakage current is used, the heating effect caused by its off-state loss is not small.

However, in the actual converter circuit, the current flowing through the device cannot be exactly the sine half-wave current. Therefore, when designing the circuit and selecting the device, the effective value of the current in the actual circuit must be equal to the sine half-wave effective value. In principle, the rated current of the device is calculated by converting it into an average value.

The specific power frequency sine half-wave current waveform in one cycle is shown in the figure below:

The specific power frequency sine half-wave current waveform

Figure4. The Specific Power Frequency Sine Half-wave Current Waveform

The expression of this waveform in one cycle is:

Expression

Figure5. Expression

Forward pressure drop UF

Refers to the forward voltage drop corresponding to a specified steady-state forward current of a power diode at a specified temperature

Sometimes the parameter table also gives the maximum instantaneous forward voltage drop of the device when a certain transient forward current flows at a specified temperature

Repetitive peak reverse voltage URRM

Refers to the highest peak reverse voltage that can be repeatedly applied to the power diode

Usually 2/3 of its avalanche breakdown voltage UB

When used, it is often selected according to twice the maximum reverse peak voltage that the power diode in the circuit may withstand

Maximum operating junction temperature TJM

Junction temperature refers to the average temperature of the PN junction of the die, expressed in TJ

The maximum operating junction temperature refers to the highest average temperature that the PN junction can withstand without damage

TJM is usually in the range of 125~175ºC

Reverse recovery time trr

trr=td+tf, during the shutdown process, the current drops to 0 to restore the response blocking ability time

Inrush current IFSM

Refers to the maximum continuous overcurrent of one or several power frequency cycles that the power diode can withstand.

2.3 Ripple Coefficient and Rectification Filter

Ripple coefficient (also known as ripple factor), it is an important parameter to measure the performance of a power rectifier. At present, many electronic products that use AC power supply require AC/DC conversion through a rectifier. The pulsating DC voltage output after the conversion contains both DC components and AC components. Therefore, people often call the AC component in this fluctuating DC power "ripple." The fluctuation degree of the ripple is generally expressed by the ripple coefficient acr, 1 = the effective value of the AC component of the output voltage/the DC component of the output voltage. It can be seen that the smaller the value of 1, the smaller the ripple and the smoother the output DC voltage. Usually the ripple coefficient of single-phase half-wave resistive load rectifier circuit is 1.21; the ripple coefficient of single-phase full-wave resistive load rectifier circuit is 0.48, and its output DC voltage is twice the former, and the ripple frequency is also the former 2 Times. For example, when the frequency of the input AC power is 50 Hz, the ripple frequency after half-wave rectification is still 50 Hz, and the ripple frequency after full-wave rectification is 100 Hz. The method of adding a filter is usually used to reduce the ripple coefficient. The more common power rectifier and filter circuits are as follows:

(1) Half-wave rectifier circuit

It is the simplest rectifier circuit. The ripple in the output pulsating DC current is relatively large. It is generally not suitable for audio-visual products and linear amplifying circuits that require high voltage smoothness, but it is ideal for charging and electroplating processes, and can also be used for loads. Circuit with current below 10mA.

(2) Bridge rectifier circuit

The positive and negative half cycles of the alternating current are respectively rectified by four diodes to synthesize pulsating direct current with higher direct current content. Its ripple coefficient and ripple frequency are the same as those of a full-wave rectifier, as shown in the figure below.

Bridge Rectifier Circuit

Figure6. Bridge Rectifier Circuit

(3) Full-wave rectifier circuit

It can rectify the positive and negative half cycles of the alternating current output by the transformer with a center tap and output the pulsating direct current in turn to reduce the ripple coefficient of the direct current, as shown in the figure below.

Full-wave rectifier circuit

Figure7. Full-wave Rectifier Circuit

(4) Capacitor filter

Use capacitor charge and discharge characteristics to connect capacitors in parallel in the rectifier output circuit to filter the ripple-containing DC power and smooth the output DC power. The larger the capacity of the capacitor of the rectifier filter circuit with the same pulsating frequency, the better the filtering effect; the higher the pulsating frequency of the circuit with the same capacity, the better the filtering effect.

(5) Inductance filter

Utilizing the inductance characteristic of inductance to alternating current, an inductor is connected in series in the rectifier output circuit to block the passage of alternating current components and make the output direct current smoother. The larger the inductance, the better the filtering performance, and the higher the ripple frequency, the better the filtering effect of an inductor with the same inductance.

(6) π-type filter

The π-type filter is a kind of "LC filter", which uses the respective filtering characteristics of capacitors and inductors to form a multiple filter. The advantages of this filter are repeated filtering, small DC resistance, and low loss. It is generally suitable for large current loads. And the rectifier with large ripple coefficient.

(7) RC filter

This kind of filter is composed of two capacitors and a resistor. It uses C1's fast charge and slow discharge to maintain a certain voltage drop at the positive terminal of C1, so that the output voltage becomes smooth and stable. Relative to the load RL, R1 and C2 in the circuit become a voltage divider of the ripple voltage, so that the ripple at the upper end of the load is short-circuited (absorbed) by C2, and the DC voltage applied by the load becomes smoother. RC filter has the advantages of small size, light weight, economy and convenience, so it is widely used in rectifiers with small load current.

III Illustrate the Structure and Principle of Precision Half-wave Rectifier Circuit with Diagrams

Using the unidirectional conductivity characteristics of the diode (switching device) and the excellent amplification performance of the amplifier can be used to accurately rectify the input alternating signal (especially the small amplitude voltage signal), thereby forming a precision half-wave rectification Circuit. If this adds a simple circuit, a precise full-wave rectifier circuit can be constructed.

The turn-on voltage drop of the diode is about 0.6V. This turn-on voltage drop is also called the diode threshold voltage, which means that the diode goes from the off state to the on state after the 0.6V threshold. In a conventional rectifier circuit, since the amplitude of the rectified voltage is much higher than the conduction voltage drop of the diode, the existence of this threshold voltage can be almost ignored. But in the processing of small-amplitude alternating signals, if the signal amplitude is even less than 0.6V, even if the diode has the ability to rectify, it is completely useless.

When the diode looked around at a loss, its helper, an operational amplifier with excellent amplification performance, appeared in a timely manner, which changed this outcome. The two hit it off, and the small-signal precision half-wave rectifier circuit is about to make its debut. Please see the picture below.

Circuits and Waveform of Precision Half-wave Rectifier

Figure8. Circuits and Waveform of Precision Half-wave Rectifier

The circuit in the above figure ignores the positive half-wave of the input signal, and only rectifies the negative half-wave of the input signal, and outputs it after phase inversion.

(1) In the positive half cycle of the input signal (0~t1 moment), D1 is turned on and D2 is turned off, the circuit is equivalent to a voltage follower (circuit b in the figure):

Before D1 and D2 are turned on, the circuit is in an open loop state where the voltage amplification factor is extremely large. At this time (the positive half-wave input period of the input signal), even if the input of the amplifier becomes negative, the diode D1 is turned on. (Equivalent to short-circuit), D2 reverse bias is cut off (equivalent to open circuit), forming a voltage follower mode. Because the non-inverting terminal is grounded, the circuit turns into a voltage follower that follows the ground level, and the output terminal can still maintain zero potential.

(2) In the negative half cycle of the input signal (at t1~t2), D1 is turned off, D2 is turned on, and the circuit is equivalent to an inverter (circuit c in the figure):

During the negative half-wave period of the input signal (before D1 and D2 are turned on), even if the output end of the tiny input signal becomes positive, the diode D1 is reversely biased and D2 is forward biased, forming an inverting (amplifier) ​​circuit pattern. The negative half-wave signal is inverted and output.

In the working process, the two diodes cooperate tacitly, one is on and the other is off, and the input positive half-wave signal is connected to the outside of the door, keeping the original output state unchanged; for the input negative half-wave signal, it is put in the door to help it turn a somersault (Reversed phase) Send out afterwards. The sincere cooperation of the two diodes, coupled with the excellent amplification performance of the operational amplifier, sufficient ingredients, and authentic workmanship, have made the "big meal" of precision half-wave rectification.

If the resistance value of the feedback resistor R2 is adjusted to make R2=2R1, and then mixed with the input signal, a full-wave precision rectifier circuit is formed, as shown in the figure below.

Circuit and Waveform

Figure9. Circuit and Waveform

Increase the feedback resistance R2 of the N1 amplifier to make R2=2R1, so that the rectified signal is inverted and amplified twice and then output, and then added to the input signal. The rectified +10V is added to the negative half-wave input -5V , 10+(-5)=5, just can "eliminate" the negative half-wave and get the full-wave rectified voltage.

The so-called magic electricity (modular electricity), if you can see through its transformation technique, there are only one circuit model left, then why magic?

The prerequisite for fault detection of precision circuits is that all operational amplifiers are DC amplifiers, and even DC voltage signals can be applied to determine whether the circuit is good or bad.

(1) When the input signal voltage is zero, the output terminal (the negative terminal of D2 is the output terminal), the output voltage is also 0V;

(2) When a positive voltage signal is input, the output terminal keeps 0V;

(3) When a negative voltage signal is input, IN=-OUT.

IV Introduction to Single-phase Half-wave Rectifier Circuit

When working in electronic circuits, most of them use DC power supply. The working principle of the rectifier circuit is to use the unidirectional conductivity of the electronic components to change the alternating current with positive and negative polarities into a direct current with only one polarity. There are two types of rectifier circuits: one-way and three-phase. Here we will introduce some basic knowledge of single-phase rectifier circuits.

Single-phase rectifier circuit mainly includes single-phase half-wave rectification, single-phase full-wave rectification and single-phase bridge rectification. The following mainly introduces single-phase half-wave rectification.

The single-phase half-wave rectifier circuit is composed of transformer T, rectifier diode D and load resistance RL. As shown in Figure 1. The transformer T reduces the 220V power frequency alternating current, u2 to several volts or more than ten volts of alternating current, which is the output voltage of the secondary side of the transformer T.

Single-phase Half-wave Rectifier Circuit and Waveform

Figure10. Single-phase Half-wave Rectifier Circuit and Waveform

The circuit uses the unidirectional conductivity of the diode for rectification. During the positive half cycle of U2, diode D is conducting forward. The current flows from the upper end of the transformer secondary coil, through the diode D, flows through the load resistance RL, and flows back to the lower end of the transformer secondary coil. The output voltage UL is the positive half cycle of the sine wave.

In the negative half cycle of U2, diode D is reversely blocked. There is no current in the rectifier circuit, and the output voltage UL is zero.

The waveform of the output signal uL is shown in Figure 1(b). It can be seen from the figure that the output signal UL is a half-wave pulsating DC signal. If we ignore the forward voltage drop of the diode and the internal resistance of the transformer, its average value in one cycle is

Formula

In the formula, U2 is the effective value of the voltage U2 on the secondary side of the transformer. Average value of load current iL

Formula

The advantages of the single-phase half-wave rectifier circuit are simple structure and fewer components. However, the output voltage has a large ripple component, and only half of the wave is used. Therefore, it is only suitable for some occasions with lower requirements.

V Application of Half-wave Rectification

Connect the diode and the electrical appliances in series (the electrical appliances here are generally electric heating appliances, such as electric soldering irons, electric stoves, rice cookers, electric frying pans, electric blankets, light bulbs, hair dryers, etc.), and then add AC voltage, this It is the most commonly used diode half-wave rectifier circuit.

If a switch K is connected in parallel at both ends of the diode, when the switch is closed, the electrical appliance returns to its original state. This not only expands the functions of these electrical appliances, but also prolongs the service life of the electrical appliances, which is of great practical value.

(1) If the electrical appliance is an electric soldering iron, closing K can make the soldering iron heat up quickly, and disconnecting K can play a role in heat preservation, which can prevent the soldering iron from "burning" during continuous use, and then close K when using it. The electric soldering iron can heat up quickly and is very convenient to use.

(2) If the electrical appliance is an electric rice cooker (or electric frying pan), it is equivalent to adding a gear to the pot. Two temperatures can be provided. First, close K, and switch to low gear after the water boils. ), used for porridge, can prevent the porridge soup from overflowing; used for steaming rice, can make the rice soft.

(3) If the electrical appliance is a light bulb, the brightness of the light bulb can be reduced when K is disconnected, which has a significant effect on prolonging the service life of the light bulb.

(4) If the electrical appliance is a hair dryer, on the basis of the original cold and hot air, two kinds of hot and cold breeze are provided, and at the same time, it can effectively avoid the damage of the current-limiting resistor, the vulnerable part in the hair dryer. In fact, this is also the temperature regulation principle of our common temperature regulation electric blanket.

After the diode is installed, the actual power of the electrical appliance becomes half of the original power.

Related Articles

pinglun 0 comment

Leave a Reply

Your email address will not be published.

 
 
   
code image
Rating: poor fair good very good excellent