Circuit Diagram 10-Analysis of Rectification Circuit and Filter Circuit Principle

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Circuit Diagram 10-Analysis of Rectification Circuit and Filter Circuit Principle [Copy link]

Analysis of the Principle of Rectifier and Filter Circuit

The rectifier and filter circuit is one of the commonly used unit circuits. The main function and role of the rectifier and filter circuit is to convert AC power into DC power. The most commonly used is the power rectifier circuit, which steps down, rectifies and filters the AC 220V mains power supply into a suitable DC voltage as the working power supply of the electronic circuit. The rectifier and filter circuit usually consists of two parts: the rectifier circuit and the filter circuit.

1. Rectifier circuit

The key issue of the rectifier circuit is to use the unidirectional conductivity of the diode to convert the AC voltage into a single-phase pulsating voltage. The single-phase rectifier circuit can be divided into half-wave rectification, full-wave rectification, bridge rectification, voltage doubler rectification and other circuit forms. Since the half-wave rectifier circuit only works in half a cycle of the power supply, the power supply utilization rate is low, the output waveform pulsation is large, and the circuit is simple, it is rarely used in practice.

1. Half-wave rectifier circuit

The half-wave rectifier circuit is the simplest and most basic rectifier circuit, as shown in the figure below, which consists of a power transformer T and a rectifier diode VD, and RL is a load resistor. The primary coil L1 of the power transformer T is connected to the AC power supply voltage (usually AC 220V mains), and after passing through the transformer T, the required AC voltage is obtained at both ends of its secondary coil L2, and then rectified by the diode VD to become a DC voltage. The working process of the half-wave rectifier circuit is as follows:

1) In the positive half cycle of the AC voltage, the polarity of U2 is positive at the top and negative at the bottom, as shown in the following figure a). We know that the diode has unidirectional conductivity, that is, the current can only flow from the positive pole to the negative pole. During the positive half cycle of U2, the rectifier diode VD is applied with a forward voltage, so VD is turned on, and the current flows from the "+" of U2 through the rectifier diode VD and the load resistor RL back to the "-" of U2, forming a loop and generating a voltage drop on the resistor RL (that is, the output voltage), with a polarity of positive at the top and negative at the bottom. 2) In the negative half cycle of the AC voltage, the polarity of U2 is negative at the top and positive at the bottom, as shown in the following figure b). At this time, the rectifier diode VD is applied with a reverse voltage. Therefore, VD is cut off, the current I=0, there is no voltage drop on the load resistor RL, and the output voltage is 0. 34)]The working waveform of the half-wave rectifier circuit is shown in the figure below. It can be seen from the figure that the half-wave rectifier circuit has output voltage only in the positive half cycle of the AC voltage, and no output voltage in the negative half cycle. The DC component of the output voltage is small and the AC component is large. Since only half of the AC voltage sine wave is used, the efficiency of the half-wave rectifier circuit is low.

2. Full-wave rectifier circuit

In order to improve the rectification efficiency and reduce the pulsation component of the output voltage, a full-wave rectifier circuit is often used. The full-wave rectifier circuit is actually a combination of two half-wave rectifier circuits, as shown in the figure below.

The secondary winding of the power transformer T has twice the number of turns as half-wave rectification, and is center-tapped and divided into two parts, L2 and L3. Two rectifier diodes VD1 and VD2 are used in the circuit. When the primary coil L1 of the power transformer T is connected to the AC power supply, two AC voltages of equal magnitude and opposite phases are generated on the secondary coils L2 and L3 respectively.

1) In the positive half cycle of the AC voltage, both U2 and U3 are positive at the top and negative at the bottom, as shown in the figure a) below. U2 is a forward voltage for the rectifier diode VD1, so VD1 is turned on, and the current flows through the load resistor RL through VD1. The voltage on RL is positive at the top and negative at the bottom, while U3 is a reverse voltage for the rectifier diode VD2, so VD2 is cut off.

2) In the negative half cycle of the AC power, both U2 and U3 are negative at the top and positive at the bottom, as shown in the figure b) below. At this time, U2 is a reverse voltage for VD1, so VD1 is cut off, and U3 is a forward voltage for VD2, so CD2 is turned on, and the current flows through the load resistor RL through VD2, and the voltage on RL is still positive at the top and negative at the bottom. In summary, during the positive half cycle of AC power, the rectifier diode VD1 is turned on, and the secondary voltage U2 supplies power to the load resistor. During the negative half cycle of AC power, the rectifier diode VD2 is turned on, and the secondary voltage U3 supplies power to the load resistor. Since U2 and U3 are equal in size and opposite in phase, both the positive and negative half cycles of AC voltage are used on the load resistor. The waveform of the full-wave rectifier circuit is shown in the figure below. As can be seen from the waveform, the full-wave rectifier circuit uses the entire sine wave of the input AC voltage, so the pulsation frequency of its output current and output voltage is twice that of half-wave rectification, and the DC component is also twice that of half-wave rectification, greatly improving the rectification efficiency.853]http://p3.pstatp.com/large/433200021c61d2944f9c[/img]

3. Bridge rectifier circuit

Another circuit form of full-wave rectification is bridge rectification, as shown in the figure below. Although the bridge rectifier circuit requires four rectifier diodes, the secondary winding of the power transformer does not need to be wound twice, nor does it need to have a center tap, which makes it easier to manufacture and has therefore been widely used.

[color=rgb(34, 34, The working process of the bridge rectifier circuit is as follows: 1) In the positive half cycle of the AC voltage, the polarity of the secondary voltage U2 of the power transformer is positive at the top and negative at the bottom. Among the four rectifier diodes, VD1 and VD4 are cut off because the applied voltage is a reverse voltage; VD2 and VD3 are turned on because the applied voltage is a forward voltage. The current flows through the load resistor as shown in the figure a), and a voltage drop (that is, the output voltage) is generated on the load resistor. The voltage polarity is positive at the top and negative at the bottom. 2) When the AC voltage is in the negative half cycle, the polarity of the secondary voltage of the power transformer is negative at the top and positive at the bottom. Among the four rectifier tubes, VD2 and VD3 are cut off because the applied voltage is a reverse voltage, and VD1 and VD4 are turned on because the applied voltage is a forward voltage. The current flows through the load resistor as shown in the following figure b), and a voltage drop (that is, the output voltage) is generated on the load resistor. The voltage polarity is still positive at the top and negative at the bottom. 34)]Because the four rectifier diodes work ingeniously in turn, the positive and negative half cycles of the AC voltage are both used on the load resistor, thus achieving full-wave rectification. Its working waveform is the same as that of the full-wave rectifier circuit.

2. Filter Circuit

[color=rgb(34, 34, The output of the rectifier circuit is a pulsating DC voltage. This pulsating DC contains not only DC components but also AC components. What we need is the DC component, so the AC component in the pulsating DC must be removed. From the impedance point of view, the DC resistance of the inductor coil is very small, while the AC impedance is very large; the DC resistance of the capacitor is very large, while the AC resistance is very small. If they are combined, the AC component can be filtered out very well, leaving the required DC amount. This combination is a filter. Commonly used filters have the following forms. The AC component must be filtered out by a filter circuit to obtain a smooth and practical DC voltage. There are many types of filter circuits, such as capacitor filter circuits, inductor filter circuits, and inverted L-type LC filter circuits.

Since the inductor element is large and bulky, and when the load current suddenly changes, it will generate a large induced electromotive force, which is easy to cause damage to the semiconductor tube, so capacitor filter circuits and RC filter circuits are usually used in actual circuits. In some circuits with higher requirements, active filter circuits are also used.

1. Capacitor filter circuit

The capacitor filter circuit is shown in the figure below, which is composed of a capacitor connected in parallel at both ends of the load. The working principle of capacitor filter: It takes advantage of the characteristic that the voltage across the capacitor cannot change suddenly. Its working process can be explained by the schematic diagram shown in the following figure. U0 is the pulsating voltage output by the rectifier circuit, and Uc is the output voltage of the filter circuit (that is, the voltage on the filter capacitor C).

From the waveform diagram, it can be seen that in the initial several cycles, although the filter capacitor C is sometimes charged and sometimes discharged, the overall trend of its voltage Uc is rising. After several cycles, the circuit reaches a stable state, and the charging and discharging of the capacitor C in each cycle is the same, that is, the charge charged on the capacitor C just replenishes the charge discharged in the last discharge. It is through the charging and discharging of the capacitor C that the output voltage remains basically constant and becomes a DC with less fluctuation. The larger the capacity of the filter capacitor, the better the filtering effect.

Although the capacitor filter circuit is very simple, the filtering effect is not very ideal, and there is still an AC component in the output voltage. Therefore, the RC filter circuit is more commonly used in actual circuits.

2. Inductor filter circuit

If the output voltage is still relatively stable when the load current is large, the inductor filter circuit is used. The inductor filter circuit is shown in the figure below. The DC impedance on the inductor is very small, so the DC component of the pulsating voltage can easily pass through the inductor and almost all reach the load. However, the inductor has a large impedance to AC, so the AC component in the pulsating voltage is difficult to pass through the inductor. Since the inductor and the load are connected in series, the AC component can be regarded as a voltage divider. If the inductor is much larger than the load, then most of the AC component will be dropped on the inductor, and the AC component on the load will be very small. In this way, the original DC output with large pulsation can be changed into a more stable DC output. The waveform after filtering is shown in the figure above.

If the load resistance is constant, the larger the inductance, the smaller the output voltage fluctuation and the better the filtering effect. Therefore, inductor filtering is generally used in situations where the load changes little and the average load current is large.

3. Duplex filter

Through capacitor filtering or inductor filtering, the DC output still fluctuates more or less. In situations where higher requirements are required, a duplex filter can be used to obtain smoother DC. 1) LC filter Capacitor filters are suitable for situations where the load is large, while inductor filters are suitable for situations where the load is small. If these two circuits are combined, a filter as shown in the figure below is formed, which is suitable for general loads. In the LC filter, the pulsating voltage will undergo double filtering, so that most of the AC component will be blocked by the inductor. Even if a small part passes through the inductor, it will also be filtered by the capacitor C to bypass the AC. Therefore, the AC component on the load is very small, thereby achieving the purpose of filtering out the AC. 2) LC-∏ type filter circuit The LC-∏ type filter is composed of a C type filter and an LC filter. The filtering process is as follows: after the AC power is rectified, it is first filtered by a C type filter and then by an LC filter. Therefore, the filtering performance of this filter circuit is superior to that of both LC and C type filters, and the voltage obtained on the load will be smoother.

LC-∏ type filter has a capacitor in front, so the appearance characteristics of this filter are similar to those of capacitor filter.

3. RC filter

In some occasions, if the load current is not large, in order to reduce weight, reduce cost and reduce volume, the inductors on the above two compound filters can be replaced by a resistor to form RC filter and RC-∏ filter, as shown in the figure below. In RC filters, the larger the resistance, the better the filtering effect, but the voltage drop loss on the resistance is also large. Generally, in the case of small current, the resistance is usually tens to hundreds of ohms, and the capacitance is hundreds of microfarads.

4. Active filter circuit

The DC amplification effect of the transistor can be used to form an active filter circuit, as shown in the figure below. In the figure, VT1 is an active filter tube, R1 is a bias resistor, which provides a suitable bias current for VT1, C2 is a base bypass capacitor, which makes the base of VT1 reliably connected to AC ground and ensures that there is no AC component in the base current, and C3 is an output filter capacitor.34)]The collector-emitter current of the transistor is mainly controlled by the base current. Although the rectifier circuit outputs a pulsating DC voltage added to the collector of VT1, which contains both DC and AC components, due to the bypass filtering function of C2, the base current of VT1 has almost no AC component, so that VT1 presents a very high impedance to AC, and a relatively pure DC voltage is obtained at its output end (VT1 emitter). Because the emitter current of the transistor is several times the base current, the function of C2 is equivalent to connecting a large filter capacitor with a capacity several times that of C2 at the output end. The active filter circuit has the characteristics of small DC voltage drop and good filtering effect, and is mainly used in occasions with high filtering requirements.

After rectification, a filter capacitor needs to be added. The capacity of the filter capacitor is selected according to the size of the power load, usually tens of microfarads to hundreds of microfarads. Some DC power supplies with higher requirements also need to add integrated voltage regulators for voltage stabilization in order to obtain high-quality regulated power supplies with smaller ripples. The relationship between the filter capacitor capacity and the load current is shown in the figure below. The size of the filter capacitor can be selected according to the output current.

Secondly, the withstand voltage of the capacitor must be determined. If the withstand voltage is too small, it will break down due to overvoltage. If it is too large, the volume and cost will increase. The withstand voltage of the capacitor can be determined according to the following formula.