Rectifier circuit working principle - Solutions - Huaqiang Electronic Network

The power grid supplies AC power to users, and various radios require DC power. Rectification is the process of turning AC power into DC power. By using a device having unidirectional conduction characteristics, a current alternating in direction and size can be converted into direct current. The various rectifier circuits composed of crystal diodes are described below.

One, half wave rectifier circuit

Figure 5-1 shows the simplest rectifier circuit. It consists of a power transformer B , a rectifier diode D and a load resistor R fz . The mains voltage transformer (mostly 220 volts) is converted to the desired alternating voltage e2, D then converts AC into pulsating direct current.

Let's see how the diode is rectified from the waveform diagram in Figure 5-2.

The transformer cut-off 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 0~K time, e2 is positive half cycle, that is, the upper end of the transformer is negative at the lower end. At this time, the diode is subjected to the forward voltage surface conduction, and e2 is applied to the load resistor R fz through it. In the period of π~2π , e2 is a negative half cycle, and the lower end of the transformer is positive and the upper end is negative. At this time, D is subjected to reverse voltage, non-conducting, R fz , and no voltage on it. In the π~2π time, the process of 0 ~ π time is repeated, and in the 3π~4π time, the process of π~2π time is repeated... This is repeated, the negative half cycle of the alternating current is "cut", only positive Half-cycle through R fz , a single right-direction (upper down-down) voltage is obtained on R fz , as shown in Figure 5-2 (b), the purpose of rectification is achieved, but the load voltage U sc . And the magnitude of the load current also varies with time, so it is often referred to as pulsating DC.

This rectification method of removing the half cycle and the lower half of the figure is called half-wave rectification. It is not difficult to see that the half-wave symmetry is exchanged for the rectification effect at the expense of half of the "sacrificial" exchange, and the current utilization rate is very low (calculation shows that the average value of the half-wave voltage obtained by rectification over the entire period, that is, the load The DC voltage U sc = 0.45 e2 ) is therefore commonly used in high voltage, low current applications, and is rarely used in general radios.

Second, full wave rectifier circuit

If the structure of the rectifier circuit is adjusted, a full-wave rectifier circuit that can fully utilize the power can be obtained. Figure 5-3 shows the electrical schematic of a full-wave rectifier circuit.

The full-wave rectification circuit can be considered as a combination of two half-wave rectification circuits. A transformer secondary center tap lead requires, the secondary coil group is divided into two symmetrical windings, which leads to equal but opposite polarity voltage two E2A, E2B, constituting e2a, D1, R fz and e2b, D2, R Fz , two energized circuits.

The working principle of the full-wave rectifier circuit can be illustrated by the waveform diagram shown in Figure 5-4. In between 0 ~ π, e2a forward voltage of Dl, D1 is turned on, to obtain the positive voltage on the negative R fz; e2b reverse voltage of D2, D2 is not conducting (see Figure 5-4 ( . b) in the π-2π time, E2B forward voltage of D2, D2 is turned on, resulting in a positive R fz remains negative voltage; E2A reverse voltage of D1, D1 is not conducting ( See Figure 5-4 (C).

The full-wave splicing circuit shown in Figure 5-3 requires the transformer to have a secondary center tap that is symmetrical at both ends, which causes a lot of trouble in production. In addition, in this circuit, the maximum reverse voltage that each rectifier diode is subjected to is twice the maximum value of the secondary voltage of the transformer, so a diode capable of withstanding higher voltages is required.

Figure 5-5(a) shows the bridge rectifier circuit diagram, and (b) shows its simplified drawing.

Third, bridge rectifier circuit

The bridge rectifier circuit is the most used rectifier circuit. Such a circuit, as long as two diode ports are connected to form a "bridge" structure, has the advantages of a full-wave rectifier circuit, while at the same time overcoming its disadvantages to some extent.

Bridge rectifier circuit works as follows: e2 when the positive half cycle of D1, D3 and the direction of the voltage, Dl, D3 conduction; of D2, D4 reverse voltage, D2, D4 are turned off. Circuit configuration e2, Dl, R fz, D3 power circuit, positive negative in R fz, is formed on the half-wave rectifier wash voltage, e2 is the negative half cycle of D2, D4 forward voltage is applied, D2, D4 guide Tong; of D1, D3 reverse voltage, D1, D3 is turned off. Circuit configuration e2, D2 R fz, D4 power circuit is also formed on the positive negative on the other half-wave rectified voltage at the R fz.

The above working states are shown in Figure 5-6 (A) (B).

Repeating this way, the result is a full-wave rectified voltage at R fz . The waveform is the same as the full-wave rectified waveform. It is not difficult to see from Figure 5-6 that the reverse voltage of each diode in the bridge circuit is equal to the maximum value of the secondary voltage of the transformer, which is half the size of the full-wave rinsing circuit!

Fourth, the selection and application of rectifier components

It should be specially pointed out that the diode as a rectifying element should be selected according to different rectification methods and load sizes. . If you choose improperly, you can't work safely, or even burn the tube; or use it too much, causing waste. The parameters listed in Table 5-1 are available for reference when selecting a diode.

Figure 5-7 shows the diodes in parallel: two diodes in parallel, each sharing half of the total current of the circuit, three diodes in parallel, each sharing one-third of the total current of the circuit. In short, there are several diodes connected in parallel. "The current flowing through each diode is equal to a fraction of the total current. However, in actual parallel operation, the current cannot be evenly divided because the characteristics of the diodes are not completely consistent." It will cause some pipes to be overburdened and burned. Therefore, it is necessary to connect a small resistor with the same resistance value in series with each diode so that the current flowing through each parallel diode is nearly uniform. The current sharing resistor R generally uses a resistor of a few tenths of a ohm to several tens of ohms. The larger the current, the smaller R should be selected.

Figures 5-8 show the case where the diodes are connected in series. Obviously under ideal conditions, several tubes are connected in series, and each tube should withstand a reverse voltage equal to a fraction of the total voltage. However, because the reverse resistance of each diode is not the same, it will cause uneven voltage distribution: diodes with large internal resistance may be broken down due to excessive voltage, and thus cause a chain reaction, and the diodes will be broken down one by one. A resistor R connected in parallel on the diode allows uniform voltage distribution. The voltage equalizing resistor should take a resistor whose resistance value is smaller than the reverse resistance of the diode, and the resistance of each resistor should be equal.