Analysis of rectifier circuit types and principles

 1. Half-wave rectifier circuit

 

The picture above is the simplest rectifier circuit. It consists of power transformer B, rectifier diode D and load resistor Rfz. The transformer converts the mains voltage (mostly 220 volts) into the required alternating voltage e2, and then D converts the alternating current into pulsating direct current.

Let’s look at how the diode rectifies from the waveform diagram.

 

The transformer step-down voltage e2 is a sine wave voltage whose direction and magnitude change with time. Its waveform is shown in Figure 5-2(a). Within the time period from 0 to 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 conducts under the forward voltage side, and e2 is added to the load resistor Rfz through it. Within π to 2π time, e2 is a negative half cycle, and the lower end of the transformer secondary is positive and the upper end is negative. At this time, D is subjected to reverse voltage and does not conduct, and there is no voltage on Rfz. Within the period of π to 2π, the process of 0 to π time is repeated, and within the time of 3π to 4π, the process of π to 2π time is repeated again... If this is repeated, the negative half cycle of the alternating current will be "cut off", leaving only the positive Half a cycle passes through Rfz, and a single rightward (upper positive and lower negative) voltage is obtained on Rfz, as shown in Figure 5-2(b). The purpose of rectification is achieved, but the load voltage Usc is. And the magnitude of the load current also changes with time, therefore, it is usually called pulsating DC.

This rectification method that removes the half cycle and the lower half of the figure is called half-wave rectification. It is not difficult to see that half-wave rectification achieves the rectification effect at the expense of "sacrificing" half of the AC, and the current utilization rate is very low (calculations show that the average value of the half-wave voltage obtained by rectification during the entire cycle, that is, the average value on the load The DC voltage Usc =0.45e2) is therefore commonly used in high voltage and small current situations, but is rarely used in general radio devices.

2. Full-wave rectifier circuit

If some adjustments are made to the structure of the rectifier circuit, a full-wave rectifier circuit that can fully utilize electric energy can be obtained. The figure below is the electrical schematic diagram of the full-wave rectifier circuit.

 

A full-wave rectifier circuit can be regarded as a combination of two half-wave rectifier circuits. A tap needs to be drawn in the middle of the secondary coil of the transformer to divide the secondary coil into two symmetrical windings, thereby drawing out two voltages e2a and e2b of equal size but opposite polarity, forming e2a, D1, Rfz and e2b, D2, Rfz. Two energized circuits.

The working principle of the full-wave rectifier circuit can be explained by the waveform diagram shown in Figure 5-4. Between 0 and π, e2a is a forward voltage to Dl, D1 is turned on, and an upper positive and lower negative voltage is obtained on Rfz; e2b is a reverse voltage to D2, and D2 is not conductive (see the figure below in π-2π During the time, e2b is a forward voltage to D2, D2 is turned on, and the voltage obtained on Rfz is still a positive and downward voltage; e2a is a reverse voltage to D1, and D1 is not turned on (see the figure below and so on, because the two The rectifier elements D1 and D2 conduct electricity in turn. As a result, the load resistor Rfz has current flowing in the same direction during the positive and negative half-cycles, as shown in the figure. Therefore, it is called full-wave rectification. Full-wave rectification not only The positive half cycle is used, and the negative half cycle is also cleverly used, thereby greatly improving the rectification efficiency (Usc =0.9e2, twice as large as half-wave rectification).

 

The full-wave filter circuit shown in Figure 3 requires the transformer to have a secondary center tap that makes both ends symmetrical, which brings a lot of trouble to the production. In addition, in this circuit, the maximum reverse voltage that each rectifier diode withstands is twice the maximum secondary voltage of the transformer, so diodes that can withstand higher voltages need to be used.

3. Bridge rectifier circuit

Bridge rectifier circuit is the most commonly used rectifier circuit. This circuit, as long as two diode ports are added to form a "bridge" structure, will have the advantages of a full-wave rectifier circuit and at the same time overcome its shortcomings to a certain extent.

 

The working principle of the bridge rectifier circuit is as follows: when e2 is the positive half cycle, Dl and D3 are turned on for D1, D3 and directional voltage; when reverse voltage is applied to D2 and D4, D2 and D4 are turned off. The circuit forms e2, Dl, Rfz, and D3 energizing circuits. On Rfz, a positive and negative half-wave rectifying voltage is formed. When e2 is a negative half cycle, positive voltage is applied to D2 and D4, and D2 and D4 are turned on; Apply reverse voltage to D1 and D3, D1 and D3 are cut off. The circuit forms e2, D2, Rfz, and D4 energized circuits, and also forms another half-wave rectified voltage with upper positive and lower negative voltages on Rfz. The above working states are as shown in the figure.

 

Repeat this, and the result is a full-wave rectified voltage at Rfz. The waveform diagram is the same as the full-wave rectification waveform diagram. It is not difficult to see from Figure 5-6 that the reverse voltage endured by each diode in the bridge circuit is equal to the maximum value of the secondary voltage of the transformer, which is half less than that of the full-wave rectifier circuit!

4. Selection and application of rectifier components

It should be pointed out in particular that diodes, as rectifier components, should be selected according to different rectification methods and load sizes. If you choose improperly, it may not work safely or even burn the pipe; or it may overuse materials and cause waste. The parameters listed in Table 5-1 can be used as a reference when selecting diodes.

“In addition, in the case of high voltage or large current, if you do not have rectifier components on hand that can withstand high voltage or set large electric filters, you can use diodes in series or parallel.

The figure below shows the situation of diodes connected in parallel: two diodes are connected in parallel, each sharing half of the total circuit current; three diodes are connected in parallel, each sharing one-third of the total circuit current. In short, if several diodes are connected in parallel, the current flowing through each diode is equal to a fraction of the total current. However, in actual parallel use, since the characteristics of each diode are not completely consistent, the current flowing through them cannot be equally divided. , which will cause some pipes to be overloaded and burned. Therefore, a small resistor with the same resistance needs to be connected in series to each diode so that the current flowing through each parallel diode is close to the same. This current-sharing resistor R generally uses a resistor ranging from a few tenths of ohms to dozens of ohms. The larger the current, the smaller R should be chosen.

 

The figure shows the case of diodes connected in series. Obviously, under ideal conditions, if there are several tubes in series, the reverse voltage that each tube withstands should be equal to a fraction of the total voltage. However, because the reverse resistance of each diode is different, uneven voltage distribution will occur: diodes with large internal resistance may be broken down due to excessive voltage, which will cause a chain reaction and break down the diodes one by one. A resistor R connected in parallel with the diode can distribute the voltage evenly. The voltage equalizing resistor should be a resistor with a resistance smaller than the reverse resistance of the diode, and the resistance of each resistor should be equal.