Bridge Rectifier: Functions, Circuits and Applications

Catalog

Ⅰ Introduction

Rectifier circuits are divided into two main categories, single-phase and three-phase. In both cases, they are further divided into three categories: uncontrolled, semi-controlled, and fully controlled. If we use diodes to convert this voltage, we can call it uncontrolled, on the contrary, if we use a power electronic component like SCRS, we can call it a controlled rectifier. We can control the half-wave or the full-wave depending on the dependency of the application.

Rectifier bridge stacks are typically used in full-wave rectifier circuits, which are divided into full-bridge and half-bridge. A half-bridge is a diode bridge rectifier with two half bridges sealed together. Two half-bridges can form a bridge rectifier circuit, one half-bridge can also form a full-wave rectifier circuit with a transformer with a center tap. The full-bridge is made up of four rectifier diodes connected and packaged as one in the form of a bridge full-wave rectifier circuit.

Bridge rectifier varieties: flat, round, square, bench shape (divided into in-line and SMD), GPP, and O/J structure. The maximum rectifier current is from 0.5A to 100A, and the maximum reverse peak voltage is from 50V to 1600V.

Bridge rectifiers

The forward current of the full bridge is 0.5A, 1A, 1.5A, 2A, 2.5A, 3A, 5A, 10A, 20A, 35A, 50A and so on, and the voltage withstand value (maximum reverse voltage) is 25V, 50V, 100V, 200V, 300V, 400V, 500V, 600V, 800V, 1000V and so on.

bridge rectifier symbol in circuits

The main difference between a conventional rectifier and a bridge rectifier is that it produces almost twice the output voltage of a full-wave center tap transformer rectifier using the same secondary voltage. The advantage of using this circuit is that it does not require a center tap transformer. In a center tap rectifier, each diode uses only half of the transformer's secondary voltage, so the DC output is relatively small, it is difficult to locate the center tap on the transformer's secondary winding, and the diode used must have a high peak-reverse voltage.

bridge rectifier circuit and resultant output waveform

During the positive half-cycle of the power supply, diodes D1 and D2 conduct in series, while diodes D3 and D4 are reverse biased and current flows through the load. During the negative half cycle of the power supply, diodes D3 and D4 conduct in series, but diodes D1 and D2 switch to "OFF" because they are now reverse biased. The current flowing through the load is in the same direction as before.

Ⅱ Functions

The bridge rectifier is used in the alternator power system, and its function is first to change the alternating current generated by the alternator into direct current to supply power to the electricity-using equipment and charge the battery; secondly, it restricts the battery current to flow back to the alternator and protects the alternator from being burned out by the reverse current. Silicon diode has one-way conductive characteristics, that is, in the silicon diode plus a certain voltage at both ends of the diode (positive power supply connected to the positive diode, negative power supply connected to the negative diode), the diode on, there is a current flow through, otherwise, the diode is not on, no current through. In this way, the current can only pass from one direction. People take advantage of this property of the diode to make a rectifier. When an AC voltage is applied to the rectifier, only the positive half of the AC is allowed to pass through, while the negative half is not, so pulsating DC is output at the negative end of the rectifier.

Bridge rectifier

The bridge rectifier circuit overcomes the shortcomings of the full-wave rectifier circuit, which requires the transformer to have a central tap in the secondary and a diode with a large back voltage. At the same time, because of the power transformer positive and negative half-weekly current supply load, the power transformer is fully utilized, high efficiency, but more than two diodes are used. In the fast development of semiconductor devices, the lower cost today, this shortcoming is not prominent, so the bridge rectifier circuit is more widely used in practice.

The rectification efficiency of a full-wave rectifier is twice as efficient as that of a half-wave rectifier.

In the case of full-wave rectification, the output voltage is higher, the output power is higher, and the transformer utilization factor is higher.

In the case of a full-wave rectifier, the ripple voltage is lower and the frequency is higher, thus requiring a simple filter circuit transformer secondary without the need for a center tap. If no step-up or step-down voltage is required, the transformer can be eliminated.

For a given power output, a smaller-sized power transformer can be used in the case of a bridge rectifier because the current in the primary and secondary windings of the power transformer flows throughout the AC cycle.

Ⅲ Applications

Using Bridge Rectifiers to Convert AC to DC Many electronic applications often require regulated DC power supplies. One of the most reliable and convenient methods of converting available AC power to DC power is to use a rectifier, which is a diode system, for this conversion from AC to DC signals. It can be a half-wave rectifier, which rectifies only half of the AC signal, or a full-wave rectifier, which rectifies two cycles of the AC signal. A full-wave rectifier can be a center tap rectifier with two diodes or a bridge rectifier with four diodes.

Ⅳ Bridge rectifier principle circuit

The bridge rectifier circuit (as shown in the figure) is the most widely used type of rectifier circuit. As long as two diode ports are connected to form a "bridge" structure, this circuit has the advantages of a full-wave rectifier circuit while overcoming its shortcomings to a certain extent.

        u2>0: D1,D3 conduction, D2,D4 stop, current path: A->D1->R->D3->B

        u2<0: D2,D4 on, D1,D3 off, current path: B->D1->R->D4->A

When the input voltage u2 is positive half-cycle, add positive voltage to D1, D3, then Dl and D3 conducted; add reverse voltage to D2 andD4, then D2 and D4 cut-off. The circuit constitutes u2, D1, RFz, D3 energized circuit, forming of positive and negative half-wave rectifier voltage on the Rfz;

When the input voltage u2 is negative half-cycle, add positive voltage to D2, D4, then D2, D4 conducted; add reverse voltage to D1, D3, then D1, D3 cut-off. The circuit is composed of u2, D2, Rfz, D4 circuit, also on the Rfz forming another half-wave of positive and negative rectifier voltage. Repeatedly, the result is a full-wave rectified voltage on Rfz. The waveform diagram is the same as that of the full-wave rectifier. It is also easy to see from the figure that the reverse voltage for each diode in the bridge circuit is equal to the maximum voltage of the transformer's secondary, which is half smaller than that of the full-wave rectifier. The bridge rectifier is an improvement on the diode half-wave rectifier.

The Bridge rectifier circuit can also be considered as a kind of full-wave rectifier circuit, the transformer winding according to the above diagram to connect four diodes. D 1 ~ D 4 for four of the same rectifier diode connected to the bridge form, so it is called the bridge rectifier circuit. The use of diode leads, so that in the negative half-week when the secondary output can also lead to the load. The specific connection is shown in the figure, from the figure can be seen, in the positive half-week by D1, D3 guide current from top to bottom through RL, negative half-week by D2, D4 also guide current from top to bottom through RL, to achieve the full-wave rectification. In this structure, if the output of the same DC voltage, transformer secondary winding, and full-wave rectification compared to only half of the winding can be, but if you want to output the same size of the current, the diameter of the winding should be thickened accordingly.

Analysis 1: Analysis of the process of power supply filtering.

Power filtering is to connect a capacitor of larger capacity in parallel at both ends of the load RL. Since the voltage at both ends of the capacitor cannot be mutated, the voltage at both ends of the load will not be mutated, so that the output voltage can be smoothed to achieve the purpose of filtering.

Waveform formation process: when the output is connected to the load RL, the power supply provides current to the load and at the same time to charge capacitor C. The charging time constant &tau; charge = (Ri∥RLC) &asymp; RiC, generally Ri〈〈RL, ignoring the influence of Ri voltage drop, the voltage on the capacitor will rise rapidly with u2. When &omega;t = &omega;t1, there are u2 = u0, thereafter u2 is lower than u0, all diode cut-off. Then capacitor C discharges through RLC with a time constant of RLC. When &omega;t=&omega;t2, u2=u0, after &omega;t2, u2 changes to be larger than u0, and starts the charging process again, u0 rises rapidly. When &omega;t=&omega;t3, u2=u0. after &omega;t3, the capacitor discharges through RL. So it is repeated, periodic charge and discharge. Due to the energy storage effect of capacitor C, the voltage fluctuation on RL is greatly reduced. The capacitor filter is suitable for occasions where the current variation is not large. the LC filter circuit is suitable for occasions where the current is large and the voltage pulsation is small.

Analysis 2: Calculation of capacity of filter capacitor and selection of voltage withstand value

The output voltage Uo of the capacitor filter rectifier circuit is between &radic;2U2~0.9U2, and the average value of the output voltage depends on the magnitude of the discharge time constant.

The capacitance RLC ≧ (3~5)T/2 where T is the period of the AC supply voltage. In practice, it is often further approximated as Uo &asymp; 1.2U2. The maximum reverse peak voltage of the rectifier URM = &radic;2U2 and the average current per diode is half the load current.