Diode bridge: device, principle of operation, purpose, circuits

Definition

A diode bridge is a circuitry solution for rectifying alternating current. Another name is a full-wave rectifier. It is built from semiconductor rectifier diodes or their varieties - Schottky diodes.

The bridge connection scheme assumes the presence of several (for a single-phase circuit - four) semiconductor diodes to which the load is connected.

It can consist of discrete elements soldered on a board, but in the 21st century, connected diodes in a separate package are more common. Outwardly, it looks like any other electronic component - legs are removed from the case of a certain standard size for connecting to the tracks of the printed circuit board.

It is worth noting that several valves combined in one case, which are not connected according to a bridge circuit, are called diode assemblies.

Depending on the scope and connection diagram, diode bridges are:

• single-phase;
• three-phase.

The designation on the diagram can be performed in two versions, which UGO to use in the drawing depends on whether the bridge is assembled from individual elements or a ready-made one is used.

Operating principle

Let's understand how a diode bridge works. To begin with, diodes pass current in one direction. The rectification of the alternating voltage occurs due to the one-sided conduction of the diodes. Due to their correct connection, the negative half-wave of the alternating voltage is supplied to the load in the form of a positive one. In simple words, it reverses the negative half-wave.

For simplicity and clarity, we will consider its operation using the example of a single-phase full-wave rectifier.

The principle of operation of the circuit is based on the fact that the diodes conduct current in one direction and is as follows:

• An alternating sinusoidal signal is supplied to the input of the diode bridge, for example, 220V from a household power supply (in the connection diagram, the input of the diode bridge is designated as AC or ~).
• Each of the half-waves of sinusoidal voltage (figure below) is passed by a pair of gates located diagonally on the diagram.

The positive half-wave is passed by the diodes VD1, VD3, and the negative half - by VD2 and VD4. You can see the signal at the input and output of the circuit below.

Such a signal is called a rectified ripple voltage. In order to smooth it, a filter with a capacitor is added to the circuit.

Main characteristics

Let's consider the main characteristics of semiconductor diodes. Latin letters give their designation in the English-language technical documentation (the so-called Datasheet):

• V_rpm - peak or maximum reverse voltage. When this voltage is exceeded, the pn junction is irreversibly destroyed.
• V_{r (rms)} - average reverse voltage. Normal for work, same as U_arr in the characteristics of domestic components.
• I_o - average rectified current, the same as I_NS from domestic.
• I_fsm - peak rectified current.
• V_fm - voltage drop in forward bias (in the open conducting state) is usually 0.6-0.7V, and more for high-current models.

When repairing electronic equipment and power supplies or designing them, beginners ask: how to choose the right diode bridge?

In this case, the most important parameters for you will be reverse voltage and current. For example, to select a 220V diode bridge, you need to look at models with a nominal voltage greater than 400V and the required current, for example, KBPC106 (or 108, 110). Its technical characteristics:

• maximum rectified current - 3A;
• peak current (short-term) - 50A;
• reverse voltage - 600V (800V, 1000V for KBPC108 and 110, respectively).

Remember these characteristics and you can easily determine which option to choose from the catalog.

Rectifier circuits

Rectifying current in power supplies is the main purpose; among other components of the circuit, an input filter can be distinguished, which is connected after the rectifier - it is designed to smooth out ripples. Let's take a closer look at this issue!

First of all, it is worth noting that a diode bridge is called a single-phase rectifier circuit of 4 diodes or a three-phase one of 6. But amateurs often refer to this as a midpoint rectifier circuit.

In a full-wave rectifier, two half-waves are supplied to the load, and in a half-wave rectifier, one.

To avoid confusion, let's understand the terminology.

Below you see a single-phase full-wave circuit, its correct name is "Gretz circuit", it is this that is most often meant under the name "diode bridge".

Larionov's circuit is a three-phase diode bridge, the output is a full-wave signal. The diodes in it transmit half-waves, opening to line voltage, i.e. alternately: upper phase A diode and lower phase B diode, upper phase B and lower phase C, etc.

For the sake of completeness, you should talk about other AC voltage rectifier circuits.

Half-wave rectifier of 1 diode connected in series with the load. It is used in ballast power supplies, low-power miniature power supplies, as well as in devices that are undemanding to the ripple coefficient. Only one half-wave is supplied to the load.

Full-wave with a midpoint - this is what is mistakenly called a bridge of 2 diodes. Here, only one diode conducts each half-wave. Its advantage is higher efficiency than that of the Gretz circuit, due to the smaller number of semiconductor gates. However, its use is complicated by the fact that a transformer with a midpoint tap is needed, which is reflected in the schematic diagram. It cannot be used to rectify the 220V mains voltage.

Rectifier made of Schottky assemblies. Used in switching power supplies, because Schottky diodes have less reverse recovery time, small barrier capacitance (faster transition from open to closed state) and low forward voltage drop (less losses). Most often, Schottkies are found in assemblies, with a common anode or cathode, as shown in the figure below.

Therefore, several assemblies are required to assemble the bridge circuit. Below is an example of 3 common cathode Schottky assemblies.

Of 4 assemblies with a common cathode. It differs from the previous one in that it can withstand a higher current, with the same components, because the Schottkys are connected in parallel in it.

Of the 2 Schottky assemblies, one with a common anode and one with a common cathode. Learn about what is anode and cathode, you can in our separate article.

How to solder and connect

It is not difficult to study and know the circuits, the main difficulties arise when a beginner decides to solder a diode bridge with his own hands. To solder a rectifier of 4 Soviet copies of the kd202 type, use the illustration below.

To assemble a diode bridge from modern discrete diodes such as low-power 1n4007 (and others - they all look the same and differ only in size), carefully look at the following illustration.

But if you do not assemble it from separate parts, but use a ready-made bridge, then see below how to properly connect it to the circuit.

Also, beginners will be interested in watching a video on how to make a simple 12V power supply:

Scope and purpose

Most often, diode bridges are used in power supplies. In transformer power supplies, they are connected to the secondary winding of the transformer

In impulse power supplies - to the input of the 220V network. In this case, the electronic control circuit and the power circuit of the UPS are powered from a rectified and smoothed (not always) mains voltage (reaches about 300-310 volts).

There is a high-frequency alternating voltage at the terminals of the secondary winding of the switching power supply. In order to straighten it, assemblies of double Schottky diodes are installed. In this regard, a midpoint rectification circuit is often used.

In cars and motorcycles, three-phase diode bridges are used, assembled according to the Larionov scheme with three additional valves, because a three-phase is used to power the on-board network generator. The bridge in the generator is made in the form of a sector of a circle and is installed on its rear part.

The exception is some modern Toyota cars and other brands, they use a 6-phase generator to implement a twelve-pulse rectification circuit of 12 valves. This is to reduce ripple and increase the output current.

Verification methods

A multimeter in diode test mode is best for testing a diode bridge.

To do this, you need to ring the input for a short circuit, then the output (the diode bridge must be soldered out).

Without soldering directly on the board, you can measure the voltage drop across the diode junctions. To do this, you need to determine the pinout of the bridge, usually it is indicated directly on the case, which we discussed above.

The multimeter screen should display numbers in the forward bias in the range of 500-800 mV, and in the reverse - above 1500 and to infinity (depending on the specific component and the measuring device). The same can be done in Ohmmeter mode, as shown in the figure below.

This process is described in more detail in the article "how to check a diode bridge", Where, in addition to the test method, we also talked about the symptoms of a malfunction. Also check out the video on how to test a single-phase rectifier and diode bridge of a car generator:

This concludes our detailed explanation. We hope you now understand what a diode bridge is for and what it does in an electrical circuit. If you have any questions, ask them in the comments below the article!

Related materials:

• How to solder radio components from boards
• How to use a multimeter - instructions for dummies
• How to lower the voltage in the network

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