Overview

DC power supplies are an application of AC rectification. Their purpose is to provide a continuous voltage with as little variation as possible. This is because circuits using semiconductor devices require a supply voltage that remains relatively constant and has minimal ripple. Because of this, unfiltered rectifiers are generally rarely used. By direct current power source is meant a set of devices and circuits that have a sinusoidal alternating voltage at the input and a fixed or adjustable direct voltage at the output.

Any stabilized power supply is composed of the following main blocks:

- the transformation block;

- the rectifier block;

- smoothing filter;

- the stabilization block.

A common block diagram of a power source is as follows (https://i.pinimg.com/originals/9e/07/91/9e07917f7ec1e740cd3cb10d80e5a1bb.jpg):

Describing functional blocks

1 The transformation block: its role is to reduce the variation of the sinusoidal voltage supplied by the electrical network. The electrical transformer is a static electromagnetic device, having two or more magnetically coupled electrical windings that transform the parameters (usually current and voltage but also the number of phases) of alternating current electricity.

The basis of the operation of the electric transformer is the phenomenon of electromagnetic induction, for this reason it is necessary to obtain intense magnetic fields with the help of iron cores, on which are located the electrical windings made of copper, aluminum or alloy conductors.

Rectification block

The voltage rectifier has the role of converting the alternating voltage applied to its input into a pulsating direct voltage, which does not contain negative alternations. The rectifier is made of rectifier diodes. The alternating voltage, with the frequency of the network from the secondary of the network transformer is rectified by the diodes rectifiers, obtaining a pulsating DC voltage at the output of the rectifier.

There are several AC rectifier circuits:

- monoalternation rectification - case in which the signal from the output of the rectifier is extracted from a single half-alternation of the alternating voltage applied to the input;

- bialternation rectification - situation in which the signal from the output of the rectifier is extracted from both semi-alternations of the alternating voltage applied to the input.

Bialternation rectification can be of several types:

- bialternation rectification with median socket;

- bialternation rectification with diode bridge;

- bialternation rectification with voltage doubling.

The smoothing block has the role of reducing the ripple factor of the rectified voltage to a value as low as possible, if possible zero. There are several types of smoothing filters, but the most used are the capacitive ones formed by a capacitor or a group of capacitors mounted in parallel.

If the rectifier is running at idle, the capacitor will be fully charged.

As the peak voltage begins to decrease, the capacitor tends to discharge and, due to this fact, the current remains constant.

The pulsations of the rectified voltage do not allow the operation of the electronic device in normal mode. For example, the pulsations of the supply voltage of the network, radio transmitters, radio receivers, amplifiers cause the appearance of low-frequency ripples. In the cathode-ray tubes, the voltage pulsations lead to the variation of the brightness of the luminescent screen. in remote control systems pulsations can cause errors in the steering system.

For these reasons, smoothing filters are used at the output of the rectifiers, which ensure an extremely low level of voltage pulsations.

Apart from the basic requirement - low level of pulsations at the output - the filter must ensure the following:

- the filter must not introduce disturbances in the operation of the used load;

- the filter must not form overvoltages and current pulses when switching on and off;

- to avoid resonance effects, the natural frequency of the filter and its stages must differ from the basic frequency of the pulsations;

- power losses and the value of the voltage falling on the filter must be reduced to a minimum;

- the filter must have high reliability;

- the filter must have the mass and dimensions reduced to a minimum

The stabilization block has the role of maintaining a constant voltage value at its output terminals, regardless of variations in the input voltage, the value of the current required by the supplied circuit or variations in the working temperature.

The voltage at the output of the rectifier depends on two factors: the variations in the voltage of the supply network and the change in the value of the load current. There are cases when the load current can vary from zero to its nominal value.

All variations of the supply voltage of electronic circuits lead, to a more or less significant extent, to the modification of electrical performances. In some situations, when the voltage increases, some active components in the circuit can be destroyed. There are other arguments that argue for the introduction of the voltage stabilizer into the power supply scheme, first of all the possibility of realizing overcurrent and overvoltage protection circuits in the stabilizer scheme.

Also, in the stabilizer there is the possibility to adjust the value of the output voltage, an important fact in the case of laboratory sources.

Any stabilized DC voltage source can be characterized by two visual parameters:

- the stability factor;

- the internal resistance Ri.

In practice, it is desirable for the internal resistance to be as low as possible, and the stability factor to be as high as possible.

The simplest stabilizer is made with the help of a Zener diode that works in the breakdown region. High-performance stabilizers are made with the help of transistors or special integrated circuits.

The load represents the powered circuit; it can be represented by means of a

equivalent resistances calculated between the input terminals of the respective circuit.

According to the method of ordering the stabilization element, the stabilizers can be:

 - with continuous (linear) action;

 - with discontinuous action (switching).