Brief description of the operation of a tension source
Due to its use in the analysis of the electronic circuits and the importance of its reliability, characteristics and way of operation for the obtaining of correct results, in this section we will take care of the analysis and design of the same.
For the development of a power supply defined specifications are needed very, based on which we will obtain the most suitable solution.
These specifications are: tension and intesidad of exit, regulation with the load, regulation with the entrance, curled and temperature coefficient.
For the synthesis of a tension source, we can analyze it by means of block diagrams, like set of independent blocks, whose parameters are related, but that nevertheless can be analyzed separately. The blocks in which a source can be divided are: Transformer; rectifier - filter and regulator series.
The transforming block: It is the element that provides the energy to the source. They are reducing of tension and they are formed generally by a primary winding and one or secondary. The operation is the following one: When it is made circulate an AC voltage around the primary winding, a magnetic flux in the nucleus is originated, which as well creates a tension induced in the secondary one, as the amount of these is this tension induced directly proportional to the number of turns of the winding modifying can be changed the induced tension of exit in the secondary one, of way like if a transformer it has equal number of turns in the primary winding that in the secondary one, its relation will be 1:1 that is, that the exit tension will be the same that the one of entrance.
Another function that carries out a transformer is the one galvanically to isolate the circuit of load of the alternating network.
The block rectifying-I leak: This block can be considered like two sub individual blocks, each of them fulfilling the function of rectified one and filtered the other.
A rectifier is a disposition of diodes (he only can be one or several), which have the function to turn the continuous alternating current voltage into pulsating. This is thanks to the fact that positive the diode is an element that only leads in a sense, leaving to only pass the half-cycle, or negative in case of placing the diodes in opposite sense, but not that it changes of sign.
The function of the filter is to filter the pulsating tension that comes from the rectifier with the purpose of to attenuate the variations, so that their S-value most continuous possible, with a component minim of alternating.
Everything what fitro makes eliminate the component of alternating is beneficial for the source because the objective of this one is to give possible purest the continuous tension. Also it limits the power to dissipate in the transistor series.
Regulating series: It provides a stabilized tension of not regulated exit from one that comes from rectifying-I leak.
It is made up of several blocks: Comparator, shows, reference, Y amplifier control element.
It is of importance of knowing the power that will dissipate, which will define the dissipater to choose and the energy efficiency of the source.
It is to hope that if the source will have the capacity to take low values from tension, it will have a bad yield because to obtain the low values a great power will have to dissipate in the power transistor that is wasted.
Also it is necessary to insist on the thermal stability of the reference tension and its margin of work.
The voltage regulator is a subgroup of the source that produces a constant tension to determined loads, from a variable tension of entrance, within very precise limits.
In order to analyze the operation of the regulator, we will study the mentioned blocks already, separately:
The regulation obtains by the comparison of the tension of exit through a splitter and a tension of reference. The difference, or signal of error, is amplified and acts more or less on the power commando, leading, varying, consequently, the tension from exit to the wished value.
The sample element is generally a splitter of tension made up of two resistance and a potentiometer connected in parallel when coming out.
So that the results are the hoped ones in the design, he is advisable that the resistance that form part of the splitter are made with the same material and temperature coefficient.
The reference tension is obtained from a diode to zener, taking advantage of the characteristic inverse rupture to obtain a constant tension in an ample margin of temperature and current.
The comparator produces an exit tension that depends on the difference between the tension of reference and the sample. Following the wished regulation and of the thermal stability a stage to common emitter or a differential amplifier also to common emitter is used generally.
The sample tension is applied to the base of the transistor and the one of reference to the emitter. When the exit tension tends to grow the base-emitting tension of the transistor increases and also the collector current. In the opposite case, a slope of the exit tension will produce a diminution of the collector current. These variations of the current of collector in the transistor comparator are not in phase with the transistor series, therefore, if the exit tension tends to grow, the amplified difference of potential falls, as well as the basic current of the power transistor, leads which us the consequence of which the low exit to its normal value.
For the thermal compensation of a block comparator in common emitter we will have to consider that all varición of the exit tension is attributable to the increases of tension of the base-emitting union of the transistor and zéner. Because the base-emitting union has a negative temperature coefficient will be able to be compensated choosing the diodes of reference with positive temperature coefficient.
The amplifier of continuous gathers the exit of the comparison differential amplifier, amplifying it the sufficient thing like being able to attack the control element or transistor series of power. A single stage will be sufficient to fulfill its assignment, but it is required of a great current of attack or is necessary to diminish the resistance of exit of the regulator will be possible to be had three stages. It is possible to clarify that the number of stages has to be uneven to obtain the suitable phase.
By means of the control element the signal is interpreted that comes from the Y amplifier the corrections necessary are realized to maintain the tension of constant exit.
Also a good source of tension must have protection of short circuits and limitor of current, which is easy to obtain if it is not required of high accuracy of power failure, by means of a resistance series with the regulator and connecting the base and the emitter to his ends, so that when the tension of the resistance, that is proportional to the intensity of step, surpasses the 0.7 approximately V of the Vbe of the transistor, this one will lead. As the collector usually is placed on the base of the power transistor, all the current is derived to the limitor auxiliary transistor of current, so that the tension source happens to work like source of constant intensity.
Now series appears a problem in the transistor because if the source works to constant intensity, with the tension of exit zero due to a short circuit, all the power it will dissipate in the same, being able to surpass the permissible maximum power.
In order to avoid this problem a variant in the protective circuit of short circuits can be made in such a way that when falling the exit tension, also diminishes the intensity of cuts of the transistor (ISC), arriving at the point of short circuit in the exit with an intensity that is a fraction of the original one (typically four or five times inferior).
The circuit is based on a splitter of tension in the exit, in whose central part the base of the transistor is connected that limits the current, and the emitter when coming out of the source where the resistance has been connected that serves as sample of the step current. When the source works to constant tension, the limitor transistor of current is polarized in inverse due to the fall of the resistance of the tension splitter.
As the intensity in the exit begins to increase, the fall of the short circuit resistance also begins to increase, so that when it reaches the value of the sum of the Vbe tensions of the transistor and the tension of the resistance of the splitter the transistor will begin to lead, avoiding of this form that the exhaust stream increases.
Design of the power supply: Once known the criteria and operation the sources we will have the elements sufficient as to realize the selection and the design of the power supply.
We have already seen that to assure a perfect thermal compenzación and stable operation to us he is preferable to use an integrated circuit before discreet elements. Besides obtaining better results, this option in the majority of the cases, she will be economic.
As we wished to obtain an adjustment from tension zero our possibilities of selection become narrower, but anyway an integrated circuit that fulfills these considerations well is the MC1466L of Motorola.
