The power supply (F.A) it has the function to provide to the tension and suitable and stable current to each block of an electronic system (computer, television, alarm system, etc).
In order to begin the subject, we are going to propose the analysis of a circuit, that with a simple transistor and zener, we are going to have a stable tension when coming out
We have seen that this type of diode is useful when he works in the inverse zone, where we have a zone (zone to zener) in which the tension stays very stable.
Diodes for some make specific tensions, so that a diode of 5.1 volts polarized in inverse, cause that the tension between anode and cathode remains to that tension, although vary the current that circulates around the same
Sources with transistor with variable exit
The sources can also be designed so that they work between certain rank.
In the case of the figure, we have a source that we can regulate it between a rank of 0 to 12 Volts, following the value of the resistance that there is in parallel with zener of 12 volts.
In fact, the source will not arrive at the 12 volts, since we must reduce the tension of the base-emitting union.
1º Line source.
They are the conventional sources, composed by the modules of:
Transformer (that reduces the tension of 220 volts to a tension next to the one of use)
Rectifier (it obtains the positive cycles of the wave in a terminal)
Filtrate system (it eliminates the elevated frequencies, tips of tension, etc)
Curled reducer of (it reduces the variations of the tension). It is the condenser that is put when coming out of the diodes
Regulator (it stabilizes the exit tension).
We show a general scheme of the same:
These sources use transforming operating to the frequency of the network (50 Hertz) with great disadvantages since these components usually are expensive, with great volume and weight. In addition they generate losses in the iron and copper, giving rise to lost global of yield of f.a
The development of the integrated electronics has caused that the circuitry that before occupied a considerable space, is integrated in circuits with the suitable sideburns to provide solutions in f.a. A case very known is the family 78XX, whom with three terminals (entered, exit and control), different tensions from exit of stable form offer. The value of the exit depends on the completion of its numeration, therefore the 7805, have an exit of 5 volts or the 7812 of 12 volts. Each of them has a rank of values of entrance tension, that in the case of the 7805 is 7 volts of minim and 25 of principle. In the following image we have a circuit, that connected the common terminal to a voltage divider, can offer a tension of different exit.
Exercise 2. To realise the suitable analysis to obtain the value of the exit tension. The typical current by the common terminal is of 80 mA. To consider that the regulator lm 7805 has a tension between the exit and the value of reference of 5 volts. As we have a connected potentiometer of 1 K, to calculate maximum and minimum the tension that we have when coming out.
Another important block is all source are the systems of protection of tips you tension.
Power supply Exchanged
In this case, we have a component (transistor) that it exchanges to elevated frequencies to obtain squared signals of high frequency that they will be rectified and leaked
As a result of the low efficiency given by the linear systems (to see exercise 1), these systems allow us to improve this factor since the transistors are not working all along, as it happens in the previous ones.
In this case, the device is going to work in commutation way (happening of the cut to the saturation and vice versa).
We will see that in a conventional regulator, whose tension of exit is 5 V and 1 To of exit, if the tension of entrance of 30 volts, the voltage drop is 25V, that when happening through the control transistor, is dissipating a power of (25 V * 1 25 A) = W, which they are lost in the form of heat. In this case, we have for a delivery of 5 watts, have a total consumption of 30 W (5 given and 25 consumed). The efficiency is:
We have only one efficiency of 16.6%
In the case of the exchanged ones, we have yields of 80%.
In the switched-mode power supplies, the ones in charge of the power control are also transistors, but when working in way conduction - he cuts, the product of BCE * IE (in the case of a bipolar one) we will always have the product of a high value by one low one, thus, the power that consumes the transistor is low.
The commutation frequency (times that the transistor happens of the cut to saturation) usually is limited in 40 bipolar transistors about KHz., but MOSFET of power are used, the frequency raises about 200 KHz, which supposes a considerable saving in size of transforming. In the following image we showed a basic scheme of one f.a exchanged
In general, the control block (the one that indicates the 30 Khz) does that the transistor happens of cuts to saturation vice versa and, controlling the time that the semiconductor is active and, therefore, the energy that goes to the primary one of the transformer, that is gathered by the secondary one and rectified secondly.
Several forms exist to control the device to control the current flow. We will see one of them, denominated Step-Down (Buck Converter).
In this configuration, the regulation is similar to which makes a reducing transformer, with a tension of smaller exit of the delivery tension. We show the scheme of this circuit (similar to explain the real one)
If we observed the previous figure, when the transistor (represented by the interrupting S) is closed, we have the state ton, moment at which a current one happens through inducer L. Of all the current that happens, a part is dedicated to the load and another one is used for load condenser C. In this time, D is polarized in inverse and it does not lead.
If the switch opens himself, we have the new Toff state.
An inducer is against to the abrupt changes of current and, therefore, it is going to change the tension of its tips to cause that the current continues flowing in the same sense.
How we regulated the exit tension?
Since we have shelp before, the regulation of the voltage when coming out is obtained modifying the time ton of the transistor, and one of the ways to obtain it is by means of the PWM (Wide Modulation or Modulación of Ancho de Pulso Beats).
We show a general scheme to secure this intention. In the scheme of the previous figure, Vo (exit tension) has a voltage divider formed by R1 and R2. The tension in that point is compared with the tension that we want to obtain and that normally it comes from a potentiometer.
A1 compares those two values and generates to its exit a tension differential. We suppose that the Vo tension is increases and therefore the tension that enters to him the comparator. The exit of A1 now is greater. This does that the time that takes in reaching the new value is increased and, therefore, the pulse that is smaller, with a ton of smaller time.
This makes reduce the current of inducer L and therefore the amount of transferred energy, thus, Vo will become minor.
If by some reason it happens the opposite, with a Vo but reduced, the process is the same:
V0↓ >> Tension differential in A1↓ >> Time in reaching the value of firing the triangular signal ↓ >> A2 generates a pulse before >> Too ↑ >> Aumenta the current by coil L >> the V0 tension ↑
A typical source of use domestic imagines next.
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