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8.8 Voltage
As we have seen, the smoothing filter gives improved dc output voltage, but in general the changes in the load have a significant effect on this output voltage. To improve still further the behavior of a power supply and make the output voltage as possible unaffected by any changes in workload, we use a special circuit called a voltage stabilization circuit. Patrakato, we see the simplest stabilization circuit, which is the stabilizer with Zener diode and then more complex circuits. Previously, however, we will consider the general principles governing the process of stabilization.
8.8.1 Principles of Stabilization
Any power source can be represented by the equivalent circuit, Thevenin, Sch.8.14 with RL load connected to the exit. The voltage VQ at the ends of the load is:
(8.8.1)
where V is the output voltage of the smoothing filter and Rs is the internal resistance of the power source.
source voltage
Vo
Figure 8.14. Power source with Thevenin equivalent circuit
If for any reason (eg change of voltage) change the output voltage V of the smoothing filter will change the current IL and therefore, according to Eq. (8.8.1) will change the output voltage Vo. But if you change the load RL, will change again in direct current IL, so
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and Vo, and the effect on voltage Vo depends on the relative price
of RL and RS. Therefore, to maintain constant voltage Vo at the edges
the burden is necessary to compensate for these changes in voltage or current or both simultaneously.
One way to stabilize according to the above, the voltage Vo is to introduce a variable resistor in series with RV in
load Rl, as shown in Sch.8.15. Then the output voltage will be:
Figure 8.15. Operating principle of stabilizing trend: series (a) parallel (b)
V0 = V - IL (RS + RJ (8.8.2)
The resistance R "should be changed automatically to compensate for changes in V and IL. There are several stabilization circuits that can change automatically active resistance Rv of the item number, so that they can regulate stabilizing
the output voltage by comparing it to a stable reference voltage.
When the active element in series with the load to stabilize said stabilizing series Sch.8.15a and controlled by
Eq. (8.8.2). The cost of stabilizing the series is very low and
for it is used quite often.
Another method of stabilization is the parallel stabilization Sch.8.15 B. It turns out that, in this case the voltage
output Vo of the circuit are:
(8.8.3)
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8.8.2 Simple Zener diode stabilization
Electronics in the previous year as we have seen reverse rupture of a typical diode Zener. Diodes Zener <avalanche must have a high impurity level if we want to have low voltages contusion <collapse.
The Sch.8.16a shows the characteristic of the diode Zener, and Sch.8.16v shows a low power parallel stabilizer using an active Zener resistance to RV.
(A) (b)
Figure 8.16. Vi characteristic of Zener (a) and circuit
stabilization voltage Zener (b)
If the voltage Zener rz = Vz / Iz remains almost constant for large ¬ ol current range, the equivalent active resistance Rv = V0 / Iz be varied to compensate for changes in output voltage of the smoothing filter, V, or the load current IL .
This circuit with Zener diode stabilizer is perfect, as shown in Sch.8.16a, since the voltage, VZ, increases slightly with increasing the flow of IZ (the characteristic is not perfectly vertical). Under the law of Ohm, the total current of I, is given by:
(8.8.4)
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where, R = Rs + RL. Therefore, for a given input voltage V, current I, is stable. But under the law of Kirchhoff currents This stream will be:
I = Iz + IL (8.8.5)
From this relationship it seems that if we increase the load current IL the current of Zener Iz reduced. It is therefore, the maximum load current to pass a current from the Zener such that this work left by the "knee» VZO the characteristic.
Moreover, when there is no burden on the ends of the stabilizer output, all stabilizer stream will pass through the Zener. Consequently, the respective strengths of losses Zener (which is then maximum) will be:
Pz, max VzIz = Vz I (8.8.6)
(It minimizes the dynamic resistance of the Zener, rz = AVz / AIz).
The election of Zener, in addition to the appropriate value of the voltage Vz = V0 should be based on the above maximum power losses. The usual inputs of small Zener losses in the order of 400 mW, but there are high power Zener reaching up to 10 W. But as we shall see later, usually avoid high power Zener for stabilization.
8.8.3 Stabilization with high load current
Most often in practice, the load current needs to be large. We must therefore make use of high power Zener. Instead, we adopt this solution, the cost is large, we can use a Zener and a low-power transistors in the following wiring voltage (CC), Figure 8.17, which would give a current output increased by a factor hFE.
Figure 8.17. Zener stabilization and tranzator (CC)
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The transistor is elected, that the operation is within the limits of power loss. The Zener diode is chosen for the desired output voltage Vo, by the relation:
Vo = Vz-Vbe (with Vbe 0.7 V) (8.8.7)
Example 8-3
We compute a voltage stabilizer in the form of Sch.8.17 with V0 = 7.5 V, IL = 30 mA and an input voltage V = 17.5 V, derived from the output of the smoothing filter.
Solution
From Eq. (8.8.7), we have:
Vz = V0 + Vbe = 7.5 + 0.7 = 8.2V
Therefore, the Zener should be of 8.2 V. We choose a test type transistors BFY50 the following characteristics: VC, max = 35 V, PC = 800 mW and hFE = 30.
C, max Ft
The current base is:
The collector voltage of the transistor will be developed are:
Vc = V - V0 = 17.5 - 7.5 = 10 V The resistor R1 is:
VC R1 f I1
where the elect I1 = 10 Ib (insensitive to the external direct current I1 to changes in current Ib). Therefore,
10
R = 1 KW
1 10
The strength loss of the transistor will be:
Pc = VC IL = 10 x 30 = 300 mW.
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From the characteristics of the manufacturer can be seen that the above operation is within the operating limits of the transistor. So the trial choice of the transistor BFY50 proved successful.
8.8.4 Improved stabilization Anasyzefxi
The stabilization of the output voltage of a power supply can be further enhanced if the previous use an amplifier circuit, which compares the reference voltage which gives a Zener with a fraction (sample) output voltage.
This sample feeds to the input of the amplifier, having been compared to the voltage of Zener, Sch.8.18a. In practical circuit, Sch.8.18v, making the circuit of the sample is a voltage divider and the reference voltage is directly the voltage Zener. The transistor T2 is the support element and the transistor T1 is as follows syndesmologimeno voltage (CC).
Amplifier
Comparators
Zener voltage reference
Sample Output
(A)
(B)
Figure 8.18. Circuit stabilization anasyzefxi:
Principle (a), practicality (b)
Because the current of Zener almost anaxartito the load current, the stabilization of this circuit is very good.
VV00
8.9 Stabilization voltage TE
Modern power supplies using operational amplifiers (TE) in order to achieve stabilization using anasyzefxis. The
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stabilization in this way is very popular because it provides a steady voltage precision. The circuit of Sch.8.19 shows the typical circuit voltage stabilization TE. This is done through anasyzefxi Rr
Figure 8.19. Stabilize voltage TE
RL V0
Because TE does not give high output current, make the circuit with a transistor power pnp, as shown. The power transistor has the potential to give the desired load current IL connected to the voltage V that we want to stabilize. The transistor is behaving like a big RV resistance similar to that of b Sch.8.15
Demonstrated, the analysis of TE (Chapter 4) that the output voltage is given by:
(8.9.1)
Therefore, the steady-state output voltage V0 can be adjusted at will through the Rf of the Ri
