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8.1 Introduction
Almost all modern electronic devices fed by continuous working voltage (dc). The power supply is achieved by taking the alternating (ac) voltage of 220 V/50 Hz and usually relegated to convert it to dc. The devices that perform this function are called simply the feeder power provisions.
Some converters such as photomultipliers and ionization detectors, require dc voltages from hundreds to thousands of volts (kV), but very low currents, typically less than mA (milliamper). Other provisions, such as transistors and integrated circuits (IC), usually work with supply voltages from 3 to 30 V and currents of several mA to several A (ampere).
The stability of supply voltage, powering the electronic device, often plays an important role because it determines the stability of the device.
Today there are several types of feeders. Some of them are based on the classic shape combined with a rectifier smoothing filters. Development of this scheme is done using specific integrated circuit power. Also used in various applications called palmotrofodotika. Finally, when the required inputs of not great, used both converters DC / DC converters and AC / DC. It is also worth noting that for a stable output voltage dc, is currently in use various stabilizers. These issues will be dealt with in the following sections.
8.2 Basic steps of a power supply
The Sch.8.1 shows the block diagram of a normal power supply and the various stages required to convert the ac voltage to dc. The figure shows the different waveforms of each step.
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Separable. voltage (optional)
Figure 8.1. Power conversion block diagram ac voltage to dc
The first step is a power transformer that converts the width of the available input voltage ac, eg the 220 V network, the desired ac voltage (with the demotion elevation). The second stage is a rectifier which converts the ac signal is already modified to wire input voltage pulse rectified voltage motasi chil-dc. Then the dc palmotasi fully converted by a dc smoothing filter. The next level is the voltage stabilizer, which improves the dc output voltage, which reduces the ripple. Finally, several power electronic devices have a voltage divider that provides several levels of dc voltage, which are used in different parts of the powered devices.
8.2.1 Quality Characteristics of Supply Voltage
To perfect the dc voltage generated by a power supply must have the following characteristics:
1. Dc steady state independent of the required changes in load current, ie, good regulation and low output impedance.
2. Time constant value independent of any changes of temperature, voltage ac network, the age of components, etc. (good stability).
3. Do not present a noise voltage or other ac component of the ripple frequency (generally small ripple).
Moreover, the volume of power supply should be as short as possible
smaller and the performance of as much as possible.
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8.3 Imianorthosi
The simplest possible process that can lead to a dc output voltage ac input is imianorthosi. The typical basic circuit shown in imianorthosis Sch.8.2.
Figure 8.2. Circuit imianorthosis
The analysis of the operation of this circuit is as follows:
The secondary of the transformer T provides alternating current, 50 Hz amplitude value appropriate to eventually produce the desired dc voltage. During the positive half period, the voltage of the secondary makes the rise of the diode D more positive than the cathode, so the diode conducts. During the negative half, the secondary voltage is such that the anode of the diode to be negative to the cathode and the diode does not conduct (consider the negligible reverse current). So, the current that circulates through the diode and the load resistance RL circulates in only once and therefore dc current (the general sense), though not continuous but steady palmorefma. The palmorefma that develops at the edges of a dc palmotasi RL (imia-northomeni), as shown in Sch.8.2. Because each negative half, it takes time to 10 ms (T = 1 / f = 1/50 = 20 ms), no circulating current, the power developed in load is small enough.
Active resistance to wear proper rectifier diode, rf, is not constant but depends on the instantaneous current of the diode ID, when the relationship rf 25/ID (the rf to Z if the ID in mA). Thus, we can calculate the instantaneous power loss of the type imianorthoti
(8.3.1)
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The main advantages of imianorthoti is the low cost and simplicity of construction and disadvantages of large size transformer, strict filtering requirements, low output and poor ability to regulate the output voltage.
The circuit shows the full Sch.8.3 rectifier. The circuit is usually used most of the circuits of the rectifiers. M O adapter is making tools and adding an extra diode allows to develop (rectified) voltage at the load during both half-of the input signal. Because the middle making entrances on both channels have phase difference between the 180 °.
During the positive half of the input signal, the diode D1 conducts and
s
thus passing the burden of its own current I1. During the negative half conducts the diode D2 and the load current I2 passes. The total load current IL is the sum I1 + I2 of the two currents.
The dc output voltage of the rectifier is fully equal to the width of the voltage developed in each half of the secondary of the transformer. To achieve peak output voltage equal to V0p, we should use a transformer with a medium that has been receiving an output voltage (rms value)
Full recovery 8.4
Figure 8.3. Full rectifier circuit
(8.4.1)
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The extra 1 V is added to take into account the voltage drop of rectifier diode.
With the circuit complete recovery develops better (denser) pulse dc voltage, which needs more easily filter ¬ term to become normal dc voltage, but the circuit has a higher cost. Indeed, the ripple frequency, as we shall see in the next section, is twice the frequency than in imianorthosi so filtering is easier. We also note that the transformer is smaller than that of imianorthosis.
8.5 Rehabilitation of Bridge
O in bridge rectifier looks in several places with full rectifier seen in the previous section. As to the frequency and ripple performance, the two circuits are identical and differ in the number of channels and the output voltage level. The Sch.8.4 shows a typical rectifier with bridge. The bridge is connected at the ends of the secondary of the transformer and
voltage reaches the load is the voltage across the secondary, if there is no middle shot.
Ui
(A) (b)
Figure 8.4. A bridge rectifier circuit
The operation of this rectifier is as follows: When the voltage at point A is positive, ie during the first half (first half) of the input signal, the diodes agoun D2 and D4, after rises' are positive, and current I1 follows the road shows
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shape, ie through the diode D2, the load RL, the diode D4 and the end point b to point A. The output voltage then has the form of the waveform shown.
During the second half (2 nd half) of the input signal as it develops the secondary of the transformer, a point is negative and the diodes D1 agoun and D3, since the anodes' are now more positive than the corresponding cathodes. The current I2 follows the path from point B through the diode D3, the load RL, the diode D1, a point and finally to point B. The currents I1 and I2 have the same direction as they pass through the load RL. The output voltage has the form corresponding to the pulse "b" of the waveform shown. The main advantage of a bridge rectifier that is used throughout the secondary voltage of the transformer, while
also have a full recovery. The disadvantage is that it uses four instead of two channels, thus increasing the cost and complexity.
8.6 Voltage Doubler
Often, electronic devices to our advantage to produce a rectified dc voltage from a starting as low as possible ac voltage. A simple circuit that can be used for this purpose is the voltage doubler.
O voltage doubler rectifier circuit is one that has syndesmologithei so that the output voltage is twice the amplitude of the ac input voltage, Sch.8.5.
Figure 8.5. Circuit Doubler voltage
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We will analyze the operation of the circuit starting from the initial behavior of the diode D1 and capacitor C,, as shown in subsidiary Sch.8.6. Suppose that initially, the input voltage has the polarity shown. Then the diode is conductive so that the capacitor will charge up to a voltage equal to
peak voltage of the input signal. This load is very fast, because the good wear resistance of rf diode is very small (so we will have a small time constant rf C1).
Figure 8.6. Charging the capacitor C1 through D1 and the capacitor voltage
Once the capacitor is charged diode D1 is reverse polarized now, since the cathode is now more positive than the anode, and the entrance is now beginning to dwindle and driven towards zero. However, the capacitor can be discharged, and the diode D1 is reverse biased and the reverse current is negligible. Therefore, the voltage across C1 is maintained constant at a value equal to the peak input voltage. The polarity of this voltage makes the conducting diode D2 so the trend has moved to the load RL.
When the input polarity is changed, the situation changes and switched capacitor C2, and diode D1 remains non-conductive, as shown in Sch.8.7. O capacitor C2 "sees" two voltage sources, which is the ac input voltage and the voltage of the capacitor C1. These two sources are in series so that the capacitor C2 is charged with the particle-diode D2 (which is still conductive), the most positive input voltage plus the constant ac voltage acquired by the capacitor C1. So, eventually the capacitor C2 will be charged at twice the price of the input voltage and the voltage transfer that directly to the load RL.
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Figure 8.7. Charging the capacitor C2
The voltage across the capacitor C2 is twice the input voltage provided that the current through the load RL is very small. If the stream is too large, ie the small resistance RL, the capacitor C2 will be discharged rapidly through, and the voltage across it remains enough time to double the expected level. For this reason we have the desired effect with a normal load, the capacitors C1 and C2 are chosen large. Typical values of these capacitors is approximately 100 mg or even higher.
