If it is necessary to obtain constant voltages that are multiples of the AC supply voltage supplying them, in many areas of radio engineering, voltage multiplier rectifiers (VF) are used. They are classified into half-wave and full-wave, sequential and parallel types.
Half-wave rectifier circuit
Figure 1 shows a half-wave rectifier circuit with voltage doubling. The circuit can be used both independently and as a constituent element of multi-tier sequential multipliers.
Figure: 1. Diagram of a half-wave rectifier with voltage doubling.
Figure 2 shows a parallel circuit of a full-wave voltage doubling rectifier (Latour circuit). This UN as a rectifier can be considered as two half-wave, switched on (secondary winding of the transformer T1 - diode VD1 - capacitors C1, C3; secondary winding of the transformer - diode VD2 capacitors C2, C4) in series. The doubled voltage at its output is obtained as a result of the addition of separately rectified bipolar voltages.
Figure: 2. Parallel circuit of a full-wave rectifier with voltage doubling (Latour circuit).
Series ladder half-wave rectifier
Sequential multi-tier half-wave rectifier (Fig. 3) with voltage multiplication is most often used at low (up to 10 ... 15 mA) load currents.
Its circuit consists of half-wave rectifiers - links, in the following algorithm - one link (diode and capacitor) - just a one-half-wave rectifier consisting of a diode and a capacitor (rectifier and filter), two links - a voltage multiplier twice, three - in three times, etc.
In most cases, the capacitance values \u200b\u200bof each link are the same and depend on the frequency of the supply voltage to the VL and the current consumption.
Figure: 3. Scheme of a multi-link half-wave voltage multiplier.
It is convenient to consider the physical processes of voltage increase in a multi-link half-wave (Fig. 3) VN when applying an alternating sinusoidal voltage to it. UN works as follows.
With a positive half-wave of the voltage at the lower terminal of the secondary winding T1, a current flows through the diode VD1, charging the capacitor C1 to the amplitude value.
With a positive half-wave of the supply voltage at the lower terminal of the secondary winding T1, the sum of the voltages on the secondary winding and the voltage on the capacitor C1 are applied to the anode VD2; as a result of which a current passes through VD2, the potential of the right plate C2 relative to the common wire increases to double the input voltage, etc. It follows from this that the more links there are, the more constant voltage (theoretically) can be obtained from the UN.
For a correct understanding of the formation and distribution of the potentials arising on radioelements during the operation of the UN, we assume that one input pulse (VI) fully charges the capacitor C1 (Fig. 3) to the voltage + U.
Imagine the second positive pulse arising at the upper terminal of T1 and entering the left plate C1 according to the scheme in Fig. 3, also in the form of a capacitor charged to voltage + U (Cu).
Their joint connection (Fig. 4) will take the form of series-connected capacitors. The potential on C1 relative to the common wire will increase to + 2U, VD2 will open, and the capacitor C2 will be charged to + 2U.
Figure: 4. Voltage multiplier circuit.
When a pulse of + U appears on the lower terminal of T1 and sums it up in the same way with a voltage of + 2U on the capacitor C2, a voltage of + 3U will appear on C3 through the opened VD3, and so on.
From the above reasoning, we can conclude that the voltage value relative to the "common" wire (Fig. 3) only at C1 will be equal to the amplitude value of the input voltage, i.e. + U, on all other capacitors of the multiplier, the voltage will increase stepwise with a step of + 2U.
However, for the correct choice of the operating voltage of the capacitors used in the CN, it is not the voltage across them relative to the "common" wire that matters, but the voltage applied to their own terminals. This voltage only on C1 is + U, and for all the others it is + 2U regardless of the multiplication step.
Now we represent the end of the duration of the action of the VI pulse, as closing the capacitor C (Fig. 4) with a jumper (S1). Obviously, as a result of the short circuit, the potential at the anode VD2 will decrease to + U, and the potential 2U will be applied to the cathode. Diode VD2 will be closed with reverse voltage 2U-U \u003d U.
Hence, we can conclude that a reverse voltage is applied to each UN diode relative to its own electrodes, no more than the amplitude value of the supply voltage pulse. For the UN output voltage, all diodes are connected in series.
Practical VHF circuits for HF and VHF
Shortwave radio amateurs who make their own radio equipment are familiar with the problem of making a good power transformer for the output stage of a transmitter or transceiver.
The diagram shown in Figure 2 will help solve this problem. The advantage of practical implementation is the use of a ready-made, not in short supply due to the departure of old technology, a power transformer (ST) from a unified lamp TV (ULT) of the second class, which can be used as a power transformer to power a power amplifier (PA) of a radio station of the 3rd category.
The recommended technical solution makes it possible to obtain from the CT all the required output voltages for the PA without any modifications. ST is made on a PL type core, all windings are structurally made symmetrically and have half turns on each of the two coils.
Such a ST is convenient both for obtaining the required anode voltage and heating voltage, since allows the use as an output in the PA as a lamp with a 6-volt filament (type 6P45S) and a lamp (type GU50) with a 12-volt filament, for which it is only necessary to connect the filament windings in parallel or in series. The use of a doubler will make it possible to easily obtain a voltage of 550 ... 600 V at a load current of about 150 mA.
This mode is optimal for obtaining a linear characteristic for the GU50 lamp when operating on SSB. By connecting the filament windings in series (used in TV to power the incandescence of lamps and a kinescope) and applying the UN according to the diagram in Fig. 3, you can obtain a source of negative bias voltage for the control grids of the lamps (about minus 55.65 V).
Due to the small current consumption along the control grid, non-polar capacitors of 0.5 μF at 100.200 V can be used as capacitors of such a UN.
The same windings can be used to obtain the switching voltage of the "receive-transmit" mode. When constructing an output stage with a grounded grid, the control grid is connected to a negative voltage source (UN 55.65 V), the cathode is connected through a choke (015 mm, n \u003d 24, PEV-1 00.64 mm) to -300 V, and + 300 V, the excitation voltage is applied to the cathode through a capacitor.
You can connect the control grid directly to -300 V, the cathode is connected to -300 V through two parallel-connected chains, each of which consists of a D815A zener diode and a 2-watt 3.9 ohm resistor. In this case, the excitation voltage is applied to the cathode through a broadband transformer.
If the output stage of the PA is made according to the scheme with a common cathode, then +600 V is supplied to the anode, and +300 V to the screen grid from the connection point C1, C2, C3, C4 (the -300 V output is connected to the "common" RXTX wire), which allows you to get rid of powerful damping resistors in the screen grid circuit, on which a lot of heat power is uselessly released. A negative bias of -55.65 V is fed to the control grid from the previously mentioned UN.
To reduce the level of ripple of the supply voltage in the rectifier, you can also use standard chokes (L1, L2, Fig. 2) of the power supply filter of the same ULT type DR2LM with a primary winding inductance of about 2 H. The winding data for ST and DR2LM are given in.
Lighting engineering
An example of using a voltage multiplier by four is the daylight lamp (LLS) starterless starting circuit shown in Figure 5, which consists of two voltage doublers connected in series DC and parallel to AC.
Figure: 5. Circuit of a voltage multiplier by four for a starless start of a daylight lamp.
The lamp ignites without heating the electrodes. The breakdown of the ionized gap of the "cold" LDS occurs when the LDS ignition voltage is reached at the UN output. LDS is ignited almost instantly.
A lighted lamp shunts with its low input impedance the high output impedance of the UN, the capacitors of which, due to their small value, cease to function as sources of increased voltage, and the diodes begin to work as ordinary valves.
2-winding choke L1 (or two 1-winding chokes) serves to smooth out the rectified voltage ripple. The voltage drop in the supply network is approximately evenly distributed over the ballast capacitors C1, C2 and LDS, which are connected in series with alternating current, which corresponds to the normal operating mode of the LDS.
When used in this scheme, LDS with a cylindrical part of 36 mm in diameter ignite without any problems, LDS with a 26 mm diameter ignite worse, since due to the peculiarities of their design, the ignition voltage even for new lamps without heating the filament can exceed 1200 V.
TV
It is known that the line scan output transformer (TVS) is one of the strained nodes in the TV set. As the evolution of the development of the circuitry of this node shows, with the transition from tube TVs to color ones, due to the increase in power consumption from a high voltage source (the current consumption of a black-and-white picture tube with a diagonal of 61 cm along the second anode is about 350 μA, and a color one is already 1 mA !), TV designers were constantly looking for ways to improve its reliability.
Circuitry solutions for obtaining a high voltage for powering the second anode of the kinescope, which were used in all models of tube TVs, took place only in the first modifications of the ULPCT, and then instead of the step-up winding of the fuel assembly (almost equal in the number of anode turns), they began to use UN, which by their electrical strength , and hence the reliability significantly exceeded similar parameters of the winding unit.
Figure: 6. Voltage multiplier circuit with trebling, from the Yunost TV.
UN almost immediately began to be used in domestic black and white portable TVs. For example, in TV "Yunost 401", a voltage tripling circuit is used, shown in Fig.6.
In the implementation of practical circuits of the UN it matters with which point of the circuit of the UN (1 or 2, Fig. 3) the "common" wire of the circuit in which it will be used is connected, i.e. "phasing" UN. This is easy to verify with an oscilloscope.
When carrying out measurements on an unloaded CD (Fig. 3), it can be seen that on odd links the value of the variable component is almost equal to the supply voltage, and on even links it is practically absent.
Therefore, when using in real structures voltages only from even or only from odd multiplication links, this fact should be taken into account by connecting the VN to the power source accordingly.
For example, if the "common" wire (Fig. 3) is connected to point 2, then the operating voltages are removed from the even links, if with point 1 - from the odd ones.
When using both even and odd links of one CN, to obtain a constant voltage from a link in which an alternating component is present, it is necessary (especially with a capacitive load) to turn on another link (diode and capacitor) between the multiplier link and the load (Fig. 7).
The diode (VDd) in this case will prevent a short circuit across the AC component, and the capacitor (Cdf) will act as a filter. Naturally, the capacitor Cdf must have an operating voltage equal to the full constant output voltage.
Figure: 7. Inclusion of one more link to the voltage multiplier.
One should also not forget about the negative impact on the reliability of the operation of multi-link UL leaks, which are always present in radioelements and materials when they operate under high voltages, which imposes certain restrictions on the actually achievable value of the output voltage.
A practical version of the UN circuitry with multiplication by three is shown in Fig. 6; four - in Fig. 4; by five - in Fig. 8, Fig. 9; by six - in Fig. 10.
Figure: 8. Voltage multiplier circuit with multiplication by four.
Figure: 9. Voltage multiplier circuit multiplied by five.
Figure: 10. Voltage multiplier circuit with multiplication by six.
This article discusses only a part of the UN circuitry that was previously used and is currently used in household appliances and amateur radio design. Some varieties of UN circuitry, the operating principles of which are similar to those considered, are published in.
In the literature and in communication with radio amateurs, one often encounters confusion regarding UN in terms of. For example, it is argued that if the marking is 8.5 / 25-1.2 or 9 / 27-1.3 on the UN, then this is a voltage tripler. According to the circuitry, these CNs are multipliers by five.
The marking carries information only that when a voltage with an amplitude of 8.5 kV is applied to the input of the ULV, it ensures that an average constant (positive) voltage of 25 kV is obtained at its output (at a current consumed by its load, about 1 mA), i.e. e. the marking speaks only of its input and output parameters.
To obtain a high voltage in the TV, a pulse voltage is used that occurs in the secondary winding of the fuel assembly during the return path of the beam, following with a frequency of 15625 Hz, with a (positive) pulse duration of about 12 μs and a duty cycle of about five.
With a large multiplication factor, a significant value is also the voltage drop in the forward direction on the rectifier posts, which are the UN rectifiers. For example, for a 5GE600AF column, when it is used as a single rectifier, the voltage drop in the forward direction is 800 V!
It follows from the above that the UN elements also serve for the supplying pulse voltage as an integrating circuit, which reduces the average DC voltage (at a load current of 1 mA) relative to the input voltage to a value of approximately 5 kV per link. It is these factors that are the main ones that affect the magnitude of the output voltage of the CN, and not approximate arithmetic.
Historically, the use of selenium diodes as rectifiers in the first UN samples for TV was determined by the level of technology achieved at that time, their low cost, as well as a soft electrical characteristic, which makes it possible to connect in series an almost unlimited number of diodes.
It is obvious that selenium rectifiers, due to their high internal resistance, withstand short-term overloads better than silicon ones. With the improvement of the technology for the manufacture of silicon diodes in UN TV, silicon poles of the KTs106 type began to be used.
When repairing TV, even a preliminary assessment of the possible presence of defects in the rectifier elements of the UN using an autometer is impossible. The physical meaning of this phenomenon is that in order to open one silicon diode, it is necessary to apply a potential difference of the order of 0.7 V in the forward direction.
If, for example, instead of the KTs106G column, an equivalent of individual KD105B diodes (uobr \u003d 400 V) is used, then to obtain a reverse voltage of 10 kV, a chain of 25 diodes connected in series will be required, as a result of which the required voltage to open them will be 17.5 V and the avometer only allows 4.5V!
The only thing that can be unambiguously stated after measuring the UN with an autometer is that when checking a working UN, the ohmmeter needle should not deviate when measuring the resistance between any of its electrodes.
A simple solution for a preliminary test of the performance of the UN elements by the voltmeter method was proposed in. The essence of the proposal is to use for this purpose an additional source (A1) of direct voltage (PSI) 200 ... 300 V and an avometer operating in the mode of a direct current voltmeter at the limit of 200.300 V. Measurements are made as follows.
The autometer is switched on (Fig. 11) in series with the same-name PSI pole and the tested rectifier column or VN. Verification algorithm.
Figure: 11. Scheme of switching on the avometer to the rectifier post.
If, when measuring the diode in opposite directions, the voltmeter readings:
- differ significantly, then it is serviceable;
- equal to the maximum voltage of the PPI, then it is broken;
- small, then it is torn off;
- intermediate values \u200b\u200bindicate the presence of significant leaks in it.
The suitability of the elements of the tested rectifier is determined empirically for a specific brand by a statistical method of comparison with the voltage drop values \u200b\u200bobtained practically during measurements in the forward and reverse directions of a serviceable, similar in brand, post or UN diode.
For radio amateurs who are engaged in the repair of television equipment at the customer's home, it is more convenient to use the circuit shown in Fig. 12 and proposed in the circuit, which is powered through current-limiting capacitors from a 220 V network, for a preliminary check on the performance of the UN elements using the voltmeter method (based on the weight and dimensions).
Figure: 12. Power circuit with current-limiting capacitors.
The circuit has proven itself well in practice, and in terms of circuitry it is a voltage doubling rectifier. The measurement algorithm is the same. The same scheme can be used to eliminate some types of interelectrode short-circuits ("lumbago") in the kinescope.
Quite often they ask whether it is possible to install UN9 / 27-1.3 instead of UN8.5 / 25-1.2? One advice: you can, but be careful! It all depends on the severity of the problem and the modification of the TV. For comparison, consider the schemes
UN8.5 / 25-1.2 (Fig. 8) and UN9 / 27-1.3 (Fig. 9). It is clear from the UN circuits that, in principle, a direct replacement is possible, but the reverse is not, since they have a different number of incoming radio components.
Therefore, when installing UN9 / 27-1.3 in TV ULPCT, proceed as follows: short-circuit the input terminals for the pulse voltage and the "V" output; the wire from the fuel assembly is soldered to the corresponding input UN9 / 27; the wire with the sign "earth" is connected at the shortest distance to the second contact of the fuel assembly; the wire going to the focusing varistor is connected to the "+ F" terminal, and the standard focusing filter capacitor C23 * (according to the factory diagram on TV) can be turned off, since its function can be performed by the capacitor C1 (Fig. 10), which is installed inside the VN. A high-voltage wire with a "suction cup" and a limiting resistor Rf is connected to the "+" terminal.
The resulting improvement in image quality on the TV screen as a result of such a replacement does not mean that this is the result of replacement!
The reason is, first of all, that in UN9 / 27-1.3 silicon poles of the KTs106G type are used as valves, the voltage drop across which in the forward direction (as mentioned earlier) is significantly less than on the poles of the 5GE600AF type, which are part of UN 8.5 / 25-1.2.
It is by the magnitude of this difference that the voltage at the output of the UN increases, and therefore at the second anode of the kinescope, which is observed visually as an increase in brightness!
In addition, in TV ULPCT, when installing UN9 / 27-1.3, it is necessary to replace the standard "suction cup" with a high-voltage resistor 4.7 kOhm installed inside it. Rf) "suction cup" from TV 3UTSST with a 100 kOhm resistor. Rf performs three functions: it is part of the link of the smoothing RC filter for the high voltage circuit formed by it and the capacitance of the ak-wadag of the kinescope Ca (Fig. 9, 10), as well as a protective DC resistor that limits its value in the UN circuit in case short-term interelectrode breakdowns inside the kinescope (which occurs very often and unpredictably in old kinescopes).
It is also a "burning fuse" that protects fuel assemblies in case of breakdown of VL diodes, when the alternating voltage coming from the fuel assembly is practically closed to the body through Ca, the reactive resistance of which is quite small for line frequency currents.
Therefore, it should be borne in mind that a significantly lower value of the total internal resistance of UN9 / 27-1.3 with a small value (or absence for one reason or another) Rf in cases of replacing the UN is undesirable, since it can lead to the appearance of the above malfunctions as a way out building the fuel assembly, and to the fire of the TV itself.
With a certain skill and accuracy, you can "get" (if you are lucky) high-voltage capacitors from those inoperable in TV UN with a certain skill and accuracy, which can still serve for urgent repair of TV modifications of ULPCTI or UPIMTST or for experiments with other designs.
To do this, first, carefully break the UN body with a hammer and release the capacitor bodies from the compound, and then separate their leads from the interconnections and the rest of the compound by sequentially chipping them off using side-cuts. Practical disassembly of three copies of each brand of UN showed that in UN8 / 25-1.2 the capacitors are marked K73-13 2200x10 kV on the case.
In UN9 / 27-1.3 (Fig. 10), which, in comparison with UN8 / 25-1.2, has a larger number of elements, but smaller overall dimensions, capacitors are used (judging by the manufacturing technology and material from which they are made) of the same type (no markings on the cases), which are structurally made in the form of a three-pin (16 mm in diameter) assembly (C2, C4 - Fig. 10) of 1000 pF capacitors, and a four-pin (C1, C3, C5 - Fig. 10) assemblies with a diameter of 18 mm. Moreover, C1 has a capacity of 2200 pF, and C3, C5 - 1000 pF each. Both assemblies are 40 mm long.
Medicine
One of the "exotic" examples of the use of CN in medical equipment is its use in the construction of an electro-effluvial chandelier (EL), which is designed to produce a flow of negative ions that have a beneficial effect on the human respiratory tract.
To obtain a high negative potential for the radiating part of the aero-ion generator, a CN with a negative output voltage was used. Due to the rather large amount of auxiliary information, recommendations on the design and use of EL are beyond the scope of this article, therefore EL is mentioned only informatively.
Details for diagrams
Specification for figures:
- to Fig. 2: C1-C4 - K50-20;
- to Fig. 6: C1-C2 - KVI-2;
- to Fig. 7: C1, C2 - MBGCH; C3-C5 - KCO-2;
- to fig. 10: C1-C6 - K15-4;
- to fig. 12: C1, C2 - K42U-2, C3, C4 -K50-20.
S.A. Elkin, Zhitomir, Ukraine. Electrician-2004-08.
Literature:
- Elkin S.A. Starless start of fluorescent lamps // E-2000-7.
- Ivanov B.S. Electronics in homemade products. M .: DOSAAF, 1981.
- Kazansky I.V. HF radio station power amplifier // To help radio amateurs. - Issue 44 .-- M .: DOSAAF, 1974.
- Kostyuk A. Power amplifier for a CB radio station // Radio amateur. -1998. - No. 4. - P.37.
- Kuzinets L.M. and other TV receivers and antennas: Ref. - M .: Communication, 1974.
- Polyakov V.T. To radio amateurs about the direct conversion technique. - M .: Patriot, 1990.
- O. M. Plyats Handbook of electrovacuum, semiconductor devices and integrated circuits. -Minsk: Higher School, 1976.
- Sotnikov S. Malfunctions of the voltage multiplier and focusing circuits // Radio. - 1983. - No. 10. - P.37.
- Sadchenkova D Voltage multipliers // Radioamator. - 2000. - No. 12. -S.35.
- Fomenkov A.P. To radio amateurs about transistor TVs. - M .: DOSAAF, 1978.
- Shtan A.Yu., Shtan Yu.A. On some features of the use of air ionizers // Radioamator. - 2001. - No. 1. - P.24.
- 12. Yashchenko O. Device for checking and restoring picture tubes // Radio. - 1991. - No. 7. - P.43.
Among the various circuits of rectifying devices, a special group is made up of circuits in which, by means of the appropriate connection of rectifying elements and capacitors, not only rectification is carried out, but also the multiplication of the rectified voltage.
The advantage of such circuits is the possibility of constructing high-voltage transformerless rectifiers and rectifiers with transformers, only for powering the heating circuits of the kenotrons. The absence of a step-up winding in the power transformer greatly facilitates its manufacture and improves the performance of the rectifier. The disadvantages of these circuits include the relatively strong dependence of the rectified voltage on the current in the load and the relative difficulty of obtaining high powers.
Rectifier circuits with voltage multiplication are most widely used in X-ray equipment. In radio engineering practice, they are used mainly to power low-power equipment that consumes no more than 50-70 mA at a voltage of about 200 V. However, here, too, the scope of their application can be significantly expanded by building, for example, according to a circuit with tripling or quadrupling of the voltage, sufficiently powerful transformerless rectifiers. Such rectifiers with an alternating current voltage of 110, 127 or 220 V allow obtaining a constant voltage of 300-400 V at a current of up to 100-150 mA, which provides power to the anode circuits of receivers, low-frequency amplifiers of average power.
A feature of the operation of rectifiers with voltage multiplication is the use of the properties of capacitors to accumulate and store electrical energy for some time. When the rectifier operates from a conventional 50-period alternating current network, the time during which the capacitor must maintain its charge does not exceed 0.02 sec. The larger the capacity (included in the circuit of the capacitors, the greater the amount of electrical energy they store and the higher at the same load the rectified voltage is obtained. Therefore, in such rectifiers it is most convenient to use electrolytic capacitors, which, being small in size, have a significant capacity! ...
A number of practical rectifier circuits with voltage multiplication are described below, and for most of them the load characteristics are given, taken at different capacities of the storage capacitors. Such characteristics make it possible to fairly fully judge the possible areas of application of a particular circuit, as well as to select the rectifier circuit for the given rectified current, rectified voltage and supply voltage and determine the basic data of its parts.
DIAGRAMS OF RECTIFIERS WITH VOLTAGE MULTIPLICATION
Voltage doubling circuits. The rectifier circuits with voltage doubling, which are most widely used in amateur radio practice, are shown in Fig. 1.
FIG. 1. Schematic diagrams of rectifiers with voltage doubling.
a - a full-wave rectifier circuit; b - half-wave rectifier circuit.
In order to be able to fully compare and evaluate the advantages and disadvantages of both schemes, in Fig. 2 shows their load characteristics. The characteristics were taken at different capacities of capacitors C1 and C2. The rectifiers used selenium columns B1 and B2, each assembled from 13 washers with a diameter of 45 mm. The supply voltage was maintained at 120 V. To limit the starting current, which, due to the capacitive nature of the load, can reach significant values, a resistance R equal to 20 ohms was connected in series to the power circuit. This created more favorable conditions for the operation of rectifiers.
FIG. 2. Load characteristics of rectifiers with voltage doubling (taken at a supply voltage of 120 V).
a - characteristics of a full-wave rectifier; b - characteristics of a half-wave rectifier.
Comparing the load characteristics of both rectifiers, taken at the same (the same values \u200b\u200bof the capacitance of the capacitors C1 and C2, it can be noted that for the full-wave rectification circuit they lie noticeably higher than for the half-wave circuit. Therefore, the rectified voltage across the load at the same current turns out to be large for the first scheme (Fig. 1, a) than for the second (Fig. 1, b).
The given characteristics also make it possible to judge the real operating voltages at which the capacitors of the circuit operate.
Due to the fact that the ripple frequency with full-wave rectification is twice as high as with half-wave rectification, for the first circuit (Fig. 1, a), further filtration of the rectified voltage is greatly facilitated, and in addition, the ripple coefficient showing how much of the rectified voltage at the output rectifier is the amplitude of the variable component of this voltage) for the same load and the same values \u200b\u200bof the capacitance of capacitors C1 and C2 turns out to be slightly smaller. So, for example, with a load resistance of 2000 Ohms and a capacitance of capacitors C1 and C2 of 48 μF each, the ripple factor for the first circuit was 6.5%, and for the second - 7.6% (despite the fact that in the first circuit the total capacitance at the output rectifier is two times less than in the second).
It should also be noted that the operating voltages across the capacitors in the first circuit are the same and equal to half the rectified voltage, that is, they do not exceed 150 V (unless the rectifier is operating without load), while in the second circuit, only capacitor C1 a the capacitor C2 is at full rectified voltage and therefore must be rated for an operating voltage of at least 300 V.
When the rectifiers operate with voltage doubling without load, i.e., idle, the rectified voltage is approximately equal to the doubled peak value of the supply mains voltage, and therefore can exceed 350 V (if the effective mains voltage is 127 V). This increase in voltage can lead to breakdown of capacitors, selenium washers, or insulation between the filament and cathode in the kenotrons. Therefore, if, according to technical conditions, the rectifier must operate without load or at a very high-resistance load, then the parts used in it must be designed for the appropriate operating voltage. The latter condition also applies to the diagrams in the following sections of the brochure.
Some advantage of a half-wave circuit is the ability to very easily switch it to power from a 220 V network. To make such a switch, it is necessary to connect in series the rectifier elements B1 and B2 and short-circuit the capacitor C1. In this case, the rectifier will operate according to the half-wave rectification circuit without voltage doubling. In this case, the load characteristics of the rectifier will hardly change.
The scope of the rectifier circuits described above is the power supply of 4 ... 5 tube receivers (with an output power of no more than 2-3 W), low-power low-frequency amplifiers and small-lamp measuring equipment.
In all these cases, it is most convenient to use the 30Ts6S kenotron as a rectifier element, the filament of which is connected in series with the filaments of other lamps of the apparatus. A rectifier with this kenotron and capacitors C1 and C2 with a capacity of 20-40 microfarads gives a voltage of 200-220 V at a current of about 70 mA. Using selenium columns assembled from washers with a diameter of 35 or 45 mm, and capacitors of a larger capacity instead of the 30Ts6S kenotron, it is possible to slightly increase the rectified voltage and obtain a current twice (for washers with a diameter of 35 mm) and three times (for washers with a diameter of 45 mm) more. Rectifiers in this case can power more powerful receivers (up to 4 W of output power), low-frequency amplifiers, small-tube TVs, etc.
FIG. 3. Schematic diagram of a rectifier with voltage tripling.
FIG. 4. Load characteristics of the rectifier with voltage tripling (taken with the supply voltage equal to 120 V).
Voltage tripling circuit. The rectifier circuit with voltage tripling is shown in Fig. 3. It is a combination of two half-wave rectifier circuits: voltage doubling circuit and non-multiplying circuit. Both circuits are connected to the supply network in parallel, and their outputs (rectified voltages) are connected in series with each other. Thus, the voltage at the output of the rectifier, equal to the sum of the rectified voltages (twice the mains voltage on the capacitor C2 and the single one on the capacitor C3), turns out to be approximately equal to three times the mains voltage.
The load characteristics of the rectifier shown in Fig. 4 show that at a current of about 200 mA, such a rectifier can deliver a voltage in excess of 300 V. The characteristics were taken with a resistance R \u003d 10 Ohm from a rectifier in which (as rectifier elements B1, B2 and B3, identical selenium columns were used, each assembled in 13 washers with a diameter of 45 mm.
The supply voltage was maintained at 120 V, and the capacitances of capacitors C1, C2 and C3 varied from 32 to 100 μF.
The nature of the ripple of the rectified voltage of this circuit with equal values \u200b\u200bof the capacitance of all three capacitors is the same as in the full-wave rectification circuit, and the ripple factor with a rectifier load with a resistance of 2000 ohms and capacitance of 50 microfarads is about 7%. Operating voltages on capacitors C1 and C3 do not exceed 150 V, and on capacitor C2 - 300 V.
It should be borne in mind that in a circuit with a tripling of voltage in the absence of a load and a supply voltage of 120-127 V, the rectified voltage exceeds 500 V.
The data above shows that a triple voltage rectifier can be used even more widely than a double voltage rectifier. The choice of rectifier elements for such a rectifier will be discussed below.
Quadruple voltage circuits. The rectifier circuit with voltage quadrupling can be of two types: symmetrical and asymmetrical.
The symmetrical circuit shown in FIG. 5 is a combination of two half-wave rectifier circuits with doubling, operating in different half-periods of the supply voltage. The operation of this circuit is as follows - During a half-cycle of one sign, capacitors C1 and C4 are charged, and the voltage on capacitor C1 reaches approximately single, and on capacitor C4 - twice the effective value of the mains voltage (capacitor C4 is charged using the existing charge on capacitor C2). During a half-cycle of the opposite sign, capacitors C2 and C3 are charged in the same way. The rectified voltage is removed from the corresponding poles of the capacitors C3 and C4, connected in series with each other. Thus, it is doubled a second time.
FIG. 5. Symmetrical rectifier circuit with voltage quadrupling.
The voltage to which the capacitors C1 and C2 are charged turns out to be the greater, the greater the load resistance, or, in other words, the less power supplied by the rectifier. The charging voltage reaches its maximum value in case of disconnection from the load rectifier, becoming equal to the peak value of the mains voltage (1.41 times the effective value) on the capacitors C1 and C2 and double the amplitude value (2.82 times the effective value) - on the capacitors C3 and C4.
FIG. 6. Load characteristics of the rectifier with a quadrupling of voltage (taken at a supply voltage of 120 V).
In order to be able to quickly determine the required capacities of the capacitors C1, C2, C3 and C4, in Fig. 6 shows the load characteristics taken from the rectifier at various values \u200b\u200bof these capacities (in all cases C1 \u003d C2 and C3 \u003d C4). The above characteristics show that even with capacitors C1 and C2 with a capacity of 60 microfarads and C3 and C4 - 16 microfarads each, the voltage at the output of the rectifier at a current of 150 mA reaches 400 V.
Capacitors C1 and C2 must be rated for an operating voltage of no less than 150 V, and C3 and C4 - no less than 250 V.
The ripple factor of the rectified voltage in the case of a 3000 Ohm rectifier load is approximately 6%, and the voltage waveform across the load is the same as in full-wave rectification.
It should be borne in mind that in symmetrical voltage multiplying rectifier circuits, the chassis is at a relatively high potential with respect to ground and the supply source.
FIG. 7. Asymmetrical rectifier circuit with voltage quadrupling.
An unbalanced quadruple voltage rectifier circuit is shown in FIG. 7. It works according to a slightly different principle than the previous one. Here, in a half-cycle of the corresponding sign, capacitor C1 is charged through the rectifier element B1 and resistance R, approximately to the mains voltage. In the next half-cycle, capacitor C3 is charged through the rectifier element B2 and resistance R, using the charge on the capacitor C1, to approximately double the mains voltage. To the same voltage, the capacitor C2 is charged in the next half-cycle through the rectifier element B3. At the same time, capacitor C1 is charged again. Then the charge of the capacitor C2 through the rectifier element B4 charges the capacitor C4. The rectified voltage is removed from the series-connected capacitors C3 and C4. The whole circuit works on the principle of half-wave rectification.
FIG. 8. Load characteristics of an asymmetric quadrupler rectifier (taken at a supply voltage equal to 120 V).
The load characteristics taken from the rectifier (Fig. 8) have a significant slope. This shows the impossibility of using such schemes for high-power radio devices. The operating voltage is distributed across the capacitors in a very peculiar way, and the nature of the distribution depends on the magnitude of the load. Table 1 shows the operating voltages across the capacitors at two different loads and no load.
Table 1
| The capacitors in the diagram of FIG. 7 | Capacity, microfarad | Operating voltage at a load of 2000 Ohm, v | Operating voltage at a load of 7500 Ohm, in | No-load voltage, V |
| C1 | 60 | 100 | 125 | 170 |
| C2 | 48 | 125 | 220 | 340 |
| C3 | 48 | 175 | 240 | 340 |
| C4 | 48 | 100 | 105 | 340 |
Note. Supply voltage 120 V.
Such an uneven distribution of voltage is accompanied by a very uneven form of ripple, and therefore the ripple coefficient at the output of the rectifier is about 10% with a load resistance of 5000 Ohm, and with a load resistance of 1700 Ohm it rises to 23%. As a result, an asymmetric rectifier circuit with a quadruple voltage can be used only at high load resistances or, in other words, at low consumed currents.
Rectifiers assembled according to a symmetrical quadruple scheme, in which selenium rectifier elements are used, can be widely used to power various radio engineering devices that require sufficiently high voltages at currents of 150-200 mA.
Circuits with multiple voltage multiplication. The principle of voltage quadrupling rectified above is valid for any even multiplication factor. For each subsequent increase in rectified voltage by twice the mains voltage, the rectifier circuit needs to be supplemented with only two rectifier elements and two capacitors, as shown in FIG. nine.
The diagram shown in Fig. 9, works well only at a very low current consumption, but it can produce a very high rectified voltage. It is convenient to use it in televisions to power the anode of a kinescope, etc. As rectifier elements, selenium washers of the smallest diameter, collected in columns so that the permissible reverse voltage is equal to the double amplitude of the voltage given by the source of alternating voltage, can be used here. All capacitors of the circuit must be designed for the same operating voltage, except for (capacitor C1, which is under a single amplitude voltage of the source. Since the circuit is designed for low operating currents,
FIG. 9. Asymmetrical rectifier circuit with multiple voltage multiplication.
capacitances of capacitors can be small, ranging from 0.25 to 0.5 μF. Due to the high resistance of the load, the ripple coefficient at the output of the rectifier is negligible even with such small values \u200b\u200bof the capacitance of the capacitors. The total voltage supplied by the rectifier is calculated for the unloaded rectifier by multiplying the ac voltage amplitude by the number of pairs of circuit elements. A capacitor and a rectifier element are taken as one pair of elements.
FIG. 10 shows a symmetrical voltage multiple multiplication circuit having, in comparison with the circuit
FIG. 10. Symmetrical rectifier circuit with multiple voltage multiplication.
fig. 9 offers the same advantages as a quadruple voltage balanced versus single-ended. This circuit can be recommended for rectifiers feeding the output stages of amateur shortwave transmitters and devices requiring high voltages and relatively high currents. In this case, of course, the rectifier elements and rectifier capacitors must be appropriately selected.
For the above rectifier circuits, the nature of the load characteristics is determined by the capacitances of the capacitors used. The larger these capacities, the lower the slope of the characteristic, and therefore, the greater the voltage at a given load.
For the case of operation of the rectifier without load, there are certain minimum values \u200b\u200b\\ u200b \\ u200bof the capacitances of the capacitors, if they are underestimated, the circuits with voltage multiplication stop working. In those cases when it is necessary to obtain a current of several tens or hundreds, milliamperes from the rectifier, capacitors should be taken with the largest possible capacity. This also contributes to improved filtering of the rectified voltage. In addition, by selecting the capacitances of the capacitors, it is possible to effectively set the anode voltage required for the power supply mode.
In industrial and amateur TV sets, the voltage multiplication circuit shown in Fig. 11. This circuit differs from the ones given earlier by the presence of additional resistances and capacities. It works as follows. During the positive half-cycle of the supply voltage through the rectifier element B1, capacitor C1 is charged to the amplitude value of the voltage, and during the negative half-cycle, capacitor C2 is charged through the resistance R1.
FIG. 11. The circuit for multiplying voltage with resistances.
In the next positive half-cycle, the voltage across the capacitor C2 is added to the supply voltage, and this capacitor is discharged through the rectifier element B2 to the series-connected capacitors C1 and C3, from the ends of which the resulting doubled rectified voltage is supplied to the load. Building up the links in the scheme as shown by the dotted line in FIG. 11, voltage multiplication of any magnitude can be obtained.
The advantages of such a circuit are to facilitate the operating conditions of the rectifier elements and capacities, since the reverse voltage on each rectifier element does not exceed double, and on each capacitor - a single amplitude voltage, we supply it from the rectifier. Resistances R1, R2, etc. allow, in the case of using selenium columns, to have a significant spread of their reverse resistances.
The considered circuit is suitable only for the operation of the rectifier with a large load resistance. Capacitors can have a capacity of the order of 500 ... 1000 nF, and resistances of about 2 ... 4 mOhm. Corresponding selenium columns or kenotrons can be used as rectifier elements, however, to power the filaments of the latter, separate well-insulated windings must be provided on the power transformer.
Many electronics engineers often use power supply circuits based on the voltage multiplication principle. After all, the use of a multiplier can significantly reduce the weight and dimensions of the device. To understand the physics of the operation of such an electronic device, we will consider the main circuitry options for building such structures. They can be conventionally divided into symmetric and asymmetric multipliers. Asymmetrical, in turn, are divided into two types: the first and second kind
All designs usually consist of capacitors and diodes; to obtain values \u200b\u200bover a kilovolt, special high-voltage diodes and non-polar capacitors must be used.
These designs are widely used in laser technology, in various high-voltage structures, for example, in air ionizers,
Single-phase unbalanced multiplication circuits are a series connection of several identical single-ended rectification circuits with a capacitive load.
In the circuit, each subsequent capacity is charged to a higher value. If the EMF of the secondary winding of the transformer is directed from point a to point b, then the first diode opens and the charge C1 goes. This capacitor is charged to U equal to the amplitude on the secondary winding of the transformer U 2m... When the EMF of the secondary winding changes, the charging current of the second capacitor will flow through the circuit: point a, C1, VD2, C2, point b. In this case, the capacitance C2 is charged to UC2 \u200b\u200b\u003d U2m + UC1 \u003d 2U2m, since the secondary winding of the transformer and C1 turned out to be connected consistently and in series. With the next change in the direction of the EMF of the secondary winding, the charge C3 begins along the circuit: point b, C2, VD3, C3 point a of the secondary winding. Capacitor C3 will be charged to the voltage UC3 \u003d U2m + UC2≈ 3U2m and so on. That is, on each subsequent capacitor, the multiplicity corresponds to the formula:
The required value of the multiplied U is removed from one container C n
During the negative half-wave, the capacitance C1 is charged through the open diode VD1 to the amplitude value U. When a positive half-cycle wave comes to the input, the capacitance C2 is charged through the open diode VD2 to the value 2Ua. During the next cycle of the negative half-cycle through the diode VD3 to a value of 2U, the capacitance C3 is charged. And as a result, with the next positive half-wave up to 2U, the capacitor C4 is charged.
It is very clearly seen that the multiplier will be launched in several half-wave periods. The constant output voltage is summed up from the voltages on the series-connected and constantly rechargeable capacitors C2 and C4 and is equal to 4Ua.
The multiplier shown in the upper diagram is of the serial type. There are also parallel ones that require lower capacitor ratings per doubling step.
The most commonly used radio amateurs use serial multipliers. They are more versatile, the voltage across the diodes and capacitors is divided approximately evenly, and a larger number of multiplication steps can be implemented. But parallel constructions also have their advantages. However, their huge disadvantage, like an increase in the voltage across the capacitors with an increase in the number of multiplication steps, limits their use to 20 kV ratings.
The advantages of the parallel circuit, the one in the center of the figure, include the following: only the amplitude voltage comes to the capacitance C1, C3, the load on the diodes is the same, a decent stability of the output voltage is achieved. The second multiplier, the schematic of which is shown below. are distinguished by such characteristics as the ability to produce high power at the output of the structure, ease of assembly with your own hands, the same load distribution between the elements, a large number of conversion steps.
This is a bridge circuit in which diodes VD1 VD2 are connected to two arms of the bridge, and capacitors C1 C2 are connected to the other two arms. The secondary winding is connected to one of the diagonals of the bridge, the load to the other. The doubling circuit can be represented in the form of two half-wave circuits connected in series and operating from one secondary winding. In the first half-cycle, when the potential of point a of the secondary winding is positive relative to b, the valve VD1 will open and the charge C1 begins. The current at this moment goes through the secondary winding, VD1 and C1.
In the second half cycle, C2 is charged. The charging current of this capacitor goes through the secondary winding, C2 and VD2. C1 and C2 in relation to Rn1 (load resistance) are connected in series, and U at the load is equal to the sum of UC1 + UC2. The main advantage of this circuit is the increased ripple frequency compared to the two-phase circuit and the fairly complete use of the transformer.
Increasingly, radio amateurs have become interested in power circuits, which are built on the principle of voltage multiplication. This interest stems from the emergence of high-capacity miniature capacitors on the market and the increased cost of copper wire used to wind transformer coils. An additional advantage of these devices is their small size, which significantly reduces the final dimensions of the designed equipment. What is a voltage multiplier? This device consists of capacitors and diodes connected in a certain way. Basically, it is a converter of low voltage AC voltage to high DC voltage. Why do you need a DC voltage multiplier?
Application area
Such a device has found wide application in television equipment (in sources of anode voltage of picture tubes), medical equipment (when powered by powerful lasers), in measuring equipment (radiation measuring instruments, oscilloscopes). In addition, it is used in night vision devices, in electric shock devices, household and office equipment (photocopiers), etc. The voltage multiplier has gained such popularity due to the ability to generate voltages up to tens or even hundreds of thousands of volts, and this is with small sizes and weight of the device. Another important plus of the mentioned devices is the ease of manufacture.
Types of schemes
The considered devices are divided into symmetrical and unbalanced, multipliers of the first and second kind. A symmetrical voltage multiplier is obtained by connecting two single-ended circuits. One such circuit changes the polarity of the capacitors (electrolytes) and the conductivity of the diodes. The symmetric multiplier has the best performance. One of the main advantages is the doubled value of the ripple frequency of the rectified voltage.
Principle of operation
The photo shows the simplest diagram of a half-wave device. Let's consider how it works. Under the action of a negative half-cycle of the voltage through the open diode D1, capacitor C1 begins to charge to the amplitude value of the applied voltage. At the moment when the period of the positive wave begins, the capacitor C2 is charged (through the diode D2) to double the value of the applied voltage. At the beginning of the next stage of the negative half-cycle, the capacitor C3 is charged - also up to twice the voltage value, and when the half-cycle is changed, the capacitor C4 is also charged to the specified value. The device starts up over several full periods of AC voltage. At the output, a constant physical quantity is obtained, which is the sum of the voltages of successive, constantly charged capacitors C2 and C4. The result is a value four times greater than the input. This is how the voltage multiplier works.
Scheme calculation
When calculating, it is necessary to set the required parameters: output voltage, power, AC input voltage, dimensions. Some restrictions should not be neglected: the input voltage should not exceed 15 kV, its frequency fluctuates within 5-100 kHz, the output value should not exceed 150 kV. In practice, devices with an output power of 50 W are used, although it is realistic to design a voltage multiplier with an output indicator approaching 200 W. The value of the output voltage directly depends on the load current and is determined by the formula:
U out \u003d N * U in - (I (N3 + + 9N2 / 4 + N / 2)) / 12FC, where
I - load current;
N is the number of steps;
F is the frequency of the input voltage;
C - generator capacity.
Thus, if you set the value of the output voltage, current, frequency and number of steps, it is possible to calculate the required
Until recently, voltage multipliers have been underestimated. Many designers look at these circuits in terms of tube technology and thus miss out on some great opportunities. It is well known what a successful solution was the use of voltage triplers and quadruplers in televisions. Fortunately, we don’t have to deal with X-rays in SMPS, but voltage multiplying circuitry can often be useful to further downsize after the obvious limit has been reached with conventional methods using high frequency switching and the removal of 60 Hz transformers. In other cases, voltage multipliers can provide an elegant way to obtain additional output voltage using a single transformer secondary.
Many textbooks elaborate on the disadvantages of voltage multipliers. They are said to have poor voltage stability and are too complex. The statement of these shortcomings is well founded, but it is based on the experience of using tube circuits, which have always worked with sinusoidal voltages with a frequency of 60 Hz. The properties of voltage multipliers are greatly improved when they work with rectangular rather than sinusoidal voltages, and especially when working with high frequencies. With a switching frequency of 1 kHz, and even more so at 20 kHz, the voltage multiplier deserves a re-evaluation of its capabilities. Given that the peak and root-mean-square values \u200b\u200bare equal for square wave, the capacitors in the multiplier circuit have a much longer accumulation time compared to the case of sinusoidal oscillations. This manifests itself in increased voltage stability and improved filtration. It is known that very good stability is possible with sinusoidal voltage, but only due to large capacitors. Some useful voltage multiplier circuits are shown in Fig. 16.4. Two different images of the same circuit in Fig. (A) shows that the way a diagram is drawn can sometimes be misleading.
Although stability is no longer a big issue in voltage multipliers, very good stability is not required in a system where one or more feedback loops take care of the final stabilization of the DC output voltage. In particular, some voltage multipliers perform very well at 50% duty cycle of the inverter. Appropriate voltage multipliers are recommended as an unregulated power supply, usually preceding the feedback loop. Typically, this use is associated with a DC / DC converter. For example, the mains voltage with a frequency of 60 Hz can be rectified and doubled. This DC voltage is then used in a powerful DC-to-DC converter, which can be configured as a switching regulator. Note that this method allows high output voltages to be obtained without a transformer operating at 60 Hz.
The voltage multiplier makes it easy to build a good inverter. An inverter transformer works best with a transformation ratio of about one. Significant deviations from this value, especially with increasing voltage, often lead to the appearance of a sufficiently large leakage inductance in the transformer windings, which causes unstable operation of the inverter. So, those who have experimented with inverters and converters are well aware that the most likely failure in the operation of even a simple circuit are oscillations whose frequency differs from the calculated one. Leakage inductance can easily destroy switching transistors. This problem can be avoided by using a voltage multiplier to use a transformer with a transformation ratio of about unity.
Figure: 16.4. Voltage multiplier circuits. Both circuits in Fig. (A) are electrically identical. Pay attention to the permissible and prohibited grounding options for different circuits - in some cases, the generator and load may not share the same grounding point.
When dealing with sinusoidal voltages, remember that voltage multipliers operate on a peak voltage. Thus, the so-called voltage doubler, operating with an input voltage having an effective value of 100 V, will give an open circuit voltage of 2 x 1.41 x 100 \u003d 282 V at the output. Thus, if the capacitance of the capacitors is large and the load is relatively small, then the result is more like tripling the input effective voltage value. A similar reasoning is true for other multipliers.
If we take equal capacities of all capacitors and the sinusoidal voltage at the input, then the voltage multipliers should have a value (ocr not less than 100, where (0 \u003d 2K /, the operating frequency is expressed in hertz, the capacitance is in farads, and is the effective resistance in ohms corresponding to the low impedance load that can be connected, in which case the output voltage will be at least 90% of the maximum achievable DC voltage and will vary relatively little.For a rectangular voltage, the cocr value can be significantly less than 100.
When choosing a voltage multiplication circuit, attention should be paid to grounding. In fig. 16.4, the generator symbol usually represents the secondary of the transformer. Note that if one of the terminals of the load is to be grounded, then in half-wave circuits it is possible to ground one terminal of the transformer, but in full-wave versions it is not. Full-wave circuits are useful for generating bipolar output sources in which one output is positive with respect to ground and the other negative, and each output has half of the total output voltage.
The circuits shown in Fig. 16.4 (A) are identical and are full-wave voltage doubling rectifiers. The diagram in Fig. B is a half-wave voltage doubling rectifier. Scheme fig. C works as a half-wave tripler. A full-wave quadrupler is shown in Fig. D, and the half-wave quadrupler in Fig. E. Such voltage multipliers are widely used in television flyback power supplies providing high voltage CRTs. They are also used in Geiger counters, lasers, electrostatic separators, etc.
Although full-wave voltage multipliers have better stability and less ripple than half-wave voltage multipliers, in practice the differences become small when high frequency square waves are used. By using large capacitors, voltage stability and ripple can always be improved. In general, at a frequency of 20 kHz and above, the presence of a common ground point for half-wave multipliers has a decisive influence on the choice of a designer.
By connecting a large number of elementary stages, very high DC voltages can be obtained. Although this method is not new, it is actually easier to implement it using semiconductor diodes than with the previous tube rectifiers, which complicated the tasks of isolation and cost due to heating circuits. Two examples of multistage voltage multipliers are shown in Fig. 16.5. They multiply the peak value of the input AC voltage by a factor of eight. In the diagram in fig. 16.5A, on no capacitor does the voltage exceed 2K A distinctive feature of the circuit shown in Fig. 16.5V is the common ground for input and output. However, the rated voltages of the capacitors should gradually rise as they approach the circuit's output. Although at 60 Hz this leads to an increase in size and cost, at high frequencies these disadvantages are less sensitive. The diodes in both circuits must withstand the peak input voltage E, but for reliability, diodes with a nominal voltage at least several times higher than E should be used. These circuits usually use capacitors having the same capacitance. The larger the capacitance of the capacitors, the better the stability and less ripple. However, large capacitors impose increased demands on diodes in terms of maximum current values.
The circuit shown in Fig. 16.6 has proven to be very useful for electronics applications. Note that it operates on a unipolar pulse train. This is a Cockroft-Walton voltage multiplier circuit that is often found in the literature. Although all capacitors can have the same capacitance and the same rated voltage E, it is better to use the following approach:
First, we calculate the capacity of the output capacitor
where / q is the output current in amperes, and / is the duration of the unipolar pulse in microseconds. Let \u003d 40 mA as an example. If you assume that the frequency is 20 kHz, then t is half the reciprocal of 20 kHz, or
The maximum ripple value is taken as the voltage V. A value of 100 mV can be considered reasonable, then
Figure: 16.5. Two options for multi-stage voltage multiplier. (A) In this circuit, no capacitor has a voltage higher than 2E. (B) A feature of this circuit is the common ground point for input and output.
As you approach the input of the circuit, the capacitance of the capacitors gradually increases several times in comparison with the capacitance of the last capacitor C ^. These calculations are simple, but they can be wrong if you don't pay close attention to them. Note the numbers next to the capacitors in the diagram in fig. 16.6. These are the coefficients by which the capacitance C ^ must be multiplied to obtain the actual value of the capacitance. Thus, the capacitance of the capacitor indicated by the number 2 is equal to 2C ^ or in our example 10 μF x 2 \u003d 20 μF. The capacitor has a capacity of 5C ^ or 50 μF. And the first capacitor has a capacitance IIC ^ or PO uF.
Where do these numbers come from? They represent the relative values \u200b\u200bof the currents along the circuit. If there are no numbers shown in fig. 16.6, you can define them using the expression (2 / 1-1). Here n represents the multiplication factor of the input voltage. Obviously, in the multiplier by six, n \u003d 6. You start with the input capacitor and find that 2n- \\ \u003d 11. Then continue along the bottom row of capacitors, getting 2 / 1-3, 2 / 2-5, 2/1 -7, 2 / 2-9 and finally for - (2 / 2-11). Then, following this procedure, start with the first capacitor on the left in the top row. This time, the factors C ^ are: 2 / 2-2, 2 / 2-4, 2 / 2-6, 2 / 2-8 and finally for the right closing capacitor 2 / 2-10.
Figure: 16.6. Voltage multiplier by six, powered by a unipolar pulse source. The meaning of the numbers next to the capacitors is explained in the text.
The fact that the capacitors near the input have a higher capacitance than those closer to the output is due to the transfer of charge, which should naturally be large enough at the input. During one cycle, 2 / 2-1 charge transfers occur. With each of these transfers, there is a natural loss of energy. These energy losses are minimal if the capacitances of the capacitors are calculated as described above.
The first test of any voltage multiplier should be done with a variable autotransformer or some other device that allows the input voltage to be stepped up. Otherwise, the diodes can be destroyed by a surge current. The rigidity of this rule depends on factors such as capacitance, power level, frequency, ESR of the capacitors, and of course the nominal peak current of the diodes. It may be necessary to place a thermistor at the input of the multiplier, or a resistor switched on by a relay. On the other hand, in many cases it is possible to do without protection at all, because diodes operating with high peak currents are quite available. Sometimes, the protection is "invisible", for example, the transformer at the input simply cannot provide a large current surge.
When working with high voltages, the magnitude of the forward voltage drop across the diodes is not significant. At low voltage, the accumulated voltage drop across the diodes can prevent the required output voltage from being achieved and significantly reduce efficiency. voltage multiplier. Make sure the diode reverse recovery time is compatible with the input voltage frequency. Otherwise, the calculated voltage multiplier will mysteriously be missing.