If it is necessary to obtain constant voltages that are multiples of the alternating supply voltage supplying them, voltage multiplying rectifiers (VM) are used in many areas of radio engineering. They are divided into half-wave and full-wave, series and parallel types.
Half-wave rectifier circuit
Figure 1 shows the circuit of a half-wave rectifier with voltage doubling. The circuit can be used both independently and as a component element of multi-link serial multipliers.
Rice. 1. Diagram of a half-wave rectifier with voltage doubling.
Figure 2 shows a parallel circuit of a full-wave rectifier with voltage doubling (Latour circuit). This UN as a rectifier can be considered as two half-wave, connected (secondary winding of transformer T1 - diode VD1 - capacitors C1, C3; secondary winding of the transformer - diode VD2 capacitors C2, C4) in series. Double the voltage at its output is obtained as a result of the addition of separately rectified voltages of different polarities.
Rice. 2. Parallel circuit of a full-wave rectifier with voltage doubling (Latour circuit).
Series multi-element half-wave rectifier
A serial multi-element 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) is simply a half-wave rectifier consisting of a diode and a capacitor (rectifier and filter), two links - a voltage multiplier in half, three - in three times, etc.
The capacitance values of each link are in most cases the same and depend on the frequency of the supply voltage and current consumption.
Rice. 3. Circuit of a multi-link half-wave voltage multiplier.
It is convenient to consider the physical processes of increasing voltage in a multi-section half-wave (Fig. 3) UN when applying an alternating sinusoidal voltage to it. The UN works as follows.
With a positive half-wave voltage at the lower terminal of the secondary winding T1, current flows through diode VD1, charging 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 current passes through VD2, the potential of the right plate of C2 relative to the common wire increases to double the input voltage, etc. It follows that the more links, the greater the constant voltage (theoretically) can be obtained from the UN.
For a correct understanding of the formation and distribution of potentials that arise on radioelements during the operation of a voltage source, we assume that one input pulse (IP) fully charges capacitor C1 (Fig. 3) to a voltage of +U.
Let's imagine the second positive pulse arising at the upper terminal of T1 and arriving at the left plate C1 according to the diagram in Fig. 3, also in the form of a capacitor (Ci) charged to voltage +U.
Their joint connection (Fig. 4) will take the form of series-connected capacitors. The potential at C1 relative to the common wire will increase to +2U, VD2 will open, and capacitor C2 will charge to +2U.
Rice. 4. Voltage multiplier circuit.
When a pulse of +U appears on the lower terminal of T1 and is summed in the same way with a voltage of +2U on capacitor C2, a voltage of +3U will appear through the opened VD3 to C3, etc.
From the above reasoning, we can conclude that the voltage value relative to the “common” wire (Fig. 3) only on C1 will be equal to the amplitude value of the input voltage, i.e. +U, on all other multiplier capacitors the voltage will increase stepwise in steps of +2U.
However, for the correct selection of the operating voltage of the capacitors used in the UL, what matters is not the voltage on them relative to the “common” wire, but the voltage applied to their own terminals. This voltage only at C1 is +U, and for all others it is equal to +2U, regardless of the multiplication stage.
Now let’s imagine the end of the action time of the VI pulse as the closure of the capacitor C (Fig. 4) with a jumper (S1). Obviously, as a result of the short circuit, the potential on the anode VD2 will drop to +U, and a potential of 2U will be applied to the cathode. Diode VD2 will be closed by reverse voltage 2U-U=U.
From this 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 output voltage of the UN, all diodes are connected in series.
Practical schemes for HF and VHF
Shortwave radio amateurs involved in the independent manufacture of radio equipment are familiar with the problem of making a good power transformer for the output stage of a transmitter or transceiver.
The circuit shown in Fig. 2 will help solve this problem. The advantage of practical implementation is the use of a ready-made power transformer (PT) from a unified tube television (ULT) of the second class, which is not in short supply due to the departure of old equipment, which can be used as a power transformer to power the power amplifier (PA) of a radio station of the 3rd category.
The recommended technical solution allows you to obtain from the ST all the necessary output voltages for the PA without any modifications. The 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 CT is convenient both for obtaining the required anode voltage and filament voltage, because allows the use as an output in the PA of both 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 allow you 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 filament lamps and kinescope) and using a voltage regulator according to the circuit 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 in the control grid, non-polar capacitors of 0.5 µF at 100-200 V can be used as capacitors of such a voltage control unit.
The same windings can also 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=24, PEV-1 00.64 mm) to -300 V, and + is supplied to the anode 300 V, 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. The excitation voltage in this case is supplied to the cathode through a broadband transformer.
If the output stage of the PA is made according to a circuit with a common cathode, then +600 V is supplied to the anode, and +300 V is supplied to the screen grid from the connection points C1, C2, C3, C4 (the -300 V output is connected to the “common” wire RXTX), which allows you to get rid of powerful damping resistors in the screen grid circuit, which uselessly release large thermal power. The control grid is supplied with a negative bias of -55.65 V from the previously mentioned UN.
To reduce the level of supply voltage ripple 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. Winding data for ST and DR2LM are given in.
Lighting engineering
An example of the use of a voltage multiplier by four is the circuit for starterless starting of a daytime running light (LDS), shown in Fig. 5, which consists of two voltage doublers connected in series for direct current and in parallel for alternating current.
Rice. 5. Circuit of a voltage multiplier by four for starterless starting of a daytime running lamp.
The lamp lights up without heating the electrodes. Breakdown of the ionized gap of the “cold” LDS occurs when the ignition voltage of the LDS is reached at the output of the UN. The ignition of the LDS occurs almost instantly.
A lit lamp shunts, with its low input resistance, the high output resistance of the UN, the capacitors of which, due to their small size, cease to function as sources of increased voltage, and the diodes begin to work as ordinary valves.
The 2-winding inductor L1 (or two 1-winding inductors) is used to smooth out the ripples of the rectified voltage. The voltage drop of the supply network is approximately evenly distributed across 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 circuit, LDS with a cylindrical part diameter of 36 mm ignite without any problems; LDS with a diameter of 26 mm ignite worse, since due to the peculiarities of their design, the ignition voltage of even new lamps without filament heating can exceed 1200 V.
A television
It is known that the output horizontal scan transformer (HVT) is one of the stressed nodes in a television set (TV). As the evolution of the development of the circuitry of this unit shows, with the transition from tube TV to color TV, due to an increase in power consumption from a high voltage source (the current consumption of a black-and-white kinescope with a diagonal of 61 cm at 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.
Circuit solutions for obtaining high voltage to power the second anode of the kinescope, which were used in all models of tube TV, took place only in the first modifications of the ULPTST, and then instead of the step-up winding of the TVS (almost equal in the number of turns of the anode), they began to use UN, which in their electrical strength , and therefore reliability significantly exceeded similar parameters of the winding unit.
Rice. 6. Voltage multiplier circuit with tripling, from the Yunost TV.
UN began to be used almost immediately in domestic black-and-white portable TVs. For example, in the TV "Yunost 401" a voltage tripling circuit, shown in Fig. 6, is used.
When implementing practical UN circuits, it matters to which point of the UN circuit (1 or 2, Fig. 3) the “common” wire of the circuit in which it will be used will be connected, i.e. "phasing" UN. This is easy to verify using an oscilloscope.
When carrying out measurements on an unloaded UN (Fig. 3), it is clear 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 voltages in real designs only from even or only from odd multiplication links, this fact should be taken into account when connecting the voltage booster 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 voltage amplifier, in order to obtain a constant voltage from a link in which there is an alternating component, it is necessary (especially with a capacitive load) to include (Fig. 7) another link (diode and capacitor) between the multiplier link and the load.
The diode (VDd) in this case will prevent the AC component from shorting through the load, and the capacitor (Cdf) will act as a filter. Naturally, the capacitor Cdf must have an operating voltage equal to the full DC output voltage.
Rice. 7. Connecting another link to the voltage multiplier.
We should also not forget about the negative impact on the reliability of operation of multi-element ultrasonic voltage leakages, 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 circuit design with multiplication by three is shown in Fig. 6; four - in Fig. 4; for five - in Fig. 8, Fig. 9; by six - in Fig. 10.
Rice. 8. Voltage multiplier circuit with multiplication by four.
Rice. 9. Voltage multiplier circuit with multiplication by five.
Rice. 10. Voltage multiplier circuit with multiplication by six.
This article discusses only part of the UN circuitry that was used previously and is currently used in household appliances and amateur radio design. Some types of UN circuitry, the principles of operation of which are similar to those discussed, were published in.
In the literature and in communication with radio amateurs, one often encounters confusion regarding UN in terms. For example, it is argued that if the voltage is marked 8.5/25-1.2 or 9/27-1.3, then this is a voltage tripler. According to the circuit design, these UN 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 UN, it ensures that at its output an average value of a constant (positive) voltage of 25 kV (with the current consumed by its load being about 1 mA), i.e. e. The marking speaks only about its input and output parameters.
To obtain high voltage in the TV, a pulse voltage is used that occurs in the secondary winding of the fuel assembly during the reverse stroke 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, the voltage drop in the forward direction on the rectifier columns, such as UN rectifiers, is also significant. For example, for a 5GE600AF column, when operating as a single rectifier, the voltage drop in the forward direction is 800 V!
From the above it follows that the UN elements also serve as an integrating circuit for the supply pulse voltage, which reduces the average value of the direct voltage relative to the input voltage (at a load current of 1 mA) to approximately 5 kV per link. It is these factors that are the main ones that influence the value of the output voltage of the UN, and not approximate arithmetic.
Historically, the use of selenium diodes as rectifiers in the first samples of UN for TV was determined by the level of technology achieved at that time, their low cost, as well as their soft electrical characteristics, which made it possible to connect in series an almost unlimited number of diodes.
It is obvious that selenium rectifiers, due to their high internal resistance, can withstand short-term overloads better than silicon rectifiers. As the technology for manufacturing silicon diodes improved, silicon pillars of the KTs106 type began to be used in UN TV.
When repairing TV, even a preliminary assessment of the possible presence of defects in the rectifying elements of the UN avometer is impossible. The physical meaning of this phenomenon is that in order to open one silicon diode, a potential difference of about 0.7 V must be applied to it in the forward direction.
If, for example, instead of a KTs106G pole, we use an equivalent of individual KD105B diodes (iobr = 400 V), 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 for opening them will be 17.5 V , and the avometer allows you to apply only 4.5 V!
The only thing that can be unambiguously stated after measuring the voltmeter with an ohmmeter is that when checking a working voltmeter, the ohmmeter needle should not deviate when measuring the resistance between any of its electrodes.
A simple solution for preliminary testing of the operability of UN elements using a voltmeter method was proposed in. The essence of the proposal is to use for this purpose an additional source (A1) of direct voltage (DC) 200...300 V and an avometer operating in the mode of a DC voltmeter at a limit of 200.300 V. Measurements are made as follows.
The avometer is switched on (Fig. 11) in series with the same pole of the IPN and the tested rectifier column or UN. Verification algorithm.
Rice. 11. Scheme for connecting the avometer to the rectifier column.
If, when measuring a diode in opposite directions, the voltmeter readings:
- differ significantly, then it is serviceable;
- equal to the maximum voltage of the IPN, then it is broken;
- are small, then it is torn off;
- intermediate values indicate 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 values of the voltage drop obtained practically from measurements in the forward and reverse directions of a serviceable pole or diode of the same brand.
For radio amateurs who are engaged in repairing television equipment at the customer’s home, for a preliminary check of the operability of the UN elements using the voltmeter method, it is more convenient (based on the weight and dimensions) to use the circuit shown in Fig. 12 and proposed in, which is powered through current-limiting capacitors from a 220 V network.
Rice. 12. Power circuit with current-limiting capacitors.
The circuit has proven itself well in practice, and in terms of circuit design it is a voltage doubling rectifier. The measurement algorithm is the same. The same circuit can also be used to eliminate certain types of interelectrode short circuits (“lumbago”) in a kinescope.
Quite often they ask whether it is possible to install UN9/27-1.3 instead of UN8.5/25-1.2? One piece of 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 diagrams
UN8.5/25-1.2 (Fig. 8) and UN9/27-1.3 (Fig. 9). From the UN circuits it is clear that, in principle, 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 a TV ULPTsT, proceed as follows: connect the input terminals for pulse voltage and the “V” output to each other; the wire from the fuel assembly is soldered to the corresponding input of UN9/27; the wire with the “ground” sign 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 circuit diagram on the TV) can be disconnected, since its function can be performed by capacitor C1 (Fig. 10), which is installed inside the UN. A high-voltage wire with a “suction cup” and a limiting resistor Rph is connected to the “+” terminal.
The resulting slight improvement in image quality on the TV screen as a result of such a replacement does not at all mean that this is the result of a replacement!
The reason is primarily that in UN9/27-1.3 silicon pillars of the KTs106G type are used as valves, the voltage drop on which in the forward direction (as mentioned earlier) is significantly less than on the pillars of the 5GE600AF type, which are included in 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 visually observed as an increase in brightness!
In addition, in the ULPTsT TV, when installing UN9/27-1.3, it is necessary to replace the standard “suction cup” with a high-voltage resistor 4.7 kOhm Rf installed inside it with a “suction cup” from the 3UCST TV with a 100 kOhm resistor. Rф performs three functions: it is part of a link in the smoothing RC filter for the high-voltage circuit formed by it and the capacitance of the kinescope ac-vadag Ca (Fig. 9, 10), as well as a protective resistor for direct current, limiting its value in the UN circuit in case of random short-term interelectrode breakdowns inside the picture tube (which happens very often and unpredictably in old picture tubes).
It is also a “burning fuse” that protects the fuel assembly in the event of a breakdown of the UN diodes, when the alternating voltage coming from the fuel assembly is practically shorted to the housing through Ca, the value of the reactance of which for lower-frequency currents is quite small.
Therefore, it should be borne in mind that a significantly smaller value of the total internal resistance UN9/27-1.3 with a small value (or absence for one reason or another) of Rf in cases of replacing the UN is undesirable, since it can lead to the emergence of the above malfunctions as a way out of building a TVS, and to the fire of the TV itself.
From inoperable TV units, with a certain skill and care, you can “extract” (if you’re lucky) high-voltage capacitors, which can still serve for urgent repairs of TV modifications of ULPTsTI or UPIMCT or for experiments with other designs.
To do this, first, carefully break the UN housing with a hammer and free the capacitor housings from the compound, and then separate their leads from the mutual connections and the remains of the compound by successive chipping using side cuts. Practical disassembly of three copies of each UN brand 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 compared to UN8/25-1.2 has a larger number of elements, but smaller overall dimensions, capacitors are used (judging by the manufacturing technology and the material from which they are made) of the same type (markings are not applied to the cases), which are structurally made in the form of a three-terminal (16 mm in diameter) assembly (C2, C4 - Fig. 10) of capacitors with a capacity of 1000 pF, and a four-terminal (C1, C3, C5 - Fig. 10) assemblies with a diameter of 18 mm. Moreover, C1 has a capacitance of 2200 pF, and C3, C5 - 1000 pF each. Both assemblies are 40mm long.
Medicine
One of the “exotic” examples of the use of CN in medical equipment is its use in the design of an electroeffluvial 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 air ion generator, a voltage booster with a negative output voltage is used. Due to the fairly large amount of supporting information, recommendations on the design and use of EL are beyond the scope of this article, therefore EL is mentioned for informational purposes only.
Details for the diagrams
Specification for drawings:
- to Fig. 2: C1-C4 - K50-20;
- to Fig. 6: C1-C2 - KVI-2;
- to Fig. 7: C1, C2 - MBGCH; C3-C5 - KSO-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. Starterless starting of fluorescent lamps//E-2000-7.
- Ivanov B. S. Electronics in homemade products. M.: DOSAAF, 1981.
- Kazansky I.V. Power amplifier for HF radio // To help the radio amateur. - Issue 44. - M.: DOSAAF, 1974.
- Kostyuk A. Power amplifier for CB radio station//Radio Amateur. -1998. - No. 4. - P.37.
- Kuzinets L.M. and others. Television receivers and antennas: Reference. - M.: Communication, 1974.
- Polyakov V.T. Radio amateurs about direct conversion technology. - M.: Patriot, 1990.
- Plyats O.M. 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. -P.35.
- Fomenkov A.P. Radio amateur about transistor TVs. - M.: DOSAAF, 1978.
- Shtan A.Yu, Shtan Yu.A. About 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.
Many electronics engineers often use power circuits based on the principle of voltage multiplication. After all, the use of a multiplier can significantly reduce the weight and dimensions of the device. To understand the physics of operation of such an electronic device, let us consider the main circuit design options for constructing such structures. They can be divided into symmetrical and asymmetrical 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 above 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 asymmetrical multiplication circuits are a series connection of several identical single-ended rectification circuits with a capacitive load.
In the circuit, each subsequent capacitance 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 C1 is charged. This capacitor charges up 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, capacitance C2 is charged to UC2 = U2m+UC1 = 2U2m, since the secondary winding of the transformer and C1 are connected in concert and in series. With the next change in the direction of the EMF of the secondary winding, the charge of C3 begins along the circuit: point b, C2, VD3, C3, point a of the secondary winding. Capacitor C3 will be charged to voltage UC3 = 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 taken from one container C n
During the negative half-wave, capacitance C1 is charged through the open diode VD1 to the amplitude value U. When a positive half-cycle wave arrives at the input, capacitance C2 is charged through the open diode VD2 to the value 2Ua. During the next cycle of the negative half-cycle, the SZ capacitance is charged through the diode VD3 to a value of 2U. And as a result, with the next positive half-wave, capacitor C4 is charged to 2U.
It is very clearly seen that the multiplier will be launched over several half-wave periods. The constant output voltage is summed up from the voltages on the series-connected and constantly recharged capacitors C2 and C4 and is equal to 4Ua.
The multiplier shown in the top diagram is of the serial type. There are also parallels that require smaller capacitor values per doubling stage.
Most often, radio amateurs use serial multipliers. They are more universal, the voltage on the diodes and capacitors is divided approximately evenly, and a larger number of multiplication stages can be implemented. But parallel structures also have their advantages. However, their huge disadvantage, such as an increase in voltage on capacitors with an increase in the number of multiplication stages, limits their use to ratings of 20 kV.
The advantages of the parallel circuit, the one in the center of the figure, include the following: only the amplitude voltage comes to the capacitors C1, S3, the load on the diodes is the same, decent stability of the output voltage is achieved. The second multiplier, the diagram of which is shown below. They 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, equal load distribution between the elements, and a large number of conversion stages.
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, and the load to the other. The doubling circuit can be represented as 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, valve VD1 opens and charge C1 begins. The current at this moment flows through the secondary winding, VD1 and C1.
During 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 Rн1 (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 a two-phase circuit and fairly complete use of the transformer.
More and more often, radio amateurs have become interested in power circuits that are built on the principle of voltage multiplication. This interest is associated with the appearance on the market of miniature capacitors with high capacitance and the increasing cost of copper wire, which is used to wind transformer coils. An additional advantage of the mentioned devices is their small dimensions, 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. Essentially, it is a converter of alternating voltage from a low voltage source to high direct voltage. Why do you need a DC voltage multiplier?
Application area
Such a device has found wide application in television equipment (in the anode voltage sources of picture tubes), medical equipment (for powering high-power lasers), and in measuring technology (radiation measuring instruments, oscilloscopes). In addition, it is used in night vision devices, electroshock devices, household and office equipment (photocopiers), etc. The voltage multiplier has gained such popularity due to the ability to generate voltage up to tens and even hundreds of thousands of volts, and this with small dimensions and weight of the device. Another important advantage of the mentioned devices is their ease of manufacture.
Types of circuits
The devices under consideration are divided into symmetrical and asymmetrical, into multipliers of the first and second kind. A symmetrical voltage multiplier is obtained by connecting two asymmetrical circuits. In one such circuit, the polarity of the capacitors (electrolytes) and the conductivity of the diodes change. The symmetrical multiplier has the best characteristics. 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 circuit of a half-wave device. Let's consider the principle of operation. When a negative half-cycle of voltage is applied, capacitor C1 begins to charge through the open diode D1 to the amplitude value of the applied voltage. At the moment when the period of the positive wave begins, capacitor C2 is charged (through diode D2) to twice the applied voltage. At the beginning of the next stage of the negative half-cycle, capacitor C3 is charged - also to twice the voltage value, and when the half-cycle changes, capacitor C4 is also charged to the specified value. The device starts up over several full periods of alternating current voltage. The output is a constant physical quantity, which is the sum of the voltage indicators of successive, constantly charged capacitors C2 and C4. As a result, we obtain a value four times greater than at the input. This is the principle on which a voltage multiplier works.
Circuit calculation
When calculating, it is necessary to set the required parameters: output voltage, power, alternating input voltage, dimensions. Some restrictions should not be neglected: the input voltage should not exceed 15 kV, its frequency ranges from 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 value approaching 200 W. The value of the output voltage directly depends on the load current and is determined by the formula:
U out = N*U in - (I (N3 + +9N2 /4 + N/2)) / 12FC, where
I - load current;
N - number of steps;
F - input voltage frequency;
C is the 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
DEFINITION
Voltage multiplier is a system that is designed to convert the alternating current voltage of a small voltage source into a high voltage direct current.
They are used in radio electronics: medical and television equipment, measuring equipment, household appliances, etc. The voltage multiplier consists of diodes and capacitors, which are connected in a special way. Multipliers are capable of generating voltages up to volts, while having a small mass and size. Multipliers are easy to manufacture and easy to calculate.
Half-wave multiplier
Figure 1 shows the circuit of a half-wave sequential multiplier.
During the negative half-cycle of the voltage, the capacitor is charged through the diode, which is open. The capacitor is charged to the amplitude value of the applied voltage. During the positive half-cycle, the capacitor is charged through the diode to a potential difference. Then, during the negative half-cycle, the capacitor is charged through the diode to a potential difference. During the next positive half-cycle, the capacitor is charged to voltage. In this case, the multiplier is started over several periods of voltage change. The output voltage is constant and it is the sum of the voltages on the capacitors and , which are constantly charging, that is, it is a value equal to .
The reverse voltage on the diodes and the operating voltage of the capacitors in such a multiplier are equal to the full amplitude of the input voltage. When implementing a multiplier in practice, attention should be paid to the insulation of the elements in order to prevent a corona discharge, which can damage the device. If it is necessary to change the polarity of the output voltage, then change the polarity of the diodes when connecting.
Series multipliers are used especially often, since they are universal and have uniform voltage distribution across diodes and capacitors. With their help, you can implement a large number of multiplication stages.
Parallel voltage multipliers are also used. They require a smaller capacitor capacity per multiplication stage. But their disadvantage is considered to be an increase in the voltage on the capacitors with an increase in the number of multiplication stages, which creates a limitation in their use to an output voltage of about 20 kV. In Fig. Figure 2 shows a diagram of a half-wave parallel voltage multiplier.
In order to calculate the multiplier, you need to know the basic parameters: input AC voltage, output voltage and power, required dimensions (or size restrictions), conditions under which the multiplier will operate. It should be taken into account that the input voltage must be less than 15 kV, frequency from 5 to 100 kHz, output voltage less than 150 kV. The temperature range is usually -55. Typically, the multiplier power is up to 50 W, but more than 200 W are also found.
For a series multiplier, if the frequency at the input to the multiplier is constant, then the output voltage is calculated using the formula:
where is the input voltage; - input voltage frequency; N is the number of multiplication stages; C is the capacitance of the stage capacitor; I is the load current.
Examples of problem solving
EXAMPLE 1
| Exercise | What should be the capacitance (C) of the series voltage multiplier stage if it is required to obtain an output voltage of 800 V, at a frequency of 50 Hz, with a current of 10 A, using 4 multiplier stages? |
| Solution | For a series voltage multiplier we will use a calculation formula of the form: |
Until recently, voltage multipliers were underappreciated. Many designers view these circuits from a tube technology perspective and therefore miss some great opportunities. It is well known what a successful solution was the use of voltage triplers and quadruplers in televisions. Fortunately, we do not have to solve X-ray problems in the SMPS, but a voltage multiplier circuit can often be useful for further size reduction once the obvious limit has been reached by conventional methods using high frequency switching and the 60 Hz transformers have been removed. In other cases, voltage multipliers can provide an elegant way to produce additional output voltage using a single transformer secondary.
Many textbooks dwell in detail on the disadvantages of voltage multipliers. They are said to have poor voltage stability and to be too complex. The statement of these shortcomings has a basis, 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 operate with square wave rather than sinusoidal voltages, and especially when operating at high frequencies. At a switching frequency of 1 kHz, and even more so at 20 kHz, the voltage multiplier deserves a reassessment of its capabilities. Considering that for a square oscillation the peak and root mean square values are equal, the capacitors in the multiplier circuit have a much longer charge accumulation time compared to the case of sine wave oscillations. This results 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 the diagram is drawn can sometimes be misleading.
Although stability is no longer a big issue in voltage multipliers, very good stability is not necessary 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 percent inverter duty cycle. Suitable voltage multipliers are recommended as an unregulated power supply, usually preceding the feedback loop stabilization circuit. Typically this use is associated with a DC/DC converter. For example, a 60 Hz mains voltage can be rectified and doubled. This DC voltage is then used in a high-power DC-DC converter, which can be designed as a switching regulator. Note that this method allows for high output voltage without a transformer operating at 60 Hz.
A voltage multiplier makes it easier to create a good inverter. The inverter transformer works best with a transformation ratio of about unity. Significant deviations from this value, especially with increasing voltage, often lead to the appearance of a fairly large leakage inductance in the transformer windings, which causes unstable operation of the inverter. Thus, those who have experimented with inverters and converters know well that the most likely failure in the operation of even a simple circuit is oscillations whose frequency differs from the calculated one. And leakage inductance can easily lead to the destruction of switching transistors. This problem can be avoided by using a voltage multiplier to use a transformer with a transformation ratio of about unity.
Rice. 16.4. Voltage multiplier circuits. Both diagrams in Fig. (A) are electrically identical. Pay attention to the acceptable and prohibited grounding options for various circuits - in some cases, the generator and load may not share the same grounding point.
When we are dealing with sinusoidal voltages, we must remember that voltage multipliers operate on the peak voltage value. Thus, a so-called voltage doubler operating with an input voltage having an effective value of 100 V will produce an output open circuit voltage of 2 x 1.41 x 100 = 282 V. Thus, if the capacitor value is large and the load is relatively light, then the result is more like tripling the input effective voltage value. Similar reasoning is valid for other multipliers.
If we take the capacitance of all capacitors and the sinusoidal voltage at the input equal, then the voltage multipliers must have a value (ocr of at least 100, where (0 = 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 this case, the output voltage will be at least 90% of the maximum achievable DC voltage and will vary relatively little. For a square wave voltage, the cocr value can be significantly less than 100.
When choosing a voltage multiplier circuit, attention should be paid to grounding. In Fig. 16.4, the generator symbol usually represents the secondary winding of a transformer. Note that if one of the load terminals must be grounded, then in half-wave circuits it is possible to ground one terminal of the transformer, but in full-wave circuits it is not possible. Full-wave circuits are useful for producing bipolar output sources in which one output is positive to ground and the other is negative, and each output has half the full output voltage.
The circuits shown in Fig. 16.4(A) are identical and are full-wave rectifiers with voltage doubling. Scheme in Fig. B is a half-wave rectifier with voltage doubling. 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 that provide picture tubes with high voltage. They are also used in Geiger counters, lasers, electrostatic separators, etc.
Although full-wave voltage multipliers have better stability and lower ripple than half-wave voltage multipliers, practically the differences become small when high-frequency square waves are used. Using large capacitors can always improve voltage stability and reduce ripple. In general, at frequencies of 20 kHz and higher, the presence of a common grounding point for half-wave multipliers has a decisive influence on the choice of the designer.
By connecting a large number of elementary stages, very high DC voltages can be obtained. Although this method is not new, actually implementing it using semiconductor diodes has proven easier than with previous tube rectifiers, which were complicated by insulation issues and cost due to filament circuits. Two examples of multistage voltage multipliers are shown in Fig. 16.5. They multiply the amplitude value of the input AC voltage by eight times. In the diagram in Fig. 16.5A, the voltage on no capacitor does not exceed 2K. A distinctive feature of the circuit shown in Fig. 16.5V is the common ground point for input and output. However, the voltage ratings of the capacitors must be gradually increased as they approach the circuit's output. Although at a frequency of 60 Hz this leads to increased 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 voltage rating at least several times higher than E should be used. These circuits typically use capacitors that have the same capacitance. The larger the capacitor capacity, the better the stability and less ripple. However, high-capacity capacitors impose increased requirements on diodes in terms of maximum current values.
The diagram shown in Fig. 16.6 has proven to be very useful for electronics applications. Note that it operates from 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 nominal voltage E, it is better to use the following approach:
First we calculate the capacitance of the output capacitor
where /q is the output current in amperes, and / is the duration of the unipolar pulse in microseconds. Let = 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
Rice. 16.5. Two options for a multistage 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 the input and output.
As you approach the input of the circuit, the capacitance of the capacitors gradually increases several times compared to the capacitance of the last capacitor C^. These calculations are simple, but can be incorrect if you don't pay close attention to them. Mark the numbers next to the capacitors in the circuit in Fig. 16.6. These are the coefficients by which the capacitance C^ must be multiplied in order to obtain the actual value of the capacitance. Thus, the capacitance of the capacitor designated by number 2 is equal to 2C^ or in our example 10 μF x 2 = 20 μF. The capacitor has a capacity of 5C^ or 50 µF. And the first capacitor has a capacitance IIC^ or PO μF.
Where do these numbers come from? They represent the relative values of currents along a circuit. If there are no numbers next to the capacitors shown in Fig. 16.6, You can determine them using the expression (2/1-1). Here n represents the input voltage multiplication factor. Obviously, in a six-fold multiplier, l = 6. You start with the input capacitor and find that 2n-\ = 11. Then continue along the bottom row of capacitors, getting 2/1-3, 2/2-5, 2/1 in sequence -7, 2/2-9 and finally for – (2/2-11). Then, following this procedure, we start with the first capacitor on the left in the top row. This time, the C^ multipliers are: 2/2-2, 2/2-4, 2/2-6, 2/2-8 and finally for the right end capacitor 2/2-10.
Rice. 16.6. Voltage multiplier by six, operating from a source of unipolar pulses. The meaning of the numbers next to the capacitors is explained in the text.
The fact that capacitors near the input have a larger capacity than those closer to the output is due to the transfer of charge, which naturally should be quite large 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 mentioned above.
The first test of any voltage multiplier should be with a variable autotransformer or some other device that allows the input voltage to be gradually increased. Otherwise, the current surge may destroy the diodes. The strictness of this rule depends on factors such as capacitor capacitance, power level, frequency, capacitor ESR and, of course, the peak current rating of the diodes. It may be necessary to place a thermistor or a resistor switched on using a relay at the multiplier input. On the other hand, in many cases you can do without any protection at all, because diodes that handle high peak currents are readily available. Sometimes, the protection is “invisible”, for example, the input transformer 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 voltages, the accumulated voltage drop across the diodes can prevent the required output voltage from being achieved and significantly reduce efficiency. voltage multiplier. Make sure that the reverse recovery time of the diodes is compatible with the frequency of the input voltage. Otherwise, the calculated voltage multiplication factor will be “mysteriously” missing.