Doubling the diode voltage. Voltage multiplier circuit

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:

Voltage multipliers- these are special circuits that convert the voltage level towards an increase. Such circuits usually combine two functions: rectification and voltage multiplication. The use of multipliers is most justified in cases where the presence of an additional step-up transformer is undesirable (a step-up transformer is a rather complex element, especially at high voltage frequencies, and is large) or cannot provide the required voltage level (at high voltages there is a high probability of breakdown between the turns of the secondary winding of the transformer) .

Multiplier circuits are typically built using the properties of a single-phase, half-wave rectifier driving a capacitive load. During its operation, this rectifier can create a voltage between certain points, the value of which is greater than the value of the input voltage. If we consider the analysis of the operation of a single-phase, half-wave rectifier with a capacitive load given in the previous section, we can understand that the named “certain points” are the terminals of the rectifier diode. If another single-phase half-wave rectifier is connected to these points, the circuit shown in Fig. will be obtained. 3.4-16 (so-called single-ended voltage doubler).

Rice. 3.4-16. Circuit diagram of an asymmetrical voltage doubler (a) and timing diagrams explaining its operation (b)

Another voltage doubler circuit, composed of two single-phase half-wave rectifiers with a capacitive filter, is shown in Fig. 3.4-17. They call her symmetrical voltage doubler(or Latour's scheme). The rectifiers included in the circuit are connected in parallel at the input and in series at the output.

Rice. 3.4-17. Symmetrical voltage doubler (Latour circuit)

With a positive half-wave of the input voltage, the rectifier on diode VD1 operates, charging capacitor C1, and with a negative half-wave, the rectifier on diode VD2 operates, charging capacitor C2. As a result, both C1 and C2 are charged to the input voltage level, and when they are connected in series, the total voltage is equal to twice the input voltage.

The main advantage of the Latour circuit over an asymmetrical voltage doubler (Fig. 3.4-16) is that the operating voltage of both capacitors is \(U_(in max)\).

The multiplication coefficient of such circuits can be increased by increasing the number of multiplication units. In Fig. 3.4-18 shows a circuit of an asymmetrical multiplier with the number of links of the “two diodes - two capacitors” type equal to \(n\).

Rice. 3.4-18. Circuit of an asymmetrical n-link voltage multiplier

When there is no load, the voltage \(U_(out1) = 2nU_(in max)\) or \(U_(out2) = (2n‑1)U_(in max)\) is generated at the output of this circuit. When a load is connected, the capacitors will periodically discharge and charge. As a result, the voltage at the output of the circuit will be slightly lower and will not remain constant. In general, the following relation is observed:

\(U_(out1) = 2 n U_(in max) - \cfrac(I_н)(fC) \left(\cfrac(2)(3) n^3 + \cfrac(1)(4) n^2 - \cfrac(1)(6) n \right) \),

where \(f\) is the frequency of the input voltage.

The above formula is also true for the asymmetrical voltage doubler circuit described above.

\(U_(out) = 2nU_(in max) - \cfrac(I_н)(fC) \left(\cfrac(1)(6) n^3 + \cfrac(1)(4) n^2 + \cfrac (1)(3)n\right)\)

Rice. 3.4-19. Symmetrical n-bar voltage multiplier circuit

It can be noted that at small values of n, the output voltage increases almost proportionally to the number of stages. As n increases, this growth slows down and then stops altogether. Obviously, it makes no sense to make multipliers with a number of stages greater than that at which the maximum multiplication is achieved. This limiting value of n for a symmetrical multiplier circuit can be found using the formula:

\(n_max = 2 \sqrt(\cfrac(fCU_(in max))(I_n)) \)

All other things being equal, for an asymmetrical multiplier circuit the maximum number of stages will be half as large. To increase the efficiency of voltage multipliers, it is advisable to increase the frequency of the supply voltage and the capacitance of the capacitors used in the multiplier. In the considered circuits, during operation, all diodes are subject to reverse voltage \(U_(rev max) = 2U_(in max)\).

Using the principles described above, it is possible to construct a large number of different voltage multiplying circuits. Several examples of such schemes are shown in Fig. 3.4‑20...3.4-23, and in Fig. 3.4-24 shows a diagram of a low-power DC-DC converter using a diode multiplier.

Rice. 3.4-20. Multiplication by three schemes

Rice. 3.4-21. Multiplication by four schemes

Rice. 3.4-22. Multiplication by six schemes

Application area

Types of circuits

Principle of operation

Circuit calculation

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

After miniature capacitors with large capacitance appeared on the modern electronics market, it became possible to use voltage multiplication techniques in electronic circuits. For these purposes, a special device has been developed - a voltage multiplier, the basis of which is diodes and capacitors connected in a certain order. The essence of the operation of this device is to convert alternating voltage received from a low-voltage source into high direct current voltage.

Thanks to the small dimensions of these devices, the final dimensions of the designed electronic devices have also been significantly reduced. There are various options for these devices, including a Schenkel voltage multiplier and other circuits designed for specific equipment.

General information about voltage multipliers

In electronics, voltage multipliers are special circuits that convert the incoming voltage level upward. At the same time, these devices also perform a straightening function. Multipliers are used in cases where it is undesirable to use an additional step-up transformer in the overall circuit due to the complexity of its design and large size.

In some cases, transformers cannot raise the voltage to the required level because a breakdown may occur between the turns of the secondary winding. These features should be taken into account when solving the problem of how to make various options for doublers with your own hands.

Multiplier circuits typically use the properties and characteristics of single-phase, half-wave rectifiers driving a capacitive load. During the operation of these devices, a voltage is created between certain points with a value exceeding the value of the input voltage. The terminals of the diode included in the circuit act as such points. When you connect another identical rectifier to them, you get an asymmetrical voltage doubler circuit.

Thus, each voltage multiplier as a boosting device can be symmetrical or asymmetrical. In addition, they are all divided into categories of the first and second kind. A symmetrical multiplier circuit consists of two asymmetrical circuits connected to each other. One of them changes the polarity of the capacitors and the conductivity of the diodes. Symmetrical multipliers have better electrical characteristics; in particular, the rectified voltage has double the ripple frequency.

Various types of such devices are widely used in electronic equipment and equipment. With the help of these devices, it became possible to carry out multiplication and obtain voltages of tens and hundreds of thousands of volts. The voltage multipliers themselves are lightweight, small in size, and easy to manufacture and further operate.

Principle of operation

In order to imagine how a voltage multiplier works, we consider the simplest device shown in the figure. When the negative half-cycle of the voltage begins to operate, diode D1 opens and capacitor C1 is charged through it. The charge must be equal to the amplitude value of the applied voltage.

When a period with a positive wave occurs, the next capacitor C2 is charged through diode D2. In this case, the charge acquires a high double value compared to the applied voltage.

Next comes a negative half-cycle, during which capacitor C3 is charged to double the value. In the same way, during a further half-cycle change, capacitor C4 is charged, again with double the value.

In order to start the device, complete voltage periods of several cycles are required, creating voltages on the diodes. The voltage value obtained at the output consists of the sum of the voltages of capacitors C2 and C4, connected in series and constantly charged. Ultimately, an output alternating voltage is generated that is 4 times higher than the input voltage. This is the principle of operation of a voltage multiplier.

The very first capacitor C1, fully charged, has a constant voltage value. That is, it performs the function of a constant component Ua used in calculations. Consequently, it is possible to further increase the potential of the multiplier by connecting additional links made according to the same principle, since the voltage on the diodes in each of these links will be equal to the sum of the input voltage and the constant component. Due to this, any multiplication coefficient with the required value is obtained. The voltage on all capacitors except the first one will be equal to 2x Ua.

If the multiplier uses an odd factor, capacitors located at the top of the circuit are used to connect the load. If it is even, on the contrary, the lower capacitors are used.

Approximate calculation of the multiplier circuit

Before starting the calculation, the main characteristics of the device are set. This is especially important when you need to make a voltage multiplier yourself. First of all, these are the input and output voltage values, power and overall dimensions. Some restrictions regarding voltage parameters should also be taken into account. Its input value should be no more than 15 kV, the frequency range ranges from 5 to 100 kHz.

The recommended value of the output high voltage voltage is not higher than 150 kV. The output power of the voltage multiplier is within 50 W, although it is possible to create a device with higher parameters, in which the power reaches even 200 W.

The output voltage is directly related to the current loads and can be calculated using the formula: Uout = N x Uin - (I (N3 + +9N2 /4 + N/2)) / 12FC, in which N corresponds to the number of stages, I - current load, F - input voltage frequency, C - generator capacity. If you set the required parameters in advance, this formula will help you easily calculate what capacity the capacitors used in the circuit should have.