voltage multipliers. Scheme, description

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Voltage multipliers

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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The article describes the main variants of voltage multipliers used in a wide variety of electronic devices, and provides the calculated ratios. This material will be of interest to radio amateurs involved in the development of equipment that uses multipliers.

Multipliers are widely used in modern electronic devices. They are used in television and medical equipment (anode voltage sources for kinescopes, power supply for low-power lasers), in measuring equipment (oscilloscopes, devices for measuring the level and doses of radioactive radiation), in night vision devices and electroshock devices, household and office electronic devices (ionizers, "Chizhevsky's chandelier", photocopiers) and many other areas of technology. This happened due to the main properties of multipliers - the ability to generate high, up to several tens and hundreds of thousands of volts, voltage with small dimensions and weight. Another important advantage is the ease of calculation and manufacture.

The voltage multiplier consists of diodes and capacitors connected in a certain way and is a converter of the AC voltage of a low voltage source into a high DC voltage.

The principle of its operation is clear from Fig. 1, which shows a diagram of a half-wave multiplier. Consider the processes taking place in it in stages.

Voltage multipliers

During the action of the negative half-cycle of the voltage, the capacitor C1 is charged through the open diode VD1 to the amplitude value of the applied voltage U. When a positive half-cycle voltage is applied to the input of the multiplier, the capacitor C2 is charged to a voltage of 2Ua through the open diode VD2. During the next stage - the negative half-cycle - the capacitor C3 is charged through the diode VD2 to a voltage of 3U. And finally, at the next positive half-cycle, the capacitor C2 is charged to a voltage of 4U.

It is obvious that the start of the multiplier occurs in several periods of alternating voltage. The constant output voltage is the sum of the voltages on the series-connected and constantly recharged capacitors C2 and C4 and is 4Ua.

Shown in fig. 1 multiplier refers to serial multipliers. There are also parallel voltage multipliers that require less capacitance per multiplication stage. On fig. 2 shows a diagram of such a half-wave multiplier.

Voltage multipliers

The most commonly used serial multipliers. They are more versatile, the voltage across the diodes and capacitors is evenly distributed, and a greater number of multiplication steps can be implemented. Have their own advantages and parallel multipliers. However, their disadvantage, as an increase in the voltage on the capacitors with an increase in the number of multiplication stages, limits their use to an output voltage of about 20 kV.

On fig. Figures 3 and 4 show diagrams of full-wave multipliers. The advantages of the first one (Fig. 3) include the following: only amplitude voltage is applied to capacitors C1, C3, the load on the diodes is uniform, and good stability of the output voltage is achieved. The second multiplier, the circuit of which is shown in Fig. 4. are distinguished by such qualities as the possibility of providing high power, ease of manufacture, uniform distribution of the load between the components, a large number of multiplication steps.

Voltage multipliers

The table shows the typical values of the parameters and the scope of voltage multipliers.

Voltage multipliers

When calculating the multiplier, you should set its main parameters: output voltage, output power, input AC voltage, required dimensions, operating conditions (temperature, humidity).

In addition, some restrictions must be taken into account: the input voltage can be no more than 15 kV, the frequency of the alternating voltage is limited within 5 ... 100 kHz. output voltage - no more than 150 kV, operating temperature range from -55 to +125 * C, and humidity - 0 ... 100%. In practice, multipliers with an output power of up to 50 W are developed and used, although values of 200 W or more are realistically achievable.

The output voltage of the multiplier depends on the load current. Provided that the input voltage and frequency are constant, it is determined by the formula: Uout = N · Nin - [1 (N3 + 9N2/4 + N/2)]/12FC, where I is the load current. A; N is the number of multiplier stages; F is the input voltage frequency. Hz; C is the capacitance of the stage capacitor, f. Setting the output voltage, current. frequency and number of steps, from which the required capacitance of the step capacitor is calculated.

This formula is given to calculate the serial multiplier. In parallel, to obtain the same output current, the required capacitance is less. So, if the capacitance in series is 1000 pF, then a three-stage parallel multiplier will require a capacitance of 1000 pF / 3 = 333 pF. In each subsequent stage of such a multiplier, capacitors with a large rated voltage should be used.

The reverse voltage on the diodes and the operating voltage of the capacitors in the series multiplier is equal to the full swing of the input voltage.

In the practical implementation of the multiplier, special attention should be paid to the choice of its elements, their placement and insulating materials. The design must provide reliable insulation to avoid corona discharge, which reduces the reliability of the multiplier and leads to its failure.

If you want to change the polarity of the output voltage, the polarity of the diodes should be reversed.

Author: D.Sadchenkov, Moscow

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