Voltage multiplier

A high-voltage cascade, known as Cockcroft -Walton generator, Villard or Siemens - multiplier circuit is an electric circuit which converts a supplied alternating voltage into a high DC voltage of up to several megavolts. It is one of the charge pump and is based on the Greinacher circuit, which is a voltage doubler in the basic form. The high voltage by cascading, i.e. achieved multiple series connection of the Greinacher circuit.

John Cockcroft and Ernest Walton developed in the early 1930s on the basis of such a cascade later named after them particle type and were able to demonstrate for the first time triggered by artificially accelerated particles nuclear reaction.

General

The cascade provides depending on the number of diodes and capacitors a theoretically as high output voltage ( rule of thumb: output voltage = peak voltage U0 of the transformer multiplied by the number of diodes). In practice, however, a limit is thus set so that the capacitors are connected in series so that with increasing number of capacitors, the capacitance is always smaller. Thus, the output voltage eventually breaks already together with minimum energy consumption. One advantage is that despite the high output voltage of each capacitor need only have a dielectric strength of 2U0.

For the supply of larger high voltage cascade as for the Cockcroft -Walton accelerator testing transformers of equivalent power can be used.

Function and structure

The operation will be explained in a two-stage cascade. The colors symbolize the polarity ( red = positive, blue = negative). The capacitors with odd numbers form the so-called thrust column packed with an even number, the smoothing column.

The voltages are relative to the lower terminal of the transformer, ie, the 0 V is always. At the output of the transformer a peak voltage of 100 V is assumed Us. The following statement is a simplification of the process for a better understanding is:

1 The first (negative ) half- wave charges C1 to 100V. In this case, the upper end of C1 is positive with respect to the bottom, which, therefore, is -100 volts.

2 In the second half cycle the output voltage of the transformer swap them, its upper end has now 100 V. Together with the 100 V of the capacitor now be evident to 200 V at the upper end of C1, that is, the voltage of this point was set to 200 V " pushed ". These 200 V load C2.

3 In the following half-wave of the upper end of C1 is reset to 0 V, therefore, can now be loaded by C3 C2 200V.

4 In the next cycle, the 200 V of C3 are now pushed up to 400 V, so there are 200 V between the upper and lower end of C4 and reload this since the lower end of C4 is already at 200 V to 200 V, appear now at the output 400 V.

In practice, the capacitors are discharged naturally when charging other capacitors, as well as losses occur through the diodes. Thus, after four half- waves far from the full output voltage is reached.

Often, the diodes are also drawn diagonally and installed as well.

High voltage cascade use stages, each pass on only the peak values ​​of the positive half-wave upward. Therefore, the frequency of the ripple of the DC voltage is equal to the AC supply voltage. However, the AC power source must supply current during both the half waves.

High voltage cascade also work with very asymmetrical, rectangular AC voltages. One example is the cascade used in televisions for generating the anode voltage of the picture tube (about 27 kV). Here one takes advantage of the very high voltage pulse of the line transformer, which occurs during the line flyback, in order to reduce the magnetic field of the horizontal deflection. Thus, the cascade fed with relatively few coil turns. In newer television sets with a cathode ray tube, there are somewhat modified high-voltage cascade: The high-voltage winding of the flyback transformer used to generate the accelerating voltage is divided into several part windings, each of which supplies a single rectifier circuit. These individual DC voltage sources are connected in series together with the transformer in a potted enclosure. The entire component is called diode split transformer (DST). The advantage of DST compared to a conventional cascade consists in the lower intrinsic capacitances within the sub- windings, the short-circuit resistance, and in a lesser Isolierstoffbelastung and size of the coil. However, the process continues to the high voltage side increasingly better against the ferrite core isolated partial windings advance. This is achieved by a potting with a synthetic resin under vacuum.

More problematic are cascades with air insulation. Here, the arrangement of the components depends on the clearances and creepage distances between the terminals. Frequently discoidal capacitors are stacked and the diodes are in a zigzag shape in between. This design can also be designed spatially (3 thrust columns) for operation with a three-phase transformer.

If two switches at the high voltage end of the cascade in parallel, which are supplied respectively with one another by 180 ° rotated phase position of two windings will result in a smaller ripple of double mains frequency.

Air-insulated cascade require from about 40 kV with rounded edges in the area of ​​upper voltage level and from about 100 kV, further measures for field control, such as rounded hollow body at the high- voltage end.

If instead of high DC voltages short, high voltage pulses needed to be so-called shock generators such as the Marx generator with applications in the field of electromagnetic compatibility, used.

Application

High voltage cascade are used wherever very high DC voltages are required at relatively low power:

• In high voltage laboratory and test facilities for insulation measurement and ensure the insulation resistance of high-voltage components which are used in the electrical industry, such as for voltage testing of power transformers, high-voltage switches or isolators.
• For picture tubes in older TVs and monitors to produce the anode voltage in combination with the line transformer. Prior to the replacement of cathode ray tubes, the cascade connection of the line transformers was replaced by diode -split transformers ( DST), which has a lower tendency to predischarges.
• High voltage for electrostatic paint spraying / painting ( 30 - 40 kV) and for electrostatic dust filter, and ionizers.
• High voltage for operation of Pockels cells, laser printers, secondary electron multipliers and image converter tubes
• Cockcroft -Walton accelerator, electron guns, electron beam sources
• X-ray tubes in different fields of medicine, material analysis (XRF - X-ray fluorescence analysis), X-ray Investigation of components such as Welds and other technical applications.