Voltage multiplier

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Villard cascade voltage multiplier.
Villard cascade voltage multiplier.

A voltage multiplier is an electrical circuit that converts AC electrical power from a lower voltage to a higher DC voltage by means of capacitors and diodes combined into a network.

Voltage multipliers can be used to generate bias voltages of a few volts or tens of volts or millions of volts for purposes such as high-energy physics experiments and lightning safety testing.

The most common type of voltage multiplier is the half-wave series multiplier, also called the Villard cascade. Such a circuit is shown opposite.

Assuming that the peak voltage of the AC source is +Us we can describe the (simplified) working of the cascade as follows:

  1. negative peak (−Us): The C1 capacitor is charged through diode D1 to 0V (potential difference between left and right plate of the capacitor is Us)
  2. positive peak (+Us): the potential of C1 adds with that of the source, thus charging C2 to 2Us through D2
  3. negative peak: potential of C1 drops to 0V thus allowing C3 to be charged through D3 to 2Us.
  4. positive peak: potential of C1 rises to 2Us (analogously to step 2), also charging C4 to 2Us. The output voltage (the sum of voltages under C2 and C4) raises till 4Us.

In reality more cycles are required for C4 to reach the full voltage. Adding more segments analogous to C1-D1-D2-C2, we can increase output voltage by 2Us.

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[edit] Breakdown Voltage

While the multiplier can be used to produce thousands of volts of output, the individual components do not need to be rated to withstand the entire voltage range. Each component only needs to be concerned with the relative voltage differences directly across its own terminals and of the components immediately adjacent to it.

Typically a voltage multiplier will be physically arranged like a ladder, so that the progressively increasing voltage potential is not given the opportunity to arc across to the much lower potential sections of the circuit.

Note that some safety margin is needed across the relative range of voltage differences in the multiplier, so that the ladder can survive the shorted failure of at least one diode or capacitor component. Otherwise a single-point shorting failure could successively over-voltage and destroy each next component in the multiplier, potentially destroying the entire multiplier chain.

[edit] Alternative diagrams

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Please improve this article if you can. (October 2006)

Alternatives:

T
The images in this article show the diodes at right angles to the capacitors, stressing that stray capacity between the columns should be minimized.
W
The diodes are arranged diagonally, this stresses that the diodes hold DC-voltage.
Z
Sometimes the capacitors of the two columns are drawn next to each other (no shift), but this makes less sense, as the capacitors are on different potential.
X
Two cascades can be driven by a single center-tapped transformer to get full-wave rectification leading to less ripple.
Stack
A second cascade can be stacked onto the first one driven by a high voltage isolated second secondary winding connected with 180° phase shift to get full wave rectification.

A single secondary winding of a transformer can drive to cascades of different polarity at the same time. Two of these can in turn be stacked. The goal is to reduce the ripple and the capacity at the same time. To further reduce ripple, an even number of stages is used and the connecting column gets bigger capacitors.

[edit] Applications

The high-voltage supplies for cathode ray tubes often use voltage multipliers with the final-stage smoothing capacitor formed by the interior and exterior aquadag coatings on the CRT itself.

A common type of voltage multiplier used in high-energy physics is the Cockcroft-Walton generator (which was designed by John Douglas Cockcroft and Ernest Thomas Sinton Walton for a particle accelerator, for use in research that won them the Nobel Prize in Physics in 1951).

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