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 (but actually invented by Heinrich Greinacher). Such a circuit is shown opposite.

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

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

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 2U_s.

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.

Other circuit topologies

Two cascades driven by a single center-tapped transformer. This configuration provides full-wave rectification leading to less ripple.

Stacking

A second cascade stacked onto the first one driven by a high voltage isolated second secondary winding. The second winding is connected with 180° phase shift to get full wave rectification. The two windings need to be insulated against the large voltage between them.

A single secondary winding of a transformer driving two cascades of opposite polarities at the same time. Stacking the two cascades provides an output of twice the voltage but with better ripple and capacitor charging characteristics than would be achieved with a single long cascade of the same voltage.

An even number of diode-capacitor cells is used in any column so that the the cascade ends on a smoothing cell. If it were odd and ended on a clamping cell the ripple voltage would be very large. Larger capacitors in the connecting column also reduce ripple but at the expense of charging time and increased diode current.

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).

See also

• Marx generator (a device that uses spark gaps instead of diodes as the switching elements and can deliver higher peak currents than diodes can)
• Cockcroft–Walton generator
• Rectifier
• Charge pump
• Voltage doubler

External links

• Basic multiplier circuits
• Cockcroft Walton multipliers
• Schematic of Kadette brand (International Radio Corp.) model 1019. A 1937 radio with a vacuum tube (25Z5) voltage multiplier rectifier.