Creating higher voltages, part 2: Voltage multipliers

There are several ways to boost a relatively low voltage by a factor of two or three, or even hundreds—but there are also many risks.

Part 1 of this high-voltage article series discussed configurations that boost the voltage, but they can only go so far. Instead, what are called voltage “multipliers” can push voltages to much-higher values. The design and considerations change dramatically when the output voltage starts going about 100 V and then into the >1000 V range.

Fortunately, many of these higher-voltage systems need only modest current (100 mA or less) which simplifies the problem somewhat. Applications for these truly high voltages include X-ray machines, physics experiments, beam steering, electric-field shaping, lightning simulation, electrostatic tests, and many more – it’s more common than many people realize.

The obvious question is why can’t you just use a transformer with suitable turns ratio to multiple the input voltage by 10× or 1000×, or whatever is needed? In theory, you could. But as the turns ratio and secondary voltage increases, there are issues of additional inefficiency due to transformer losses (perhaps acceptable), and high-voltage breakdown of the wire insulation, among others. This does not mean it can’t be done, but it can be a difficult approach due to practical considerations.

The transformer approach been used commercially in many applications with a configuration called a flyback converter (Reference 1), such as for the cathode-ray tube (CRT) of the mass-market TVs that dominated the video world for over 50 years until the LED/LCD flat panels took over) The flyback design uses a special transformer which is a wound as a combination of a transformer, plus an inductor to store energy.

However, there are various topologies which use passive components to multiply an input voltage. In general, they rely on a combination of diodes and capacitors in a series of stages to leverage the voltage of the previous stage. These voltage multipliers are specialized rectifier circuits which develop an output voltage which is (in theory) an integer times the AC peak input, for example, ×2, ×3, or ×4 times the AC peak input value. They are usually configured as a stack of half or full-wave rectifiers.

For example, the circuit of Figure 1 shows a three-stage multiplier. The action is as follows: each cycle of the AC input charges the various capacitors, and each capacitor is eventually charged by successive AC cycles to the peak value of the input sine wave. Thus, for a 120 VAC rms line voltage, the peak is 170 V. By doing this across the multiple capacitors – each charging independently – and then allowing these capacitors to be stacked in series, the output voltage is the sum of the individual capacitor voltages.


Figure 1 In the basic voltage-multiplier configuration, a ladder of capacitor-diodes arranged as rectifiers is used with the capacitor charging from the AC line and then adding their voltages in series at the output. (Image source: Spellman High Voltage Electronics Corporation)

There is no tangible or visible switching in this arrangement, yet it goes from charging to stacking of stored voltage in a standard rectifier arrangement. The diodes act as passive switches and prevent discharge of the capacitors when the input voltage starts to drop. They allow current flow to the capacitors, but not flow from the capacitors back to the input source. And if you need a negative high voltage from a multiplier, that’s only easy thing about high-voltage multipliers: simply reverse the polarities of all the multiplier diodes and capacitors to produce a negative version.

These multiplier configurations seem easy enough to implement but they are not. First, the diodes and capacitors must have a minimum reverse-breakdown voltage rating of at least twice the peak voltage across them. Even if you are starting with a 120 VAC line (170 V peak), you’d need components rated for at least 1700 V if you are building a 10× multiple (plus a factor of ×2 or more for safety margin, of course).

There are also other issues in a high-voltage designs such as these. First, they can deliver very little current at that high voltage: once you begin to draw current for the for the load, the stored charge and thus the voltage drops. In many cases, this is too restrictive, since many higher-voltage applications need only small amount of current, for example, to create an electric field.

Also, the physics of the high-voltage world has many points at which “strange” things happen; increasing the voltage is not a linear situation. Think of it as analogous to water crossing the temperatures where freezing or boiling occurs. For example, high voltage circuits will have corona discharges from sharp points. These are both hazardous in themselves, and they also cause pitting of metal contacts and can punch through insulators.

Even “small things” such solder joints are an issue for corona. You might assume a standard, well-made solder joint would be OK, but it will act as a starting point for corona discharge. Therefore, “ball soldering” is frequently used instead in the high-voltage implementation using large, spherical solder connections (Figure 2). The large-diameter, smooth spherical shape of the solder connection helps to control the local E-field, thus suppressing the generation of corona.


Figure 2 In the high-voltage word, every aspect of the physical construction is critical: a standard solder joint will encourage corona discharge, so the joints must be made as balls without corners. (Image source: Maker Forums)

If you are tempted to build your own high-voltage circuit, think about it very carefully. It’s probably not a good idea, because there is so much more to it than just the schematic diagram and the bill of materials (BOM), and that BOM itself has truly unique parts.

It is a truly dangerous world where just “being careful” is not enough, and even the slightest mistake or oversight has deadly consequences. Physical spacing, component ratings, insulation properties, pinhole imperfections, and material breakdown are just some of the many issues. Circuit protection is also unique: specialty fuses and other overvoltage/overcurrent devices are essential.

Not surprisingly, that doesn’t stop people from doing so, as seen in the posting which shows someone building a multiplier going into the tens of kilovolts (Reference 5).  Don’t try this at home (or even in the lab) unless you have someone with you is experienced in high-voltage construction (and can call for help – although it usually too late at that point!). Finally, there are the complicated regulatory issues if you intend to produce and sell the product.

There are many examples of high-voltage multipliers used in attention-getting projects. For example, a team at MIT recently built and tested an aircraft with a wingspan of a few meters but with no moving parts (Figure 3, details are in References 6 and 7). Instead, it uses the flow of negative ions attracted by high-voltage, positive electrostatic field across the wing rear edge.


Figure 3 High voltages are key to some very unusual applications, such as this MIT aircraft, which is propelled by a stream of ions across the wing; the ions are attracted to the 20-kV “fence” at the trailing edge of the wing. (Image source: MIT)

This non-tethered, free-flying aircraft has an on-board battery (nominally at 200 V DC) for its 600-W power supply (Figure 4, designed by the Power Electronics Research Group in the MIT Research Laboratory of Electronics). It uses a switching regulator followed by a 1:15 step-up transformer, and then a six-stage, full-wave “Cockcroft-Walton” voltage multiplier to deliver 20 kV DC (see References 8 through 10).


Figure 4 Within the MIT aircraft is a DC/DC power subsystem which converts 200 V DC from on-board batteries into an AC waveform, then transforms it to 20 kV DC using the Cockcroft-Walton multiplier topology. (Image source: MIT)

Cockroft-Walton multipliers play a very large role in advanced physics experiments, often used as initial “injectors” for various particle accelerators. The 750-kV unit of Figure 5 was commissioned in 1975 at CERN and was recently retired. Larger units can now reach up to the single-digit megavolt range.


Figure 5 This Cockroft-Walton generator at CERN produces up to 750 kV; it’s definitely not a “desktop” unit, as it is about 12 feet (~3 ½ meters) tall. (Image source: CERN)

The world of high-voltage boosters and multipliers is a strange yet important one. Some of these architectures result in modestly higher voltages, and some reach into kilovolt range. As voltages go beyond about 60 V, issues of safety become a concern. As they go into the hundreds and thousands of volts, additional issues of material characteristics and electrical phenomena show that design and construction for these levels is a very different world.

References:

  1. Wikipedia, “Flyback converter”
  2. Spellman High Voltage Electronics Corporation, “High-Voltage Reference Manual”
  3. All About Circuits, “Voltage Multipliers (Doublers, Triplers, Quadruplers, and More)”
  4. Instructables, “High Voltage Multiplier”
  5. MIT, “MIT engineers fly first-ever plane with no moving parts”
  6. MIT Electric Aircraft Initiative, “MIT engineers fly first-ever plane with no moving parts in propulsion system”
  7. Georgia State University Hyperphysics, “Cockroft-Walton Accelerators”
  8. Wikipedia, “Cockcroft–Walton generator”
  9. Wenzel Associates, TechLib, “Cockcroft-Walton voltage multipliers”
  10. CERN, “The 750kV Cockcroft-Walton generator and Faraday cage”

—Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors and signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing, and he also developed significant mechanical-engineering insight while designing control electronics for large materials-testing systems.

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