Multistage HV generators create high voltage by connecting a number of lower voltage sources in series to produce the required voltage. At some voltage level, it becomes impractical to generate the required voltage in one circuit, usually due to component construction and insulation problems, so a multistage approach is the only way to get the voltages required. There are also some applications, linear particle accelerators and photomultiplier dynode bias supplies being two notable ones, where you actually want a series of stepped voltages. The fact that the power supply creates them without needing some sort of external voltage divider (which will potentially dissipate some power) is an advantage. The best known form of a multistage HV generator is the Cockroft-Walton voltage multiplier, which will be discussed in a later section.
In general, multistage generators are built as a stack of basically identical stages. Power is fed in at the bottom (low voltage) end of the stack and transmitted up the stack by some means either mechanical or electrical to power a series of independent power supplies. Economies of scale can be realized because all the stages are identical (or nearly so). Further economies result from the generally lower cost of lower voltage components, even in multiple quantities.
Start with a crude, but effective, system that uses mechanical means to transmit the power up the stack. A single large motor turns an insulating shaft which runs up the stack. At each stage, a generator is connected to the shaft and takes mechanical power from the shaft. The generator powers an appropriate supply which is connected in series with the supplies in the stages above and below. This system would potentially be heavy and the mechanical problems of transmitting significant power up an insulating shaft would need to be addressed. Also, electrical stability might be a problem, although with a sufficiently sophisticated central control and fiber optics it could probably be made to work quite well.
The technique of transmitting power by mechanical means has been used to generate isolated electrical power at high common mode voltages. For instance, the ion supply in the collector of a Van deGraaf accelerator is often powered from a generator driven by the charging belt. This technique has also been used to provide filament power to rectifier tubes in a more conventional Cockroft Walton or Greinacher stack.
The next system to be considered transmits the power between the stages by means of transformers with a high isolation voltage. This system, described by Greinacher in 189x (?), requires that the transformers be large enough to carry the power for all the stages above. So, either you give up having identical stages, or the transformers have a lot of excess capacity in the upper stages. It is a simple generalization to do the same thing using a third winding on the power supply transformer of the stage, essentially sharing the iron core. This system has the advantage that the output voltage of the entire stack can be easily adjusted by changing the input voltage to the first stage. company name here manufactures high voltage power supplies based on this principle.
Later design improvements use air core transformers operating at RF which has the multiple advantages of eliminating the weight of the iron and making the required filter capacitors much smaller, although rectifiers and other parasitic losses become larger at higher frequencies.
The Dynamitron takes this approach to the ultimate, using the accelerator tube itself as one plate of the filter capacitor.
The Cockroft Walton configuration uses a chain of capacitors to transmit the power up the stack.
Analysis of the Cockroft-Walton configuration
optimum number of stages
ripple
Copyright 1997, Jim Lux / staged.htm / Back to HV Home / Back to home page / Mail to Jim
