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 Voltage Multipliers - Micapliers
What do they do?
A voltage multiplier converts an AC waveform into a DC output which is
N times greater than the peak-to-peak voltage of the input waveform or
2 x N times the peak of the input voltage. N is the number of stages
in the multiplier and is usually between two and ten.
Other terminology used for multipliers include:
• Voltage Doubler = 2 x multiplier
• Voltage Tripler = 3 x multiplier
• Quadrupler, Pentupler, Hextupler = 4, 5, 6 x multipliers
Figure 1: Representation of sine wave and square wave
for electrical and hydraulic models. |
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Figure 2: Showing same format but different frequency
and voltage for Household Current and typical Multiplier Input. |
How Do They Work?
The input AC waveform to a multiplier can be sinusoidal (sine wave), rectangular
(square wave) or indeed almost any shape. Household current available
from a wall socket is sinusoidal and has a frequency of 60 Hertz. This
can be applied to a voltage multiplier to produce higher values of DC
voltage.
Applications for Reynolds Micapliers usually use a much higher frequency,
from 10 kHertz to 100 kHertz and typically at voltages from 500 to 5000V
peak-to-peak. |
Figure 3: Electrical diode and its hydraulic equivalent. |
Another Component - The DIODE
From Figure 3 it should be clear that the diode operates as an electrical
flap valve, permitting free flow in the forward direction when the anode
voltage (pressure) is higher than the cathode. In the opposite direction
the diode cuts-off (valve closes), stopping the flow.
Electrical diodes behave almost identically in every respect to the model
except that they are capable of operating much faster. Many diodes can
easily control the unidirectional flow of current in a circuit which
would otherwise reverse billions of times a second. The diode used in a
typical Micaplier closes in less than 1/10,000,000th of a second when the
voltage across it reverses.
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Figure 4: Initial positive half cycle of input waveform. |
AC + Capacitor + Diode = DC of Sorts
In Figure 4, the first positive half cycle of the input waveform does not
cause any flow into the capacitor (diaphragm), because the diode (flap
valve) “closes” to prevent this.
The pressure (voltage) at the output follows the input and no pressure
difference occurs across the diaphragm (capacitor). The diaphragm is therefore
not stressed. All the voltage is across the diode and the flap valve is
firmly closed.
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Figure 5: Capacitor charge is changed during the first
negative half cycle. |
Conversely, the first half of the negative half cycle produces no voltage
charge at the output because the flap valve (diode) opens, permitting
free flow into the capacitor (diaphragm) as shown in Figure 5. The
input voltage actually appears across the capacitor as indicated by
the stressed condition of the elastic diaphragm. Once the input voltage
has passed beyond its negative peak, the diode (flap valve) is held
closed by the voltage (pressure) stored on the capacitor and no further
flow occurs. |
Figure 6: Subsequent input cycles are reflected at the
output. |
During the second half of the negative half cycle, the input waveform
returns to zero but the output waveform rises to Vpeak. This is due
to the Vpeak charge stored during the first half of the negative half
cycle and “trapped” by the reverse biased diode. The diode
(or flap valve) is always kept shut from this point on, due to the
pressure (voltage) from the stressed diaphragm (charged capacitor).
However, the output waveform is interesting. Notice that it has the same
AC form as the input waveform but is “stood up” on its bottom
peaks such that its voltage varies from 0V to the Vpeak-to-peak input value.
It therefore has an average value of Vpeak. The input waveform, of course,
is still split equally about the zero line with a consequent average value
of zero.
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Figure 7: Complete one stage multiplier. |
Adding More Parts
When the additional diode and capacitor shown in figure 7 are added to
the circuit to form a new output node, there is a marked improvement
in the form of the new output.
Whenever the pressure at the output of the previous Figure (0V to Vpeak-to-peak)
exceeds the pressure stored in the new diaphragm a flow occurs through
the new flap valve into the new diaphragm. However, because of the presence
of the new flap valve, no reverse flow occurs when the original output
pressure goes down again. The new output thus is charged to a relatively
steady DC level at 1 x Vpeak-to-peak (2 x Vpeak). This is a one stage multiplier
and is the basic building block of many multiplier designs.
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Figure 8: Adding the original circuit components a second
time. |
A Logical Leap
The voltages of the previous Figure are reproduced in Figure 8. It will
be noticed that the lower right point is a DC level of Vpeak-to-peak,
while the upper right node is the original AC input waveform shifted
upwards in voltage by Vpeak.
Connecting the components of the very first circuit to these two points
(the added components), the new output node will appear as shown to the
right of Figure 8 - still the same AC input waveform but now stood on 2
x Vpeak to produce a peak voltage of 4 x Vpeak. Completing the second stage,
by adding a further diode and capacitor, produces a DC output of 2 x Vpeak-to-peak
(4 x Vpeak). This is now a two stage multiplier or peak-to-peak doubler
and is shown electrically in Figure 9 on the next page.
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More of the Same…
Figure 9 shows the completed two stage multiplier. This general configuration
is known as the Cockcroft-Walton type. Several other popular configurations
exist and the Micaplier technology can be successfully employed to implement
all of them.
Figure 10 indicates the generalized configuration for a multi stage multiplier.
Additional features including voltage feedback divider networks, surge
limiters and filter capacitors can readily be built into the Micaplier
packages.
Figure 9: Two stage Cockcroft-Walton multiplier |
Figure 10: Multistage Cockcroft Walton configuration. |
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Practicalities and Applications
Voltage multipliers, especially those with three or more stages, are generally
used in low current applications. One micro-ampere to a few milli-amperes
of output current is typical, but of course, at quite high voltages.
A 25kV output at 2mA is a power output of 25,000 x 0.002 = 50 watt for
example. Although there are technical constraints on output current capability,
the most notable being high effective output resistance, this rarely
disqualifies any new application. After all, most ciruitry designed to
operate at tens of thousands of volts requires only a small current.
As a matter of interest, voltage multipliers are the only realistic way
to produce high voltages in many situations. A low current transformer
for example, producing 25 kV directly, has to be physically large to satisfactorily
handle the voltage stresses, especially if used at altitude. Using a 5
stage Micaplier, the transformer only needs to produce voltages of 2.5
to 3.5kVpeak to do the same job. This alone makes it possible to reduce
both the size and weight dramatically. Further, the high speed diodes used
in the Micaplier permit a much higher operating frequency to be used than
is possible for the direct transformer design, leading to further considerable
space
and
weight
economies.
Practically all airborne CRT displays and most stroke written CRTs use
multipliers. Other common applications are listed in the Micaplier Data
Sheets.
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Specifying a Micaplier
To make an initial determination as to whether an application can be
met with a Micaplier, please complete the following questionnaire.
- Input Voltage: Maximum input voltage for a micaplier is 22kVpeak-to-peak.
Required input voltage ________ kVpeak-to-peak
- Output Voltage: Maximum output voltage for a micaplier is 100kVDC.
Required output voltage ________ kVDC
- Output Power: Maximum for a single Micaplier is 100W. This
is the product of the output voltage and the output current,
volts x amperes (equal to Kv x mA). Output power required ________
W
Continue with question 4 unless any of the above requirements exceed
the stated limits. In this case, please consult the factory.
- Operating Frequency: Typically in the range of 10 kHz to 100
kHz ________ kHz
- Output Ripple: Vp-p at Maximum Current ________ Vp-p
- Input waveform: May be flyback, push-pull, PWM push-pull, Sinusodial,
Square, etc. Describe as fully as possible_______________________________
_____________________________________
_______________________________________________________________
- Additional features: Check any features to be incorporated
in the assembly. DC F/B divider; AC F/B divider; Additional filtering;
Surge limiting resistor; Other (describe) ______________________________
- Available Space: Length____”; Width____”; Height____”;
- Termination Type (Pee Wee, Turret, Flying Lead, etc.): ________
- Mounting Means (Studs, Inserts, Lugs, etc.) ________
- Required Quantity ________
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Glossary
anode n. A positively charged electrode, as of a
storage battery, electron tube, or semiconductor diode.
average value n. the arithmetic mean of the voltage
with respect to time.
cathode n. A negatively charged electrode, as
of a storage battery, electron tube, or semiconductor diode.
diode n. an electronic device that restricts
current flow chiefly to one direction, a two terminal semiconductor
device used as a rectifier.
feedback divider n. two or more resistors
connected in series across a voltage source from the junction
of which, a sample
of the output voltage can be measured; AC voltages can be sampled
using two or more capacitors similarly connected.
filter capacitor n. a decoupling, or reservoir
capacitor connected close to the output of a multiplier to reduce
ripple voltage
peak voltage n. the largest voltage difference,
positive or negative, measured between two points in a circuit
peak-to-peak voltage n. the sum of the positive
and negative voltage differences between two points of a circuit.
reverse bias n. application of a voltage to
a diode or other rectifier, in such a direction as to not permit
current
flow
ripple n. usually measured as the peak-to-peak
voltage excursion of a DC node from its mean value; sometimes
peak value
or even rms value is specified
rms value n. root-mean-square value of an AC
waveform is equal to the DC voltage which would produce an equal
amount of
power.
stroke writing n. a CRT display technique of
wrtiting discrete lines, or strokes, by moving the electron beam
from one
end point to the other as opposed to a scanned raster display
used in television sets.
surge limiter n. a resistor located at the output
of a multiplier to limit the peak output current in a default
condition to a known value
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