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Home»Miscellaneous
Circuits»Voltage Multiplier

Voltage
Multiplier

9:31 am Wayne Storr

Voltage Multiplier Circuit

In the tutorial about Rectifiers,
we saw that the DC output voltage being controlled by the rectifier is at a
value below that of the mains input voltage. The Voltage
Multiplier, however, is a special type of diode rectifier circuit which
can potentially produce an output voltage many times greater than of the applied
input voltage.

Although it is usual in Electronic Circuits to use a voltage transformer to increase a
voltage, sometimes a suitable step-up transformer or a specially insulated
transformer required for high voltage applications may not always be available.
One alternative approach is to use a diode voltage multiplier circuit which
increases or “steps-up” the voltage without the use of a transformer.

Voltage multipliers are similar in many ways to rectifiers
in that they convert AC-to-DC voltages for use in many electrical and electronic
circuit applications such as in microwave ovens, strong electric field coils for
cathode-ray tubes, electrostatic and high voltage test equipment, etc, where it
is necessary to have a very high DC voltage generated from a relatively low AC
supply.

Generally, the DC output voltage (Vdc) of a
rectifier circuit is limited by the peak value of its sinusoidal input voltage.
But by using combinations of rectifier diodes and capacitors together we can
effectively multiply this input peak voltage to give a DC output equal to some
odd or even multiple of the peak voltage value of the AC input voltage. Consider
the basic voltage multiplier circuit below.

Full Wave Voltage Multiplier

The above circuit shows a basic symmetrical voltage multiplier circuit made
up from two half-wave rectifier circuits. By adding a second diode and capacitor
to the output of a standard half-wave rectifier, we can increase its output
voltage by a set amount. This type of voltage multiplier configuration is known
as a Full Wave Series Multiplier because one of the diodes is
conducting in each half cycle, the same as for a full wave rectifier circuit.

When the sinusoidal input voltage is positive, capacitor C1 charges up through diode D1 and when the sinusoidal voltage is negative,
capacitor C2 charges up through diode,
D2. The output voltage 2VP is taken across the two series connected
capacitors.

The voltage produced by a voltage multiplier circuit is in theory
unlimited, but due to their relatively poor voltage regulation and low current
capability there are generally designed to increase the voltage by a factor less
than ten. However, if designed correctly around a suitable transformer, voltage
multiplier circuits are capable of producing output voltages in the range of a
few 100’s to tens’s of 1000’s of volts, depending upon their original input
voltage value but all with low currents in the milliamperes range.

The Voltage Doubler

As its name suggests, a Voltage Doubler is a voltage
multiplier circuit which has a voltage multiplication factor of two. The circuit
consists of only two diodes, two capacitors and an oscillating AC input voltage
(a PWM waveform could also be used). This simple diode-capacitor pump circuit
gives a DC output voltage equal to the peak-to-peak value of the sinusoidal
input. In other words, double the peak voltage value because the diodes and the
capacitors work together to effectively double the voltage.

DC Voltage Doubler Circuit

So how does it work. The circuit shows a half wave voltage doubler. During
the negative half cycle of the sinusoidal input waveform, diode D1
is forward biased and conducts charging up the pump capacitor, C1
to the peak value of the input voltage, (Vp). Because
there is no path for capacitor C1 to discharge into,
it remains fully charged and acts as a storage device in series with the voltage
supply. At the same time, diode D2 conducts via D1 charging up capacitor, C2.

During the positive half cycle, diode D1 is reverse
biased blocking the discharging of C1 while diode
D2 is forward biased charging up capacitor C2. But because there is a voltage across capacitor C1 already equal to the peak input voltage, capacitor C2 charges to twice the peak voltage value of the input
signal.

In other words, V(positive peak) + V(negative
peak) as on the negative half-cycle, D1 charges
C1 to Vp and on the positive
half-cycle D2 adds the AC peak voltage to Vp
onC1 and transfers it all to C2. The voltage across capacitor, C2 discharges through the load ready for the next half
cycle.

Then the voltage across capacitor, C2 can be
calculated as: Vout = 2Vp, (minus of course
the voltage drops across the diodes used) where Vp is
the peak value of the input voltage. Note that this double output voltage is not
instantaneous but increases slowly on each input cycle, eventually settling to
2Vp.

As capacitor C2 only charges up during one half
cycle of the input waveform, the resulting output voltage discharged into the
load has a ripple frequency equal to the supply frequency, hence the name half
wave voltage doubler. The disadvantage of this is that it can be difficult to
smooth out this large ripple frequency in much the same way as for a half wave
rectifier circuit. Also, capacitor C2 must have a DC
voltage rating at least twice the value of the peak input voltage.

The advantage of “Voltage Multiplier Circuits” is that it allows higher
voltages to be created from a low voltage power source without a need for an
expensive high voltage transformer as the voltage doubler circuit makes it
possible to use a transformer with a lower step up ratio than would be need if
an ordinary full wave supply were used. However, while voltage multipliers can
boost the voltage, they can only supply low currents to a high-resistance
(+100kΩ) load because the generated output voltage quickly drops-off as load
current increases.

By reversing the direction of the diodes and capacitors in the circuit we can
also reverse the direction of the output voltage creating a negative voltage
output. Also, if we connected the output of one multiplying circuit onto the
input of another (cascading), we can continue to increase the DC output voltage
in integer steps to produce voltage triplers, or voltage quadruplers circuits,
etc, as shown.

DC Voltage Tripler Circuit

By adding an additional single diode-capacitor stage to the half-wave voltage
doubler circuit above, we can create another voltage multiplier circuit that
increases its input voltage by a factor of three and producing what is called a
Voltage Tripler Circuit.

A “voltage tripler circuit” consists of one and a half voltage doubler
stages. This voltage multiplier circuit gives a DC output equal to three times
the peak voltage value (3Vp) of the sinusoidal input
signal. As with the previous voltage doubler, the diodes within the voltage
tripler circuit charge and block the discharge of the capacitors depending upon
the direction of the input half-cycle. Then 1Vp is
dropped across C3 and 2Vp
across C2 and as the two capacitors are in series,
this results in the load seeing a voltage equivalent to 3Vp.

Note that the real output voltage will be three times the peak input voltage
minus the voltage drops across the diodes used, 3Vp –
V(diode).

If a voltage tripler circuit can be made by cascading together one and a half
voltage multipliers, then a Voltage Quadrupler Circuit can be
constructed by cascading together two full voltage doubler circuits as
shown.

DC Voltage Quadrupler Circuit

The first voltage multiplier stage doubles the peak input voltage and the
second stage doubles it again, giving a DC output equal to four times the peak
voltage value (4Vp) of the sinusoidal input signal.
Also, using large value capacitors will help to reduce the ripple voltage.

Voltage Multiplier Summary

THe
Diode Handbook

List Price: $19.95

Price: $19.95

Price
Disclaimer

Then we have seen that Voltage Multipliers are simple
circuits made from diodes and capacitors that can increase the input voltage by
two, three, or four times and by cascading together individual half or full
stage multipliers in series to apply the desired DC voltage to a given load
without the need for a step-up transformer.

Voltage multiplier circuits are classified as voltage doubler’s, tripler’s,
or quadrupler’s, etc, depending on the ratio of the output voltage to the input
voltage. In theory any desired amount of voltage multiplication can be obtained
and a cascade of “N” doublers, would produce an output voltage of 2N.Vp
volts.

For example, a 10-stage voltage multiplier circuit with a peak input voltage
of 100 volts would give a DC output voltage of about 1,000 volts or 1kV,
assuming no losses, without the use of a transformer.

However, the diodes and capacitors used in all multiplication circuits need
to have a minimum reverse breakdown voltage rating of at least twice the peak
voltage across them as multi-stage voltage multiplication circuits can produce
very high voltages, so take care. Also, voltage multipliers usually supply low
currents to a high-resistance loads as the output voltage quickly drops away as
the load current increases.

The Voltage Multiplication Circuits shown above, are all
designed to give a positive DC output voltage. But they can also be designed to
give negative voltage outputs by simply reversing the polarities of all the
multiplier diodes and capacitors to produce a negative voltage doubler.

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Tags: DC Theory Diode Voltage

18 Responses to “Voltage Multiplier”

Older
comments

yazdani Manouchehr

your posts is always useful and well designed, i am very grateful to the
owners of this site, i live in Iran and have a site and sometimes use of your
posts and translate them into persian and also use of your i mages, i tried to
contact you to allow me to do that but i did not receive any answer please
inform me that it is OK or not!
February
12th, 2016 Reply

Wayne
Storr

Hello Yazdani, I do not recall receiving any message. Please try
again.
February
12th, 2016 Reply

Adelusi Samuel

Thanks for your innovative ideals. Can you please give exaltly diode name
and numbers. Together with voltage capacity with capacitor UF. Thanks. Best
regards.
December
16th, 2015 Reply

Wayne
Storr

Hello Adelusi, the values and the types of the diodes and capacitors will
depend ultimately depend on the voltage rating of the output and sections
and what is available at your location.
December
16th, 2015 Reply

CBK

THANKS
November
25th, 2015 Reply

Lulamile

This is actually a good site
October
24th, 2015 Reply

Dhananjay

Need to develop a voltage tripler circuit. please let me know how to select
the component values based on load, load regulation required etc.
July
10th, 2015 Reply

Fab

Hi,
I need to size C for a charge pump, with a C load of 100uF. How
can I do it?
October
12th, 2015 Reply

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2016,
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