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Ripple voltage is the magnitude of fluctuation in DC output voltage at a specific output current.  This assumes the AC input voltage and frequency are kept constant. 

A close approximation for series half-wave multipliers can be expressed as follows:

 

[ I * (N2+N/2)]
Vrip =
8*f*c

N = number of stages

f = AC input frequency (Hz)

c = capacitance per stage (farads)

I= DC output current (amps)

 

 

 

 

 

 

 

 

Example
Calculate the ripple voltage of a 6 stage multiplier with 1000pF capacitors, 50kHz operating frequency (sine wave), 1mA DC output current, 20kV DC output voltage as follows:

Vrip =
[ 1*10-3 (62+6/2)) ]

where N = 6, f = 50kHz, c = 1000pf, I = 1mA.
8 (50,000) (1*10-9)
Vrip = 97.5V
An equivalent parallel multiplier would require each capacitor stage to equal the total series capacitance of the AC capacitor bank. 

In the above case, the 3 capacitors in the DC bank would equal 1000pF/3 or 333pF. The parallel equivalent would require 333pF capacitors in each stage. However, each successive stage would require a higher voltage capacitor.
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