Diode as a voltage multiplier:
A very sophisticated method of obtaining high voltage with the help of simple circuit is Cockcroft-Walton voltage multipliers. It consists of diodes and capacitors only. There are many possible ways to design voltage multipliers, with the help of diodes and capacitors. This article is intended to provide basic information about Cockcroft-Walton voltage multipliers.
Voltage multipliers are used to produce high voltages ( hundreds and thousands of volts). They are commonly used in high voltage low current device like cathode ray tubes, Lasers etc.
Diode capacitor voltage multiplier takes AC as an input, and produce DC output which is multiple (2,4,6) of AC peak voltage.
VO = 2*N*VP Equation 1
VO = 2*N*VP - Vdrop Equation 2
Where
N = number of stages,
VP = peak input voltage
Equation 2 >> we consider the diode drop.
For example from a three stage multiplier, we get
VO = 2*3*VP
VO = 6*VP
A single stage Cockcroft Walton multiplier consists of 2 diodes and 2 capacitors. Each stage produce an output which is multiple of 2. Hence, the number of diodes and capacitors stages increase as the voltage increases.
For example at input if you have 10V (AC) at input, each stage produce 20 V. For example, we have 3 stages we get 60 V (DC) at output. It means each stage produce 20V. (20+20+20 = 60). And the voltage multiplier circuit contains 6 capacitors and 6 diodes.
VO = 2*N*VP
VO = 2*3*10 = 60V (neglect diode drop)
Basically voltage multiplication depends on charging and discharging of capacitors. Each capacitor charges upto 2VP except C1 , which charges upto VP. The upper rail of capacitors clamps the signal, while the bottom rail of capacitors smooths the DC.
Diode Capacitor Voltage Doubler | Single Stage Cockcroft Walton Multiplier:
During negative half cycle D1 is forward biased and charges the capacitor C1 to peak voltage Vp, while D2 is reverse biased. During positive half cycle D2 is forward biased. C2 will try to charge up to 2Vp, because C1 and source are in series. See figure below, C2 tries to charge upto 2VP, after several cycles it will reach upto 2VP. The output is taken across capacitor C2.
Figure 1 Single stage multiplier |
Look a figure 2. Input 10V (AC)
Output 20V (DC) approximately (we are considering practical diodes)
Voltage at capacitor C1
Figure 2 Output of Single stage multiplier (ideally output is 20 V, practically it is a little bit less because of diode drop as shown in the figure) |
Voltage Quadrupler | Two Stage Cockcroft Walton Multiplier:
Figure 3 Two stage voltage multiplier |
The voltage quadrupler circuit consists of two stages of Cockcroft-Walton multiplier. The upper rail of capacitors stores and clamps the signal. The output is observed across lower rail of capacitors.
During first negative half cycle D1 and D3 are forward biased. D1 charges the capacitor C1 to peak voltage Vp, while D2 and D4 remain reverse biased. During first positive half cycle D2 and D4 are forward biased. C2 will try to charge up to 2Vp, because C1 and source are in series.
During second negative half cycle, again D1 and D3 are forward biased. C1 already charged upto VP. Apply KVL and calculate the voltage across capacitor C3. It is 2VP. Look at figure 4. During second positive half cycle D2 and D4 are forward biased. Capacitor C4 charges upto 2VP. C2 has already charged upto 2VP. The output is taken across the two capacitors.
Figure 4 Working of voltage quadrupler |
Look a figure 5: Waveforms obtained at various points. You can visualize easily. Voltage across C1 is shifted or clamped. Also, voltage across C3 is also clamped. I discussed earlier, upper rail of capacitors clamp or shift the waveforms. While, the lower rail of capacitors produce DC at the output.
Input 10V (AC)
Output 20V (DC) approximately (we are considering practical diodes)
Voltage at capacitor C1
Voltage at capacitor C3
Figure 5 Output of voltage quadrupler |