Cascade circuit (Voltage Multiplier)

A cascade voltage multiplier circuit converts the lower AC voltage to a higher DC voltage. Typically, this circuit is using a network of capacitors and diodes. The number of capacitor-diode stages can vary depending of the circuit needs. On Picture 1 is shown a 4-stage cascade circuit which multiplies the input AC voltage and provides four times higher DC voltage than the amplitude of the input AC voltage. The input voltage in this circuit is sinusoidal with amplitude of 100 V at frequency of 50 Hz. The output DC voltage is 400 V, and it reach this value after about 1 second. This type of half-wave voltage multiplier circuit is also called Villard cascade.

Picture 1: The 4-stage Cascade Circuit (Half-wave Voltage Multiplier)

The voltage peak of the input AC voltage source is Us. As we can see from the circuit configuration, the capacitors can be charged through different current paths depending on that which diode is on or off, while the input voltage source changes it's values from positive to negative peaks +/- Us. After few cycles the circuit output will reach the full voltage of 4Us. In this circuit the full voltage is reached after about 1 second, as we can see from the Picture 2. At this state, the voltage of the capacitor C1 is 100 V (Us). The voltage of the capacitors C2, C3 and C4 is 200 V (2Us). So, after capacitor C1 the voltage of the circuit is 100 V (Us), after C2 is 200 V (2Us), after C3 is 300 V (3Us) and after C4 is 400 V (4Us). The wave forms of the input and output voltage of this circuit are shown on the Picture 2. The input voltage is represented with the blue line plot and the output voltage is represented with the green line plot.

Picture 2: The Input and Output Voltage wave forms of the 4-stage Cascade Circuit

So, the cascade circuit is a special rectifier circuit which produces DC output voltage which is many times greater than the AC input voltage. These AC to DC voltage converters are used in many applications, such as cathode-ray tube (CRT) field coils, high voltage test equipment, microwave ovens, etc. In theory, the maximum output voltage of this circuit can be infinity, since we can add more and more capacitor-diode stages. In practice, the most multiplier circuits are designed to produce voltages in the range of about 10 kV to 30 kV and low current of several miliamperes. The circuit shown here provides a positive DC voltage output. If we need a negative DC voltage output, we can use the same circuit configuration, but with reversed polarities of the diodes in all stages.

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