Villard circuit
A voltage doubler is an electronic circuit which charges capacitors from the input voltage and switches these charges in such a way that, in the ideal case, exactly twice the voltage is produced at the output as at its input.
The simplest of these circuits are a form of rectifier which take an AC voltage as input and outputs a doubled DC voltage. The switching elements are simple diodes and they are driven to switch state merely by the alternating voltage of the input. DC-to-DC voltage doublers cannot switch in this way and require a driving circuit to control the switching. They frequently also require a switching element that can be controlled directly, such as a transistor, rather than relying on the voltage across the switch as in the simple AC-to-DC case.
Voltage doublers are a variety of voltage multiplier circuit. Many, but not all, voltage doubler circuits can be viewed as a single stage of a higher order multiplier: cascading identical stages together achieves a greater voltage multiplication.
The Villard circuit, due to Paul Ulrich Villard, consists simply of a capacitor and a diode. While it has the great benefit of simplicity, its output has very poor ripple characteristics. Essentially, the circuit is a diode clamp circuit. The capacitor is charged on the negative half cycles to the peak AC voltage (Vpk). The output is the superposition of the input AC waveform and the steady DC of the capacitor. The effect of the circuit is to shift the DC value of the waveform. The negative peaks of the AC waveform are "clamped" to 0 V (actually −VF, the small forward bias voltage of the diode) by the diode, therefore the positive peaks of the output waveform are 2Vpk. The peak-to-peak ripple is an enormous 2Vpk and cannot be smoothed unless the circuit is effectively turned into one of the more sophisticated forms. This is the circuit (with diode reversed) used to supply the negative high voltage for the magnetron in a microwave oven.
The Greinacher voltage doubler is a significant improvement over the Villard circuit for a small cost in additional components. The ripple is much reduced, nominally zero under open-circuit load conditions, but when current is being drawn depends on the resistance of the load and the value of the capacitors used. The circuit works by following a Villard cell stage with what is in essence a peak detector or envelope detector stage. The peak detector cell has the effect of removing most of the ripple while preserving the peak voltage at the output. The Greinacher circuit is also commonly known as the half-wave voltage doubler.
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