Voltage multiplier

A voltage multiplier is an electrical circuit that converts AC electrical power from a lower voltage to a higher DC voltage, typically using a network of capacitors and diodes.
Voltage multipliers can be used to generate a few volts for electronic appliances, to millions of volts for purposes such as high-energy physics experiments and lightning safety testing. The most common type of voltage multiplier is the half-wave series multiplier, also called the Villard cascade.

Operation

Assuming that the peak voltage of the AC source is +U_s, and that the C values are sufficiently high to allow, when charged, a current to flow with no significant change in voltage, then the working of the cascade is as follows:
Image:Voltage amplifier explain.png|right|thumb|400px|Illustration of the described operation, with +U_s = 100 V

going from positive peak to negative peak : The C₁ capacitor is charged through diode D₁ to U_s V.
going from negative peak to positive peak: The voltage of C₁ adds with that of the source, thus charging C₂ to 2U_s through D₂ and discharging C₁ in the process.
positive to negative peak: Voltage of C₁ has dropped to 0 V by the end of the previous step, thus allowing C₃ to be charged through D₃ to 2U_s.
negative to positive peak: Voltage of C₂ rises to 2U_s, also charging C₄ to 2U_s. The output voltage rises until 4U_s is reached.

Adding an additional stage will increase the output voltage by twice the peak AC source voltage.
In reality, more cycles are required for C₄ to reach the full voltage, and the voltage of each capacitor is lowered by the forward voltage drop of each diode on the path to that capacitor. For example, the voltage of C₄ in the example would be at most since there are 4 diodes between its positive terminal and the source. The total output voltage would be. In a cascade with stages of two diodes and two capacitors, the output voltage is equal to. The term represents the sum of voltage losses caused by diodes, over all capacitors on the output side. For example, if we have 2 stages like in the example, the total loss is times. An additional stage will increase the output voltage by twice the source voltage, minus the forward voltage drop over diodes:.

Voltage doubler and tripler

A voltage doubler uses two stages to approximately double the DC voltage that would have been obtained from a single-stage rectifier. An example of a voltage doubler is found in the input stage of switch mode power supplies containing a SPDT switch to select either 120 or 240 V supply. In the 120 V position, the input is typically configured as a full-wave voltage doubler by opening one AC connection point of a bridge rectifier and connecting the input to the junction of two series-connected filter capacitors. For 240 V operation, the switch configures the system as a full-wave bridge, reconnecting the capacitor center-tap wire to the open AC terminal of a bridge rectifier system. This allows 120 or 240 V operation with the addition of a simple SPDT switch.
A voltage tripler is a three-stage voltage multiplier. A tripler is a popular type of voltage multiplier. The output voltage of a tripler is, in practice, below three times the peak input voltage due to their high impedance, caused in part by the fact that as each capacitor in the chain supplies power to the next, it partially discharges, losing voltage doing so.
Triplers were commonly used in color television receivers to provide the high voltage for the cathode-ray tube.
Triplers are still used in high voltage supplies such as copiers, laser printers, bug zappers and electroshock weapons.

Breakdown voltage

While the multiplier can be used to produce thousands of volts of output, the individual components do not need to be rated to withstand the entire voltage range. Each component only needs to be concerned with the relative voltage differences directly across its own terminals and of the components immediately adjacent to it.
Typically, a voltage multiplier will be physically arranged like a ladder, so that the progressively increasing voltage potential is not given the opportunity to arc across to the much lower potential sections of the circuit.
Note that some safety margin is needed across the relative range of voltage differences in the multiplier, so that the ladder can survive the shorted failure of at least one diode or capacitor component. Otherwise, a single-point shorting failure could successively over-voltage and destroy each next component in the multiplier, potentially destroying the entire multiplier chain.

Frequency Response

The open loop frequency response of a voltage multiplier behaves as a single pole followed by a high-frequency zero. The single pole is the result of the energy transfer characteristic of the multiplier, while the high frequency zero is the result of direct a.c. coupling to the load through the capacitors This allows for a simple integrator in the high-voltage feedback loop for output regulation. More complicated control schemes can be used if a faster response is required, or if the addition of output overcurrent protection is desirable.

Other circuit topologies

;Stacking:
An even number of diode-capacitor cells is used in any column so that the cascade ends on a smoothing cell. If it were odd and ended on a clamping cell the ripple voltage would be very large. Larger capacitors in the connecting column also reduce ripple but at the expense of charging time and increased diode current.

Dickson charge pump

The Dickson charge pump, or Dickson multiplier, is a modification of the Greinacher/Cockcroft–Walton multiplier. There are, however, several important differences:

The Dickson multiplier takes a DC supply as its input so is a form of DC-to-DC converter. In addition to the DC input, the circuit requires a feed of two clock pulse trains with an amplitude swinging between the DC supply rails. These pulse trains are in antiphase.
The Dickson multiplier is intended for low-voltage applications, unlike Greinacher/Cockcroft–Walton, which is commonly used in high-voltage applications. This is because the final capacitor has to hold the entire output voltage, whereas in the Greinacher/Cockcroft–Walton multiplier, each capacitor holds at most twice the input voltage.

To describe the ideal operation of the circuit, number the diodes D1, D2, etc., from left to right, and the capacitors C1, C2, etc. When the clock is low, D1 will charge C1 to V_in. When goes high, the top plate of C1 is pushed up to 2V_in. D1 is then turned off and D2 turned on and C2 begins to charge to 2V_in. On the next clock cycle, again goes low and now goes high, pushing the top plate of C2 to 3V_in. D2 switches off, and D3 switches on, charging C3 to 3V_in and so on with charge passing up the chain, hence the name charge pump. The final diode-capacitor cell in the cascade is connected to ground rather than a clock phase and hence is not a multiplier; it is a peak detector which merely provides smoothing.
There are a number of factors which reduce the output from the ideal case of nV_in. One of these is the threshold voltage, V_T of the switching device, that is, the voltage required to turn it on. The output will be reduced by at least nV_T due to the voltage drops across the switches. Schottky diodes are commonly used in Dickson multipliers for their low forward voltage drop, amongst other reasons. Another difficulty is that there are parasitic capacitances to ground at each node. These parasitic capacitances act as voltage dividers with the circuit's storage capacitors reducing the output voltage still further. Up to a point, a higher clock frequency is beneficial: the ripple is reduced and the high frequency makes the remaining ripple easier to filter. Also, the size of capacitors needed is reduced since less charge needs to be stored per cycle. However, losses through stray capacitance increase with increasing clock frequency, and a practical limit is around a few hundred kilohertz.
Dickson multipliers are frequently found in integrated circuits to increase a low-voltage battery supply to the voltage needed by the IC. Because IC designers and manufacturers benefit from using the same technology and basic device throughout a chip, CMOS Dickson multipliers often wire MOSFETs to behave as diodes.
The diode-wired MOSFET version of the Dickson multiplier does not work very well at very low voltages because of the large drain-source voltage drops of the MOSFETs. Frequently, a more complex circuit is used to overcome this problem. One solution is to connect in parallel with the switching MOSFET another MOSFET biased into its linear region. This second MOSFET has a lower drain-source voltage than the switching MOSFET would have on its own, and consequently, the output voltage is increased. The gate of the linear biased MOSFET is connected to the output of the next stage, so that it is turned off while the next stage is charging from the previous stage's capacitor. That is, the linear-biased transistor is turned off at the same time as the switching transistor.
An ideal 4-stage Dickson multiplier with an input of would have an output of. However, a diode-wired MOSFET 4-stage multiplier might only have an output of. Adding parallel MOSFETs in the linear region improves this to around. More complex circuits still can achieve an output much closer to the ideal case.
Many other variations and improvements to the basic Dickson circuit exist. Some attempt to reduce the switching threshold voltage such as the Mandal-Sarpeshkar multiplier or the Wu multiplier. Other circuits cancel out the threshold voltage: the Umeda multiplier does it with an externally provided voltage and the Nakamoto multiplier does it with internally generated voltage. The Bergeret multiplier concentrates on maximising power efficiency.