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 (but actually invented by Heinrich Greinacher).
Operation Assuming that the peak voltage of the AC source is +Us, and that the C values are sufficiently high to allow, when charged, a current to flow with no significant change in voltage, then the (simplified) working of the cascade is as follows:
going from positive peak (+Us) to negative peak (−Us): The C1 capacitor is charged through diode D1 to Us V (potential difference between left and right plate of the capacitor is Us). going from negative peak to positive peak: The voltage of C1 adds with that of the source, thus charging C2 to 2Us 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 2Us. negative to positive peak: Voltage of C2 rises to 2Us (analogously to step 2), also charging C4 to 2Us. The output voltage (the sum of voltages of C2 and C4) rises until 4Us is reached. Adding an additional stage will increase the output voltage by twice the peak AC source voltage (minus losses due to the diodes ‒ see the next paragraph). 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 (Uf) of each diode on the path to that capacitor. For example, the voltage of C4 in the example would be at most 2Us - 4Uf since there are 4 diodes between its positive terminal and the source. The total output voltage would be U(C2) + U(C4) = (2Us - 2Uf) + (2Us - 4Uf) = 4Us - 6Uf. In a cascade with n stages of two diodes and two capacitors, the output voltage is equal to 2n Us - n(n+1) Uf. The term n(n+1) Uf represents the sum of voltage losses caused by diodes, over all capacitors on the output side (i.e. on the right side in the example ‒ C2 and C4). For example, if we have 2 stages like in the example, the total loss is 2+4 = 2*(2+1) = 6 times Uf. An additional stage will increase the output voltage by twice the source voltage, minus the forward voltage drop over 2n+2 diodes: 2Us - (2n+2)Uf.
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 (CRT, picture 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:
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