page_title: Voltage multiplier
section_title: Dickson charge pump
hierarchical_section_title: Voltage multiplier / Other circuit topologies / Dickson charge pump
context_page_description: 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.
context_section_description: The Dickson charge pump, or Dickson multiplier, is a modification of the Greinacher/Cockcroft–Walton multiplier.  Unlike that circuit, however, the Dickson multiplier takes a DC supply as its input so is a form of DC-to-DC converter.  Also, unlike Greinacher/Cockcroft–Walton which is used on high-voltage applications, the Dickson multiplier is intended for low-voltage purposes.  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.
  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 Vᵢₙ.  When  goes high the top plate of C1 is pushed up to 2Vᵢₙ.  D1 is then turned off and D2 turned on and C2 begins to charge to 2Vᵢₙ.  On the next clock cycle  again goes low and now  goes high pushing the top plate of C2 to 3Vᵢₙ.  D2 switches off and D3 switches on, charging C3 to 3Vᵢₙ 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 nVᵢₙ.  One of these is the threshold voltage, VT of the switching device, that is, the voltage required to turn it on.  The output will be reduced by at least nVT due to the volt 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 (ICs) where they are used to increase a low-voltage battery supply to the voltage needed by the IC.  It is advantageous to the IC designer and manufacturer to be able to use the same technology and the same basic device throughout the IC.  For this reason, in the popular CMOS technology ICs the transistor which forms the basic building block of circuits is the MOSFET.  Consequently, the diodes in the Dickson multiplier are often replaced with MOSFETs wired 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 volt 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 (because the switching MOSFET is driven hard on) 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 (5× multiplier) with an input of 1.5 V would have an output of 7.5 V.  However, a diode-wired MOSFET 4-stage multiplier might only have an output of 2 V.  Adding parallel MOSFETs in the linear region improves this to around 4 V.  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 th
caption: Dickson charge pump using diode-wired MOSFETs (4 stages : 5× multiplier)