By Surbhi Prakash
The simplest of the voltage doubler circuit are a type of rectifier which takes the input in the form of Alternate Current (AC) voltage and produces a double magnitude of (DC) voltage as the output.
Simple diodes are used as switching elements and an input in the form of mere alternating voltage is used to drive these diodes in a switching state. An additional driving circuit is required in order to control the switching rate in case voltage doublers being used are of DC to DC type since they cannot be switched in the above manner.
The DC to DC voltage circuit most of the times require another additional device called switching element which can be easily and directly controlled such as in a transistor. Thus, when it uses switching element, it does not have to depend on the voltage present across the switch as is the case in a simple form of AC to DC.
The voltage doubler is a type of the voltage multiplier circuit. Most of the voltage doubler circuits with few exceptions can be viewed in the form of a higher order multiplier at a single stage. Also, a greater amount of voltage multiplication is achieved when there are cascading identical stages which are being used together.
The Villard circuit has a simple composition consisting of a diode and a capacitor. On the one hand where the Villard circuit provides benefit in terms of simplicity, on the other hand it is also known to produce output which has ripple characteristics which are considered very poor.
|Figure 1. Villard circuit|
The Greinarcher voltage doubler has proved to be better than the Villard circuit by improving itself significantly by adding some additional components for a small cost. Under the condition of open-circuit load the ripple is found to be reduced very much, most of the times to a state of zero; but the resistance of the load and the value of the capacitor which is being used play an important role and affect the current being drawn.
|Figure 2. Greinacher circuit|
Heinrich Greinacher was the first person to invent this circuit in 1913 (which was published in 1914) in order to provide the voltage of 200-300V which was needed by him for his ionometer which was again a new invention by him. The requirement of inventing this circuit to get that much voltage arose because the power supplied by the Zurich power stations was of only 110V AC and thus was insufficient. Heinrich developed this idea more in 1920 and extended it to make a cascade of multipliers. Most of the times, people refer this cascade of multipliers invented by Heinrich Greinacher as a Villard cascade which is inaccurate and not true. This cascade of multipliers is also known as Cockroft-Walton after the scientists John Cockroft and Ernest Walton who had built the particle accelerator machine and had rediscovered the circuit independently in 1932.
The use of two Greinacher cells which have polarities opposite to each other but being driven from the same AC source can extend the concept of this kind of topology to a voltage quadrupler circuit. The two individual outputs are used in order to take down the output across them. The grounding of the input and output simultaneously in this circuit is quite impossible as is the case with a bridge circuit.
The kind of topology used by a Delon circuit in order to have voltage doubling is known as bridge topology. One of the common uses of this type of delon circuit was found to be in the television sets with cathode ray tube. The delon circuit in these television sets was used in order to provide the e.h.t. voltage supply.
|Figure 3. Voltage quadrupler – two Greinacher cells of opposite polarities|
|Figure 4. Bridge (Delon) voltage doubler|
Switched Capacitor Circuits
The voltage of a DC source can be doubled by using the diode-capacitor circuits which are simple enough and have been described in the above section by preceding the voltage doubler with the use of a chopper circuit. Thus, this is effective in converting the DC to AC before it goes through the voltage doubler. In order to attain and built circuits which are more efficient, the switching devices are driven from an external clock which is proficient in functioning both in terms of chopping and multiplying and can be achieved on a simultaneous basis.
|Figure 5. Switched capacitor voltage doubler achieved by simply switching charged capacitors from parallel to series|
Thus, the diagram in Figure 5 displays schematically the simplest form of switched capacitor configuration. In this diagram, there are two capacitors which have been charged to the same voltage simultaneously in a parallel. Post this; capacitors are switched into series after switching off the supply. Thus, the output voltage produced is twice the supply or input voltage in case the output is derived from the two capacitors in series. There are various different kinds of switching devices which can be used in such circuits, but MOSFET devices are the most frequently used switching devices in the case of integrated circuits.
|Figure 6. Charge-pump voltage doubler schematic|
The diagram in Figure 6 displays schematically one of the other basic concepts of the “Charge Pump”. The input voltage is used to first charge the Cp, the charge pump capacitor. After this, the output capacitor, C0 is charged by switching in series with the input voltage which results in charging the C0 double the amount of input voltage. In order to successfully charge C0 fully, the charge pump may be required to take many cycles. But once a steady state has been acquired, the only essential thing for the charge pump capacitor, Cp is to pump charge in small amounts which is equivalent to the charge being supplied from the output capacitor, C0 to the load. A ripple is formed on the output voltage when C0 gets discharged partially into the load while it is being disconnected from the charge pump. This ripple formed in this process has the characteristic of shorter discharge time and easy to be filtered and thus these characteristics make them smaller for frequencies for higher clock frequencies. Thus, for any given specific ripple, the capacitors can be made smaller. The maximum amount of clock frequency for all practical purposes in the integrated circuits typically falls in the range of hundreds of kHz.
Dickson charge pump
The Dickson charge pump, also known as Dickson multiplier consists of a cascade of diode/capacitor cells where a clock pulse train drives the bottom plate of each of the capacitor. The circuit is considered to be a modification of the Cockcroft-Walton multiplier but with the only exception of switching signal being provided by the DC input with clock trains instead of an AC input as is the case with Cockcroft-Walton multiplier. The basic requirement of a Dickson multiplier is that the clock pulses of phases opposite to each other should drive the alternate cells. But, in the case of a voltage doubler, depicted in the Figure 7, only a single clock signal is required since there is only one stage of multiplication.
|Figure 7. Dickson charge-pump voltage-doubler|
The circuits where Dickson multipliers are mostly and frequently used are the integrated circuits where the supply voltage such as from any battery is less than what is required by the circuitry. The fact that all the semiconductors used in this are basically similar acts as an advantage for the manufacturers of the integrated circuit. The standard logic block which is most commonly found and used in numerous integrated circuits is the MOSFET devices. This is one of the reasons why the diodes are many a times replaced by the transistor of this type, but are also wired to a function in the form of a diode. This arrangement is also known as a diode-wired MOSFET. The diagram in Figure 8 depicts a Dickson voltage doubler using this kind of diode-wired n-channel enhancement type MOSFET devices.
|Figure 8. Dickson voltage doubler using diode-wired MOSFETs|
The basic form of Dickson charge pump has undergone a lot many improvements and variations. Most of these improvements are in the area of the reduction of the effect produced by the transistor drain source voltage. This improvement is considered as significant in case the input voltage is small as is in the case of a low-voltage battery. The output voltage is always an integral multiple of the input voltage (twice in case of a voltage doubler) when ideal switching elements are used. But in case where a single-cell battery is used as the input source along with MOSFET switches, the output in such cases are far lesser than this value because there will a drop in the voltage across the transistors. Due to the extremely low drop in the voltage in the on-state of a circuit which is using discrete components, the Schottky diode is considered a good choice as a switching element. But the designers of integrated circuit mostly prefer MOSFET to use as it is more easily available which more than compensate for the presence of inadequacies and high complexity in the circuit which is present in MOSFET devices.
To illustrate this, let us take an example: a nominal voltage of the tune of 1.5V is present in an alkaline battery. The output in this can be doubled to 3.0V by using a voltage doubler along with ideal switching elements which has a voltage drop of zero. But the diode-wired MOSFET’s voltage drop of drain-source when it is in the state of on must be at the minimum equal to the gate threshold voltage which is typically in the tune of 0.9V. The output voltage can be raised by the voltage doubler successfully only by approximately 0.6V to 2.1V. The increase in the voltage by the circuit cannot be achieved without using multiple stages in case the drop across the final smoothing transistor is also considered and taken into account. On the other hand, the onstage voltage of a typical Schottky diode is of 0.3 V. the output voltage produced by a voltage doubler will be in the range of 2.7V if it uses Schottky diode, or 2.4V if it uses smoothing diode.
Cross-coupled switched capacitors
The cross-coupled switched capacitor circuits are known for the input voltage being very low. A single-celled battery can be required in the equipments which are driven by wireless battery such as pagers and Bluetooth devices in order to supply power continuously when it has discharged to under a volt.
|Figure 9. Cross-coupled switched-capacitor voltage doubler|
The transistor Q2 is turned off in case the clock is low. At the same time, the transistor Q1 is turned on if the clock is high and this results in the charging of the capacitor C1 to the voltage Vn. the top plate of C1 is pushed up to double Vin in case the Ø1 goes high. In order to enable this voltage to appear as an output, the switch S1 closes at the same time. Also, at the same time C2 is allowed to charge by turning on the Q2. The roles of the components are reversed in the next half cycle: Ø1 will be low, S1 will open, Ø2 will be high, and S2 will close. Thus alternatively from each side of the circuit, the output voltage is supplied with 2Vin. the loss incurred in this circuit is low since there is a lack of diode-wired MOSFETs and the threshold voltage problems associated with it.
One of the other advantages of the circuit is that it doubles the ripple frequency since there are two voltage doublers present which supply the output effectively from the phase clocks.
The basic disadvantage of this circuit is that the stray capacitances of the Dickinson multiplier is found to be much less significant than this circuit and thus accounts for the most of the losses which are incurred in this circuit.