When solving circuit problems, there are times when it is necessary to get away from the use of transformers to increase the output voltage. The reason for this most often turns out to be the impossibility of including step-up converters in devices due to their weight and size indicators. In such a situation, the solution is to use a multiplier circuit.
Voltage Multiplier Definition
A device, which means an electricity multiplier, is a circuit that allows you to convert AC or pulsating voltage to DC, but of a higher value. The increase in the value of the parameter at the output of the device is directly proportional to the number of stages of the circuit. The most elementary voltage multiplier in existence was invented by scientists Cockcroft and W alton.
Modern capacitors developed by the electronics industry are characterized by small size and relatively large capacitance. This made it possible to rebuild many circuits and introduce the product into different devices. A voltage multiplier was assembled on diodes and capacitors connected in their own order.
In addition to the function of increasing electricity, multipliers simultaneously convert it from AC to DC. This is convenient because the overall circuitry of the device is simplified and becomes more reliable and compact. With the help of the device, an increase of up to several thousand volts can be achieved.
Where the device is used
Multipliers have found their application in various types of devices, these are: laser pumping systems, X-ray wave radiation devices in their high voltage units, for backlighting liquid crystal displays, ion-type pumps, traveling wave lamps, air ionizers, electrostatic systems, particle accelerators, copying machines, televisions and oscilloscopes with kinescopes, as well as where high, low-current DC electricity is required.
The principle of the voltage multiplier
To understand how the circuit functions, it is better to look at the operation of the so-called universal device. Here the number of stages is not exactly specified, and the output electricity is determined by the formula: nUin=Uout, where:
- n is the number of circuit stages present;
- Uin is the voltage applied to the input of the device.
At the initial moment of time, when the first, say, positive half-wave comes to the circuit, the input stage diode passes it to its capacitor. The latter is charged to the amplitude of the incoming electricity. With a second negativehalf-wave, the first diode is closed, and the semiconductor of the second stage lets it go to its capacitor, which is also charged. Plus, the voltage of the first capacitor, connected in series with the second, is added to the last one and the output of the cascade is already doubled electricity.
The same thing happens on each subsequent stage - this is the principle of a voltage multiplier. And if you look at the progression to the end, it turns out that the output electricity exceeds the input by a certain number of times. But as in a transformer, the current strength here will decrease with an increase in the potential difference - the law of conservation of energy also works.
Scheme for constructing a multiplier
The whole chain of the circuit is assembled from several links. One link of the voltage multiplier on the capacitor is a half-wave type rectifier. To obtain a device, it is necessary to have two series-connected links, each of which has a diode and a capacitor. Such a circuit is a doubler of electricity.
The graphical representation of the voltage multiplier device in the classic version looks with the diagonal position of the diodes. The direction of switching on the semiconductors determines which potential - negative or positive - will be present at the output of the multiplier relative to its common point.
By combining circuits with negative and positive potentials, a bipolar voltage doubler circuit is obtained at the output of the device. A feature of this construction is that if you measure the levelelectricity between the pole and the common point and it exceeds the input voltage by 4 times, then the magnitude of the amplitude between the poles will increase by 8 times.
In the multiplier, the common point (which is connected to the common wire) will be the one where the output of the supply source is connected to the output of a capacitor grouped with other series-connected capacitors. At the end of them, the output electricity is taken on even elements - at an even coefficient, on odd capacitors, respectively, at an odd coefficient.
Pumping capacitors in the multiplier
In other words, in the device of the constant voltage multiplier, there is a certain transient process of setting the output parameter corresponding to the declared one. The easiest way to see this is by doubling electricity. When the capacitor C1 is charged to its full value through the semiconductor D1, then in the next half-wave, it, together with the source of electricity, simultaneously charges the second capacitor. C1 does not have time to completely give up its charge to C2, so the output does not initially have a double potential difference.
At the third half-wave, the first capacitor is recharged and then applies a potential to C2. But the voltage on the second capacitor already has an opposite direction to the first. Therefore, the output capacitor is not fully charged. With each new cycle, the electricity on the C1 element will tend to the input, the C2 voltage will double in size.
Howcalculate multiplier
When calculating the multiplication device, it is necessary to start from the initial data, which are: the current required for the load (In), the output voltage (Uout), the ripple coefficient (Kp). The minimum capacitance value of capacitor elements, expressed in uF, is determined by the formula: С(n)=2, 85nIn/(KpUout), where:
- n is the number of times the input electricity is increased;
- In - current flowing in the load (mA);
- Kp – pulsation factor (%);
- Uout - voltage received at the output of the device (V).
Increasing the capacitance obtained by calculations by two or three times, one obtains the value of the capacitance of the capacitor at the input of circuit C1. This value of the element allows you to immediately get the full value of the voltage at the output, and not wait until a certain number of periods have passed. When the work of the load does not depend on the rate of rise of electricity to the nominal output, the capacitance of the capacitor can be taken identical to the calculated values.
Best for the load if the ripple factor of the diode voltage multiplier does not exceed 0.1%. The presence of ripples up to 3% is also satisfactory. All diodes of the circuit are selected from the calculation so that they can freely withstand a current strength twice its value in the load. The formula for calculating the device with high accuracy looks like this: nUin - (In(n3 + 9n2/4 + n/2)/(12 f C))=Uout, where:
- f – voltage frequency at the device input (Hz);
- C - capacitor capacitance (F).
Benefits anddisadvantages
Speaking of the advantages of a voltage multiplier, we can note the following:
The ability to get significant amounts of electricity at the output - the more links in the chain, the greater the multiplication factor will be
- Simplicity of design - everything is assembled on standard links and reliable radio elements that rarely fail.
- Weight – the absence of bulky elements such as a power transformer reduces the size and weight of the circuit.
The biggest drawback of any multiplier circuit is that it is impossible to get a large output current from it to power the load.
Conclusion
Choosing a voltage multiplier for a particular device. it is important to know that balanced circuits have better parameters in terms of ripple than unbalanced ones. Therefore, for sensitive devices it is more expedient to use more stable multipliers. Asymmetrical, easy to make, contains fewer elements.
