When solving circuitry 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 is most often the inability to include boost converters in the devices because of their overall dimensions. In such a situation, the solution is to use a multiplier circuit.
Voltage Multiplier - Definition
A device by which an electricity multiplier is meant is a circuit that allows you to convert an alternating current voltage or a pulsating voltage to a constant, but higher in value. The increase in the parameter value at the output of the device is directly proportional to the number of cascades of the circuit. The most elementary of the existing voltage multipliers was invented by scientists Cockcroft and Walton.
Modern capacitors developed by the electronics industry are characterized by their small size and relatively large capacity. This allowed us to rebuild many schemes and implement the product in different devices. Assembled voltage multiplier on diodes and capacitors connected in their own order.
In addition to the function of increasing electricity, multipliers simultaneously convert it from alternating to constant. This is convenient because the general circuitry of the device is simplified and becomes more reliable and compact. Using the device, you can achieve an increase of up to several thousand volts.
Where is the device used
Multipliers have found their application in various types of devices, such as: laser pumping systems, X-ray wave emission devices in their high voltage units, for illuminating displays of a liquid crystal structure, ion-type pumps, traveling-wave lamps, air ionizers, electrostatic systems, particle accelerators, copy machines, televisions and oscilloscopes with picture tubes, and also where high constant electricity of small current strength is required.
The principle of operation of the voltage multiplier
To understand how the circuit works, it is better to see the operation of the so-called universal device. Here, the number of cascades is not exactly specified, and the output electricity is determined by the formula: n * Uin = Uout, where:
- n is the number of cascades of the circuit present;
- Uin is the voltage supplied to the input of the device.
At the initial moment of time, when the first, say, positive half-wave arrives at the circuit, the diode of the input stage passes it to its capacitor. The latter is charged to the amplitude of the incoming electricity. In the second negative half-wave, the first diode is closed, and the semiconductor of the second stage sends it to its capacitor, which is also charged. Plus, the voltage of the first capacitor, connected in series with the second, is summed with the last and the output of the cascade already doubles the electricity.
At each subsequent cascade the same thing happens - this is the principle of the 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 the transformer, the current strength here will decrease with increasing potential difference - the law of conservation of energy also works.
Multiplier construction scheme
The entire circuit of the circuit is assembled from several links. One link of the capacitor voltage multiplier is a half-wave type rectifier. To obtain the device, it is necessary to have two series-connected links, each of which has a diode and a capacitor. This circuit is a doubler of electricity.
The graphic image of the voltage multiplier in the classic version looks with the diagonal position of the diodes. The direction of the inclusion of semiconductors determines which potential, negative or positive, will be present at the output of the multiplier relative to its common point.
When combining circuits with negative and positive potentials at the output of the device, a bipolar voltage doubler circuit is obtained . A feature of this construction is that if you measure the level of electricity 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 the capacitor, combined in a group with other series-connected capacitors. At the end of them, the output electricity is taken on even elements - at an even coefficient, at odd capacitors, respectively, at an odd coefficient.
Pumping capacitors in a multiplier
In other words, in the device of the constant voltage multiplier, there is some transient process of establishing a parameter at the output corresponding to the declared one. The easiest way to see this is by the example of a doubling of electricity. When the capacitor C1 is charged to its full value through the semiconductor D1, then in the next half-wave it simultaneously with the source of electricity charges the second capacitor. C1 does not have time to completely give up its charge C2, therefore, at the output, at first there is no doubled potential difference.
At the third half-wave, the first capacitor recharges and then applies a potential to C2. But the voltage on the second capacitor already has a counter direction to the first. Therefore, the output capacitor is not fully charged. With each new cycle, the electricity on element C1 will tend to the input, the voltage C2 to double in value.
How to calculate the multiplier
When calculating the multiplication device, it is necessary to build on the initial data, which are: the current required for the load (In), the output voltage (Uout), the ripple factor (Kp). The minimum capacitance of capacitor elements, expressed in microfarads, is determined by the formula: C (n) = 2.85 * n * In / (Kp * Uout), where:
- n is the number of times the input electricity increases;
- In - current flowing in the load (mA);
- Kp is the pulsation coefficient (%);
- Uout is the voltage received at the output of the device (V).
By increasing the capacitance obtained by the calculations by two or three times, the value of the capacitance of the capacitor at the input of circuit C1 is obtained. This element rating allows you to immediately get the full voltage value at the output, and not wait until a certain number of periods have passed. When the load does not depend on the rate of rise of electricity to the nominal output, the capacitor capacitance can be taken identical to the calculated values.
It is best for a load if the ripple factor of the voltage multiplier on the diodes does not exceed 0.1%. Ripple up to 3% is also satisfactory. All diodes of the circuit are selected from the calculation so that they can easily withstand the current strength, twice its value in the load. The formula for calculating the device with great accuracy looks like this: n * Uin - (In * (n3 + 9 * n2 / 4 + n / 2) / (12 * f * C)) = Uout, where:
- f is the voltage frequency at the input of the device (Hz);
- C is the capacitor capacitance (F).
Advantages and disadvantages
Speaking about the advantages of a voltage multiplier, the following can be noted:
- The ability to receive significant amounts of electricity at the output - the more links in the circuit, the greater the multiplication coefficient will be.
- Simplicity of design - everything is assembled on standard links and reliable radio elements, rarely failing.
- Mass and size indicators - 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 obtain with its large output current to power the load.
Conclusion
Choosing a voltage multiplier for a particular device. it is important to know that symmetric circuits have better parameters in terms of ripple coefficient than asymmetric ones. Therefore, for sensitive devices it is more expedient to use more stable multipliers. Asymmetric easy to manufacture, contain fewer elements.