Aug 192012
 

 


By definition, the Power Factor ( PF) of an AC power source – that is the ratio of the real power in Watts to the apparent power, calculated as the product of current and voltage. PF = Real Power / Apparent Power. Hence, we can see that the power factor can take values ​​only in the range between 0 and 1. Therefore, when the current and voltage are sinusoidal and are in phase – PF = 1. If the current and voltage are sinusoidal and out of phase, the apparent power will be greater than the real power and in this case, the power factor is equal to the cosine of the angle between current and voltage. The power factor is equal to 1 in the ideal case when the load is purely resistive and linear. In real electronic offline power supply systems of the type AC / DC, the power source has a certain pulse nature and is characterized as a non-linear load. Nowadays, namely the switch power supplies, distorting the sinusoidal shape of the input voltage and current and changing the phase angle between them, are the most commonly used ones.

Whenthe phases of thecurrent and voltageare not the same, the power factor is lowerthan 1.Besideslosses, the power factorwhich is lower than 1,causesthe appearance ofharmonicsthat shift the voltageneutralandbadlyaffectthe operation ofother devicesconnected to the network. Thelowerpower factor, the greaterthe presence ofharmonicsin the networkand vice versa.For this reason,there are strictrules limitingthe level ofnon-linear distortionspermitted inACnetworks. For example,in Europe there isastandardEN61000-3-2,defining the allowancefor letting harmonicsofelectronic devicesback intothe network.It applies toall ClassD electronic systems (computers, laptops, monitors, TVsandtuners) consuming more than75W.Nowadays, this standardis accepted at international level.

To meet the requirements of the standards regarding the level of nonlinear distortions and to maintain the high value of the power factor in AC/DC conversion devices for supplying  electronic devices consuming more than 75 W, it is necessary to provide power factor correction (PFC). The presence of such a corrector provides large values of the power factor and ensures a reduction of harmonics in the AC circuit. There are many passive and active circuits for power factor correction.

Passive power factor correctors

The simplest way to control the current harmonics is using a passive filter, transmitting only electricity with network frequency (50-60Hz). This filter reduces the harmonic components of the current and thus the plugged in non-linear device begins to resemble by characteristics a linear one. Using the filter, composed of capacitors and inductors, the power factor can be brought close to 1. However, the disadvantage of this solution is the need to use high-ampere coils and high-voltage capacitors, which occupy much space and are quite expensive.

fi

Figure 1. The comparison of the curves shows that the active controller of the power factor in the power source significantly excels the passive correction and with a large stock meets the requirements of the European standard EN / IEC61000-3-2 in terms of the level of harmonics in the AC network.

 

Figure 1 displays the input harmonics of three different computer sources of 250 W in relation to the restriction of the standard EN / IEC61000-3-2 for class D devices. The amplitudes of the harmonics are proportional to the input power of these devices. The passive power factor corrector provides correspondence with the standard only regarding the level of the third harmonic. The power supply, where there is a circuit with an active power factor corrector not only meets the standard EN / IEC61000-3-2, but significantly exceed its requirements. Regardless of the simple technical solution and the frequent use the circuits which have passive power factor correctors have a variety of disadvantages.

 

First, the dimensions of the coil with the inductance create some limitations regarding its application in many devices. Second, to make sure you can use the device in different countries you need to obtain an input voltage switch (110 / 220V). This in turn increases the risk of burning the unit due to misuse of the user. And finally, the supply voltage cannot be adjusted, which affects the cost and efficiency of the DC / DC converter.

 

Active power factor correctors

 

Along with the good features, the growth in the prices of copper and the material of magnetic cores, combined with the constant decrease in the value of semiconductor components, the scales tilt in favor of the use of active power factor correctors. There are three main types of microcircuits for active cosφ correctors: 1. Critical conductivity mode. 2. Continuous conductivity mode. 3 Interrupted conductivity mode. Correctors of this type are offered by different manufacturers, each using their own arguments to justify the appropriateness and the scope of the various correctors. The control circuit in the critical conductivity mode holds the current in the inductive coil on the boundary between continuous and interrupted conductivity.

 

Some manufacturers prefer to call this mode – boundary conductivity mode. To correct the power factor, microcircuits use a method of managing the switching time, allowing the simultaneous fulfillment of the functions of cos φ corrector and regulation of the input voltage. Due to the peak level reduction, the leveling out of the pulsations and the filtering simplification, the continuous conductivity mode is widely used when the power is medium or high. The interrupted conductivity mode is preferred for devices with medium and low power.

 

When creating an active controller of cos φ, digital technologies are embedded which allow the exclusion of some external components that are required in analog devices and searching for new solutions with little losses for supplying power to laptop computers and digital TV receivers. Microcircuits use algorithms with switching frequency control and timing control in order to achieve a power factor close to 1 and to reduce electromagnetic radiation. In conclusion we can say that due to the strict requirements of standards such as EN / IEC61000-3-2 and its international analogues, the number of manufacturers of power factor correctors has increased in recent years, giving developers greater opportunities to create solutions to improve the power factor at low losses and with minimal number of components.

 

Коментари от Google+

 Leave a Reply

(required)

(required)


You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>