Power factor correction circuit

Details Power factor correction circuit Power factor correction circuit

2001-03-15

2002-10-15

Patel, Rajnikant B. (Department: 2838)

Electricity: power supply or regulation systems

In shunt with source or load

Using choke and switch across source

C363S040000

Reexamination Certificate

active

06465990

ABSTRACT:

TECHNICAL FIELD
This invention relates to an active Power Factor Correction (“PFC”) circuit. More particularly, the present invention describes several methods and an apparatus for supporting a Parallel Charge, Series discharge (“PCSD”) system, to be used in a Power Factor Correction process between a generator and a load, which includes reactive components.
BACKGROUND
In the process of transferring AC electrical power from a generator to a load, in addition to the efficiency, the power factor is a very important parameter. The Efficiency (&eegr;) is technically the ratio of Output Power (measured in Watts) to the consumed Input Power (measured also in Watts) and often expressed as a percentage (0 to 100%).
&eegr;(%)=100×Output Power (
W
)/Input Power (
W
); 0%<&eegr;<100%
This above expression illustrates that the Efficiency (&eegr;) of an electrical system is determined only by the parameters of Output Power and Input Power. The ideal efficiency is 100%, meaning there is absolutely no loss in the generator-load circuit. In real AC/DC converter situations, the efficiency is often as low as 60%. In the most modem designs, by using new system architectures and suitable parts, converters with efficiencies of greater than 95% are possible.
Power Factor (“PF”) is traditionally known as the cosine of the phase difference between a sinusoidal source voltage and the corresponding load current waveform. In fact, this thinking is only valid when the input current waveform is sinusoidal. For the general case and valid for all situations, PF is the ratio of Real Input Power consumed to Apparent Input Power and is expressed as a decimal fraction between 0 and 1. The Real Power (in some other publications may be called “True Power”, “Average Power” or “RAM Watts (sic) Power”) is measured in Watts (W) and the amount is equal to the time integral of the input voltage and input current product. The Apparent Power is measured in Volt-Amperes (VA) and the amount is equal to the product of the rms input voltage and the rms input current.
PF
=Real Input Power (
W
)/Apparent Input Power (
VA
); 0
<PF
(
W/VA
)<1.
This above expression shows that the PF of an electrical system refers only to characteristics of the Input Power. Since the Output Power is not involved in the PF expression, a low Power Factor is not necessarily related to low efficiency, but rather with a low utilization of the Apparent Input Power. The ideal value for the Power Factor is PF=1, which means that the Real Input Power is equal in amount to the Apparent Input Power. This happens only when the entire load circuit is purely resistive, because resistors do not change the current phase or current shape from the source voltage. For example, for a simple AC generator-resistive load circuit (FIG.
1
A), the shape and phase of the current waveform of the generator's circuit (
FIG. 5B
) is identical to the shape and phase of the voltage waveform generated into the circuit (FIG.
5
A), assuming the voltage drop of the bridge rectifier is negligible. For this case the Real Input Power is equal to the Apparent Input Power, so PF=1.
However, when the load is non-linear, time varying, or contains reactive elements (e.g., capacitors and inductors), the current waveform in the generator's circuit becomes very different in shape and/or phase than the voltage waveform and PF decreases. In the “bridge rectifier-bulk capacitor” case (FIG.
1
B), the capacitor Cload acts a storage device (exactly the desired filtering function), keeping the DC voltage across the load close to the peak of the rectified input voltage. This means that the capacitor charges only for a small part of the AC cycle, i.e., the capacitor only charges during the time when the AC peak voltage (minus the small voltage lost on the bridge rectifier) exceeds the instantaneous capacitor voltage. The capacitor stops charging just as the AC voltage reaches its maximum peak value, because after that moment, there is no more current through the rectifier bridge (as long the load's voltage is higher than the generator's voltage, BR's diodes are reverse-biased). This results in a current pulse lasting typically for only 1 or 2 mS of the 8.33 mS half cycle (based on a 60 Hz source). This pulse waveform (see
FIG. 5C
) is a dramatically different shape from the incoming sine wave voltage and many harmonics are created in the current's waveform spectrum. Since only the fundamental 60 Hz component of the current can contribute to the Real Power, the current from each harmonic increases the Apparent Power amount. This also increases THD (Total Harmonic Distortion), typically to over 100%, and decreases the PF down to typically less than 0.65. Thus, the rms input current is higher than otherwise necessary, so the electrical utilities need to have more generating and distributing capacity. In addition, because the impedance of the power lines and distribution transformers is not zero, the harmonic currents distort the voltage waveform and can cause problems with other equipment.
Virtually all existing electrical devices supplied from AC power sources have Power Factors less than 1.0 (with the obvious exception of pure resistive loads, such as heating devices and incandescent lamps). Because of the problems that low PF creates, PF correction is desirable for a wide range of electrical devices. However, the extra cost and complexity of the PF circuit have to be weighed against the advantages of improved PF. Also, in a given system, by attaching a PF circuit, the overall efficiency of the entire system will almost always decrease due to the less than 100% efficiency of the PF circuit itself. (Exceptions exist for cases where the addition of PF Correction enables improved efficiency in the rest of the system, by increasing the internal DC operating voltage or by other effects.) Therefore, PF, Harmonic Distortion, cost and efficiency are the most important parameters of a PFC circuit.
Presently, worldwide, the number of electrical devices containing internal electronic circuits (such as computers, TV sets and computer monitors, stereos, industrial equipment, telecommunication equipment, etc.) is increasing dramatically every day. These devices require an internal DC voltage supply, obtained by converting the AC current available from the AC power line. Often a simple circuit like the one illustrated in
FIG. 1B
is used for this AC to DC conversion. Therefore, there is a worldwide need to resolve low PF issues efficiently and cost effectively. As an example of the seriousness of the problems caused by low PF equipment, Japan and the European Union have set standards for PF and Harmonic Distortion. These standards cover a wide variety of electrical devices.
Although there are methods to increase PF and lower THD using passive components (inductors and capacitors), generally the size and cost of the passive components is prohibitive for most common electronic equipment. Considerable effort has been expended over the last 15 years to develop so-called active methods of power factor correction.
FIG. 1C
illustrates the core of most classic active PFC systems and contains an inductor, L, a switching diode, D, and an electronic switch, SW. Four terminals, Vin
0
, Vin
1
, Vout
0
, and Vout
1
connect this block to the external circuit. Commonly, an AC voltage generator, Gac, provides energy to the system through a Low Pass Filter, LPF, and a bridge rectifier, BR. (The purpose of LPF is to reduce substantially the amount of electrical high frequency noise generated in the PFC circuit that appears as high frequency currents in Gac.) The unfiltered full-wave rectified output from BR appears with the positive pole at Vin
1
with respect to Vin
0
. A continuous series of high frequency pulses is delivered through D to the reactive load, Zload, that includes a resistive load and a large value bulk storage capacitor. The polarity is positive at Vout
1
with respect to Vout
0
. The electronic switch SW (usually a power MOSFET) switch

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