Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Patent
1994-10-21
1997-06-10
Wong, Peter
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
363 81, H02M 542
Patent
active
056382650
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates, generally, to methods and apparatus for implementing a power supply having a power factor substantially equal to one while substantially eliminating line harmonics, and more particularly to a technique involving the generation of an ideal synthesized current reference waveform which is independent of the AC line voltage waveform and which is employed in the control loop for controlling the duty cycle of the switch mode DC power supply as used in AC-DC, AC-AC, and DC-DC converter applications.
BACKGROUND ART AND TECHNICAL PROBLEMS
For an ideal utility power delivery system characterized by an AC line voltage V.sub.in, a line impedance Z.sub.in, and an AC current I.sub.in, the power delivered to a load is the dot product of the voltage across the load and the current running through the load, or P.sub.out =V.sub.out .multidot.I.sub.out cos(.theta.) where .theta. represents the phase difference between the voltage across the load and the current running through the load. Maximum power is thus most efficiently delivered to the load when the phase angle of the current coincides with the phase angle of the voltage at the load (cos(.theta.)=1), corresponding to a power factor of one.
Power distribution systems typically supply power for loads which are both purely resistive as well as loads which exhibit impedances having substantial reactive components, for example electric motors, power supplies (converters), fluorescent and HID lighting, and the like. The reactive component of load impedance, whether capacitive or inductive, shifts the phase angle of the current running through the load with respect to the supply voltage, resulting in a proportional decrease in power factor at the load and corresponding reduction in the efficiency of the power distribution system. Stated another way, for power factors less than one, an electric utility power company must provide more "power" than is actually consumed by the various loads connected to the power distribution system.
Power factor correction techniques are generally well known. Typically, electronic devices having a reactive component equal in magnitude but opposite in sign to the reactive component of the impedance exhibited by the load are placed in parallel with the load; for example, the common technique is to place a bank of capacitors in parallel across an inductive motor to cancel the inductive reactance produced by the motor. In this way, the power factor is corrected to unity, and the overall impedance of the load appears purely resistive from the perspective of the source (e.g. the power company).
Reactive loads which draw current in a nonlinear fashion are considerably more problematic, however, particularly to the extent that the line current harmonics resulting from the nonlinearities are reflected back to the power source.
Presently known power converters, for example AC-DC converters, and in particular those employing silicon rectifiers, tend to exhibit discontinuous supply line current drawing characteristics, i.e., nonlinear load characteristics. In such systems, current flowing through the load is typically zero until the AC supply voltage exceeds a first DC conduction threshold voltage defined by, inter alia, the rectifier circuit. Thereafter, the current through the load increases sharply, limited primarily by line impedance. The current level again returns to zero as the AC supply voltage drops below a second DC conduction threshold voltage, typically defined by the filter capacitor and the rectifier circuit. Consequently, the diode conduction angle is restricted to a relatively small angular region centered around .pi./2 radians in the AC line voltage, which constitutes a comparatively small fraction of the total potential conduction angle provided by a rectified sine wave. As a result of this reduced conduction angle, substantially all of the power consumed by the load is drawn during a small portion of the AC cycle, resulting in very high current peaks and, hence, very high peak-to-RMS curr
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Berhane Adolf
Lecher Michael A.
Phillips James H.
Wong Peter
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