Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2000-06-27
2001-08-14
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S037000
Reexamination Certificate
active
06275397
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to switching power supplies, and more particularly, to a dynamic power factor correction control circuit for a switching power supply that regulates the current wave shape to match the input voltage wave shape in order to obtain close to unity power factor.
2. Description of Related Art
In view of the ever-increasing number of electronic devices that require a direct current (DC) source voltage, power supply circuits are well known in the art for converting an alternating current (AC) line voltage into a DC voltage. Such power supply circuits are known to include a full wave rectifier that converts the AC line voltage to a haversine signal, and a switching converter that converts the haversine signal to a relatively high DC output voltage level (e.g., 360 volts). The DC output voltage may be further reduced to a lower DC voltage level usable by an electronic device by coupling the power supply circuit to additional DC-to-DC converter circuits. Power supply circuits of this type tend to have a poor effective power factor (i.e., the ratio of true power to apparent power in an AC circuit) since they draw input current in short pulses of high peak value such that the current waveform is not sinusoidal. In order to maximize the actual power that can be drawn from a power supply, it is known to include a power factor correction (PFC) control circuit that controls the magnitude and phase of the input current to be sinusoidal and match the line voltage.
One conventional type of PFC control circuit derives a reference signal for the input current from a multiplier that scales the reference signal according to the deviation of the output voltage from its desired value. The output voltage is sampled by a voltage divider stage to provide an output reference signal that is feed to an error amplifier. The output of the error amplifier is then sampled by a sample/hold stage to scale the multiplication process. A drawback of this type of PFC control circuit is that the output power is dependent upon the square of the input voltage. This is undesirable since the purpose of the PFC control circuit is to either make the load appear to the line as a resistor (i.e., current proportional and in phase to voltage) or to supply power to a load that is relatively constant and independent of line fluctuations while improving its power factor. To address this drawback, it is also known to include a feed forward loop that divides the output of the error amplifier by the square of the input voltage. These circuits have limitations associated with the use of the arithmetic devices (i.e., multipliers and dividers), such as scaling errors, offsets and drifts, as well as increased complexity and associated cost due to the multiplicity of circuit components.
Accordingly, it would be very desirable to provide a simplified power factor correction control circuit for a switching power supply that regulates the current waveshape to match the input voltage waveshape.
SUMMARY OF THE INVENTION
The present invention is directed to a simplified power factor correction circuit that regulates the current waveshape to match the input voltage waveshape, while avoiding the complexity of the prior art power factor correction circuits.
In an embodiment of the invention, a switching power supply comprises an AC rectifier adapted to receive an AC line voltage and provide an input voltage (V
IN
) therefrom having a haversine waveform. A switching power converter is connected to the AC rectifier and provides a DC output voltage (V
OUT
) for a load. The switching power converter comprises an inductor and a power switch adapted to control current in the inductor. A pulse width modulator provides a drive signal to the power switch having a variable duty cycle to regulate current provided to the load by the switching power converter. A power factor correction circuit is adapted to control operation of the pulse width modulator so that a waveshape of the current from the AC rectifier matches the output voltage (V
OUT
) with changes in the input voltage (V
IN
) and load.
More particularly, the power factor correction circuit further comprises a voltage error circuit providing a voltage error signal corresponding to a difference between the output voltage (V
OUT
) and a reference voltage, a differential amplifier circuit generating a current program signal based in part on the voltage error signal, including an amplifier circuit to amplify a scaled input voltage (V
IN
) signal with gain determined by the voltage error signal, and a current error circuit controlling the pulse width modulator based on the current program signal and a current sense signal corresponding to the current from the AC rectifier. The amplifier circuit further comprises a field effect transistor (FET) biased in a saturation condition to thereby provide a resistance that varies in accordance with the voltage error signal. The gain of the amplifier circuit is determined by the resistance. The differential amplifier circuit further comprises a second amplifier circuit adapted to amplify a difference between the scaled input voltage (V
IN
) signal and the amplified scaled input voltage (V
IN
) signal from the first amplifier circuit.
A more complete understanding of the power factor correction control circuit will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings that will first be described briefly.
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Berhane Adolf Deneke
O'Melveny & Myers LLP
Power-One, Inc.
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