Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2000-04-28
2001-08-28
Patel, Rajnikant D. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S084000, C363S037000
Reexamination Certificate
active
06282109
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to power conversion and, more specifically, to a non-isolated power factor corrector and method of regulating the non-isolated power factor corrector.
BACKGROUND OF THE INVENTION
A power converter is a power processing circuit that converts an input voltage or current source waveform into a specified output voltage or current waveform. A switched-mode power converter is a frequently employed power converter that converts an input voltage waveform into a specified output voltage waveform. A boost converter is one example of a switched-mode power converter that is typically employed in off-line applications wherein power factor correction and total harmonic distortion (THD) reduction at the input and a stable, regulated voltage at the output are desired.
A non-isolated power factor correction (PFC) boost converter generally includes a boost inductor and a power switch coupled to the boost inductor. The boost converter further includes a rectifying diode coupled to a node between the boost inductor and the power switch. The boost converter still further includes an output capacitor coupled across an output of the boost converter. The output capacitor is usually large to ensure a constant output voltage. A load is then connected in parallel across the output capacitor. The output voltage (measured at the load) of the boost converter is always greater than the input voltage.
The boost converter generally operates as follows. The power switch is closed (conducting) for a first interval D (D interval). The rectifying diode is reverse-biased, isolating the output capacitor and, therefore, the load from the input of the boost converter. During this interval, the input voltage supplies energy to charge the boost inductor and the inductor current increases. Since the load is isolated from the input voltage, a stored charge in the output capacitor powers the load. Then, for a second interval 1-D (1-D interval), the power switch is opened (non-conducting). The inductor current decreases as energy from both the boost inductor and the input flows forward through the rectifying diode to charge the output capacitor and power the load. By varying a duty cycle of the power switch, the output voltage of the boost converter may be controlled.
The boost converter may be operated in three modes: continuous conduction mode (CCM), discontinuous conduction mode (DCM) or critical mode (CM). The modes are defined by characteristics of the inductor current. More specifically, in CCM, the inductor current is unidirectional and is always greater than zero. In DCM, the inductor current is unidirectional and is equal to zero for a period of time during each switching cycle. In CM, the inductor current is unidirectional and reaches zero only for an instant during each switching cycle.
As previously mentioned, the boost converter, when employed as a power factor corrector, generally provides adequate power factor correction. The power factor is defined as a ratio of the actual power delivered to the load to a product of the voltage and current at the input of the boost converter. The conventional boost converter, however, cannot directly process the AC power available from the AC line. An input full wave rectifier bridge is required at the input of the boost converter to rectify the AC voltage from the AC line. The rectified AC voltage may then be processed by the boost converter. The rectifier bridge is subject to dissipative losses, particularly at low AC line voltages (e.g., 85 to 100 VAC). Power dissipation in the bridge diodes of the rectifier bridge may be as high as 2 to 3% of the total power processed by the power converter. Further, the rectifier bridge may contribute to electromagnetic interference noise generated by the power converter.
As discussed above, the boost converter also contains its own rectifier circuitry, namely, the rectifying diode coupled between the boost inductor and the output capacitor. The rectifying diode may be subject to conduction losses that reduce the efficiency of the boost converter. The combination of AC line rectification (by the rectifier bridge) and switching frequency rectification (by the rectifying diode of the boost converter) reduces the efficiency of the overall power conversion process. Further, while the boost converter provides adequate power factor correction, the output voltage of the boost converter is necessarily greater than the input voltage. The resulting high output voltage may adversely affect the efficiency of other devices to which the boost converter may be connected.
Accordingly, what is needed in the art is a power converter and a controller and method for operation the power converter that overcomes the deficiencies of the prior art.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present invention provides a controller or method for regulating a non-isolated power factor corrector, and a power factor corrector employing the controller or the method. The power factor corrector is adapted to provide a DC output voltage at an output thereof. The power factor corrector has first and second power switches, coupled to an input thereof, that receive unrectified AC power. In one embodiment, the controller includes: (1) a sensor, coupled proximate the input, that senses a polarity of the unrectified AC power and (2) a drive circuit, coupled to the sensor, that: (2a) closes the first power switch and modulates the second power switch to regulate the DC output voltage when the polarity is negative, and (2b) closes the second power switch and modulates the first power switch to continue to regulate the DC output voltage when the polarity is positive.
The present invention introduces, in one aspect, the concept of closing one of the first and second power switches of a power factor corrector during alternate half-cycles of the unrectified AC power and modulating the other power switch to regulate the DC output voltage. By alternately closing one of the first and second power switches, the power factor corrector may be continually adapted to operate with both the positive and negative polarities of the AC power. The power factor corrector may thus avoid, for instance, the use of an input full wave rectifier bridge (or other rectifier topologies) for AC line rectification and the inefficiencies associated therewith.
In one embodiment of the present invention, the sensor is a voltage sensor that senses a polarity of the AC voltage. Those skilled in the pertinent art are familiar with a variety of sensors for sensing voltage polarities, e.g., a comparator or a Schmidt trigger device.
In one embodiment of the present invention, the controller further includes an input current sensor that develops an input current signal representative of an input current to the power factor corrector. The controller operates the first and second power switches based on the input current signal to correct the power factor (i.e., the shape of the input current). Power factor correction is generally desired in off-line applications.
In one embodiment of the present invention, the controller further includes an output voltage sensor that develops an output voltage signal representative of the DC output voltage. The controller operates the first and second power switches based on the output voltage signal to regulate the DC output voltage of the power factor corrector. In a related embodiment, the controller is a pulse-width modulated controller. The drive circuit may thus provide pulse-width modulated drive signals to drive the first or second power switches. In an alternative embodiment, the drive circuit provides a single pulse-width modulated drive signal. The controller further includes a steering circuit that steers the drive signal to an appropriate one of the first and second power switches based on the polarity. Those skilled in the pertinent art are familiar with pulse-width modulation. Of course, other schemes for driving the first and second power s
Fraidlin Simon
Polikarpov Anatoliy
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