Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
1999-12-08
2001-10-16
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C363S089000, C323S222000
Reexamination Certificate
active
06304465
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power regulator for supplying power to a load efficiently, and more particularly to a power regulator that achieves this by using an active filter.
2. Description of Related Art
As shown in
FIG. 6
, a power supply for various electrical appliances acquires a DC voltage from a commercial AC line using a rectifier circuit
91
and a large capacity smoothing capacitor
92
. The rectifier circuit
91
instantaneously grabs power from the AC line when the AC voltage exceeds a capacitor charged voltage. An inverter
93
operating as a switching circuit converts the DC voltage charged by the capacitor to an AC voltage, and supplies it to a load
94
. As clearly seen from
FIG. 7
, the power is captured only around the peaks of the AC line voltage as illustrated by a solid waveform. Therefore, the effective power is considerably lower than the apparent power of the AC power supply, resulting in a rather low power factor of the electrical appliances of about 50-70%.
In view of this, a power regulator is proposed whose power factor is improved using an active filter
100
as shown in FIG.
8
. In
FIG. 8
, the reference numeral
101
designates an AC power supply;
102
designates a DC power source using a full-wave rectifier;
103
designates a switching device connected in parallel with the DC power source
102
via a coil
104
of the active filter
100
at the output side of the DC power source
102
;
105
designates a charge-and-discharge capacitor connected in parallel with the switching device
103
through a backflow preventing diode
106
;
107
designates an inverter for supplying a fluorescent lamp
108
with an AC voltage generated by switching the charges of the charge-and-discharge capacitor
105
;
109
designates a zero-cross detector for detecting zero-cross points of the AC current;
110
designates an input current detector for detecting consumed current;
111
designates a microcomputer that receives detection signals from the zero-cross detector
109
and input current detector
110
; and
112
designates a driver for bringing the switching device
103
into conduction in response to a control signal from the microcomputer
111
.
Next, the operation of the conventional power regulator will be described.
The DC power source
102
converts the AC output of the AC power supply
101
into a DC output to charge the charge-and-discharge capacitor
105
via the coil
104
and backflow preventing diode
106
. Then, the inverter
107
generates a high frequency AC voltage by switching the voltage charged in the charge-and-discharge capacitor
105
, and supplies it to the fluorescent lamp
108
.
During charging of the charge-and-discharge capacitor
105
, no current will flow from the AC power supply
101
to the charge-and-discharge capacitor
105
in the periods in which the output voltage of the AC power supply
101
is lower than the voltage charged in the charge-and-discharge capacitor
105
. In other words, the charge current flows only when the output voltage of the AC power supply
101
is higher than the voltage charged in the charge-and-discharge capacitor
105
. This will result in a lower power factor and higher frequency noise.
In view of this, Japanese patent application laid-open No. 5-300794/1993 proposes a technique in which the microcomputer
111
carries out its control as follows. First, the zero-cross detector
109
detects the zero-cross of the AC input, and the microcomputer
111
detects the instantaneous values of the input current successively over the half wave from the zero-cross point. The microcomputer
111
compares the instantaneous values with prestored reference values, regulates the ON time of the switching signal of the switching device
103
in response to the difference of these values, and carries out the ON/OFF control of the switching device
103
through the driver
112
such that the input current (that is, the inductor current) can take a sinusoidal waveform in phase with the input voltage as shown in FIG.
9
.
Alternatively, active filter control (for improving power factor) is carried out by a circuit composed of a switching device controlled by a dedicated IC, an inductance and a diode such that the charging is performed even in the duration in which the AC power supply voltage is less than the DC power supply voltage.
In the conventional power regulator with the foregoing arrangement, the periods of the control pulses for bringing the switching device
103
into conduction are predetermined values. This presents a problem of requiring a high performance microcomputer
111
that can instantaneously calculate the consumed current and supply voltage to vary the pulse periods of the control pulses in accordance with the temperature change, circuit constant errors and the like. Furthermore, because bringing the switching device
103
into conduction while the residual field of the coil
104
is not yet sufficiently reduced will cause a short circuit which will cause energy loss and harmonic noise, the pulse periods must be set with considerable margin. This presents a problem of reducing the effect of the power factor improvement.
SUMMARY OF THE INVENTION
The present invention is implemented to solve the foregoing problems. It is therefore an object of the present invention to provide a power regulator capable of providing power to a load efficiently.
According to one aspect of the present invention, there is provided a power regulator comprising: a coil connected to an output of a DC power source for rectifying an AC voltage; a charge-and-discharge capacitor for charging a current supplied from the DC power source through the coil and a backflow preventing diode; an inverter for converting a DC voltage charged in the charge-and-discharge capacitor to an AC voltage to be supplied to a load; a switching device connected, in parallel with the DC power source, to a connection point of the coil and the backflow preventing diode; and a microcomputer for detecting a zero-cross point of one of magnetic field energy generated in the coil and current flowing through the coil, and for bringing the switching device into conduction in response to detection of the zero-cross point.
Here, the microcomputer may comprise a one-shot timer for counting a time period designated by the microcomputer and for supplying the switching device with a control signal to bring it into conduction for the time period, and an A/D converter for converting the voltage charged in the charge-and-discharge capacitor and a voltage at the connection point of the coil and the backflow preventing diode into digital values, wherein the microcomputer may activate the one-shot timer when the voltage at the connection point of the coil and the backflow preventing diode drops below the voltage charged in the charge-and-discharge capacitor to bring the switching device into conduction.
The microcomputer may control the time period to be counted by the one-shot timer in response to a difference between the voltage charged in charge-and-discharge capacitor and a preset reference voltage.
Timing for setting the time period to be counted by the one-shot timer may be synchronized with zero-cross points of the AC voltage.
The microcomputer may inactivate the one-shot timer when the voltage charged in the charge-and-discharge capacitor exceeds a maximum reference voltage.
REFERENCES:
patent: 5793623 (1998-08-01), Kawashima et al.
patent: 5-300794 (1993-11-01), None
patent: 8-35712 (1996-02-01), None
Burns Doane , Swecker, Mathis LLP
Mitsubishi Electric Semiconductor System Corporation
Patel Rajnikant B.
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