Electronic power supply device

Details Electronic power supply device Electronic power supply device

1997-11-18

2001-04-17

Berhane, Adolf Deneke (Department: 2838)

Electric power conversion systems

Current conversion

Using semiconductor-type converter

Reexamination Certificate

active

06219263

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an electronic supply device. More particularly, it relates to a device with a power factor correction circuit.
2. Description of the Prior Art
There is a known circuit for the correction of power factor without line inductance. The circuit is connected to the output of a bridge rectifier and uses rectifier diodes for the series charging and parallel discharging of two filtering capacitors. An electrical diagram of such a circuit is shown in
FIG. 1. A
bridge rectifier
1
receives a periodically varying voltage V
AC
at a pair of input terminals. A power factor correction circuit
3
is connected between the rectifier
1
and a load
2
at a pair of output terminals of the rectifier. This circuit
3
uses two filtering capacitors, C
1
and C
2
, having the same capacitance.
A first rectifier diode D
1
is connected directly between the two capacitors C
1
and C
2
. The assembly is connected between two output terminals of the bridge rectifier.
A second rectifier diode D
2
is reverse-connected in parallel with the combination of the first capacitor C
1
in series with the first diode D
1
.
A third rectifier diode D
3
is reverse-connected in parallel with the combination of the second capacitor in series with the first diode D
1
.
The working of such a circuit shall now be explained with reference to the curves shown in FIG.
2
.
In a steady operating state, at a start of a half-wave of the line voltage V
in
, the diode D
1
is off. The diodes D
2
and D
3
are on. This corresponds to the end of the period of the discharging of the capacitors C
1
and C
2
. When the line voltage V
in
exceeds the charging voltage of the capacitors, the diodes D
2
and D
3
go to the off state. Then, when the line voltage V
in
exceeds the sum of the charging voltages of the two capacitors (Vc1+Vc2), the diode D
1
becomes conductive (T
1
) and the two capacitors are charged in series until the line voltage reaches its peak value Vc (T
2
). The diode D
1
then goes back to the off state. The two capacitors are each charged at Vc/2 (being identical capacitors).
The line voltage, which then decreases, becomes lower than this charging voltage Vc/2: the diodes D
2
and D
3
therefore come on, while D
1
remains off (T
3
). The capacitors are again parallel-connected. The capacitor C
1
supplies the load through the diode D
3
and the capacitor C
2
supplies the load through the diode D
2
. This process stops as soon as the line voltage V
in
again starts increasing (at the next half-wave) and becomes greater than the voltage of each capacitor: the diodes D
2
and D
3
go back to the off state, the diode D
1
remains off. The system is then at T
0
, and the cycle then repeats. The current waveform I
in
shown in
FIG. 2
is obtained.
Between T
0
and T
1
, it is the mains supply system (V
AC
) that directly supplies the load (with D
1
, D
2
and D
3
off). The shape of the current waveform for a value of power P
out
consumed in the load
2
is given by the relationship:
I
in(t)
=P
out
/V
in(t)
For Pout constant, between T
0
and T
1
, V
in
increases and I
in
decreases.
Between T
1
and T
2
, the capacitors are charged. On top of the current consumed in the load
2
(shown in dashes), there is superimposed the charging current for the capacitors.
Between T
2
and T
3
, the charging of the capacitors, each at half of the peak voltage Vc, is over. The current I
in
is only the current consumed in the load
2
and the waveform of the current is given by the relationship:
I
in(t)
=P
out
/V
in(t)
.
The line voltage decreases and I
in
decreases (with P
out
constant).
Finally, between T
3
and T
0
, it is the capacitors C
1
and C
2
that supply the load
2
. The current I
in
drawn from the rectifier is zero.
The circuit
3
therefore makes it possible to increase the angle of flow of the bridge rectifier. The waveform of the current I
in
is spread over the voltage half-wave with three phases of conduction: [T
0
-T
1
], [T
1
-T
2
] and [T
2
-T
3
]. In this way, the power factor of the device (namely the ratio of the actual power to the apparent total power) is improved since the line is forced to consume current during the most significant part of the voltage wave, namely when the instantaneous value of the line voltage exceeds half of the peak value Vc.
However, for the charging of the capacitors, there is a drawing of charging current which gives rise to a steep leading edge of the line current. There is therefore a current peak. This corresponds to non-negligible low frequency harmonic contents that limit the value of the power factor (with a supply of power at harmonic frequencies different from the line frequency).
SUMMARY OF THE INVENTION
An object of the invention is to improve the afore-mentioned power factor correction circuit.
An object of the invention is to reduce the low frequency harmonic contents of the waveform of the current drawn from the rectifier.
As characterized, the invention relates to an electronic supply device for a load comprising a bridge rectifier receiving a periodic voltage at a pair of input terminals and a power factor correction circuit connected to a pair of output terminals of the rectifier. The power correction circuit includes two capacitors, a rectifier diode to charge them in series and two rectifier diodes to discharge them in parallel. According to the invention, the correction circuit further includes a resistor that is series-connected to the first rectifier diode to limit the current drawn in the capacitors and reduce the low frequency harmonic contents of the current conducted by the rectifier.
The addition of a resistor in series with the diode that enables the control of the charging of the capacitors in series makes it possible to attenuate the charging current. This results in a more rounded-out waveform of the line current: the low frequency harmonic contents of such a waveform are highly attenuated. The power factor of this device is thus appreciably improved.
Furthermore, when the voltage is turned on, the capacitors are charged immediately. However, the resistor, in addition to attenuating low frequency harmonic contents, will limit the drawn current which, if excessively high, damages the diodes and the capacitors.
In one improvement of the invention, a particular three-diode structure of the power factor correction circuit according to the invention is used to protect the circuitry downline with respect to the rectifier against overvoltages on the mains supply system.
According to the invention, zener diodes are used as rectifier diodes. In the event of overvoltage in the mains supply system, the three zener diodes are series-connected. The circuitry is therefore protected against overvoltages greater than three zener voltages. Each capacitor is protected against overvoltages greater than two zener voltages.
One variant uses a current-controlled power switch parallel-connected with the resistor and the diode which controls the series charging of the capacitors. A zener diode is used for each of the two diodes that controls the discharging of the capacitors. In this way, it is possible to protect the circuitry against overvoltages greater than two zener voltages and the capacitors against overvoltages greater than one zener voltage.


REFERENCES:
patent: 4186418 (1980-01-01), Seiler
patent: 5345164 (1994-09-01), Lesea
patent: 0 602 908 (1994-06-01), None
patent: 0 600 340 (1994-06-01), None
patent: WO 91/02400 (1991-02-01), None
patent: WO 92/22953 (1992-12-01), None

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