Electricity: power supply or regulation systems – In shunt with source or load – Using choke and switch across source
Reexamination Certificate
2000-11-13
2001-09-25
Patel, Rajnikant B. (Department: 2838)
Electricity: power supply or regulation systems
In shunt with source or load
Using choke and switch across source
C363S065000, C363S097000
Reexamination Certificate
active
06294900
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention relates to the field of electrical power supplies, and particularly although not exclusively to a bi-directional AC or DC voltage regulator.
2. Background Art
A conventional AC variable transformer (a variac) for stepping down a mains voltage, for example 230 volts AC to a reduced Ac voltage, comprises AC voltage input terminals, across which is connected an inductive winding, and AC output terminals, which draw power from the winding at a selectable voltage, depending upon where a wiper blade is positioned along the winding. The wiper is typically a rotating wiper which rotates across the winding which is formed in a substantially cylindrical or ring shape. The wiper may be driven by a servo motor, in order to automatically move the wiper, thus varying the output voltage in response to a control signal.
However, the conventional variac has the problems of high weight, large size and poor response time in moving the wiper blade, and produces noise which is fed back onto the mains supply and through to the output terminals.
An apparatus which has been used in DC power systems to transform and regulate voltage is the Ćuk converter. Such a device is described in U.S. Pat. No. 4,186,437 and in a paper entitled “Topologies of Bi-directional PWM DC—DC Power Converters” from the 1993 IEEE National Aerospace and Electronics Conference. A basic topology Ćuk converter, as illustrated in
FIG. 1
of the drawings, has a circuit comprising input and output choke inductances L
1
and L
2
, an energy-transfer capacitor C
1
, an output smoothing capacitor C
2
, a diode D
1
and a switching transistor Q
1
.
This arrangement permits the DC output voltage to be stepped up or stepped down for a given input voltage depending on the proportion of time the transistor Q
1
conducts during a period of its operation. This ratio is known as the duty cycle of the transistor.
During a first time interval when the transistor Q
1
is off, the diode D
1
is forward biased and the capacitor C
1
is charged in the positive direction through the inductor L
1
. During a second time interval, the transistor Q
1
is turned on, and the capacitor C
1
becomes connected across the diode D
1
, reverse biasing it. Thus the capacitor C
1
discharges through the load and the output inductance L
2
, charging the output capacitor C
2
to a negative potential. The circuit operation is repeated when the transistor Q
1
is turned off again.
The DC output voltage V
out
is dependent upon a number of parameters. Firstly the input voltage V
in
naturally effects the voltage value across the output terminals of the converter. If all other parameters are kept constant and the input voltage V
in
is increased, the DC output voltage of the converter will also increase. As discussed previously, the duty cycle (&dgr;) of conduction of the transistor Q
1
is another parameter which effects the DC output voltage V
out
. A high duty cycle (&dgr;) may yield a stepped up voltage at the output terminals, while a low duty cycle (&dgr;) will produce an output voltage V
out
which is smaller in magnitude than the input voltage V
in
. The remaining principal parameter which controls converter performance is the converter circuit efficiency (&egr;).
It has been evaluated that the voltage relationship between the output signal and the input signal is as follows:
V
out
/V
in
=&dgr;&egr;/(1−&dgr;)
Further extensions of the converter, illustrated in
FIGS. 2
to
4
, have a similar operation to that discussed above.
FIG. 2
shows a Ćuk converter in which the input and output choke inductors L
1
and L
2
are coupled by a common core. There are obvious advantages in developing the converter in this way, namely, reductions in converter size, weight and component numbers, while the basic DC-to-DC conversion properties of the converter remain unchanged. Further, it has been shown that a significant reduction in ripple current magnitudes can be achieved by magnetic coupling of the choke inductances L
1
and L
2
.
FIG. 3
illustrates how an isolating transformer TX
1
can be introduced to the Ćuk converter to provide galvanic isolation between the output and the input voltages V
out
and V
in
. As the transformer TX
1
is isolated by the two energy-transfer capacitors C
1
a
and C
1
b,
no DC transformer core magnetization can take place.
The Ćuk converter illustrated in
FIG. 4
has coupling of the input and output inductances L
1
and L
2
and an isolating transformer TX
1
. This converter benefits from the features described above but its basic operation remains unchanged.
The Ćuk converters discussed so far permit only DC voltage/current transformation and allow power to flow in one direction only. In order to fully understand the invention, a further possible extension of the Ćuk converter is described below, with reference to
FIGS. 5 and 6
of the drawings.
Although the converter illustrated in
FIG. 5
is similar to a basic topology Ćuk converter and is essentially a DC regulator, the additional components, a second transistor Q
2
and second diode D
2
, permit bi-directional operation of the converter.
The controlling signals supplying the base of the transistors switch each of the transistors on and off alternately, in anti-phase with each other.
During a first time interval, when the first transistor Q
1
is off and the second transistor Q
2
is on, the first diode D
1
is forward biased and the energy-transfer capacitor Q
1
is charged in the positive direction through the input inductor L
1
.
During a second time interval, when the first transistor Q
1
is on and the second transistor Q
2
is off, the energy-transfer capacitor C
1
is connected across the first diode D
1
, reverse biasing it. Therefore, the energy-transfer capacitor C
1
discharges through the output load and inductance L
2
, and in the process charges the output capacitance C
2
b
to a negative potential.
The circuit operation described above is similar to that of a basic topology Ćuk converter. However, the converter of
FIG. 5
is symmetrical in respect of the inputs and the outputs, and therefore will permit power flow in either direction.
As described previously, use can be made of a common core to couple the input and output choke inductances L
1
and L
2
to reduce ripple, and/or an isolating transformer TX
1
to provide galvanic isolation.
FIG. 6
illustrates the addition of such an isolating transformer TX
1
to the circuit of FIG.
5
.
Ćuk converter technology has been used exclusively to convert a DC input voltage to a DC output voltage, and is essentially uni-directional in terms of power flow. A further example of the prior art is given in U.S. Pat. No. 5,321,597 which discloses a complex Ćuk-like circuit which is primarily used as a galvanic isolation device for DC electrical signals.
The present Application addresses the problem of providing an apparatus which permits bi-directional power flow so as to be able to accommodate regenerative load currents. In a preferred aspect, the invention provides an AC or DC voltage regulator/converter which, while functionally analogous to conventional iron/copper AC transformers, benefits from solid state control so as to permit a reduction in weight, size and cost while improving performance when compared to conventional means.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided a bi-directional AC or DC voltage regulator having a controller, an input circuit and an output circuit, the input and output circuits being capacitively coupled one to the other and being symmetrical one relative to the other, wherein each circuit comprises two terminals across which are connected a capacitor and, in parallel with the capacitor, a series connection of an inductor and a switching network controlled by the controller wherein;
each switching network has two branches in anti-parallel, and
each branch comprises a switching means (Q
1
,Q
2
; Q
3
,Q
4
) for pe
Greenwood Simon Richard
Soar Stephen
Greenwood Simon R.
Patel Rajnikant B.
Zavis Katten Muchin
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