Electricity: power supply or regulation systems – External or operator controlled – Phase controlled switching using electronic tube or a three...
Reexamination Certificate
1999-11-22
2001-03-27
Sterrett, Jeffrey (Department: 2838)
Electricity: power supply or regulation systems
External or operator controlled
Phase controlled switching using electronic tube or a three...
C323S239000
Reexamination Certificate
active
06208126
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims priority from prior French Patent Application No. 98-15150, filed Nov. 27, 1998, the entire disclosure of which is herein incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to switches for a load supplied by an AC voltage, and more specifically to a circuit for controlling an AC load using a low voltage control signal.
2. Description of Related Art
The control of an AC load from an AC voltage power system implies the use of a switch which is bidirectional both in current and voltage, that is, a four-quadrant switch. The present invention more specifically applies to such a switch made by means of two distinct switches for the two directions of the current, conversely to a triac, which is bidirectional. In an assembly with two one-way switches, the two switches may be openable (i.e., blockable) according to a constraint of opening under a non-zero current.
A problem which arises in any assembly using two semiconductor switches to make a four quadrant switch is the fact that one of the two switches must be controlled with a floating potential. This problem is illustrated by
FIG. 1
which shows a conventional example of a four-quadrant switch
1
in series with a load
2
between two terminals E
1
and E
2
of an AC voltage source Vac (e.g., a main of 240V/50 Hz or 110V/60 Hz). In the example of
FIG. 1
, switch
1
is formed of two thyristors
3
and
4
mounted in antiparallel between a terminal A of load
2
and terminal E
2
, for example, the neutral of the AC power supply forming the assembly ground.
The use of thyristors implies an application in which it is not necessary to open switch
1
under a non-zero current, since thyristors are turned off by the disappearance of the current flowing therethrough. To have a possibility of opening under a non-zero current, blockable components, for example, MOS transistors, are used, as will be seen hereafter. Thyristors
3
and
4
are controlled from a control block
5
supplied, generally, by a low DC voltage Vdc provided by an auxiliary power supply. Control block
5
provides, in conventional assemblies, two control signals s
1
and s
2
intended, respectively, for thyristors
3
and
4
.
The control of thyristor
3
by signal s
1
raises no particular reference problem, since the anode of this thyristor is connected to the same ground as control block
5
. However, the control of thyristor
4
must be performed via an isolation block
6
, since the thyristor cathode is connected to node A. In the example of
FIG. 1
, isolating block
6
is an optocoupler, the emitting diode
7
of which receives, on its anode, signal s
2
provided by control block
5
and the cathode of which is connected to the ground E
2
common to control block
5
and to AC load
2
(via switch
1
). Phototransistor
8
of optocoupler
6
is connected, by its collector, to gate g
4
of thyristor
4
and, by its emitter, to node A.
The biasing of phototransistor
8
requires a DC auxiliary supply Vdc′, different from supply Vdc of control block
5
. Indeed, the emitter of phototransistor
8
must be connected to a reference potential M′, which is necessarily different from the ground E
2
of the rest of the assembly, and thus from the reference potential of voltage Vdc. Isolation block
6
thus is generally supplied by a voltage Vdc′ referenced to potential M′. The operation of a bidirectional switch
1
such as shown in
FIG. 1
is well known. Control block
5
organizes the respective conduction states of thyristors
3
and
4
according to the halfwave of AC supply Vac and according to a control reference which depends on the application.
A disadvantage of a circuit such as that shown in
FIG. 1
is that the use of an optocoupler requires two galvanically isolated auxiliary power supplies Vdc and Vdc′ having different references. To replace optocoupler
6
of
FIG. 1
, a pulse transformer
9
may also be used, as illustrated in FIG.
2
. For simplification,
FIG. 2
does not show all of the elements of the assembly of FIG.
1
. Only transformer
9
is shown therein, the rest of the assembly is similar. A first winding
10
of transformer
9
is connected between terminal A and gate g
4
of thyristor
4
(FIG.
1
), that is, similarly to the connection of phototransistor
8
. A second winding
11
of transformer
9
is connected between the assembly ground E
2
and the terminal providing control signal s
2
.
A disadvantage of the solution partially illustrated in
FIG. 2
is the use of a pulse transformer. Further, the pulse transformer used is a transformer operating at a high frequency as compared to the frequency of voltage Vac, to reduce the bulk. Control block
5
of the assembly must then provide high frequency control pulses. Further, a solution using a transformer such as illustrated by
FIG. 2
excludes the use of transistors to form the switch. Indeed, to use transistors (for example, MOS power transistors) instead of the thyristors, the control signal cannot be an AC signal but must be a DC signal, which then requires rectifying the transformer signal and complicates the circuit. Another disadvantage common to the two conventional examples hereabove, be it an optocoupler or a pulse transformer assembly, is that neither of the two assemblies is integrable.
A control circuit for a bidirectional switch that has two unidirectional switches associated in antiparallel is disclosed in U.S. Pat. No. 5,796,599. More specifically, the disclosed control circuit controls the switches using two DC voltages that are generated at the terminals of two capacitors, each of which is connected to one of the switches. However, such a circuit requires two distinct DC voltages for controlling the thyristors of the unidirectional switches. Further, these voltages are referenced to different potentials that correspond to the respective power terminals of the switches. Thus, there is the absence of a common reference, and this makes it compulsory to include an insulating device (e.g., an optocoupler or transformer).
SUMMARY OF THE INVENTION
In view of these drawbacks, it is an object of the present invention to overcome the above-mentioned drawbacks and to provide a switch which is bidirectional in current and voltage, made and formed by two distinct semiconductor components. The switch is preferably formed by two one-way semiconductor switches mounted in antiparallel.
Another object of the present invention is to provide a switch that avoids the need for two auxiliary power supplies to control the switch, without having to use a high-frequency transformer.
A further object of the present invention is to provide a control circuit which is integrable.
Yet another object of the present invention is to provide a device of the level shifter assembly-type used for DC power supplies that can be used to supply an AC load.
One embodiment of the present invention provides a circuit for supplying a load from an AC voltage supply. The circuit includes a control circuit and a bidirectional switch coupled in series with the load. The bidirectional switch includes two one-way switches connected in antiparallel, and the control circuit controls the bidirectional switch based on a relatively low DC voltage. Further, the bidirectional switch is connected to a first terminal of the AC voltage supply, and the DC voltage is referenced to the first terminal of the AC voltage supply. In a preferred embodiment, the two one-way switches are controlled by two signals that are referenced to the reference potential of the DC voltage.
Another embodiment of the present invention provides an apparatus connected to an AC voltage supply and a relatively low DC voltage. The apparatus includes a control circuit, a load to be supplied by the AC voltage supply, and a bidirectional switch coupled in series with the load. The bidirectional switch includes two one-way switches connected in antiparallel, and the control circuit controls the
Bongini Stephen C.
Fleit Kain Gibbons Gutman & Bongini P.L.
Galanthay Theodore E.
Sterrett Jeffrey
STMicroelectronics S.A.
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