Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Silicon controlled rectifier ignition
Reexamination Certificate
1998-11-17
2001-04-24
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Silicon controlled rectifier ignition
C315S219000, C315S276000, C315SDIG005
Reexamination Certificate
active
06222325
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of fluorescent lamps supplied from the high voltage a.c. mains power system (for example, 220 volts/50 Hz or 115 volts/60 Hz). The present invention more specifically relates to the lamp control, essentially, in current limitation in nominal operation and in turn-on triggering.
2. Discussion of the Related Art
In nominal operation, it is necessary to provide in the lamp supply circuit a current limiting element due to the structure of fluorescent lamps. Indeed, this type of lamp behaves, in nominal operation, as a voltage limiting component, that is, the voltage across the lamp is independent from the supply voltage, and is determined by the power of the lamp itself. Accordingly, to supply a fluorescent lamp on the mains voltage, it is necessary to provide a current limiting component, generally called a “ballast”.
At turn-on, it is necessary to provide a triggering or starting component, generally called a “starter”, meant to, first, heat up the filaments of the lamp, then start the lamp with an overvoltage.
FIG. 1
shows an example of a diagram of a conventional fluorescent lamp power supply circuit. A lamp
1
is generally formed of a tubular piece T filled with gas and at both ends of which are provided two excitation filaments f, f′. Each filament is meant to be electrically connected by both its ends and is thus associated with two supply terminals
1
,
2
, respectively,
1
′,
2
′. The two filaments f, f′ are meant to be supplied by an a.c. voltage Vac, for example, the mains voltage applied between two supply terminals
3
,
4
, of the lamp circuit.
In the example shown in
FIG. 1
, the current limiting component is formed of a high value iron inductance L interposed between a first a.c. supply terminal
3
and a first terminal
1
of one of the filaments f of lamp T. The second terminal of filament f is connected, via a starting component
5
, to a terminal
2
′ of the second filament f′, the second terminal
1
′ of which is connected to the second mains supply terminal
4
. A capacitor C interconnects terminals
3
and
4
.
Triggering or starting element
5
is most often a thermal switch meant to heat up filaments f and f′, of lamp T by short-circuiting terminals
2
and
2
′ as long as the filaments are cold. The thermal switch opens as soon as it has reached a given temperature, which causes an overvoltage which triggers the lamp by means of the power storage formed by inductance L.
The function of inductance L is, in nominal operation, to limit the current in lamp T so that its voltage does not exceed the value for which it is designed. The function of capacitor C is to compensate the dephasing associated with the inductive assembly in order to improve the power factor and to make it acceptable for a connection to the network.
A disadvantage of a conventional supply system such as shown in
FIG. 1
is that the use of a high inductance (generally on the order of 1 Henry) results in a bulky and heavy system. Further, the inductive nature of the assembly which requires a compensation of the dephasing by capacitor C requires a capacitor of high value (generally of more than 10 &mgr;F), which thus has to be an electrolytic capacitor.
Another disadvantage of such a system is that there exists no light dimmer for use with this system.
FIG. 2
shows an example of a conventional diagram of a so-called “electronic” limiting circuit, that is, a circuit using active components to limit the current of the fluorescent lamp in nominal operation.
Such a circuit is formed by a diode bridge
10
, having two terminals that receive an a.c. voltage connected to two terminals
3
,
4
, that receive mains voltage Vac. A first rectifying output terminal
11
of bridge
10
forms a ground terminal of the circuit. A second rectifying output terminal
12
of bridge
10
provides, by means of a high value electrolytic capacitor C′, a d.c. supply to a switched-mode converter
13
used to supply fluorescent lamp T. Switched-mode converter
13
generally is formed by a control circuit
14
associated with two MOS power transistors M
1
, M
2
(or two bipolar transistors) connected in series between terminal
12
of bridge
10
and the ground, capacitor C′ being connected in parallel to this series association. A terminal
15
of the switched-mode converter is connected to a first terminal of a high frequency inductance L′ mounted, as in the case of
FIG. 1
, in series with one of the filaments f of lamp T. A capacitor C″ of low value interconnects filaments f and f′ and contributes to the lamp triggering. The second terminal
1
′ of filament f′ is grounded via a capacitor
16
. Another capacitor
17
connects terminal
1
′ to an input terminal
18
of switched-mode capacitor
13
. Capacitors
16
and
17
are used to filter the d.c. component in lamp T. Terminal
18
receives the d.c. voltage provided by capacitor C′. Transistor M
1
is connected between terminal
18
and terminal
15
and transistor M
2
is connected between terminal
15
and the ground. Transistors M
1
and M
2
are controlled by circuit
14
which also includes a feedback input connected to terminal
15
and which is supplied from terminal
18
via a resistor R. A capacitor
19
interconnects terminals
15
and
18
and contributes to the generation of an auxiliary power supply necessary for the control of transistor M
1
.
Circuit
14
may include other configuration and parametering terminals, not shown. The operation of an electronic limiting circuit such as shown in
FIG. 2
is perfectly well known. Bridge
10
and capacitor C′ provide, for a 220 -volt a.c. voltage, a power supply on the order of 300 d.c. volts to the switched-mode converter which is of “symmetrical half-bridge” type. This converter provides an alternating current at a frequency which is generally approximately 30 kHz to fluorescent lamp T via the high frequency (series) inductance L′, which may be of low value (on the order of one mH).
A system such as shown in
FIG. 2
eliminates the use of a high inductance (L, FIG.
1
).
However, a disadvantage of a circuit such as shown in
FIG. 2
is that it still requires an electrolytic capacitor C′ of high value (generally higher than 10 &mgr;F) to filter the voltage rectified by bridge
10
. The use of electrolytic capacitors may result in a reduced circuit lifetime.
Another disadvantage of the system shown in
FIG. 2
is that it requires two high voltage MOS power transistors which operate at high frequency.
Another disadvantage of such a system is that it is required to add to bridge
10
a power factor correction circuit
20
. Without circuit
20
, the harmonics of the supply current strongly adversely affect the power factor.
SUMMARY OF THE INVENTION
The present invention aims at overcoming the disadvantages of known fluorescent lamp supply systems.
The present invention aims, in particular, at providing a novel system for controlling a fluorescent lamp which limits the nominal current while being of low bulk and at least partially integrable.
The present invention also aims at providing a solution which allows addition of a light dimming function to the control system.
The present invention also aims at improving the reliability of the control system by avoiding the use of electrolytic capacitors.
A characteristic of the present invention is to provide a supply of the fluorescent lamp by a sine current at a low frequency corresponding to the mains frequency (for example, 50 hertz) while avoiding the use of a high value inductance by means of an active circuit operating at a higher frequency (for example, on the order of 100 kHz).
Thus, according to the present invention, an active device is used to supply the fluorescent lamp while controlling its current. This device provides a low frequency a.c. current to the fluorescent lamp of a shape similar to that of a ferromagnetic-type limitin
Bardouillet Michel
Wuidart Luc
Galanthay Theodore E.
Morris James H.
STMicroelectronics S.A.
Vo Tuyet T.
Wolf Greenfield & Sacks P.C.
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