Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Impedance or current regulator in the supply circuit
Reexamination Certificate
2002-05-31
2003-09-30
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Impedance or current regulator in the supply circuit
C315S2090SC, C315S276000, C315S291000, C315SDIG005
Reexamination Certificate
active
06628090
ABSTRACT:
TECHNICAL FIELD
This invention relates generally, but not exclusively, to a resonant driving system for a fluorescent lamp having at least one end connected to a primary winding of a transformer.
The invention is particularly, but not exclusively, directed to a resonant driving system for a fluorescent lamp having at least one end connected to a primary winding of a transformer. The system includes an inductor inserted between an input section of the system and an internal circuit node, a converter inserted between the internal node and a voltage reference and comprising a first transistor and a second transistor of the complementary type, inserted, in series to each other, between the internal node and the voltage reference, and a control circuit connected to a secondary winding of the transformer and to the converter as well as to the control terminals of the first and second transistors of the converter.
BACKGROUND OF THE INVENTION
As is well known, fluorescent lamps are generally made with a tube filled with a mercury-based gas mixture at low pressure. The inner side of the tube is covered by phosphorus or other similar fluorescent elements. When a lamp is turned on, its two electrodes starts to warm up and to emit ions that contribute to fully ionize the gas mixture inside the lamp, facilitating the strike of the arc across the two electrodes.
From an electrical point of view, the resistance of the lamp falls from about one mega ohm down to few hundreds ohm as illustrated in
FIG. 1
by the I-V characteristic of a typical low pressure fluorescent lamp at the start up of the lamp.
Once the arc is established, the mercury gas emits radiation in the ultraviolet spectrum that excites the phosphorous coating of the inner side of the lamp. At this time the phosphorous starts to fluoresce producing the light in the visible spectrum.
It should be noted that fluorescent lamps usually require about 600V as peak voltage to strike the arc. Once the arc is established about 100V is enough to sustain it.
In order to avoid the so-called cataphoresis effect, the fluorescent lamps are usually supplied with sinusoidal waveforms by means of an inductor, usually indicated as the ballast. The ballast is connected to a DC-to-AC converter. A capacitive filter is also included to further remove any DC components of the relevant waveforms.
From the I-V characteristic shown in
FIG. 1
it is also evident that the voltage across a fluorescent lamp is never equal to the main voltage. In particular, at the start up such a voltage is much higher than the main voltage, and while in steady state it is quite lower. Consequently, the ballast has the important function of generating the high voltage needed to strike the arc and after to generate the inductive reactance needed to reduce the voltage across the electrodes of the lamp. Furthermore the value of inductance L of the ballast must be chosen so that it does not saturate when the lamp is in its operation voltage condition, even at high temperatures.
However, it should be noted that the I-V characteristic shown in
FIG. 1
is valid only at the lamp start up. Moreover, the shown characteristic is valid for any traditional magnetic ballasts working at a main frequency. In particular, the negative slope in the characteristic occurs at every cycle near the current zero crossing and it is due to the gas de-ionization. This causes a visible 50 Hz or 60 Hz flicker effect that can be eliminated by increasing the switching frequency of the lamp about three orders of magnitude. In fact at frequencies higher than 20 kHz there is no time for the de-ionization of the gas, and the I-V characteristic of the lamp is linear, as shown in FIG.
2
.
The use of a higher frequency also provides two other important benefits: the power consumption is lowered, typically by about 70% due to the no deionization of the gas, with consequent longer life time of the fluorescent lamp;
a smaller inductor in series with the fluorescent lamp can be used, with consequent reduction in the weight of the fluorescent-lamp system.
A simplified block schematic diagram of a system
1
comprising a fluorescent lamp connected to a ballast according to a voltage fed topology is shown in FIG.
3
. In particular, the system
1
has a first IN
1
and a second input terminals IN
2
, connected to a main voltage supply and to a filtering block
2
. The filtering block
2
in turn comprises a bridge circuit connected to the main supply and a filter, in turn connected to a conversion block
3
.
The conversion block
3
substantially comprises a DC to AC converter and is connected to a tube
4
of the fluorescent lamp. In particular, the tube
4
has a first end connected to the conversion block
3
by means of a series of an inductor L (the ballast) and a first capacitor C
1
and a second end directly connected to the conversion block
3
.
Finally, the tube
4
has a second capacitor C
2
connected in parallel to its ends.
According to the topology of the system
1
for fluorescent lamps shown in
FIG. 3
, the tube
4
is fed by generating an over voltage across the second capacitor C
2
through the circuit formed by the series of the inductor L, the first capacitor C
1
and the second capacitor C
2
itself. At the start up, the system
1
is an open circuit and, by using a second capacitor C
2
which is much smaller than the first capacitor C
1
, such a second capacitor C
2
imposes the resonant frequency of the system
1
. In such a way, the generated over voltage is high enough to ionize almost instantaneously the gas in the tube
4
of the fluorescent lamp. Once the fluorescent lamp is on, the second capacitor C
2
will be short circuited by the fluorescent lamp itself and the natural frequency of the system
1
will be mainly determined by the first capacitor C
1
. So, the working frequency of the system
1
will be higher than this natural frequency and determined by the DC-AC converter
3
.
From all the above, it is evident that core of the system
1
is the DC-to-AC converter
3
. In order to clarify the operation of such a DC-to-AC converter
3
reference will be made to
FIG. 4
, which shows the system
1
of
FIG. 3
in a greater detail.
It should be noted that the first capacitor C
1
shown in
FIG. 3
has been substituted by the two capacitors C
5
and C
6
(with identical capacitance) to better balance the system
1
as a whole.
In particular, the filtering block
2
of the system
1
comprises a filter
5
connected between the first IN
1
and second input terminals IN
2
as well as to a diode bridge circuit
6
in turn connected to the conversion block
3
.
Moreover, the conversion block
3
has a first IN
3
and a second input terminal IN
4
and comprises a first resistor R and a third capacitor C
3
connected in series between such input terminals IN
3
and IN
4
. The interconnecting node between the first resistor R and the third capacitor C
3
is connected to the control terminals of a first Q
1
and a second transistor Q
2
, respectively by means of a series of a diode D
0
and a second resistor R
0
and by means of a diac D.
In the example shown in
FIG. 4
, the transistors Q
1
and Q
2
are bipolar transistors and their control terminals are the base terminals. In particular, the transistors Q
1
and Q
2
are both of the NPN type, the emitter terminal of the first transistor Q
1
and the collector terminal of the second transistor Q
2
being connected to an internal node X.
More particularly, the series of the diode D
0
and the second resistor R
0
is connected to the control terminal of the first transistor Q
1
through a first winding of a transformer T and a third resistor R
3
, while the diac D is connected to the control terminal of the second transistor Q
2
through a fourth resistor R
4
and to a voltage reference, for instance a ground GND, through a second winding of the transformer T.
The first transistor Q
1
is connected to the first input terminal IN
3
of the conversion block
3
and to the internal node X by means of a fifth resistor R
5
. Moreover, a fourth capaci
Graybeal Jackson Haley LLP
STMicroelectronics S.r.l.
Vo Tuyet T.
Wong Don
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