Electric lamp and discharge devices: systems – With cathode or cathode heater supply circuit – Pulsating or a.c. supply to the cathode or heater circuit
Reexamination Certificate
2002-01-31
2003-04-22
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
With cathode or cathode heater supply circuit
Pulsating or a.c. supply to the cathode or heater circuit
C315S107000, C315S177000, C315S046000, C315S049000, C315S094000
Reexamination Certificate
active
06552494
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention refers to a fluorescent lamp driver circuit.
Fluorescent lamps are generally composed of a glass tube
1
which contains fluorescent gas
2
and at the ends of which there are two electrodes F
1
and F
2
, as shown in FIG.
1
. Under normal conditions the lamp is assimilable to an open circuit and presents an infinite impedance between its electrodes, as shown in diagram I(V) of FIG.
2
. If the voltage between its electrodes exceeds a threshold voltage Vth the lamp ignites as there is an ionization of the gas
2
by means of emission of electrons by the two electrodes. The value of the voltage Vth depends on the temperature of the two electrodes given that, at equal voltage applied, as the temperature increases the quantity of electrons emitted increases and therefore at a higher temperature of the electrodes corresponds a voltage Vth of lower value. Once the threshold voltage Vth is exceeded the gas
2
changes state and the tube
1
becomes assimilable to a resistive load; so that said condition remains it is necessary to supply a small current Imin.
Fluorescent lamps must be driven by circuits that are capable of permitting their turning on and off, like the circuit shown in
FIG. 3. A
first block
31
converts an alternating voltage, generally a mains voltage, into a direct voltage between Vdd and ground Gnd. To a terminal of the voltage Vdd is connected a resistor R
2
connected in turn to an inductance L
1
connected with a terminal P
1
of the electrode F
1
of the fluorescent lamp. The other terminal P
2
of the electrode F
1
is connected to a terminal of a capacitance C
3
connected in turn to a terminal P
4
of the electrode F
2
; the other terminal P
3
of the electrode F
2
is connected to a condenser C
1
connected to the voltage Vdd and to a condenser C
2
connected to ground Gnd. The inductance L
1
and the condenser C
3
form a circuit L-C series. Two secondaries Ls
1
and Ls
2
are wound on the inductance L
1
that transfer the state of the circuit L-C to the control logic formed by two blocks
32
and
33
. The block
32
comprises a diode DIAC
34
connected to a resistor R
1
connected to the voltage Vdd and connected to a condenser C
4
having the other terminal grounded Gnd; the DIAC
34
is capable of giving a first impulse to a system of switches
35
then disabling itself. The system of switches
34
acts so that the circuit L-C series begins to oscillate at a resonance frequency given by
f
r
1
2
Σ
L1C3
,
which generally has a value comprised between 60 Khz and 70 Khz, and there will be a square wave signal of frequency fr and amplitude Vdd in a node PC on the terminals in common of the inductance L
1
and of the resistor R
2
. The resonant circuit L-C shall determine overvoltages on the condenser C
3
such that after a few cycles of oscillation the value of the threshold voltage Vth is exceeded causing the ignition of the fluorescent lamp. Between the nodes PC and P
3
there will no longer be the resonant circuit L-C but a circuit R-L
1
where R is the resistor of the fluorescent lamp and the control logic inside the blocks
32
and
33
will determine the working frequency, generally between 30 Khz and 50 Khz. The block
33
is similar to the block
32
but does not comprise the diode Diac
34
and instead comprises a system of switches
101
similar to the system of switches
35
of the block
32
.
In the place of the two blocks
32
and
33
and of the respective inductances Ls
1
and Ls
2
an integrated circuit
41
, as shown in
FIG. 4
, that controls the operations described above, can be inserted in the circuit of FIG.
3
.
THE SUMMARY OF THE INVENTION
To increase the life of the fluorescent lamp a function of preheating of the electrodes F
1
and F
2
is required, in the phase prior to the ignition of the lamp; said preheating of the electrodes F
1
and F
2
enables them to be more emissive and to obtain a threshold voltage Vth of lower value. A circuit
51
that implements the preheating function is shown in
FIG. 5
; said circuit
51
comprises a condenser C
6
inserted between the terminal P
4
and a terminal of the condenser C
3
and placed in parallel with a block PTC that comprises a resistor variable with the temperature. The preheating function is carried out by passing a current through the electrodes F
1
and F
2
that is the same as the current that passes through the oscillating circuit L-C where at the beginning, seeing the PTC is a low impedance, the C corresponds to C
3
which is chosen sufficiently big so as not to generate high voltages near the threshold voltage Vth. The heating of the PTC causes an increase of its resistance and after a certain time it presents an infinite impedance at the limit. In this case the capacitor C of the resonant circuit L-C is given by the series of condensers C
3
and C
6
and the value of C
6
must be chosen much lower than C
3
. The capacitive impedance is high and such as to generate at its ends a higher voltage than the threshold voltage Vth and therefore ignites the lamp.
In particular applications where the replacement of the worn fluorescent lamp is envisaged, a protection function of the lamp driver circuit called “End of life” is required. In fact if the gas in the lamp is depleted, the lamp will never ignite and the driver circuit will remain in perpetual free oscillation with high overvoltages and overcurrents that lead to the destruction of the driver circuit of the lamp.
In
FIG. 6
a driver circuit is shown in which the End of Life function is made by means of a block
61
comprising a series of elements connected in series between the terminal P
1
and ground Gnd: a condenser C
7
, a diode Zener Dz
1
, a resistor R
3
, a condenser C
8
. A condenser Cp is placed in parallel with the elements Dz
1
, R
3
and C
8
, while a resistor R
4
is placed in parallel with only the condenser C
8
. A diode Zener Dz
2
is connected to the terminal in common with the elements R
3
and C
8
, connected to a turn-off block of the type SCR
62
connected to the block
32
, to the terminal in common of the elements R
1
and C
4
and a ground. With the lamp ignited the voltage value on the terminal P
1
is low and therefore the voltage value on the element Cp does not exceed the breakdown voltage BVDz
1
of the diode Dz
1
. In the phase prior to the ignition of the lamp the terminal P
1
reaches high voltage values, generally between 0,8 KV and 1 KV, and therefore the voltage on the element Cp manages to exceed the voltage BVDz
1
; in that case current flows through the condenser C
8
whose value is limited to the resistor R
3
. The time constant of R
3
and C
8
is a very few seconds, generally 2 or 3 seconds, therefore in conditions of normal ignition with or without preheating the voltage at the ends of the element C
8
does not exceed the breakdown voltage BVDz
2
of the diode Dz
2
. In condition of gas depleted in the lamp the driver circuit remains in perpetual free oscillation and that implies that on the condenser C
8
current continues to flow; the voltage at the ends of C
8
exceeds the voltage BVDz
2
and that enables the activation of the turn-off block
62
. This block causes the turn-off of the block
32
causing the stop of the oscillations and impedes the voltage on the element C
4
from reaching an ignition value of the block
32
.
In view of the state of the technique described, object of the present invention is to present a driver circuit for fluorescent lamps which is simpler than known circuits and carries out the functions of preheating and end of life.
In accordance with the present invention, said object is reached by means of a driver circuit of a fluorescent lamp having a first and a second electrode and igniting when the voltage between said first and second electrode exceeds a given threshold voltage, said driver circuit comprising a inductance coupled to a supply voltage and to a terminal of said first electrode, a first condenser coupled to the other termin
La Barbera Atanasio
Randazzo Vincenzo
STMIcroelectronics S.r.l.
Vo Tuyet T.
Wong Don
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