Discharge lamp lighting device

Details Discharge lamp lighting device Discharge lamp lighting device

2002-02-14

2003-06-17

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, C315S246000, C315S276000, C315S291000, C315S324000, C363S037000, C363S034000, C363S017000

Reexamination Certificate

active

06580229

ABSTRACT:

TECHNICAL FIELD
The present invention is directed to a discharge lamp driving device with an abnormality detection and protection function of detecting a lamp life end for circuit protection.
BACKGROUND ART
(First Prior Art)
FIG. 1
s a circuit diagram showing one example of a prior discharge lamp driving device which is identical in circuit configuration to that shown in
FIG. 36
of Japanese Patent Publication No. 8-251942. A rectifier DB composed of a diode bridge is connected to an AC power source AC through a surge absorption element ZNR and a filter circuit F. Connected across a pulsating output terminals are a high frequency bypassing capacitor C
2
, a series combination of switching elements Q
1
and Q
2
in the form of field effect transistors through a series circuit of diodes D
5
and D
6
, a series combination of a smoothing capacitor C
10
and a diode
13
, and a high frequency bypassing capacitor C
11
. A series circuit of an inductor L
2
and a diode D
12
is connected between a connection point of switching elements Q
1
and Q
2
and a connection point of smoothing capacitor C
10
and diode D
13
. A leakage transformer LT
1
has a primary winding N
1
that is connected in series with a DC blocking capacitor C
3
between the cathode of diode D
5
and the connection point of switching elements Q
1
and Q
2
. A secondary winding N
2
of the leakage transformer LT
1
has its one end connected through a DC blocking capacitor C
9
to one of filaments of one discharge lamp La
1
, and has its other end connected to one of filament of the other discharge lamp La
2
. The other filaments of the two discharge lamps La
1
and La
2
are connected at one ends thereof to each other through an auxiliary winding N
3
of the leakage transformer LT
1
and a DC blocking capacitor C
6
. The other ends of the filaments of the discharge lamps La
1
and La
2
are connected to each other through a resonant inducing capacitor C
7
. Further, a harmonic distortion improving capacitor C
4
is connected across diode D
6
.
The two switching elements Q
1
and Q
2
are driven by a control circuit CNT to turn on and off alternately. The leakage transformer LT
1
includes an auxiliary winding N
4
for detection of lamp voltage of the discharge lamps La
1
and La
2
. The detected voltage induced at the auxiliary winding N
4
is rectified by means of a diode D
8
and is fed to a detection circuit
20
for detection of the lamp voltage. Based upon thus detected lamp voltage, the control circuit CNT varies a switching frequency of the switching elements Q
1
and Q
2
. In short, the source AC voltage is rectified through rectifier DB of which pulsating output is partially smoothed out by a valley-filling power source in the form of a step-down chopper circuit composed of switching element Q
2
, diode D
12
, inductor L
2
, smoothing capacitor C
10
and a parasitic diode of switching element Q
1
. The partially smoothed DC output is converted into a high frequency output by means of an inverter circuit in the form of a half-bridge type including the switching elements Q
1
and Q
2
. The high frequency output is fed through the leakage transformer LT
1
to the discharge lamps La
1
and La
2
as a load for driving the same. Further, in this prior art, the harmonic distortion improving capacitor C
4
compensates for a voltage difference between the rectifier DB and the valley-filling power source, while an input voltage is switched on and off by utilization of a high frequency voltage appearing within the inverter circuit so as to draw in the input current from the rectifier DB through a resonant circuit composed of leakage transformer LT
1
, capacitor C
3
, discharge lamps La
1
and La
2
, and capacitor C
7
, and through capacitor C
4
for improving harmonic distortion of the input current. The operation of this prior art is known and therefore not discussed herein.
When the above prior art sees that the discharge lamps La
1
or La
2
reaches to the lamp life end, a protective action is made as follows. That is, when the lamp reaches its lamp life end as a result of the depletion of the negative thermion radiating material (emitter) coated on the filaments, the lamp voltage of the discharge lamps La
1
and La
2
increases than in a normal condition. With this result, the voltage induced at the auxiliary winding N
4
of the leakage transformer LT! increases so that the detection circuit
20
gives an abnormality detection signal to the control circuit CNT in response to the voltage induced at the auxiliary winding N
4
exceeds a threshold. The control circuit CNT responds to the abnormality detection signal for activating the inverter circuit to intermittently oscillate, thereby effecting a protective action of reducing the stress on the circuit.
(Second Prior Art)
FIG. 2
shows a circuit diagram of another prior art which is identical in configuration to the circuit disclosed in
FIG. 15
of a Japanese Patent Publication 2000-100587. The second prior art differs from the first prior art in that the inductor L
2
forming the step-down chopper circuit is omitted, that diode
12
has its anode connected to a connection point of smoothing capacitor C
10
and diode
13
and has its cathode connected to a connection point of the primary winding N
1
of the leakage transformer LT
1
and capacitor C
3
in order to share the leakage transformer LT
1
with the step-down chopper circuit, and that an output regulation circuit
21
is added in compensation for a large characteristic variation of a driving transformer T
2
. The output regulation circuit
21
includes a switching element Qb realized by a bipolar transistor connected across a control voltage source E through a variable resistor VR and a collector resistor Re. The switching element Qb has its base connected through a resistor Rd to a point between a resistor Rc and a capacitor Cb which are connected in series between the connection point of the switching elements Q
1
, Q
2
and the negative pole of the control voltage source E. Connected between the output terminal of the control circuit CNT and the negative pole of the control voltage source E is a series combination of a diode Da, a resistor Ra, and a switching element Qa of bipolar transistor. The switching element Qa has its base connected through a base resistor Rb to a connection point of collector resistor Re and variable resistor VR. Further, a capacitor Ca and a diode Db are connected in parallel across the series combination of the switching element Qb and the collector resistor Re, while a diode Dc is connected in a base-emitter path of the switching element Qb. While the one switching element Q
2
is off, capacitor Cb is charged through resistor Rc so that switching element Qb is caused to turn on in response to the voltage increase across capacitor Cb, thereby turning off the switching element Qa and giving no influence on the operation of the inverter circuit. When the switching element Q
2
turns on, the switching element Qb is turned off so that the control voltage source E acts to charge capacitor Ca through variable resistor VR. As the voltage across capacitor Ca increases, the switching element Qa responds to turn on, thereby causing the switching element Q
2
to turn off. Accordingly, it is made possible to regulate the on-period of switching element Q
2
by varying the resistance of the variable resistor VR to thereby maintain the output substantially at a constant level irrespective of the varying characteristic of the driving transformer T
2
. Also this prior art has the same protective action as is made in the first prior art when the lamp life end is reached.
In the second prior art, however, the inclusion of the output regulation circuit
21
brings about an asymmetry (unbalance) of the on-period of the switching elements Q
1
and Q
2
in the normal lamp operating condition, whereby a DC voltage will be applied to capacitor C
9
connected in series with the discharge lamps La
1
and La
2
. With this result, the DC voltage of the charged capacitor C
9
will be superimposed u

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