Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2003-03-28
2004-03-23
Vo, Tuyet T. (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S307000, C315S274000, C315S224000
Reexamination Certificate
active
06710555
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a discharge lamp lighting circuit, and more specifically to a discharge lamp lighting circuit provided with protection circuit to prevent breakdown of a semi-conductor switching element use in a driving transformer.
2. Description of the Related Art
A discharge lamp, especially a rare-gas discharge lamp for use in various scanners and lighting devices is often lighted by a high frequency voltage obtained by switching a DC power source. The voltage waveform induced in a transformer is oscillated by a resonant circuit composed of an inductance of the transformer and a stray capacitance at the time of switching, and the voltage to be applied to the semi-conductor for driving and the secondary voltage of the transformer rise. Especially, when a rare-gas discharge lamp is not connected or not lighted, the load is light, and therefore the primary voltage of the transformer rises further thereby possibly destroying the semi-conductor for driving, and at the same time the secondary voltage of the transformer also rises further generating continuously a high voltage equivalent to the starting voltage, which may result in dielectric breakdown of the transformer.
In order to overcome the above problem, a discharge lamp lighting device has been disclosed in Japanese Patent Application Laid-open No. Hei 10-41081. The discharge lamp lighting device disclosed utilizes resonant oscillation for pulse lighting, stabilizes a power supply voltage thereby stabilizing luminance of a fluorescent lamp, and is provided with a protection means working when the fluorescent lamp is not connected or not lighted. The protection means functions such that a current flowing in the discharge lamp is detected by a lamp current detecting means thereby stopping the driving of the switching element when the discharge lamp is not connected or not lighted.
FIG. 5
is a circuit diagram of the conventional discharge lamp lighting device above described. The discharge lamp lighting device includes a voltage oscillation type inverter of one transistor (hereinafter referred to as inverter) and is connected to both electrodes of a DC power supply
1
which outputs a voltage Vi. A discharge lamp (hereinafter referred to as fluorescent lamp)
2
has a rare gas, such as xenon, filled therein as a discharge gas and has fluorescent material coated on the inner wall of its glass tube. An equivalent circuit, when the fluorescent lamp
2
is lighted, can be shown as a series circuit consisting of a resistor and an interelectrode capacitance. Referring to
FIG. 5
, the inverter comprises: a step-up transformer
11
having a step-up ratio of N and including a primary winding
11
p
and a secondary winding
11
s
; a switching element
12
(power MOSFET) connected to the primary winding
11
p
; a resonant capacitor
13
connected in parallel to the switching element
12
; and a switching control circuit
3
. A pair of electrodes
2
a
and
2
b
of the fluorescent lamp
2
are connected to the secondary winding
11
s, that is, to respective output terminals of the inverter.
At a primary side of the step-up transformer
11
, a series resonant circuit is formed by a primary inductance (inductance of the primary winding
11
p
), and by a sum of a capacitance of the resonant capacitor
13
, an output capacitance Coss (not shown) of the switching element
12
and an interelectrode capacitance (not shown) of the fluorescent lamp
2
converted to the primary side. The series resonant circuit has its resonant cycle set to be shorter than an off-time Toff of the switching element
12
. The off-time Toff is controlled to be constant always. The switching control circuit
3
, which comprises a switching control IC
4
for the inverter, a plurality of resistors and a plurality of capacitors, is connected to a gate terminal of the switching element
12
, and the switching element
12
is driven by a switching control signal outputted from an output terminal
4
B of the switching control IC
4
, whereby the inverter is operated. A voltage detecting circuit
5
, which detects the voltage Vin of the DC power supply
1
, is connected to the switching control circuit
3
.
The structure and operation of a lamp current detecting circuit
6
and a protection circuit (comparator COMP provided in the switching IC
4
) will be described hereafter. The lamp current detecting circuit
6
detects a lamp current flowing in a capacitor
41
by causing the lamp current to flow to a resistor
43
via a capacitor
42
thereby converting into a voltage, and the voltage is rectified by a diode
44
, smoothed by a capacitor
45
, divided by resistors
46
and
47
, and inputted to a base of a transistor
48
. A resistor
49
and a capacitor
50
are connected to a collector of the transistor
48
, that is, to an output terminal
6
A, and the resistor
49
has its other end connected to the DC power supply
1
thereby supplying a voltage to the collector of the transistor
48
. When the lamp current is zero, the transistor
48
has a base voltage of zero and therefore is in an “off state”. Consequently, the capacitor
50
is charged by the DC power supply
1
via the resistor
49
, and the voltage at the output terminal
6
A increases and gets at a voltage equal to a power supply voltage Vin when a delay time Td (for example, 5 seconds) elapses, which is determined by the values of the resistor
49
and the capacitor
50
. And, when the lamp current is flowing, the transistor
48
has its base supplied with a voltage and therefore is in an “on state”, and the output terminal
6
A has a voltage of zero.
The protection circuit comprises the aforementioned comparator COMP provided in the switching control IC
4
. The comparator COMP has its non-inverting input terminal connected to the output terminal
6
A of the lamp current detecting circuit
6
and has its inverting terminal supplied with a reference voltage. The reference voltage is lower than the power supply voltage Vin. An output of the comparator COMP is connected to a driver DB. The driver DB is controlled such that when the voltage at the non-inverting terminal of the comparator COMP is higher than the reference voltage, the output of the comparator COMP goes up to a high level so as to stop the operation of the driver DB, whereby the switching control signal is held at a low level causing the inverter to stop its operation, and such that when the voltage at the non-inverting terminal of the comparator COMP is lower than the reference voltage, the output of the comparator COMP goes down to a low level so as to have no impact on the operation of the driver DB causing the inverter to operate normally.
The operation of the circuit depending on the presence/absence of the lamp current will be described. Referring to
FIG. 5
, when the fluorescent lamp
2
is not connected or not lighted, the lamp current is zero, therefore when the delay time Td elapses, the voltage at the output terminal
6
A of the lamp current detecting circuit
6
, that is the voltage at the non-inverting input terminal of the comparator COMP, becomes equal to the power supply voltage Vin thereby causing the output of the comparator COMP to cease. At the very start of supplying power, the fluorescent lamp
2
connected is not lighted, and therefore the lamp current is zero, but due to the delay time Td of the lamp current detecting circuit
6
the driver DB does not cease its operation in the immediate wake of starting power supply, and if the fluorescent lamp
2
is lighted within the delay time Td, the inverter operates normally. That is to say, when the lamp current is flowing at the normal operation, the voltage at the output terminal
6
A of the lamp current detecting circuit
6
, that is the voltage at the non-inverting input terminal of the comparator COMP, is zero, whereby the output of the comparator COMP goes down to a low level and the inverter operates normally.
FIGS. 6A and 6B
show a voltage VF between a source terminal S and a drain terminal D o
Kamimura Shunsuke
Suzuki Shin'ichi
Terada Ryu
Minebea Co. Ltd.
Vo Tuyet T.
LandOfFree
Discharge lamp lighting circuit with protection circuit does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Discharge lamp lighting circuit with protection circuit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Discharge lamp lighting circuit with protection circuit will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3281065