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
1999-09-10
2001-04-03
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
C315S094000, C315S107000, C315S291000, C315S307000, C315S2090SC
Reexamination Certificate
active
06211620
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ballast for a fluorescent lamp using an inverter power source.
2. Description of the Prior Art
Conventionally, a ballast for a fluorescent lamp using a series inverter as shown in
FIG. 8
is known. In the series inverter as shown in
FIG. 8
, when a switch
79
is turned on, an AC voltage supplied from an AC power source
78
is rectified by a rectifying circuit
80
. The output current charges a smoothing capacitor
81
and also charges a capacitor
87
via a resistor
86
. When the voltage of the capacitor
87
reaches the breakdown voltage of a trigger element
88
, the charges of the capacitor
87
are supplied to the gate of a FET
84
so that the FET
84
turns on.
When the FET
84
turns on, the charges of the capacitor
87
are discharged via a resistor
90
, a diode
89
and the FET
84
instantly. Thus, the voltage of the capacitor
87
drops and the trigger element
88
turns off. Further, the current from the AC power source
78
flows through a loop including the rectifying circuit
80
, a capacitor
82
, an electrode
73
A of a fluorescent lamp
72
, a parallel circuit composed of a capacitor
74
and a positive characteristic thermistor
70
, an electrode
73
B of the fluorescent lamp
72
, a choke coil
75
, a primary winding
85
B of a current transformer
85
and the FET
84
. This current increases gradually. As a result, the current through the primary winding
85
B of the current transformer
85
generates a voltage in a secondary winding
85
C of the current transformer
85
, and this voltage supplies a gate voltage to the FET
84
. Thus, the FET
84
is maintained to be on.
When the current flowing through the windings of the current transformer
85
increases enough, the core of the current transformer
85
is saturated magnetically. The magnetic saturation in the core of the current transformer
85
stops the output of the secondary winding
85
C so that the FET
84
cannot be supplied with a gate voltage and thus turns off.
At this point, the energy accumulated in the choke coil
75
causes current to continue to flow through a loop including a parasitic diode
83
A of the FET
83
, a capacitor
82
, the electrode
73
A of the fluorescent lamp
72
, a parallel circuit composed of the capacitor
74
and the positive characteristic thermistor
70
, the electrode
73
B of the fluorescent lamp
72
, the choke coil
75
and the primary winding
85
B of the current transformer
85
. This current decreases gradually.
This current becomes primarily a resonance current of the choke coil
75
and the capacitor
74
. When this current reverses, the output polarity of the secondary winding
85
A reverses so that the FET
83
turns on. When the core of the current transformer
85
is saturated magnetically again, the output from the secondary winding
85
A stops, and the FET
83
cannot be supplied with a gate voltage and thus turns off. At the same time, the gate voltage supplied from the secondary winding
85
C turns the FET
84
on again. Thereafter, the above-described operations are repeated.
The resonance current of the choke coil
75
and the capacitor
74
flows through the electrodes
73
A and
73
B of the fluorescent lamp
72
and heats these electrodes. Immediately after the switch
78
is turned on, the temperature of the positive characteristic thermistor
70
is low and the resistance value thereof is small. Therefore, the charging current that flows into the capacitor
74
connected in parallel to the positive characteristic thermistor
70
is small, and the voltage across the capacitor
74
is small. Therefore, a resonant voltage sufficient to activate the fluorescent lamp
72
is not applied across the fluorescent lamp
72
.
The temperature of the electrodes of the fluorescent lamp
72
is raised to a temperature sufficient to generate thermoelectrons as time passes. Furthermore, the positive characteristic thermistor
70
rises in temperature due to Joule heat, and the resistance value thereof rises. As a result, the voltage across the capacitor
74
reaches a resonant voltage sufficient to activate the fluorescent lamp
72
. Thus, the fluorescent lamp
72
is activated and stays lit up. In the manner as described above, the electrodes
73
A and
73
B of the fluorescent lamp
72
start discharging after they are preheated and reach a state where thermoelectrons are supplied sufficiently. Therefore, the loss of active substances applied to the electrodes
73
A and
73
B due to positive ion bombardment can be reduced, so that the life of the fluorescent lamp
72
can be prolonged.
However, in the conventional ballast for a fluorescent lamp as described above, when the resistance value of the positive characteristic thermistor
70
is excessively small at room temperature, the period from the introduction of the power to the lighting of the fluorescent lamp becomes long, namely, it takes a long time to preheat the electrodes. Thus, the instant startability of the ballast is poor.
On the other hand, when the resistance value of the positive characteristic thermistor is excessively large, the initial resonance current is large, and an increase in the resistance value due to an increase in the temperature of the positive characteristic thermistor becomes steep. Therefore, the fluorescent lamp may be activated in a premature state where the electrodes have not generated thermoelectrons sufficiently yet. In this case, the active substances in the electrodes are lost readily due to positive ion bombardment, and the life of the fluorescent lamp becomes short. Since it is necessary to reduce the increase rate of the temperature of the positive characteristic thermistor in order to solve this problem, a positive characteristic thermistor having a large heat capacity, namely, a large-scale and expensive positive characteristic thermistor is required.
Furthermore, in the case where the fluorescent lamp is restarted after it is turned off and before the positive characteristic thermistor is cooled to room temperature, the following problem may arise. When the resistance value of the positive characteristic thermistor is large, the fluorescent lamp is activated in a premature state where the electrodes have not generated thermoelectrons sufficiently yet. Thus, the life of the fluorescent lamp becomes short.
SUMMARY OF THE INVENTION
Therefore, with the foregoing in mind, it is an object of the present invention to provide a ballast for a fluorescent lamp having a compact and inexpensive circuit configuration that can start with preheating and light up a fluorescent lamp instantly and hardly deteriorates electrodes of the fluorescent lamp at the start and at the restart in a short time after the fluorescent lamp is put out.
In order to achieve the object, the present invention provides an improved ballast for a fluorescent lamp including a high frequency power source circuit for supplying a preheat start type fluorescent lamp with preheating and lighting current via an inductor. The high frequency power source circuit includes at least two switching elements for respectively controlling application of voltages of different polarity to the fluorescent lamp; a self-exciting type switching element driving circuit for driving the switching elements so as to alternate on and off repeatedly; and a timer circuit for detecting the lapse of a predetermined time from the start of the ballast for the fluorescent lamp. The switching element driving circuit shortens an ON-period of at least one of the switching elements to restrict an increase of an amplitude of current flowing in the inductor during a period until the timer circuit detects the lapse of a predetermined time.
This embodiment ensures that the fluorescent lamp is preheated during a predetermined period in which duty control restricts an increase of the amplitude of current flowing in the inductor. Furthermore, after the predetermined period has passed, the amplitude of the current flowing in the inductor increases, so that
Gyoten Masayoshi
Miyazaki Koji
Murakami Masanobu
Ozawa Masataka
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
Vo Tuyet T.
Wong Don
LandOfFree
Ballast for fluorescent lamp does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Ballast for fluorescent lamp, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Ballast for fluorescent lamp will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2550344