Electronic dimming ballast for compact fluorescent lamps

Details Electronic dimming ballast for compact fluorescent lamps Electronic dimming ballast for compact fluorescent lamps

2002-06-01

2003-11-04

Vu, David (Department: 2821)

Electric lamp and discharge devices: systems

Condenser in the supply circuit

Inductance in the condenser circuit

C315S224000, C315SDIG004

Reexamination Certificate

active

06642669

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates in general to dimming gas discharge lamps and, more particularly, to electronic dimming ballasts for dimming compact fluorescent lamps.
BACKGROUND OF THE INVENTION
A gas discharge lamp converts electrical energy into visible light with high efficiency. A gas discharge lamp is generally an elongated gas-filled (usually low-pressure mercury vapor) tube having electrodes at each end. Each electrode is typically formed from a resistive filament (usually tungsten) coated with a thermionically emissive material, such as a mixture of alkaline earth oxides.
The steady-state operation of a typical gas discharge lamp is as follows. Voltage is applied across the resistive filaments, heating the electrodes to a temperature sufficient to cause thermionic emission of electrons into the discharge tube. A voltage applied between the electrodes accelerates the electrons toward the anode. En route to the anode, the electrons collide with gas atoms to produce positive ions and additional electrons, forming in the tube a gas plasma of positive and negative charge carriers. The electrons continue to stream toward the anode and the positive ions toward the cathode, sustaining an electric discharge in the tube and further heating the electrodes. If the applied power is AC, the electrodes reverse polarity each half cycle.
The discharge causes the emission of radiation having a wavelength dependent upon the particular fill gas and the electrical parameters of the discharge. Because each collision produces additional electrons and ions, increases in the arc current cause the impedance of the lamp to decrease, a characteristic known as “negative incremental impedance.” Operation of the lamp is inherently unstable, due to this negative incremental impedance characteristic, and thus the current between the electrodes must be controlled to provide stable operation of the lamp.
Gas discharge lamps, including fluorescent lamps, are designed to deliver their full rated, or “nominal”, light output at a specified RMS lamp current value. In this specification and the attached claims, the full rated light output of a lamp will be referred to as its “nominal light output”.
Fluorescent gas discharge lamps include a phosphor coating on the inside surface of the tubular glass housing, and the excitation of this coating by radiation from the discharge provides the visible light output. Conventional fluorescent lamps are generally straight elongated tubes of essentially circular cross section with varying outside diameters ranging between about five-eighths and one and one-half inches.
Compact fluorescent lamps differ from conventional fluorescent lamps in that they are constructed of smaller diameter tubing, typically having an outside diameter of less than about five-eighths of an inch. Also, the lamps are compact in part because the tubing has one or more small radius bends that allow the tube to fold back on itself in such a manner as to achieve a compact shape. Additionally, in compact fluorescent lamps wherein the tube is folded back on itself, the lamp ends typically are in close proximity to each other.
With reference to
FIG. 1
, a prior art lamp system
10
includes a source of AC power such as from a 120 volt 60 Hz. sinusoidal line voltage
100
, a phase controlled dimmer
102
, an electronic dimmable fluorescent ballast
200
, and a compact fluorescent lamp
300
.
The ballast
200
receives input power (or hot) on line
202
, a variable input dimming signal (or dimmed hot) on line
204
, and neutral on line
206
. It is understood that the voltage on line
202
is rectified by a full wave bridge rectifier
209
within the ballast
200
to yield a voltage having a positive DC average value with respect to circuit common.
The electronic dimming ballast
200
is designed to provide an amount of output power to the lamp
300
in accordance with the variable input signal on line
204
from the dimmer
102
. The dimmer
102
is a phase control dimmer which provides the variable input signal on line
204
by varying its phase firing angle which controls the RMS value of the variable input signal on line
204
.
As is known in the art, the ballast
200
typically includes a first power stage comprising a boost circuit
210
which receives a rectified voltage from rectifier
209
and produces a high DC voltage on line
214
which may reach 400 VDC or more.
The ballast
200
also typically includes a second power stage comprising an inverter circuit
216
which converts the DC voltage on line
214
into a high frequency switched voltage which is applied to a resonant tank circuit
230
which provides suitable AC voltage to drive the lamp
300
. A high voltage energy storage capacitor
212
is provided in a shunt configuration with respect to line
214
to provide a low impedance source of current to the inverter circuit
216
.
A control circuit
220
provides control signals to the boost circuit
210
and inverter circuit
216
over lines
221
and
222
, respectively. The control circuit
220
controls the boost circuit
210
to provide a desired DC bus voltage and controls the inverter circuit
216
to provide the high frequency switched voltage to the resonant tank circuit
230
. As a result, the ballast provides the desired current and voltage over line
208
to the lamp
300
responsive to the variable voltage input signal on line
204
such that the lamp
300
is illuminated at the proper intensity.
The control circuit
220
typically controls the inverter
216
, for example, by comparing a rectified version of the variable input signal on line
204
with a signal representative of the current delivered to the lamp over line
208
and (via known error signal techniques) adjusting the control signals input to the inverter
216
over line
222
to command the proper current to the lamp
300
.
As is known in the art, the control circuit
220
also commands the boost circuit
210
to produce the proper DC output voltage on line
214
. Further, the control circuit
220
typically includes circuits which perform other functions such as low voltage lockout, over-current protection, over-voltage protection and the like.
In the embodiment shown in
FIG. 1
, power is provided by a control circuit power supply
240
to drive the control circuit
220
, boost circuit
210
, and inverter circuit
216
. It is understood that the control circuit power supply
240
may be implemented using many circuit configurations.
The lamp system
10
of
FIG. 1
requires three wires between the dimmer
102
and the ballast
200
, and the ballast
200
may be located in the light fixture itself. Systems have been developed which eliminate the need for a third terminal on the ballast
200
for receiving the variable input signal on line
204
. In these systems, the variable input signal is received on line
202
. Other systems have been developed which utilize a third and a fourth terminal on the ballast
200
for receiving the variable input signal.
Ordinarily, when dimming linear fluorescent lamps down to low levels of light output (e.g., about one percent light output level), it is necessary to increase the output impedance of the electronic dimming ballast to maintain stable lamp operation and prevent visible flicker. Typically, the ballast output impedance is increased by driving the frequency of operation of the ballast close to the unloaded resonant frequency of the resonant tank circuit. The need for, and an apparatus and method for, obtaining high ballast output impedance is taught in U.S. Pat. No. B1 5,041,763, the entirety of which is hereby incorporated by reference.
In addition, the inventors have discovered that compact fluorescent lamps, in comparison to typical linear fluorescent lamps, have an additional area of lamp instability at low levels of lamp current around one percent of nominal light output. This additional region of instability manifests itself as a propensity for the lamp light output to extinguish, or “drop out”, as opposed to flickering between various low li

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