Dimmable, single stage fluorescent lamp

A dimmable fluorescent lamp including a feedback circuit for drawing sufficient current to maintain an appropriate charge across the buffer capacitor. A bypass capacitor provides a path for diverting a portion of the power which would otherwise be fed back to the buffer capacitor during low level dimming. Consequently, overboost voltages across the buffer capacitor during low level dimming are substantially reduced.

BACKGROUND OF THE INVENTION 
This invention relates generally to a ballast for a fluorescent lamp, and 
more particularly to a dimmable, single stage fluorescent lamp. 
A conventional single stage fluorescent lamp, such as disclosed in PCT 
Patent Application No. WO 96/07297, includes a ballast having an output 
(i.e. inverter) stage. No additional stage, such as a switch mode power 
supply, is required in maintaining a sufficient D.C. voltage across a 
buffer capacitor. Instead, the lamp ballast includes a feedback path for 
drawing sufficient current from the ballast input to maintain an 
appropriate charge across the buffer capacitor. 
Many lamp ballasts are not designed for dimming. Unacceptably high 
overboost voltages can occur across the buffer capacitor during low level 
triac dimming (e.g. at about 10% of full light output) due to too much 
power fed back to the buffer capacitor through the feedback path. 
Accordingly, it is desirable to provide an improved dimmable fluorescent 
lamp in which the amount of power fed back to the buffer capacitor during 
low level dimming is reduced. The improved dimmable fluorescent lamp, in 
particular, should substantially reduce overboost voltages across the 
buffer capacitor during low level dimming. 
SUMMARY OF THE INVENTION 
Generally speaking, in accordance with a first aspect of the invention, a 
ballast for powering a lamp load includes a buffer capacitor, a first 
serial combination of two switches joined together at a first junction and 
connected in parallel across the buffer capacitor and a second serial 
combination of an inductor and capacitor joined together at a second 
junction and coupled at one end to the first junction with the lamp load 
connected in parallel with the capacitor. The ballast also includes a 
feedback circuit coupled to the other end of the second serial combination 
for supplying a high frequency signal to the buffer capacitor, and a 
bypass circuit coupling the second junction to a junction joining the 
first serial combination of two switches and the buffer capacitor 
together. 
The bypass capacitor provides a path for diverting a portion of the power 
which would otherwise be fed back to the capacitor during low level 
dimming. Consequently, overboost voltages across the buffer capacitor 
during low level dimming are substantially reduced. 
It is a feature of the invention that the ballast also includes a serial 
combination of a diode and a feedback capacitor connected in parallel with 
the buffer capacitor. The feedback circuit can include the combination of 
discrete inductive and capacitive components. The ballast typically powers 
a lamp load consisting of a fluorescent lamp. 
In accordance with a second aspect of the invention, a ballast for powering 
a lamp load includes a buffer capacitor, a first serial combination of two 
switches joined together at a first junction and connected in parallel 
with the buffer capacitor and a second serial combination of an inductor 
and capacitor joined together at a second junction. The second serial 
combination has one end coupled to the first junction and another end 
coupled to a junction joining the first serial combination of two switches 
and buffer capacitor together with one end of the lamp load connected to 
the second junction. A feedback circuit is connected to the other end of 
the lamp load for supplying a high frequency signal to the buffer 
capacitor. The ballast typically powers the lamp load of a fluorescent 
lamp. 
Accordingly, it is an object of the invention to provide an improved 
dimmable, single stage fluorescent lamp in which the amount of power being 
fed back to the ballast buffer capacitor is reduced during low dim levels. 
It is another object of the invention to provide an improved dimmable, 
single stage fluorescent lamp in which overboost voltages across the 
ballast buffer capacitor are substantially reduced during low dim levels. 
Still other objects and advantages of the invention, will, in part, be 
obvious and will, in part, be apparent from the specification. 
The invention accordingly comprises several steps in a relation of one or 
more of such steps with respect to each of the others, and the device 
embodying features of construction, a combination of elements and 
arrangement of parts which are adapted to effect such steps, all as 
exemplified in the following detailed disclosure and the scope of the 
invention will be indicated in the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1, a fluorescent lamp 10 is powered from an A.C. power 
line represented by an A.C. source 20. The desired level of illumination 
of a lamp 30 is set by a controller 40 in response to a varying D.C. 
voltage applied to an input 43. 
A full bridge rectifier 25 rectifies the low frequency, sinusoidal A.C. 
voltage supplied by A.C. source 20. A pair of fast switching diodes 51 and 
54 applies this rectified, sinusoidal A.C. voltage to a buffer (e.g. an 
electrolytic) capacitor 60. Buffer capacitor 60 filters the sinusoidal 
voltage into a substantially constant D.C. voltage supplied to an 
inverter. 
An inverter is configured as a half-bridge and includes the serial 
combination of switches (e.g. power MOSFETs) 70 and 80 connected in series 
across buffer capacitor 60. Switches 70 and 80 are joined together at a 
junction 83 and commonly identified as forming a totem pole arrangement. 
The switching states of the MOSFETs, serving as switches 70 and 80, are 
controlled by controller 40. 
A serial combination of an inductor 90 and a shunt capacitor 95 are joined 
at a junction 100 and coupled at one end through a D.C. blocking capacitor 
85 to junction 83. Lamp 30 is connected in parallel across shunt capacitor 
95. A feedback circuit 110 is coupled to an end 105 of the serial 
combination of inductor 90 and capacitor 95 
Power feedback circuit 110 includes a feedback capacitor 113 connected 
between a junction 116 joining the cathode of diode 51 to the anode of 
diode 54. The serial combination of diode 54 and feedback capacitor 113 
are connected in parallel across buffer capacitor 60. Alternatively, as 
shown in FIG. 3, feedback capacitor 113 can be connected in parallel with 
diode 54. A short circuit is connected between the end 105 and junction 
116. Alternatively, as shown in dashed lines, an impedance 108 including a 
combination of one or more inductors and capacitors including, if desired, 
feedback capacitor 113 (as shown in FIG. 5), can be connected between end 
105 and junction 116 in order to increase the impedance of circuit 110 to 
substantially reduce an overboost voltage across buffer capacitor 60. A 
bypass circuit, which includes an impedance such as, but not limited to, a 
capacitor 119, is coupled between junction 100 and the junction joining 
the serial combination of switches 70 and 80 and buffer capacitor 60 
together. 
In operation, controller 40 drives the switching frequency of switches 70 
and 80 based on the illumination level corresponding to the D.C. voltage 
applied to an input 43. Power at a high frequency of between about 50 kHz 
to 85 kHz is delivered to lamp 30 by a resonant circuit formed by inductor 
90, capacitors 119, 95 and 113 and lamp 30. In an alternative embodiment, 
the resonant circuit includes impedance 108. At low dim levels of, for 
example, 10% of full lamp output, a portion of the power delivered is 
diverted away from feedback circuit 110 by capacitor 119. This same 
resonant circuit controls the amount of power fed back to buffer capacitor 
60. Consequently, overboost voltages across the buffer capacitor during 
low level dimming are substantially reduced. 
In accordance with another embodiment of the invention, as shown in FIG. 2, 
a capacitor 120 serves ;as both bypass capacitor 119 and shunt capacitor 
95. In the alternative embodiment, the common resonant circuit would also 
include impedance 108. In all embodiments, a single inductor (choke) 90 
can be used to minimize the number of parts and manufacturing cost. FIGS. 
4 and 6 are similar to FIGS. 3 and 5 except that the incorporate capacitor 
120 rather than bypass capacitor 119 and shunt capacitor 95, respectively. 
As can now be readily appreciated, bypass capacitor 119 or 120 provides a 
path for diverting a portion of the power which would otherwise be fed 
back to buffer capacitor 60 during low level dimming. Consequently, 
overboost voltages across buffer capacitor 60 during low level dimming are 
substantially reduced. 
It will thus be seen that the objects set forth above and those made 
apparent from the preceding description are efficiently attained and, 
since certain changes can be made in the above method and construction set 
forth without departing from the spirit and scope of the invention, it is 
intended that all matter contained in the above description and shown in 
the accompanying drawings shall be interpreted as illustrative and not in 
a limiting sense. 
It is also to be understood that the following claims are intended to cover 
all the generic and specific features of the invention herein described 
and all statements of the scope of the invention, which as a matter of 
language, might be said to fall therebetween.