Electronic ballast with reversely wound filament winding

An electronic ballast includes a series resonant inductor and capacitor and a filament winding magnetically coupled to said inductor. The filament winding forms a closed circuit with a filament in a gas discharge lamp, wherein the current induced in the winding opposes a portion of the current through the inductor to reduce the net voltage on the filament during normal lamp operation. In accordance with another aspect of the invention, the filament winding is reversely wound with the inductor on a common core to reverse the phase of the current induced in the filament winding from the current through the inductor.

BACKGROUND OF THE INVENTION 
This invention relates to an electronic ballast for a gas discharge lamp 
and, in particular, to the portion of the ballast supplying power to the 
filaments in a gas discharge lamp. 
A gas discharge lamp is a non-linear electrical load, i.e. the current 
through the lamp is not proportional to the voltage across the lamp. The 
current is zero until the voltage increases sufficiently for an arc to 
strike, then the current will increase rapidly through the ionized gases 
in the lamp unless there is a ballast in series with the lamp to limit 
current. 
In many gas discharge lamps, small filaments at each end of the lamp are 
made to glow and emit electrons to facilitate starting the lamp. The 
filaments are typically coated with a material having a low work function, 
that is, a material that emits electrons profusely when heated, thereby 
aiding in ionizing the gases within the lamp and reducing the voltage 
required to start the lamp. 
A "magnetic" ballast is an inductor in series with a lamp for limiting 
current through the lamp. The inductor includes many turns of wire wound 
on a laminated iron core and magnetic ballasts of the prior art are 
physically large and heavy, often accounting for more than half the weight 
of a fixture including the lamps. 
An "electronic" ballast typically includes a converter for changing the AC 
from a power line to direct current (DC) and an inverter for changing the 
DC to high frequency AC. Converting from AC to DC is usually done with a 
full wave, or bridge, rectifier. A filter capacitor on the output of the 
rectifier stores energy for powering the inverter. The inverter changes 
the DC to high frequency AC at 140-300 volts for powering one or more gas 
discharge lamps. 
It is known in the art to provide an electronic ballast having a "direct 
coupled" output, in which a lamp is connected in parallel with the 
capacitor in a series resonant LC circuit. Separate windings, magnetically 
coupled to the resonant inductor, provide current for heating the 
filaments in a lamp. Thus, the filaments are powered continuously, 
reducing the efficiency of the lamp, measured in lumens per watt. 
The windings are made by winding the resonant inductor on a suitable core, 
tying off a common lead, and then winding one of the filament windings. A 
second filament winding, or second and third filament windings for a two 
lamp system, are then wound on the core. The magnetics, i.e. the inductors 
and transformers, are one of the more expensive components in an 
electronic ballast. Winding a filament winding having a common lead with 
the resonant inductor is less expensive than a magnetic with separate 
windings for the resonant inductor and a filament. 
Many modern, high efficiency lamps, such as T2, T5, and some compact lamps 
require a relatively low filament voltage during normal operation for 
reduced temperature and long life. For example, a T2 lamp is usually 
specified as having a maximum filament voltage of three volts during 
normal operation. Unfortunately, it is also required that the filaments be 
red-hot during the pre-heat phase of lamp starting. Adequately heating the 
filaments for starting requires six to eight volts, substantially in 
excess of the three volt limit imposed during running. Some manufacturers 
specify a limit on the total current through a filament, which is the 
equivalent of a limit on voltage. Thus, the problem is to provide adequate 
voltage for starting and low voltage for running. 
In a ballast having a direct coupled output, the starting phase and the 
running phase of lamp operation are distinguished by a change in 
frequency. As more fully described in the Detailed Description, a ballast 
having a direct coupled output is typically started at a frequency well 
above resonance, causing a relatively high voltage to be applied to the 
filaments and a relatively low voltage to be applied to the lamp, and is 
run at a frequency slightly above loaded resonance, causing a high voltage 
to be applied to the lamp and a relatively low voltage to be applied to 
the filaments. Even so, the filament voltage during normal operation is 
typically greater than three volts in ballasts of the prior art. 
One could use electronic switches to control the voltage on the filaments 
but this would substantially increase the cost of a ballast and is, 
therefore, undesirable. 
In view of the foregoing, it is therefore an object of the invention to 
reduce filament voltage during normal operation of a gas discharge lamp 
without impairing starting. 
A further object of the invention is to improve the efficiency of an 
electronic ballast. 
Another object of the invention is to improve the efficiency of an 
electronic ballast without increasing cost. 
SUMMARY OF THE INVENTION 
The foregoing objects are achieved in this invention in which an electronic 
ballast includes a series resonant inductor and capacitor and a filament 
winding magnetically coupled to said inductor. The filament winding forms 
a closed circuit with a filament in a gas discharge lamp, wherein the 
current induced in the winding opposes a portion of the current through 
the inductor to reduce the net voltage on the filament during normal lamp 
operation. In accordance with another aspect of the invention, the 
filament winding is reversely wound with the inductor on a common core to 
reverse the phase of the current induced in the filament winding from the 
current through the inductor.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a preferred embodiment of the invention as applied to a 
ballast for a single gas discharge lamp. Output 10 is a direct coupled 
output including resonant inductor 11 and resonant capacitor 12 series 
connected across lines 14 and 15. Lines 14 and 15 are coupled to a source 
of pulses (not shown) in which the frequency of the pulses can be varied 
from approximately equal to the resonant frequency of inductor 11 and 
capacitor 12 to a frequency substantially higher than the resonant 
frequency. Circuitry known in the art as a half bridge inverter is but one 
example of a suitable source of such pulses. 
Lamp 17 is connected in parallel with capacitor 12 and filaments 21 and 22 
in the lamp are connected in series with capacitor 12. During normal 
operation, current i.sub.L flows through lamp 17 and current i.sub.R flows 
through capacitor 12. During starting, only current i.sub.R flows. Nodes 
19 and 20 are the output terminals of the ballast illustrated in FIG. 1 
and the sum of currents i.sub.L and i.sub.R flows through inductor 11 to 
node 19. 
During starting, filaments 21 and 22 are pre-heated by applying a frequency 
higher than the resonant frequency of inductor 11 and capacitor 12 to 
lines 14 and 15. Because the frequency is above resonance, most of the 
output voltage is across inductor 11 and is coupled to filament windings 
31 and 32. Also because of the high frequency, DC blocking capacitors 35 
and 37 have a low impedance, allowing maximum coupling to the filaments 
for heating. The filament windings are each in a small closed circuit 
through which the heater current flows. Filament windings 31 and 32 are 
magnetically coupled to resonant inductor 11. Heater current i.sub.H from 
winding 31 flows through filament 21 and capacitor 35. In accordance with 
the invention, current i.sub.R is opposed by current i.sub.H. During 
starting, current i.sub.H is considerably greater than current i.sub.R and 
there is little effect on filament voltage from the slight reduction in 
net current through filament 21. The same effects occur in the circuit 
including filament 22. 
Once lamp 17 begins conduction, the frequency is reduced to slightly above 
the loaded resonant frequency of inductor 11 and capacitor 12. As a 
result, the voltage across capacitor 12 increases considerably and the 
voltage across inductor 11 is reduced, with a commensurate reduction in 
the voltages induced in windings 31 and 32. Even so, without the 
invention, the voltage on filaments 21 and 22 can be excessive. One could 
reduce the size of resonant capacitor 12 but this increases the resonant 
frequency and makes the ballast sensitive to stray capacitances. 
Because the frequency has been reduced for running, current i.sub.H has 
decreased, current i.sub.R has increased, and the net current in the 
filament circuit is lower than during starting. The current induced by 
windings 31 and 32 does not completely cancel resonant current i.sub.R but 
merely reduces the net current and, therefore, the RMS voltage across the 
filaments. In one embodiment of the invention, in a two lamp ballast, the 
RMS voltage was reduced from 8.1 volts to 5.8 volts. 
FIG. 2 illustrates an alternative arrangement of the components in output 
10 (FIG. 1). In particular, resonant capacitor 12 is coupled between nodes 
19 and 20. Lamp current i.sub.L is opposed by heater current i.sub.H when 
lamp 17 conducts. Thus, FIG. 2 differs from FIG. 1 in that the heater 
current opposes the lamp current instead of opposing the resonant current 
during normal lamp operation. The filament, the winding, and the blocking 
capacitor can be connected in several configurations, all of which can 
benefit from the invention by having the heater current opposes a 
component of the current from the resonant inductor. 
FIG. 3 illustrates the voltage across a filament using a ballast 
constructed in accordance with the prior art. The current through 
capacitor 12 (FIG. 1) is sinusoidal but the current from lines 14 and 15 
is pulsed, producing the spikes illustrated in FIG. 3. FIG. 4 illustrates 
the voltage across a filament winding driven by a ballast constructed in 
accordance with the invention. FIGS. 3 and 4 are drawn to the same scale. 
Thus, it is clear that the sinusoidal component in FIG. 4 is reduced 
compared to FIG. 3. The output circuit illustrated in FIG. 1 enables one 
to provide a filament winding having the correct phase to cause partial 
cancellation of the current through a filament. 
In accordance with another aspect of the invention, winding 31 is a 
reversely wound extension of inductor 11 and is connected to inductor 11 
at common node 19. FIG. 5 illustrates the winding of a magnetic in 
accordance with this aspect of the invention. In particular, winding 41 
and winding 42 are wound about common core 43, illustrated in FIG. 5 as a 
simple bar. It is understood that the magnetics in an electronic ballast 
are typically wound on an E-shaped core. 
Winding 41 encircles core 43 with the turns going in a first direction from 
the left hand end of core 43 to a point above common node 19. At common 
node 19, the wire wound around core 43 is brought out away from the core 
to provide a lead or tap in the windings. Continuing from left to right, 
the wire continues to be wound around core 43, except that the direction 
of rotation is changed and winding 42 encircles core 43 by turning in the 
opposite direction to winding 41. The number of turns in winding 41, a 
resonant inductor, greatly exceeds the number of turns in winding 42, a 
filament winding. The voltage induced in winding 42 is of opposite phase 
to the voltage in winding 41. 
Filament winding 32 is a separate winding on a common core with inductor 11 
and winding 31. Inductor 32 is connected to filament 22 in such a way that 
the current from inductor 32 opposes current i.sub.R. There are only two 
ways to connect winding 32 and the winding should be connected for 
opposing currents. 
Output 50, illustrated in FIG. 6, includes lamps 51 and 52 connected in 
series across a resonant capacitor (not shown). A filament (not shown) at 
the upper end of lamp 51 corresponds to filament 21 in FIG. 1. A filament 
(not shown) at the lower end of lamp 52 corresponds to filament 22 in FIG. 
1. At the juncture of lamps 51 and 52, filaments 53 and 54 are series 
connected across winding 56. Winding 56 is coupled to filaments 53 and 54 
in such a way that the current from winding 56 opposes the lamp current 
through filaments 53 and 54. Thus, the connection of winding 56 is 
analogous to the connection of filament winding 32 in FIG. 1, except that 
lamp current rather than resonant current is being opposed. 
Thus, the invention provides a no-cost enhancement of an electronic ballast 
that improves the efficiency of the ballast and enables a ballast to drive 
T2, T5, and other lamps within the manufacturers specifications for the 
lamps. In one embodiment of the invention, a two lamp ballast, the 
invention reduced the power consumed by the ballast by approximately one 
watt without changing the luminance of the lamp coupled to the ballast. 
Having thus described the invention, it will be apparent to those of skill 
in the art that various modifications can be made within the scope of the 
invention. For example, the magnetic cores can be E-C, toroidal, or other 
shapes. The windings in FIG. 5 are exaggerated for illustration. In an 
actual ballast, the windings are closer together and winding 42 overlays 
winding 41. Although illustrated in connection with one lamp and two lamp 
ballasts, the invention applies to ballasts for any number of lamps. The 
invention also applies to ballasts having a parallel resonant output or a 
class-E output. The blocking capacitors can be replaced by a direct 
connection, a diode, or a suitable impedances. 
Reference to the direction of a current, as though the output were DC 
rather than AC, is for ease of understanding. The phase of an alternating 
current is the property that is actually being discussed. When two 
alternating currents are mixed, varying degrees of cancellation or 
addition may occur, depending upon the relative phases of the currents. If 
the two currents also differ in waveform, the reduction in voltage on the 
filaments may not be as great but the filament voltage will be reduced 
compared to ballasts of the prior art.