Current fed parallel resonant ballast

Details Current fed parallel resonant ballast Current fed parallel resonant ballast

1999-12-13

2001-12-04

Philogene, Haissa (Department: 2821)

Electric lamp and discharge devices: systems

Periodic switch in the supply circuit

Impedance or current regulator in the supply circuit

C315S2090SC, C315S287000, C315SDIG004, C315SDIG007, C363S037000

Reexamination Certificate

active

06326737

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to a ballast for fluorescent lights. More particularly, the invention is directed to a parallel resonant, current-fed ballast circuit which maintains optimum drive conditions for a wide range of input voltages, permitting improved dimming performance.
BACKGROUND OF THE INVENTION
Fluorescent lighting is a very common type of illumination. Fluorescent lamps function when an electrical arc is established between two electrodes located at opposite ends of the lamp. The electrical arc is established by supplying a proper voltage to the lamp. The lamp is filled with an ionizable gas and a very small amount of vaporized mercury. When the arc is established, collisions occur between the electrons and the mercury atoms, causing the emission of ultraviolet energy. The fluorescent lamps have a phosphorous coating on their inner surface, which transforms the ultraviolet energy into diffused, visible light. In order to establish the electrical arc, and thus turn on the lamp, a high voltage is typically required. However, once the lamp has been turned on, a lesser voltage is required to maintain the lamp's operation.
In order to start and operate a fluorescent lamp, a fluorescent lamp ballast is used. Among other functions (such as limiting the current flow through the lamp once it has already been started), a ballast is a device which provides the appropriate voltage to establish the arc through the lamps. Several different kinds of ballasts currently exist, e.g.—series mode and parallel mode. The series mode operates lamps in series across the output voltage of the ballast. The series mode ballast, while capable of performing dimming applications, usually is complex and thus, requires relatively high manufacturing cost. Parallel mode ballasts, while being less complex, and less expensive, are typically unsuitable for dimming applications, as will be explained below.
FIG. 1
shows a schematic diagram of a prior art parallel resonant current-fed circuit, coupled to a DC supply source
190
, which functions in a fluorescent lighting ballast. Transformer
101
contains a first primary winding comprising windings
111
and
112
and second primary winding comprising windings
121
and
122
. Additionally, the first primary windings of transformer
101
is connected in parallel with capacitors
161
,
162
and
163
. Primary windings
111
and
112
, and capacitors
161
,
162
and
163
form a tuned circuit, also known as an L-C parallel resonant circuit, and in conjunction with the other components of the circuit, produce an oscillating action upon the introduction of a start-up current.
Linear inductor
151
, is coupled to a center tap terminal
105
of first primary winding of transformer
101
so as to provide a substantially constant current signal to the center tap terminal. Linear inductor
151
is also coupled to a drive terminal
102
of the second primary winding of transformer
101
through a resistor
141
, so as to provide the start-up current feed to transistors
131
and
132
respectively. The current feed is sufficient to provide the minimum base drive current required by transistors
131
and
132
to start the transistors to operate in an oscillation mode. After the initial start, transistors
131
and
132
are provided a regenerative feedback current drive generated by windings
121
and
122
as explained later.
In the oscillation mode, transistors
131
and
132
are continuously turned on and off, so as to conduct current alternately through each of primary windings
111
and
112
. The alternating current flow through the primary windings creates an AC voltage signal which is applied to a series combination of capacitors
162
,
163
and lamps
181
and
182
coupled together in parallel. Capacitors
162
and
163
control the current flow through lamps
181
and
182
.
A constant current flow network
154
, comprising inductor
152
, resistor
142
and diode
171
, operates to maintain a substantially constant biasing current flow to the base terminals of transistors
131
and
132
respectively. The base-emitter junction of each transistor acts as a diode, and thus blocks any current flow from returning via windings
121
or
122
to drive terminal
102
through the transistors' base-emitter junction, provided that the voltage applied by the drive windings does not exceed the reverse base-emitter breakdown voltage of the transistors (as will be further discussed later). Diode
171
is configured so as to prevent the reverse flow of current in a direction from drive terminal
102
to constant current flow network
154
.
The switching back and forth between transistor
131
and transistor
132
is enhanced by the regenerative feedback current from drive windings
121
and
122
, and constant current flow network
154
. As shown, windings
121
and
122
are disposed between drive terminal
102
and the base terminals of transistors
131
and
132
, respectively. It is desirable to maximize the voltage level across drive windings
121
and
122
, since a higher voltage level at the base terminals turns the transistors on and off more rapidly and more efficiently than a low voltage level, and allows a wider range of applied voltage.
As previously mentioned, the voltage at the base terminal of the transistors and across the windings increases and decreases in accordance with the circuit's oscillating nature, and can be represented by a corresponding sine-wave curve. Since transistors
131
and
132
are alternately being turned on and off, the base voltage of each transistor is 180 degrees out of phase with the other. Significantly, there exists a point within each half-cycle of operation of this circuit when the voltage signal of the base terminal of a transistor and the corresponding drive winding voltage passes through zero. This point occurs when one transistor is turning on while the other transistor is turning off At this point, the switching action of the circuit may be interrupted because no current would be flowing to compel the corresponding transistor to turn on or off again. In order to prevent the interruption of the switching action and maintain a constant current flow to the drive windings and transistors, the circuit includes constant current flow network
154
previously described.
The voltages which can be utilized in this circuit are limited by the base-emitter breakdown voltage of the transistors, which is approximately 6.5 to 7 volts. This breakdown voltage limits the voltage level at drive terminal
102
to minus 3.5 volts. This follows because when one of the transistors, e.g.—
131
, is switched “on” its base-emitter junction acts like a diode to clamp the left-hand side voltage of drive winding
121
to a value near zero or to the common line negative voltage level of power supply
190
. At the same time the voltage level at drive terminal
102
and the right-hand side of winding
122
and base terminal of transistor
132
is taken to a negative level by an amount that depends on the number of turns of winding
122
, and hence the drive voltage of the windings. Thus, because of the limit imposed by the breakdown voltage of the transistors, the total voltage across windings
121
and
122
cannot exceed 7 volts. Hence only 3.5 volts will be generated at the center of the circuit. Exceeding the base-emitter breakdown voltage adversely affects the operation of the transistors and decreases the lifespan of the circuit.
Additionally, since the circuit must maintain a relatively small voltage between the base terminals of the transistors, the resistive value of resistor
142
of constant current flow network
154
is also required to be small. The current-defining resistor
142
, in order to permit an appropriate current flow into the center of the circuit, must be in the range of 10 to 20 ohms. Since voltage and current are directly related, a small change in the input supply voltage causes the drive current to change significantly and the lamp to either go out, or to be over drive

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