Electric lamp and discharge devices: systems – Current and/or voltage regulation
Reexamination Certificate
2000-12-06
2001-10-16
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Current and/or voltage regulation
C315S224000, C315S276000, C315S287000, C315S291000, C315S307000, C315SDIG004, C315SDIG007
Reexamination Certificate
active
06304041
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to a ballast for a compact gas discharge lamp, and more particularly to a ballast that uses regenerative gate drive circuitry to control a pair of serially connected, complementary conduction type switches of a d.c.- to- a.c. converter for producing a lamp driving signal.
BACKGROUND OF THE INVENTION
Gas discharge lamps such as fluorescent lamps produce light by exciting a gas with a high voltage a.c. signal generated by the drive section of a ballast circuit. The intensity of illumination is determined by the power of the excitation signal, which in turn depends on the signal frequency.
A ballast circuit for a gas discharge lamp is illustrated in U.S. Pat. No. 5,965,985, which issued to Nerone and is commonly assigned to the owner of the present application. U.S. Pat. No. 5,965,985 is incorporated herein by reference in its entirety.
FIG. 1
shows Nerone's ballast circuit
10
. A gas discharge lamp
12
is powered from a d.c. bus voltage between d.c. bus
16
and reference bus
18
that is converted to a.c. Switches
20
and
22
are serially connected between buses
16
and
18
in the form of a complementary converter circuit. The switches comprise n-channel and p-channel enhancement mode MOSFETs connected in a common source arrangement at a common node
24
. The switches may alternatively comprise other devices having complementary conduction modes, such as PNP and NPN bipolar junction transistors.
A resonant load circuit
25
including a resonant inductor
26
a
and a resonant capacitor
28
receives current through the common node
24
. Circuit
25
includes a d.c. blocking capacitor
30
and a snubber capacitor
32
. Lamp
12
includes resistively heated cathodes
12
a
and
12
b
that are supplied with heating current by windings
26
c
and
26
d
mutually coupled to inductor
26
a
. Switches
20
and
22
cooperate to provide a.c. current from the common node
24
to resonant inductor
26
a.
The gate electrodes
20
a
and
22
a
of the switches are connected at a control node
34
. Gate drive circuitry, generally designated
36
, is connected between control node
34
and common node
24
, and implements regenerative control of switches
20
and
22
. A drive winding
26
b
is mutually coupled to resonant inductor
26
a
, which carries a voltage proportional to the instantaneous rate of change of current in load circuit
25
. A transformer winding
38
a
serially connected to the driving inductor
26
b
couples a controlled voltage in series with the driving inductor
26
b
as described below.
A bidirectional voltage clamp
40
comprised of back-to-back Zener diodes cooperates with the transformer winding
38
a
such that the phase angle between the fundamental frequency component of voltage across resonant load circuit
25
(e.g., from node
24
to node
18
) and the a.c. current in resonant inductor
26
a
approaches zero during lamp ignition. A capacitor
44
is provided between nodes
24
and
34
to predictably limit the rate of change of control voltage between the nodes. This provides a dead time interval during switching of switches
20
and
22
during which neither switch is turned on.
The frequency of the a.c. signal produced by the ballast is controlled by a clamping circuit
62
.
FIG. 2
shows details of Nerone's clamping circuit. The clamping circuit controls ballast circuit frequency by varying the loading across the transformer winding
38
a
by means of a controlled impedance, implemented as a MOSFET
72
, in response to an error signal. The error signal is produced by a difference amplifier
64
that receives as input a set point signal provided by a user input potentiometer
68
, and a lamp current feedback signal provided by low pass filter
60
of FIG.
1
. The low pass filter
60
provides a time-averaged signal derived from a lamp current signal sensed by a sensing resistor
54
and rectified by p-n diode
56
. Half cycles of lamp current of the other polarity are shunted across resistor
54
by a diode
58
. The error signal provided by the difference amplifier
64
is amplified by an error amplifier
70
, powered from a node
73
, and applied to the gate of the MOSFET
72
. The MOSFET
72
determines the voltage across a control winding
38
b
, which is mutually coupled to the transformer winding
38
a
of the driving circuit of FIG.
1
.
A diode bridge network
74
a
-
74
b
enables the MOSFET
72
to conduct current through winding
38
b
in both directions, e.g., first through diodes
74
a
,
74
b
and then through diodes
75
a
,
75
b
. A capacitor
78
shunts MOSFET
72
to assist in clamping voltage across the control winding. A voltage clamp
80
such as a Zener diode shunts MOSFET
72
to limit the minimum frequency so as to set a maximum voltage across the lamp during ignition. The lower node of MOSFET
72
comprises the reference bus
18
of
FIG. 1
, and upper node
73
comprises a power supply node coupled via a resistor (not shown) to the d.c. bus
16
of FIG.
1
.
A preheat switch
82
, such as a p-channel enhancement mode MOSFET, may be provided to conduct for a preheat timing interval when the ballast circuit is first supplied with d.c. bus voltage. When conducting, switch
82
overrides MOSFET
72
by shorting its output. This allows resistively heated cathodes
12
a
and
12
b
of
FIG. 1
to reach a desired temperature, while maintaining a low voltage across lamp
12
, before lamp ignition. A circuit
84
for controlling the preheat switch
82
may be constructed as shown in FIG.
3
. As shown in
FIG. 3
, a comparator
85
receives a reference voltage from circuit
86
on its negative input, and upon bus energization, receives an increasing voltage on its positive input from a preheat capacitor
88
. The capacitor is charged by current conducted from node
73
by a preheat resistor
90
. The values of resistor
90
and capacitor
88
determine the duration of the preheat period during which switch
82
of
FIG. 2
conducts upon bus energization.
Nerone's ballast circuit thus enables control of the brightness of a fluorescent discharge lamp by means of the user input, which controls the voltage across the clamping transformer winding
38
a
of the driving circuit, thereby determining the frequency of the a.c. voltage applied to the resonant load circuit and thus controlling the current provided to the gas discharge lamp.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to compact fluorescent lamps that are compatible with standard incandescent light bulb sockets. In these lamps, ballast circuit size is a major contributor to lamp mass and volume. Therefore it is an object of the present invention to provide a dimmable ballast for a compact fluorescent lamp that significantly reduces the size and cost of components of the ballast.
The ballast of the present invention includes a resonant load circuit having a resonant inductance and a resonant capacitance. The load circuit couples a.c. current to a gas discharge lamp. A self-oscillating complementary converter circuit of the ballast induces a.c. current in the resonant load circuit. The converter circuit includes pair of switches serially connected between a d.c. voltage bus and a reference bus. respective reference nodes of the switches are interconnected at a common node through which the induced a.c. current flows, and respective control nodes of the switches are substantially directly interconnected. A gate drive circuit controls the switches. The gate drive circuit includes a drive winding connected to the control nodes. The drive winding is mutually coupled to the resonant inductance for coupling voltage therefrom, and an inductor is serially connected between the drive winding and the common node. A clamping circuit is directly coupled across terminals of the inductor of the gate drive circuit and controls voltage across the inductor in response to an error signal. The error signal represents the difference between a user-selectable set point signal and a feedback signal that represents a tim
Andarawis Emad Andarawis
Farkas Thomas
General Electric Company
Johnson Noreen C.
Tran Thuy Vinh
Vo Toan P.
Wong Don
LandOfFree
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