Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Periodic switch cut-out
Reexamination Certificate
2000-09-06
2001-11-13
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Periodic switch cut-out
C315S283000, C315S291000, C315S307000, C315SDIG004
Reexamination Certificate
active
06316882
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to electronic ballast systems and, more particularly, the invention relates to an electronic ballast system that controls the illumination of a lamp by using a reference voltage which is temperature stable and unaffected by resistance process dispersion and which enables soft dimming and ON/OFF control of the lamp via a multi-function input terminal.
2. Description of Related Technology
Generally speaking, electronic ballast systems initiate a glow discharge within a gas-filled lamp, such as a conventional fluorescent lamp, and thereafter maintain a stable supply of power to the lamp to sustain the discharge. As is well known, conventional electronic ballast systems typically include an inverter circuit that supplies alternating current (AC) power to the lamp and a lamp driver circuit, which uses a pulse-width modulated (PWM) control signal to vary the amount of power that the inverter supplies to the lamp.
FIG. 1
is an exemplary schematic diagram of a conventional lamp system
5
that uses an electronic ballast (not shown) to control the illumination of a lamp LAMP. The lamp system
5
includes a power supply unit
10
, a switching circuit
20
and a lamp unit
30
, all of which are connected as shown. The power supply unit
10
supplies direct current (DC) power to the switching unit
20
, which includes first and second power switches S
1
and S
2
that are alternately turned ON and OFF by the ballast to drive the lamp unit
30
with AC power, thereby illuminating the lamp LAMP.
As is well known in the art, when the first switch S
1
is in an ON condition (i.e., is conducting current) and the second switch S
2
is in an OFF condition, current flows from the power supply unit
10
through the first switch S
1
, an inductor L, the lamp LAMP, a first capacitor CL
1
and a second capacitor CL
2
. On the other hand, when the first switch S
1
is in an OFF condition and the second switch S
2
is in an ON condition, current flows from the power supply unit
10
through a third capacitor CL
3
, the lamp LAMP, the first capacitor CL
1
, the inductor L and the second switch S
2
. As is also well known, a resonance circuit is formed by the inductor L, the first capacitor CL
1
and the second capacitor CL
2
when the first switch S
1
is ON and the second switch S
2
is OFF. Likewise, the inductor L, the first capacitor CL
1
and the third capacitor CL
3
form a resonance circuit when the second switch S
2
is ON and the first switch S
1
is OFF.
In operation, the illumination of the lamp LAMP is controlled by varying the switching frequency of the switching unit
20
. In particular, the drive current supplied to the lamp LAMP may be increased (to increase the intensity of the amp illumination) by reducing the switching frequency of the switching unit
20
or, alternatively, may be decreased (to decrease the intensity of the lamp illumination) by increasing the switching frequency of the switching unit
20
.
The ballast (not shown) compares a feedback voltage developed across a current sense resistor Rsense to a reference voltage to control the switching frequency of the switching unit
20
. During normal operation, the ballast increases the switching frequency of the switching unit
20
, which decreases the drive current supplied to the lamp LAMP, if the feedback voltage is larger than the reference voltage and decreases the switching frequency of the switching unit
20
, which increases the drive current supplied to the lamp LAMP, if the feedback voltage is less than the reference voltage. Additionally, the reference voltage may be varied to provide a soft-start interval during initial power-up of the lamp system
5
and/or may be used to control a dimming operation of the lamp system
5
.
As is generally known, the stable operation of the lamp system
5
depends on the stability of the above-noted ballast reference voltage. Unfortunately, conventional electronic ballast circuits are typically based on integrated circuits, which are typically influenced by resistance process dispersion and temperature variations that cause the reference voltage to be far from stable. Further, when conventional electronic ballasts control the dimming of a lamp, the reference voltage typically changes abruptly, which abruptly alters the current flowing through the lamp (and the illumination intensity of the lamp) and strains the entire lamp system. Still further, because conventional electronic ballast systems are based on integrated circuits, remote control over the operation of the ballast becomes difficult, particularly because separate terminals are typically required for performing ON/OFF and dimming control functions.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, a current amplifier for use in an electronic ballast includes a current source and a differential amplifier coupled to the current source. The differential amplifier may include first and second resistors and first and second transistors. An emitter terminal of the first transistor may be coupled to the first resistor and an emitter terminal of the second transistor may be coupled to the second resistor. The current amplifier may further include a selection circuit having third and fourth transistors and the emitter terminals of the third and fourth transistors may be coupled to each other and to a base terminal of the second transistor. Additionally, the current amplifier may include a first current mirror, a second current mirror coupled to the first current mirror and a collector terminal of the first transistor and a third current mirror coupled to the first current mirror and a collector terminal of the second transistor.
In accordance with another aspect of the invention, a reference voltage generator for use in an electronic ballast includes a comparison voltage generator having a soft start current source and a soft start capacitor coupled to the soft start current source. The reference voltage generator may further include a first amplifier having a first current amplifier. The first current amplifier may have first and second non-inverting input terminals and a first inverting input terminal. Additionally, one of the first and second non-inverting input terminals may be coupled to the soft start capacitor and the other one of the first and second non-inverting input terminals may coupled to a first amplifier reference voltage. The reference voltage generator may further include a first current source having a first current mirror, first and second resistors and a first transistor. A first output terminal of the first current mirror may be coupled to a collector terminal of the first transistor and a second output terminal of the first current mirror may be coupled to the second resistor. Still further, a base terminal of the first transistor may be coupled to an output terminal of the first current amplifier and the first resistor may be coupled between a ground potential and the inverting input terminal of the first current amplifier. Still further, the reference voltage generator may include a capacitor charger that compares a voltage across the soft start capacitor to a first comparison reference voltage and charges a dimming capacitor based on the comparison. The reference voltage generator may additionally include a second amplifier having a second current amplifier. The second current amplifier may have third and fourth non-inverting input terminals and a second inverting input terminal and one of the third and fourth non-inverting input terminals may be coupled to a dimming voltage and the other one of the third and fourth non-inverting input terminals may be coupled to a second amplifier reference voltage. An output terminal of the second current amplifier may be coupled to the dimming capacitor and the second current amplifier may select the smaller of the dimming voltage and the second amplifier reference voltage to control a charging characteristic of the dimming capacitor. Still further, the voltage ref
Choi Nak-choon
Seo Maeng-ho
Fairfield Korea Semiconductor Ltd.
Marshall Gerstein & Borun
Tran Thuy Vinh
Vu David
LandOfFree
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