Electric lamp and discharge devices: systems – Pulsating or a.c. supply – With power factor control device
Reexamination Certificate
1999-11-01
2001-05-01
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Pulsating or a.c. supply
With power factor control device
C315S2090SC, C315S291000, C315S310000, C315SDIG007
Reexamination Certificate
active
06225755
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an electronic ballast system and more specifically to a ballast arrangement employed for driving High Intensity Discharge (HID) lamps with a signal having a controllable frequency.
BACKGROUND OF THE INVENTION
There is an ever increasing need for gas discharge lamps, such as fluorescent lamps, for both commercial and consumer applications. Gas discharge lamps are usually driven by a mains voltage supply source provided by power utility companies. In order to drive a discharge lamp from the mains voltage supply line, a ballast is employed that functions as an interface between the lamp and the supply line.
One main function of a ballast is to drive the discharge lamp with a signal that has an appropriate voltage and current level. Another important function of a ballast is to perform, what is known as, power factor correction. The voltage and current level necessary to operate the discharge lamp is governed, among other things, by the characteristics of the gas contained inside the lamp. Power factor correction is necessary to insure that the operation of the ballast does not contribute noise signals to the power supply line feeding the ballast. Typically, a power factor correction arrangement controls the supply current provided by the ballast such that it remains in phase with the voltage supply line waveform.
With the advent of HID lamps, the ballasts need to also ensure that the discharge lamp is driven by a low frequency current signal, in the range of 1 kHz or less. Driving an HID lamp at high frequencies is usually difficult due to arc instabilities caused by acoustic resonance. This resonance can lead to lamp failure.
FIG. 1
illustrates a circuit diagram of a typical ballast employed to drive an HID lamp. The operation of ballast
10
is very well understood and is not described in detail herein. The ballast circuit includes an upper signal line and a lower signal line each coupled to a respective terminal of mains power supply line. Ballast
10
comprises an EMI filter
36
followed by a full bridge diode rectifier
12
to rectify the ac voltage signal provided by the mains supply line. The rectified signal is then fed to a preconditioner stage, such as a boost converter
14
, which operates to shape the ballast supply current, also referred to as mains current, for power factor correction. The preconditioner is followed by an energy storage capacitor
26
, which accumulates a dc bus voltage V
bus
, which is typically larger than the peak voltage level provided by the mains power supply line. Boost converter
14
includes an inductor
20
having inductance I
L
, along the upper signal line of ballast
10
, coupled in series with a diode
24
, which in turn is coupled to storage capacitor
26
. A transistor switch
22
is coupled across inductor
20
and the lower signal line of the ballast. The duty cycle of switch
22
can be controlled so as to operate the boost converter in different operation modes.
For example, boost converter
14
can operate under, what is known as, a continuous conduction mode operation (CCM). During this mode of operation, the average voltage across capacitor
26
is
V
26
=V
in
/(1
−D
(
t
))
wherein V
in
is the voltage signal fed to boost converter
14
and D(t) a variable duty cycle of switch
22
. A controller (not shown) varies the duty cycle of switch
22
so that the current I
L
has a sinusoidal shape that is in phase with the mains voltage supply waveform. Other control operation modes for boost converter
14
include discontinuous conduction mode operation (DCM) and critical discontinuous conduction mode operation (CDCM), which may be employed based on various design considerations. For a continuous conduction mode operation, the average voltage signal across inductor
20
is substantially zero.
Boost converter
14
is followed by a buck converter
16
that is fed by the dc bus voltage signal formed across capacitor
26
. A transistor switch
28
couples capacitor
26
to an inductor
32
, which in turn is coupled to a filter capacitor
34
. A diode
30
is coupled to switch
28
and to the lower signal line of the ballast. The buck converter creates a dc current which drives the lamp through a commutator stage
18
.
Commutator stage
18
includes four transistor switches, which interchangeably operate to switch the current signal provided to lamp
36
. The commutator inverts the lamp polarity at a low frequency, typically in the 100 Hz range.
One disadvantage with the ballast circuit described in
FIG. 1
is that it suffers from a high component count and poor converter efficiency. There has been some effort to reduce the number of component parts of a ballast for driving HID lamps. One approach is to synchronize the lamp current to the power supply voltage frequency, as described in U.S. Pat. No. 5,917,290, entitled Parallel-Storage Series-Drive Electronic Ballast. The disadvantage of such a ballast circuit is that when the frequency of the power voltage supply signal is low, for example 50 Hz, there is the possibility of a visible light flicker from the lamp.
Thus, there is a need for an efficient and simple ballast circuit having a low component count and a driving current signal that has a controllable frequency to avoid possible light flicker.
SUMMARY OF THE INVENTION
In accordance with one embodiment of the invention an electronic ballast circuit includes a ballast bridge unit configured to receive a ballast supply signal from a mains voltage supply line via an input inductor. The ballast bridge unit comprises an input converter bridge having at least two transistor switches coupled in series at a common terminal, wherein an upper transistor switch is coupled to the upper signal line of the ballast bridge unit and the lower transistor switch is coupled to the lower signal line of the ballast bridge unit. The transistor switches of the input converter bridge are operated such that the input converter current waveform follows the mains voltage supply signal waveform.
The ballast bridge unit includes a rectifier bridge comprising two diodes coupled in series forming a diode bridge common terminal. The upper diode is coupled to the upper signal line of the bridge unit, while the lower diode is coupled to the lower signal line of the bridge unit.
The ballast bridge unit further comprises an output converter bridge having at least two transistor switches coupled in series at an output common terminal, wherein an upper transistor switch is coupled to the upper signal line of the ballast bridge unit and the lower transistor switch is coupled to the lower signal line of the ballast bridge unit. The transistor switches of the output converter bridge are operated such that the average voltage signal level of their common output terminal shifts between two desired values, so as to form an ac current for driving a gas discharge lamp.
The ballast bridge unit also comprises a capacitor divider bridge having at least two storage capacitors coupled in series at a reference voltage common terminal, wherein an upper storage capacitor is coupled to the upper line of the ballast bridge unit and the lower storage capacitor is coupled to the lower signal line of the ballast bridge unit. The voltage across the two storage capacitors defines a bus voltage signal, V
bus
, for the input and output converters of the ballast circuit. The average voltage signal level of the reference voltage terminal is set at half the bus voltage signal so as to form a differential voltage signal between the output common terminal and the common reference voltage terminal.
The output common terminal of the ballast bridge unit is coupled to a filter capacitor via an output inductor so as to provide a driving signal to a gas discharge lamp disposed across the filter capacitor.
During operation, the input converter bridge switches are controlled at a high frequency in a pulse width modulation arrangement to shape the input inductor current to be in phase with the mains voltage signal. The diodes in the rectifier b
Franzblau Bernard
Philips Electronics North America Corporation
Philogene Haissa
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