Electric lamp and discharge devices: systems – Pulsating or a.c. supply – With power factor control device
Reexamination Certificate
1999-12-20
2001-01-30
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Pulsating or a.c. supply
With power factor control device
C315S224000, C315S308000, C315S312000
Reexamination Certificate
active
06181079
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an electronic ballast apparatus for operation of gas discharge lamps, and more particularly to a high power electronic ballast equipped with a unique and improved circuit topology that provides electric isolation from the AC power line voltage.
A conventional electronic ballast operated from a 60 Hz AC power line usually includes a two-stage converter circuit in order to supply a high frequency AC current to a load consisting of one or more gas discharge lamps. In order to provide protection against the hazard of an electrical shock, the output stage of the converter circuit is coupled to the lamp load via an isolation transformer, thereby providing electric isolation between the ballast output terminals and the converter circuit.
One example of an improved prior art electronic ballast is described in U.S. Pat. No. 5,084,653, which issued Jan. 28, 1992. This electronic ballast provides electric isolation in the front end stage of the apparatus. This patent describes an electronic ballast for powering three series connected fluorescent lamps with a 30 kHz lamp current and comprising a half-bridge series-resonance type inverter circuit powered from a substantially constant magnitude DC supply voltage derived from ordinary 60 Hz power line voltage by way of a bridge rectifier and a single transistor DC-to-DC converter using an energy storing inductor with an isolated secondary winding from which the DC supply voltage is derived. Thus, the DC supply voltage, the inverter circuit and the ballast output terminals are all electrically isolated from the AC power line and thereby provide protection against the hazard of an electric shock.
The front end stage of this electronic ballast is based upon a flyback DC-to-DC converter. This type of converter requires an energy-storing inductor which is a rather large and relatively expensive magnetic component, especially as compared with the inductors used in a boost converter or a single-ended primary inductor converter (SEPIC converter) with the same or comparable power ratings. Another disadvantage of the flyback converter is the large magnitude of its pulsating input current which usually requires larger EMI filters in order to achieve comparable EMC performance to that of a boost converter or a SEPIC converter.
Another prior art high frequency electronic ballast for gas discharge lamps is shown in the accompanying
FIG. 1
of the drawings. This electronic ballast circuit basically consists of two building blocks. The front end is a boost converter for power factor correction and universal line voltage regulation. The main components are a transistor power switch Q
1
, an inductor L
1
, a diode D
5
and the DC storage capacitor C
1
along with an EMI filter and the diode bridge rectifier interposed between the AC supply voltage and the boost converter. The transistor switch Q
1
is periodically switched on and off by a control circuit, for example, a Motorola Corporation product MC34262, as a function of the voltage across capacitor C
1
and the current flowing the transistor switch Q
1
.
The back end is a typical voltage-fed half-bridge inverter loaded with a group of lamps via a resonant tank L
2
-C
3
. The main components are the power switches Q
2
and Q
3
, resonant components including capacitor C
3
, inductor L
2
and possibly the magnetizing inductance of the output transformer T
1
. The capacitors Clp in the secondary circuit of the transformer T
1
are usually provided in order to ballast the lamp current and to protect against possible lamp rectification at the end of lamp life. There are four magnetic components in the circuit configuration shown in
FIG. 1
, i.e. the EMI filter L
0
, the boost choke L
1
, the resonant inductor L
2
and the output isolation transformer T
1
. The operation of the power switches Q
2
and Q
3
is controlled by a high voltage control IC, for example, an IC UBA 2010 manufactured by Philips, as a function of current flow in transistor switch Q
3
and the voltage on capacitor C
3
. Here too the size and cost of the magnetic components of the apparatus make this type of high frequency electronic ballast less than an optimal choice, especially from a competitive commercial viewpoint.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a high power electronic ballast having a reduced number of magnetic components with a concomitant reduction in the size and cost of the electronic ballast.
Another object of the invention is to provide a compact and inexpensive high power electronic ballast which uses a single integrated transformer-inductor combination in the ballast input stage that exhibits reduced power losses in the magnetic components.
A further object of the invention is a new and improved method of operating a high power electronic ballast. More specifically, the coupled inductor based SEPIC converter is designed to be operated in a boundary or critical conduction mode with variable frequency during the 60 Hz line cycle. The variable frequency range cound be, for example, from 50 kHz to 150 kHz. The control circuit along with the sensing arrangement of the invention leads to close to unity power factor at the line input terminal.
The above and other objects and advantages are achieved in accordance with the present invention wherein the isolation transformer is introduced into the power factor correction front end stage of the electronic ballast as a single integrated magnetic component that provides the electric isolation function as well as the converter inductor function. As a result, the output isolation transformer common in prior art high frequency electronic ballast circuits is no longer required.
The invention herein is based upon the following observations. In a prior art electronic ballast of the type shown in
FIG. 1
, a major function of the output transformer T
1
is to provide electric isolation between the primary and the secondary in order to meet the ANSI safety requirements. In current-fed half-bridge converters, such an output transformer also serves as a resonant component and voltage gain booster. But, in a voltage-fed half-bridge inverter as shown in
FIG. 1
, the voltage gain can be obtained from the resonant tank consisting of inductor L
2
and capacitor C
3
. Therefore, it is possible to remove the output transformer if the electric isolation is introduced into the front end part of the circuit. This results in a single magnetic component AC-DC converter with a power factor correction (PFC) front end stage providing galvanic isolation in cascade with a single magnetic component half-bridge resonant converter in accordance with the present invention.
The introduction of an isolated power factor correction front end stage employing a single magnetic component eliminates the requirement for the output transformer. As a result, the total number of magnetic components of the ballast circuit is reduced to three and the overall size and weight of the magnetic components are less than those of prior art electronic ballast circuits of the type described above. The related circuit losses and cost are reduced and further circuit miniaturization is now feasible.
In a preferred form of the invention, the front end stage of the high power electronic ballast utilizes a coupled inductor isolated SEPIC converter. The invention also contemplates the use of an isolated boost circuit as the front end stage of the electronic ballast.
In another advantageous form of the invention we provide an improved control function for the transistor switch in the PFC input stage of the apparatus, i.e. the apparatus is operated in a so-called critical conduction mode in which the transistor switch is turned on as soon as the input inductor current falls to zero. Prior art converters usually operate in a continuous conduction mode or in a discontinuous conduction mode.
In a continuous conduction mode (CCM) apparatus, the circuit efficiency is generally high (low losses) but the control circuit is more complex and therefo
Chang Chin
Hernandez Adan F.
Franzblau Bernard
Lee Wilson
Philips Electronics North America Corporation
Vu David
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