Variable frequency self-oscillating half-bridge drive...

Details Variable frequency self-oscillating half-bridge drive... Variable frequency self-oscillating half-bridge drive...

2000-05-19

2002-02-12

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Current and/or voltage regulation

C315S2090SC, C315S224000, C363S132000

Reexamination Certificate

active

06346779

ABSTRACT:

TECHNICAL FIELD
This invention relates to a variable frequency self-oscillating half-bridge drive architecture, and, more particularly to a drive architecture for electric loads, such as light sources and the like, that include first and second drive circuit blocks connected in series with each other into a half-bridge configuration between first and second terminals of a rectified power supply network for the light source.
BACKGROUND OF THE INVENTION
A halogen lamp or fluorescent lamp can be driven by an electronic circuit capable of generating signals at a very high frequency compared to the frequency of the power supply network. In particular, frequencies in the 30 to 50 kHz range can be generated compared to the 50-60 Hz of the power supply network.
In this way, the quality of the emitted light and the efficiency of the emitting source can be improved substantially.
This amplified frequency is usually obtained by interposing, between the power supply network and the light emitting source or lamp, a circuit effective to perform a first conversion from AC voltage [50/60 Hz] to essentially DC voltage, with only a limited oscillation or ripple. A subsequent conversion from DC voltage to AC voltage brings the signal up to a higher frequency [30-50 kHz], as shown schematically in FIG.
1
.
In particular,
FIG. 1
shows a drive circuit
1
which comprises first
3
and second
4
stages cascade connected with each other between a supply network terminal TR and a light source
2
.
The AC voltage is rectified and filtered through the first stage
3
to produce a DC voltage which is input to the second stage
4
for conversion to a suitable high-frequency AC voltage for driving the source
2
.
In actual practice, there exist several ways of obtaining this conversion from low-frequency to high-frequency AC voltage. In general, two switches SW
1
and SW
2
are used, suitably driven and connected into a half-bridge configuration, and will be discussed with reference to
FIGS. 2A-2D
.
More particularly, the switches SW
1
and SW
2
are connected in series with each other between the terminals T
1
′ and T
2
′ of the rectified supply network, which terminals are connected together by a series of a first C′ and a second C″ capacitor. The second terminal T
2
′ of the rectified supply network is connected to a voltage reference, such as a signal ground GND.
The halogen or fluorescent source
2
is placed between a first interconnection node of the switches SW
1
, SW
2
and a second interconnection node of the capacitors C′, C″, it being connected in series with a winding or the primary winding of a transformer
4
.
Lately the trend among manufacturers of halogen or fluorescent apparatus has been toward increasingly smaller and low-cost designs. Accordingly, a primary concern has become the design of circuits which can be driven using a minimum of components, while being reliable and inexpensive.
In this framing, different design circuits are currently available for driving such apparatus, as shown schematically in
FIGS. 2A
to
2
D.
FIG. 2A
shows a conventional drive architecture
1
A which comprises an integrated circuit
5
arranged to drive both switches SW
1
and SW
2
directly.
This prior architecture is quite effective to minimize the number of on-board components, but is highly expensive on account of the high cost of the integrated circuit, and disallows feedback between the working state of the lamp and an oscillator contained in the integrated circuit
5
; the oscillator operates, therefore, at a fixed frequency regardless of the operating phase of the light source
3
.
A second conventional design is shown schematically in
FIG. 2B
, wherein a drive architecture
1
B drives the switches SW
1
and SW
2
with the intermediary of two L-C oscillators
6
and
7
which are connected in parallel with the switches SW
1
, SW
2
and triggered by first
8
and second
9
secondary windings wound around the same core of transformer
4
.
The drive architecture
1
B includes a DIAC circuit connected to the input of the second switch SW
2
, and an internal circuit node X which is formed between a resistor R and a capacitor C connected in series with each other between the terminals T
1
′ and T
2
′of the rectified supply network.
The drive architecture
1
B also includes a diode D, connected between the node X and the intermediate node of the switches SW
1
and SW
2
.
It should be noted that the DIAC circuit and diode D are only useful at startup of the drive architecture because, afterwards, the oscillations of the oscillators
6
,
7
support themselves automatically.
A prior art modification of the drive architecture
1
B is shown in
FIG. 2C
, generally at
1
C in schematic form, and comprises a single oscillator
10
having a respective trigger secondary winding
11
. The drive architecture
1
C further comprises a driver block
12
connected to the second terminal T
2
′ of the rectified supply network, and connected to the second switch SW
2
directly and the first switch SW
1
via a voltage shifter
13
.
FIG. 2D
shows another state-of-art drive architecture
1
D which is widely used because of its low cost. The drive architecture
1
D comprises first
14
and second
15
drive circuits connected to the inputs of the switches SW
1
, SW
2
and triggered by first
16
and second
17
secondary windings which are connected to a saturated-core transformer
18
, itself connected to the light source
3
by a winding
19
.
The frequency of oscillation of the drive architecture
1
D is set by the saturated-core transformer
18
, which is incapable, however, of ensuring ready repeatability of its characteristics. To achieve stable operation of this transformer, its ferrite components must be carefully selected.
In general, working frequencies are obtained, however, which differ between devices, resulting in the lamp being supplied different power levels.
There has yet to be developed a drive architecture that has adequate structural and functional features to overcome the drawbacks of conventional architectures.
SUMMARY OF THE INVENTION
Embodiments of this invention have an oscillation generated within the drive architecture using a trigger winding, rather than by a true oscillator.
Presented, therefore, is a drive architecture for electric loads, in particular light sources and the like, that includes first and second drive circuit blocks connected in series with each other into a half-bridge configuration between first and second terminals of a rectified electric power supply network. Each drive circuit block has a respective secondary winding of a transformer associated therewith, and each drive circuit block includes at least a power device and a control circuit portion for controlling the power device. In each control circuit portion of each drive is a circuit block being subjected to a trigger action directly by its associated secondary winding to generate a high-frequency AC current for driving the light source.
The features and advantages of the architecture according to embodiments of the invention will be apparent from the following description of one of the embodiments thereof, given by way of non-limitative example with reference to the accompanying drawings. Although this description covers an architecture adapted to drive light sources, e.g. halogen or fluorescent lamps the invention is not limited to this exclusively, and the description covers this field only for convenience of illustration.


REFERENCES:
patent: 5303140 (1994-04-01), Shimizu
patent: 5500792 (1996-03-01), Jeon et al.
patent: 5740021 (1998-04-01), Lecheler et al.
patent: 5828188 (1998-10-01), Lechelelr
patent: 6072710 (2000-06-01), Chang

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