Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-07-06
2001-10-16
Patel, Rajnikant (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S023000, C315S169300
Reexamination Certificate
active
06304464
ABSTRACT:
The invention relates to a circuit arrangement for operating a semiconductor light source, comprising
input terminals for connecting a supply voltage,
input filter means,
a converter provided with inductive means and with a switching element having a control circuit, which switching element is periodically in a conducting state for a period t
on
, and
output terminals for connecting the semiconductor light source.
The invention also relates to a signal light provided with such a circuit arrangement.
A circuit arrangement of the type mentioned in the opening paragraph is known from WO 99/07188. The converter serves as a switch-mode power supply of the semiconductor light source. In the case of the known circuit arrangement, a multiresonant forward/flyback converter is used as the converter. This enables a good, i.e. high, power factor to be achieved. Semiconductor light sources are increasingly used as signal lights. In such an application, the advantage of a semiconductor light source over a customary incandescent lamp is a substantial improvement in service life and a substantial reduction in energy consumption. Signal lights often form part of a complex signaling system, for example a traffic control system with traffic lights. Generally, the functioning of semiconductor light sources applied as a light source is determined by the value of the current supplied to the semiconductor. Consequently, the converter should serve as a current source (constant current generator). On the other hand, traffic light installations are often driven by a triac switching circuit. An important property of such a triac switching circuit is that a minimum current must flow through said triac switching circuit if the driver is to function properly in the conducting phase of the light source. This leads to a complex construction of both the converter and the control circuit of the known circuit arrangement. This constitutes a drawback.
It is an object of the invention to provide a circuit arrangement of the type described in the opening paragraph, wherein the above-mentioned drawback is precluded. In accordance with the invention, this object is achieved in that the circuit arrangement of the type mentioned in the opening paragraph is characterized in accordance with the invention in that the converter is a flyback converter and the inductive means are formed by a transformer, and in that the control circuit controls the period t
on
.
It has surprisingly been found that the circuit arrangement in accordance with the invention can suitably be used to reliably operate a semiconductor light source as a replacement for an incandescent lamp in a signal light, also in the case of low temperatures, and said circuit arrangement is also characterized by great simplicity, which is very advantageous. Controlling the period t
on
in such a manner that it changes only little has the important advantage that the flyback converter is capable of achieving a very good power factor as well as a very low level of harmonic distortion (THD) of mains current extracted from the supply source. The power supply at the input terminals of the circuit arrangement will generally take place from a mains voltage source and hence by means of an alternating voltage. If the alternating voltage has a period T, then, in a preferred embodiment of the circuit arrangement in accordance with the invention, the control circuit involves an integration over a period of at least 2.5 T. The realization of a very good power factor can thus be combined with a very fast feedback with respect to variations, such as temperature variations.
In the case of alternating current power supply, preferably, rectifier means are present between the input terminals and the converter. The low level of THD can be further improved by incorporating only a high-frequency filter between rectifier means and flyback converter. This also precludes an adverse effect on the power factor, which would be caused by clipping of the rectified supply voltage.
The converter can be driven into conduction at such a fixed frequency that current stops flowing simultaneously through the primary and secondary windings for some time. Such a mode of operation is commonly referred to as discontinuous mode. Preferably, however, the converter is operated in such a mode that a reduction to zero of the current through the secondary transformer winding causes the switching element to be driven into conduction. This mode of operation, which is referred to as critical mode, can be realized by means of relatively simple means and additionally has the important advantage that peak currents are minimized and switching losses counteracted.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 4561046 (1985-12-01), Kuster
Bucks Marcel J. M.
De Clercq John E. K. G.
Hontele Bertrand J. E.
Jacobs Ronny A. A. M.
Nijhof Engbert B. G.
Patel Rajnikant
U.S. Philips Corporation
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