Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2000-04-25
2001-10-30
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S089000
Reexamination Certificate
active
06310790
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a circuit arrangement for supplying a direct current or a low-frequency commutated direct current to a lamp, comprising;
supply input terminals for connection to the poles of a supply voltage source supplying an alternating current,
rectifier means coupled to the supply input terminals to rectify the alternating current and provided with a first output terminal and a second output terminal,
a buffer capacitance coupled to the output terminals of the rectifier means,
a DC-DC converter of the downconverter type coupled to the buffer capacitance and provided with
a first chain which interconnects the output terminals and comprises a series-arrangement of a first circuit element and a first unidirectional element,
a first control circuit coupled to the first circuit element to render the first circuit element high-frequency conducting and non-conducting at a frequency f
1
, and
a second chain which shunts the first unidirectional element and comprises a series arrangement of a first inductive element and a first capacitance.
Such a circuit arrangement is well-known. The known circuit arrangement can very suitably be used for supplying a direct current to, for example, a high-pressure discharge lamp. A disadvantage of the known circuit arrangement, however, resides in the fact that current is taken from the supply voltage source almost exclusively when the amplitude of the supply voltage is high. As a result, the power factor of the known circuit arrangement is low.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a circuit arrangement which can suitably be used to supply a direct current to a lamp, the power factor of said circuit arrangement being relatively high.
To achieve this, a circuit arrangement as mentioned in the opening paragraph is characterized in accordance with the invention in that the circuit arrangement is also provided with
a third chain which comprises a second circuit element and which shunts the first unidirectional element,
a second control circuit which is coupled to a control electrode of the second circuit element and which serves to render the second circuit element high-frequency conducting and non-conducting at the frequency f
1
, and
a fourth chain which shunts the first circuit element and which comprises a series arrangement of a second capacitance and a second unidirectional element, a junction point of the second capacitance and the second unidirectional element being coupled to the first output terminal of the rectifier means.
If the supply input terminals of a circuit arrangement in accordance with the invention are connected to the poles of a supply voltage source, the supplied alternating voltage is rectified by the rectifier means and a first direct voltage is present across the buffer capacitance. The first circuit element is rendered conducting and non-conducting with a frequency f
1
by the first control circuit. As a result, the first direct voltage is converted to a second direct voltage of a lower amplitude which is present across the first capacitance. A sequence of 4 successive operating states can be distinguished, which, during operation of the circuit arrangement, are repeated with a frequency f
1
. In the first operating state, the first circuit element is conducting and the second circuit element is non-conducting, and a current flows from the buffer capacitance through the first circuit element and the first inductive element to the first capacitance. In this first operating state, the amplitude of the current increases. At the end of the first operating state, the first circuit element is rendered non-conducting and the second operating state begins. In the second operating state, the first circuit element is non-conducting and current flows through the first inductive element to the first capacitance. This current decreases in amplitude. A first part of this current flows through the first unidirectional element. A second part of this current flows from the first output terminal of the rectifier means to the second capacitance. The second capacitance is charged by this second part of the current. At a certain moment during the second operating state, the second circuit element is rendered conducting by the second control circuit. As a result, a current flows from the first output terminal of the rectifier means through the second capacitance and the second circuit element. Also this current charges the second capacitance. When the amplitude of the current through the first inductive element has decreased to zero, the third operating state begins. In the third operating state, the first circuit element is non-conducting and the second circuit element is conducting. Under the influence of the direct voltage across the first capacitance, the current in the first inductive element reverses sign. During the third operating state, this current flows from the first capacitance through the first inductive element and through the second circuit element. During the third operating state, the amplitude of this current increases. At the end of the third operating state, the second circuit element is rendered non-conducting by the second control circuit, which marks the beginning of the fourth operating state. During the fourth operating state, both the first and the second circuit element are non-conducting. The current through the first inductive element flows in the same direction as in the third operating state, but the amplitude decreases. The current now flows from the first capacitance through the first inductive element, the second capacitance and the second unidirectional element to the buffer capacitance. This current discharges the second capacitance and charges the buffer capacitance. When the amplitude of the current has decreased to approximately zero, the first circuit element is rendered conducting and the first operating state begins anew. In each sequence of the four operating states, the second capacitance is charged from the supply voltage source. As a result, current is taken from the supply voltage source, even when the momentary amplitude of the supply voltage is lower than the voltage across the buffer capacitance. As a result, it has been achieved with relatively simple means that the power factor of a circuit arrangement in accordance with the invention is relatively high. Although the current through the first inductive element reverses sign during each sequence of the four operating states, the sequence-averaged current through the first inductive element is a direct current.
The fourth chain is additionally provided with preferably a second inductive element. This second inductive element limits the current with which the second capacitance is charged, so that the power dissipation in the second circuit element during charging the second capacitance remains limited.
A second inductive element may also be coupled between the first output terminal of the rectifier means and the second unidirectional element. If the second inductive element is coupled in this manner, it not only limits the charging current of the second capacitance but it also charges the buffer capacitance after the second circuit element has become non-conducting at the beginning of the fourth operating state.
It is possible to incorporate the first unidirectional element in the second circuit element. This is achieved, for example, if the circuit element is embodied so as to be a field effect transistor.
In some cases it is desirable to supply a low-frequency commutated direct current to the lamp. For this purpose, for example a commutator comprising four low-frequency controlled circuit elements may be incorporated in the circuit arrangement. If the lamp voltage is relatively low, two of said circuit elements of the commutator may be replaced by capacitors. Such a current type can also be obtained, however, with a relatively small number of components by embodiments of a circuit arrangement in accordance with the invention, comprising
a fifth chain which comprises a ser
Antic Darko
Hendrix Machiel A. M.
Sterrett Jeffrey
U.S. Philips Corporation
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