Abstract:
In a downconverter for supplying a DC current to a lamp, power feedback is achieved by adding a high frequency-operated switch and a capacitor to the topology. The power factor is improved thereby.

Description:
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 series arrangement of a third capacitance and a third unidirectional element and which shunts the second circuit element, 
     a sixth chain which comprises a series arrangement of the first inductive element and a fourth capacitance and which shunts the first circuit element, 
     a seventh chain which comprises a fourth unidirectional element and which shunts the first circuit element, and 
     a circuit part COM which is coupled to the first and the second control circuit for the low-frequency commutation of the direct current at a frequency f 0 , f 0  being smaller than f 1 . 
     In this embodiment, the functions of the first and the second circuit element alternate at the same low frequency f 0  as the frequency at which the direct current is commutated. Good results are achieved for embodiments wherein the buffer capacitance comprises a first buffer capacitor which forms part of the first capacitance, and a second buffer capacitor which forms part of the fourth capacitance. 
     These and other aspects of the invention will be apparent from and elucidated with reference to embodiments described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 shows a first example of a circuit arrangement in accordance with the invention, to which a lamp is connected, and 
     FIG. 2 shows a second example of a circuit arrangement in accordance with the invention, to which a lamp is connected. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, K 1  and K 2  are supply input terminals for connection to poles of a supply voltage source supplying an alternating voltage. Supply input terminals K 1  and K 2  are interconnected by a series arrangement of a coil L 0  and a capacitor C 0 . The coil L 0  and the capacitor C 0  jointly form a filter. A first side of the capacitor C 0  is connected to a first input of diode bridge RB which, in this example, forms rectifier means. A second side of the capacitor C 0  is connected to a second input of diode bridge RB. A first output terminal of diode bridge RB is connected to a second output terminal by means of a series arrangement of a capacitor C 2 , a coil L 2  and a diode D 1 . In this example, the capacitor C 2 , the coil L 2  and the diode D 1  form, respectively, a second capacitance, a second inductive element and a first unidirectional element. The series arrangement of the capacitor C 2  and the coil L 2  is shunted by a series arrangement of the diode D 2  and the circuit element S 1 . The diode D 2  and the circuit element S 1  form, respectively, a second unidirectional element and a first circuit element. A control electrode of the circuit element S 1  is connected to an output of a circuit part SC 1  which, in this example, forms a first control circuit. The diode D 1  is shunted by the circuit element S 2  which, in this example, forms a second circuit element. A control electrode of the circuit element S 2  is connected to an output of the circuit part SC 2  which forms a second control circuit. The second circuit element is shunted by a series arrangement of a coil L 1  and a capacitor C 1  which form, respectively, a first inductive element and a first capacitance. The capacitor C 1  is shunted by the lamp La. The series arrangement of the first and the second circuit element is shunted by a capacitor Cbuf which, in this example, forms a buffer capacitance. A first chain is formed by the series arrangement of the diode D 2 , the circuit element S 1  and the diode D 1 . A second chain is formed by the series arrangement of the coil L 1  and the capacitor C 1 . The circuit element S 2  forms a third chain, while a fourth chain is formed by the series arrangement of the coil L 2 , the capacitor C 2  and the diode D 2 . 
     The operation of the exemplary embodiment shown in FIG. 1 is as follows. 
     If the supply input terminals K 1  and K 2  are connected to the poles of a supply voltage source, an alternating voltage supplied by the supply voltage source is rectified by the diode bridge RB and a first direct voltage is present across the buffer capacitance Cbuf. The first control circuit SC 1  renders the first circuit element S 1  conducting and non-conducting at a frequency f 1 . As a result, the first direct voltage is converted to a second direct voltage of a lower amplitude which is present across the capacitor C 1  and across the lamp. A sequence of  4  successive operating states can be distinguished which, during operation of the circuit arrangement, are repeated at a frequency f 1 . In the first operating state, the first circuit element S 1  is conducting and the second circuit element S 2  is non-conducting, and a current flows from the buffer capacitance Cbuf through the first circuit element S 1  and the coil L 1  to the capacitor C 1  and the lamp La. 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 by the first control circuit SC 1  and the second operating state begins. In the second operating state, the first circuit element S 1  is non-conducting and current flows through the coil L 1  to the capacitor C 1  and the lamp La. The amplitude of this current decreases. A first part of this current flows through the diode D 1 . A second part of this current flows from the first output terminal of the diode bridge RB through capacitor C 2  and coil L 2 . This second part of the current charges the capacitor C 2 . At a certain moment during the second operating state, the second circuit element S 2  is rendered conducting by the second control circuit SC 2 . As a result, a current flows from the first output terminal of the diode bridge RB through the capacitor C 2 , the coil L 2  and the second circuit element S 2 . Also this current charges the second capacitance. When the amplitude of the current through the coil L 1  has decreased to zero, the third operating state begins. In the third operating state, the first circuit element S 1  is non-conducting and the second circuit element S 2  is conducting. Under the influence of the second direct voltage across the capacitor C 1 , the current in the coil L 1  reverses sign. During the third operating state, this current flows from the capacitor C 1  through the coil L 1  and through the second circuit element S 2 . As this current discharges the capacitor C 1  only partly, the voltage across the capacitor C 1  and the current through the lamp do not reverse sign. During the third operating state, the amplitude of the current through the coil L 1  increases. At the end of the third operating state, the second circuit element S 2  is rendered non-conducting by the second control circuit SC 2 , which marks the beginning of the fourth operating state. During the fourth operating state, both the first circuit element S 1  and the second circuit element S 2  are non-conducting. The current through the coil L 1  flows in the same direction as in the third operating state, but the amplitude decreases. The current now flows from the capacitor C 1  through the coil L 1 , the coil L 2 , the capacitor C 2  and the diode D 2  to the buffer capacitance Cbuf. This current discharges the capacitor C 2  and charges the buffer capacitance Cbuf. When the amplitude of the current has decreased to approximately zero, the first circuit element S 1  is rendered conducting, and the first operating state begins anew. During the third as well as the fourth operating state, the lamp La is fed by means of the voltage across the capacitor C 1 . In each sequence of the four operating states, the capacitor C 2  is charged from the supply voltage source. As a result, current is taken from the supply voltage source, also when the momentary amplitude of the alternating voltage is lower than the voltage across the buffer capacitance. As a result, a relatively high power factor of the circuit arrangement shown in FIG. 1 has been achieved using relatively simple means. Although the current through the coil L 1  changes sign in each sequence, a sequence-averaged current flows from the buffer capacitance through the coil L 1  to the lamp La. 
     The example shown in FIG. 2 can suitably be used to supply a low-frequency commutated direct current to a lamp. In FIG. 2, components and circuit parts which correspond to components and circuit parts shown in the example of FIG. 1 bear the same references. The differences between the example shown in FIG.  2  and the example shown in FIG. 1 are the following. In the example shown in FIG. 2, the coil L 2  connects the first output terminal of the diode bridge RB to the anode of the diode D 2 . The second circuit element S 2  is shunted by a series arrangement of the capacitor C 3  and the diode D 3 . The first circuit element is shunted by a diode D 4 . The buffer capacitance consists of a series arrangement of the capacitor Cbuf 1  and the capacitor Cbuf 2 . A series arrangement of the coil L 1  and the lamp La connects a junction point of the first circuit element S 1  and the second circuit element S 2  to a junction point of the capacitors Cbuf 1  and Cbuf 2 . The lamp La is shunted by a capacitor C 4 . An input of the first control circuit SC 1  is connected to a first output of a circuit part COM for low-frequency commutating, at a frequency f 0 , the direct current supplied to the lamp La. A second output of the circuit part COM is connected to an input of the second control circuit SC 2 . A first branch is formed by the series arrangement of the coil L 2 , the diode D 2 , the first circuit element S 1 , the diode D 1  and the diode D 3 . A second branch is formed by the coil L 1 , the capacitor C 4  and the capacitor Cbuf 2 . The series arrangement of the capacitor C 4  and the capacitor Cbuf 2  forms a first capacitance. A third branch is formed by the second circuit element S 2 . A fourth branch is formed by the series arrangement of the capacitor C 2  and the diode D 2 . A fifth branch is formed by the series arrangement of the diode D 3  and the capacitor C 3  which form, respectively, a third unidirectional element and a third capacitance. A sixth chain is formed by the series arrangement of the coil L 1 , the capacitor C 4  and the capacitor Cbuf 1 . The series arrangement of the capacitor C 4  and the capacitor Cbuf 1  forms a fourth capacitance. The diode D 4  forms a seventh branch. 
     The operation of the example shown in FIG. 2 can be described as follows. 
     In the first half period of the low-frequency commutated direct current, the average current through the coil L 1  flows in the direction of the lamp, and the first circuit element serves as a downconverter circuit element. A sequence now comprises the following operating states. The first operating state starts with the first circuit element S 1  becoming conducting. As a result, a current flows from the capacitor Cbuf 1  through the first circuit element S 1 , the coil L 1  and the parallel arrangement of the capacitor C 4  and the lamp L 1  back to the capacitor Cbuf 1 . The amplitude of this current increases during the first operating state. During the first operating state, also the capacitor C 3  is charged by means of a current which flows from the first output terminal of the diode bridge RB to the second output terminal of the diode bridge RB through the coil L 2 , the diode D 2 , the first circuit element S 1  and the capacitor C 3 . At the end of the first operating state, the first control circuit SC 1  renders the first circuit element non-conducting, whereafter the second operating state begins. The current now flows from the coil L 1  through the parallel arrangement of the lamp La and the capacitor C 4 , the capacitor Cbuf 2 , the diode D 3  and the capacitor C 3  back to the coil L 1 . The amplitude of this current decreases. This current causes the capacitor C 3  to become discharged and the capacitor Cbuf 2  to become charged. When the capacitor C 3  is discharged, the current flows via the diode D 1  instead of via the diode D 3  and the capacitor C 3 . The second operating state ends as soon as the amplitude of the current through the coil L 1  has become zero. At the beginning of the third operating state, the second circuit element S 2  is rendered conducting. During the third operating state, the second circuit element S 2  is conducting and the first circuit element S 1  is non-conducting. The capacitor C 2  is charged by a current which flows from the first output terminal of the diode bridge RB to the second output terminal of the diode bridge RB through the coil L 2 , the capacitor C 2 , the second circuit element S 2  and the diode D 3 . In addition, under the influence of the voltage across the capacitor Cbuf 2 , a current flows from the capacitor Cbuf 2  through the parallel arrangement of the lamp La and the capacitor C 4 , the coil L 1  and the second circuit element S 2  back to the capacitor Cbuf 2 . The amplitude of this current increases. At the end of the third operating state, the second circuit element S 2  is rendered non-conducting, whereafter the fourth operating state begins, in which operating state both the first and the second circuit element are non-conducting. In this fourth operating state, a current flows from the coil L 2 , through the diode D 2 , the capacitor Cbuf 1 , the capacitor Cbuf 2 , the diode D 3  and the diode bridge RB back to the coil L 2 . This current charges the capacitors Cbuf 1  and Cbuf 2 , its amplitude decreases and it flows until the energy stored in the coil L 2  has become zero. In addition, a current flows from the coil L 1  through the capacitor C 2 , the diode D 2 , the capacitor Cbuf 1  and the parallel arrangement of the lamp La and the capacitor C 4  back to the coil L 1 . This current has a decreasing amplitude, and it discharges the capacitor C 2  and charges the capacitor Cbuf 1 . When the capacitor C 2  is discharged, the current flows via the diode D 4  instead of via the capacitor C 2  and the diode D 2 . At the end of the fourth operating state, the first circuit element S 1  is rendered conducting and the first operating state starts anew. Although the current through the coil L 1  changes direction during each sequence of the first half period of the low-frequency commutated direct current, the sequence-averaged current through the coil L 1  is a direct current in the direction of the lamp La. During the third and the fourth operating state, the voltage across the capacitor C 4  is supplied to the lamp La. Since the current through the coil L 1  during the third and the fourth operating state only partly discharges the capacitor C 4 , the lamp current, unlike the current through the coil L 1 , does not reverse direction. 
     At the end of the first half period of the low-frequency commutated direct current, the circuit part COM changes the control of the first and the second circuit element in such a manner that the second circuit element starts acting as a downconverter circuit element. As a result, the direction of the average current through the coil L 1  and the lamp La is reversed. During the second half period of the low-frequency commutated direct current, the average current through the coil L 1  flows in the direction of the second circuit element S 2 . A sequence now includes the following operating states. The first operating state begins with the second circuit element S 2  becoming conducting. As a result, a current flows from the capacitor Cbuf 2  through the parallel arrangement of the lamp La and the capacitor C 4 , the coil L 1  and the circuit element S 2  back to the capacitor Cbuf 2 . The amplitude of this current increases during the first operating state. During the first operating state, also the capacitor C 2  is charged by means of a current which flows from the first output terminal of the diode bridge RB to the second output terminal of the diode bridge RB through the coil L 2 , the capacitor C 2 , the second circuit element S 2  and the diode D 3 . At the end of the first operating state, the second control circuit SC 2  renders the second circuit element non-conducting, whereafter the second operating state begins. The current now flows from the coil L 1  through the capacitor C 2 , the diode D 2  and the capacitor Cbuf 1  and through the parallel arrangement of the lamp La and the capacitor C 4  back to the coil L 1 . The amplitude of this current decreases. This current causes the capacitor C 2  to become discharged and the capacitor Cbuf 1  to become charged. As soon as the amplitude of the current through the coil L 1  has become zero, the second operating state ends. At the beginning of the third operating state, the first circuit element S 1  is rendered conducting. During the third operating state, the first circuit element S 1  is conducting and the second circuit element S 2  is non-conducting. The capacitor C 3  is charged by a current which flows from the first output terminal of the diode bridge RB through the coil L 2 , the diode D 2 , the circuit element S 1  and the capacitor C 3  to the second output terminal of the diode bridge RB. Under the influence of the voltage across the capacitor Cbuf 1 , also a current flows from the capacitor Cbuf 1  through the first circuit element S 1 , the coil L 1  and the parallel arrangement of the lamp La and the capacitor C 4  back to the capacitor Cbuf 1 . The amplitude of this current increases. At the end of the third operating state, the first circuit element is rendered non-conducting, whereafter the fourth operating state begins wherein both the first and the second circuit element are non-conducting. In this third operating state, a current flows from the coil L 2  through the diode D 2 , the capacitor Cbuf 1 , the capacitor Cbuf 2 , the diode D 3  and the diode bridge RB back to the coil L 2 . This current charges the capacitors Cbuf 1  and Cbuf 2 , its amplitude decreases and it flows until the energy stored in the coil L 2  has become zero. A current also flows from the coil L 1  through the parallel arrangement of the lamp La and the capacitor C 4 , the capacitor Cbuf 2 , the diode D 3  and the capacitor C 3  back to the coil L 1 . This current has a decreasing amplitude and it discharges the capacitor C 3  and it charges the capacitor Cbuf 2 . At the end of the fourth operating state, the second circuit element S 2  is rendered conducting, and the first operating state begins anew. Although the current through the coil L 1  changes direction during each sequence of the second half period of the low-frequency commutated direct current, the sequence-averaged current through the coil L 1  is a direct current in the direction of a junction point of the first and the second circuit element. During the third and the fourth operating state, the voltage across the capacitor C 4  is supplied to the lamp La. Since the current through the coil L 1  during the third and the fourth operating state only partly discharges the capacitor C 4 , the lamp current, unlike the current through coil L 1 , does not reverse direction. The voltage across the capacitor C 4  does reverse sign when the transition to a subsequent half period of the low-frequency commutated direct current takes place. In practice, f 0  generally is of the order of 10 Hz, and f 1  is of the order of 10 kHz.