Converter for discharge lamps with dimming means

Abstract: The invention relates to a circuit arrangement for operating a discharge lamp, comprising a DC-AC converter provided with a branch A comprising at least one switching element for generating a current of alternating polarity by being alternately conducting and non-conducting with a frequency f, a load branch B coupled to the branch A and provided with lamp connection terminals and with inductive means, a drive circuit E for rendering the switching element conducting and non-conducting with a frequency f, which drive circuit E is provided with a branch D which comprises a series circuit of further inductive means and capacitive means, and with a branch C, which comprises a variable impedance, the drive circuit E being coupled to the inductive means in the load branch B, the branch D being coupled to the switching element in branch A, and the branch C being coupled to the further inductive means in branch D. According to the invention, branch C also comprises inductive means, while the variable impedance is a variable resistor. It is achieved in this way that the lamp can be dimmed over a wide range by simple and inexpensive means.

Description: The invention relates to a circuit arrangement for operating a discharge lamp, comprising a DC-AC converter provided with a first branch comprising at least one switching element for generating a current of alternating polarity by being alternately conducting and non-conducting with a frequency f, a second branch coupled to the first branch and provided with lamp connection terminals and with inductive means, a drive circuit for rendering the switching element conducting and non-conducting with a frequency f, which drive circuit is provided with a fourth branch which comprises a series circuit of further inductive means and capacitive means, and with a third branch, which comprises a variable impedance, the drive circuit being coupled to the inductive means in the second branch B, the fourth branch being coupled to the switching element in the first branch, and the third branch being coupled to the further inductive means in fourth branch. Such a circuit arrangement is known from the Netherlands Patent Application 8701314 (=EP-A-0 294 878). In the circuit arrangement described therein, first branch comprises two switching elements which are alternately conducting and non-conducting. Third branch C shunts the further inductive means of the drive circuit. By adjustment of the variable impedance, it is possible to set the frequency f of the current of alternating polarity and thus the power consumed by a lamp connected to the lamp connection terminals. It was found, however, that a comparatively small range of the lamp power can be controlled if the third branch consists of a variable resistance, which has the advantage of being comparatively inexpensive. This is a drawback which is caused by the fact that a reduction of the power consumed by the lamp to below approximately 80% of the rated lamp power requires such a reduction of the resistance setting that the quantity of power dissipated in the resistance increases to such an extent that the drive circuit is no longer capable of rendering the switching elements of a first branch conducting. The result is that the lamp extinguishes. A variable inductance or a variable capacitance may also be chosen to form the variable impedance. A disadvantage of these options is that both a variable inductance and a variable capacitance are comparatively expensive components. The invention has for its object to provide a circuit arrangement with which the power consumed by the lamp is adjustable over a wide range by means of comparatively inexpensive components. A circuit arrangement of the kind described in the opening paragraph, according to the invention, is for this purpose characterized in that the variable impedance in third branch is a variable resistor and the third branch furthermore comprises inductive means. Since the inductive means form part of third branch, the quantity of power taken up by the variable resistor is relatively small. It was found possible to adjust the power consumed by the lamp over a comparatively wide range as a result. A particular embodiment of a circuit arrangement according to the invention is characterized in that the further inductive means are shunted by a primary winding of a transformer and third branch shunts a secondary winding of the transformer. Since the variable resistor must be readily accessible in a practical embodiment of the circuit arrangement in order to be able to dim a lamp connected to the lamp connection terminals, it is difficult to screen off the variable resistor, which may give rise to radio interference. However, if the further inductive means and third branch are electrically separated by means of a transformer, the radio interference is effectively suppressed, also if the variable resistor is screened only to a small degree. Suppression of radio interference in this manner is of particular importance if first branch comprises two switching elements which are alternately conducting with a frequency f, and which comprises ends suitable for being connected to a DC voltage source, while the fourth branch is connected to a common point of the two switching elements. Since fourth branch is connected to a common point of the two switching elements of first branch, the voltage across the further inductive means is superimposed on a square-wave voltage of frequency f and of an amplitude equal to a DC voltage supplied by the DC voltage source. If third branch shunts the further inductive means, the voltage across the variable resistor is also superimposed on this square-wave voltage. If, however, the further inductive means and third branch are coupled to one another by means of a transformer, radio interference as a result of this square-wave voltage is substantially eliminated. A further particular embodiment of the design just described of a circuit arrangement according to the invention is characterized in that an end of the secondary winding of the transformer is connected to a pole of a DC-voltage source via a branch which comprises capacitive means. A further reduction of the radio interference is achieved in this way. An embodiment of a circuit arrangement according to the invention will be described in more detail with reference to a drawing. In the drawing, the figure shows the construction of an embodiment of a circuit arrangement according to the invention. In the figure, reference numerals 1 and 2 denote input terminals suitable for connection to an AC voltage source. F is an AC-DC converter of which one output terminal is connected to input terminal 12 and of which a further output terminal is connected to input terminal 13. The series circuit of input terminal 12, switching elements 6 and 7, and input terminal 13 forms first branch A. First branch A together with capacitors 4 and 11 forms a DC-AC converter. The series circuit of coil 5, lamp connection terminal K1, capacitor 39 and lamp connection terminal K2 constitutes the second branch B. In this embodiment, coil 5 forms the inductive means of second branch B. A lamp La can be connected to the lamp connection terminals. All further components of the circuit arrangement form part of the drive circuit: the drive circuit consists of coils 19 and 45, transformer 41, zener diodes 26, 27, 29, 30 and 43, capacitors 44 and 20, resistors 23, 24, 25 and 28, variable resistor 42, switching element 22 and diodes 10 and 22a. Fourth branch D in this embodiment is formed by the series circuit of coil 19 and capacitor 20. Coil 19 and capacitor 20 in this embodiment represent the further inductive means and the capacitive means of fourth branch D, respectively. Coil 45 and variable resistor 42 together form third branch C. The drive circuit is built up as follows. Ends of fourth branch (D) are connected by portion 21 of coil 5. Coil 19 is shunted by a primary winding of transformer 41. A secondary winding of transformer 41 is shunted by third branch C. A first end of the secondary winding of transformer 41 is connected to input terminal 12 via capacitor 44. Coil 19 is also shunted by a series circuit of zener diodes 29 and 30 and resistor 28 in order to limit the voltage across the coil 19. A first end of resistor 25 is connected to a control electrode of switching element 7. Capacitor 20 connects a further end of resistor 25 to a common point P of switching element 6 and switching element 7. The point P is connected to the control electrode of switching element 7 via a series circuit of zener diode 26 and zener diode 27. The object of this is to limit the voltage between the control electrode of switching element 7 and the point P. Input terminals 12 and 13 are shunted by a series circuit of resistor 24 and switching element 22. A common point of resistor 24 and switching element 22 is connected to a control electrode of switching element 6. The control electrode of switching element 6 is connected to input terminal 13 by means of diode 22a. The control electrode of switching element 22 is connected to input terminal 12 by means of resistor 23. The control electrode of switching element 22 is connected to a common point of coil 19 and capacitor 20 via a series circuit of zener diode 43 and diode 10. The operation of the circuit arrangement shown in Fig. 1 is as follows. When input terminals 1 and 2 are connected to the poles of an AC voltage source, a DC voltage is present between input terminals 12 and 13. In a stationary operating condition, the drive circuit renders the switching elements alternately conducting with a frequency f. The result is that a substantially square-wave voltage is present between ends of the load branch with a frequency f, while a current flows through the load branch whose polarity changes with the frequency f. Since portion 21 of coil 5 interconnects the ends of fourth branch D, a periodic voltage of frequency f is present between the ends of fourth branch D. Periodic voltages whose polarities alternate with frequency f are also present between the ends of coil 19 and across capacitor 20. The periodic voltage across capacitor 20 renders switching element 7 alternately conducting and non-conducting with frequency f. Switching element 6 is also made alternately conducting and non-conducting with frequency f by the periodic voltage across capacitor 20 through the circuit elements 10, 43, 23, 24 and 22. Furthermore, switching element 7 is non-conducting when switching element 6 is conducting, and switching element 6 is non-conducting when switching element 7 is conducting. Zener diode 43 serves to give the voltage across capacitor 20 a more sinusoidal shape. Capacitor 44 and transformer 41 serve to limit radio interference. When the resistance value of the variable resistor 42 in third branch C is changed, the frequency f with which the current through the load branch changes polarity is also changed as a result. Since the lamp in the load branch is connected in series with coil 5, the power consumed by the lamp decreases with an increasing frequency f. An increase in the frequency f can be achieved in that the resistance value setting of the variable resistor 42 is reduced. Inversely, an increase in the resistance value setting corresponds to a decrease in the frequency f, so that the power consumed by the lamp increases. In a concrete embodiment of the circuit arrangement shown in the figure, the self-inductance of coil 19 was 680 µH and the capacitance of capacitor 20 was 10 nF. The self-inductance of both the primary and the secondary winding of transformer 41 was 20 mH and the self-inductance of coil 45 was 100 µH. Through adjustment of the resistance value of the variable resistor 42 between 0 Ω and 2,2 KΩ, it was possible to vary the power consumed by a lamp connected to the lamp connection terminals between 9,2 W and 12,7 W. The luminous flux in this range varied from approximately 300 lumens to 1000 lumens.

Claims: A circuit arrangement for operating a discharge lamp (LA), comprising a DC-AC converter provided with a first branch (A) comprising at least one switching element (6, 7) for generating a current of alternating polarity by being alternately conducting and non-conducting with a frequency f, a second branch (B) coupled to the first branch (A) and provided with lamp connection terminals (K1, K2) and with inductive means (5), a drive circuit (E) for rendering the switching element conducting and non-conducting with a frequency f, which drive circuit (E) is provided with a fourth branch D which comprises a series circuit of further inductive means (19) and capacitive means (20), and with a third branch (C), which comprises a variable impedance (42), the drive circuit (E) being coupled to the inductive means (5) in the second branch B, the fourth branch (D) being coupled to the switching element (6, 7) in first branch A, and the third branch (C) being coupled to the further inductive means (19) in fourth branch (D), characterized in that the variable impedance (42) in third branch (C) is a variable resistor and the third branch (C) furthermore comprises inductive means (45). A circuit arrangement as claimed in Claim 1, characterized in that the further inductive means (19) are shunted by a primary winding of a transformer (41) and third branch (C) shunts a secondary winding of the transformer (41). A circuit arrangement as claimed in Claim 2, characterized in that an end of the secondary winding of the transformer (41) is connected to a pole (12) of a DC voltage source via a branch which comprises capacitive means (44).

Inventors: SMITS EGBERTUS HENDRICUS MARIA, SMITS, EGBERTUS HENDRICUS MARIA
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: H05B41
IPC codes: H05B41