DC/AC converter with improved starter circuit

#This invention relates to a circuit arrangement for operating a discharge lamp, comprising a self-oscillating DC/AC converter provided with:

#a series arrangement of a first and a second switching element coupled between a first and a second input terminal for connection to a DC voltage source,

#which series arrangement during nominal lamp operation supplies an alternating current Ib to a load branch which comprises at least first capacitive means, inductive means, and output terminals for connection of the discharge lamp,

#These and other aspects of the circuit arrangement will be explained in more detail with reference to the drawing, in which the sole FIGURE shows an embodiment.

#The drawing shows a circuit arrangement for operating a discharge lamp I provided with a self-oscillating DC/AC converter II. The DC/AC converter II has a first and a second input terminal 2, 2' for connection to a DC voltage source III. The DC voltage source here is an AC/DC converter with input terminals for connection to poles P, N of the public mains. The DC/AC converter is provided with a series arrangement of a first and a second switching element 1, 1' between tile first and the second input terminal 2, 2'. A main electrode 1a of the first switching element is connected to the first input terminal 2. A further main electrode 1b of the first switching element 1 is connected to a main electrode 1a' of the second switching element 1'. A further main electrode 1b' of the second switching element 1' is connected to the second input terminal 2'. The series arrangement (1, 1') has a junction point P between the first and the second switching element 1, 1'. The series arrangement supplies an alternating current Ib to a load branch during nominal operation, with a frequency of approximately 3 MHz in this case. The load branch comprises first capacitive means C1 formed by a capacitive impedance 3, inductive means comprising a primary winding 4a of a transformer 4 and an inductive impedance 5, and output terminals 6a, 6b for connection of the discharge lamp I. In an embodiment, the inductive impedance 5 and the primary winding 4a of the transformer 4 are integrated. The capacitive impedance 3, the primary winding 4a of the transformer 4, and the inductive impedance 5 are connected in series with one another between the junction point P in the series arrangement (1, 1') and one of the output terminals 6a. The other output terminal 6b is connected to the first input terminal 2. One side of the capacitive impedance 3 here forms the first end 7a of the load branch which is connected to the junction point P. The point P here is the connection between the further main electrode 1b of the first switching element 1 and the main electrode 1a' of the second switching element 1'. Output terminal 6b forms the second end 7b of the load branch which is connected to an input terminal 2. A discharge lamp I is connected to the output terminals 6a, 6b. The discharge lamp I here is a low-pressure mercury discharge lamp with a discharge vessel 8, which surrounds a discharge space 10, and with a coil 9. The coil 9 generates an alternating magnetic field for maintaining an electric discharge in the discharge space 10. The output terminals 6a, 6b are shunted by a capacitive impedance 11. The capacitive impedance 11 forms a resonant circuit together with inductive impedance 5 and coil 9 of the discharge lamp I.

#In an alternative embodiment, the lamp I has electrodes instead of a coil, inside or outside of the discharge vessel, for maintaining an electric discharge. The electrodes then each have a current supply conductor to connect them to the output terminals of the DC/AC converter.

#A control circuit comprising means for generating a control signal from the alternating current Ib is connected to the control electrode 1c and the main electrode 1a of the first switching element 1. The control signal generating means are formed by a secondary winding 4b of a transformer 4 in this case. The control circuit further comprises a capacitive impedance 13 and breakdown elements 14a, 14b. An end 4ba of the secondary winding 4b is connected to the main electrode 1a of the first switching element 1. A further end 4bb of the secondary winding 4b is connected to the control electrode 1c of the first switching element 1 via a capacitive impedance 12. The capacitive impedance 12 forms part of the starter circuit ST which will be described further below. The capacitive impedance 13, which shunts the control electrode 1c and the main electrode 1a of the first switching element 1, forms a resonant circuit together with the secondary winding 4b of the transformer 4. The control electrode 1c and the main electrode 1a of the first switching element 1 are in addition shunted by the breakdown elements 14a, 14b which are connected in series in mutually opposed directions. The breakdown elements protect the switching elements 1, 1' against an excessive voltage at their control electrodes 1c, 1c'. Components of the control circuit of the second switching element 1' corresponding to those of the first switching element 1 have reference symbols to which an accent sign (') has been added. The control circuit of the second switching element 1' differs from that of the first in that the voltage generated in the secondary winding 4b' has a polarity which is opposed to that in the secondary winding 4b.