DC/AC converter for igniting and supplying a gas discharge lamp

A DC/AC converter for igniting and supplying a gas discharge lamp (1). The converter has two input terminals (C, D) intended to be connected to a DC voltage source, said input terminals (C, D) being connected together by means of a series arrangement including a load circuit comprising at least an induction coil (10) and a parallel arrangement of the lamp and a capacitor (12), as well as a first semiconductor switching element (13). The load circuit is shunted by a circuit comprising a second semiconductor switching element (14). The semiconductor switching elements (13, 14) are rendered alternately conductive and non-conductive by means of control circuits (13a, 14a). A second capacitor (11) is arranged in series with the induction coil (10) and the lamp. The second capacitor is shunted by a third switching element (15) which is non-conductive during the pre-heat period of the lamp electrodes (2, 3) and is conductive at least during ignition of the lamp.

BACKGROUND OF THE INVENTION 
This invention relates to a DC/AC converter for igniting and supplying a 
gas discharge lamp. The converter has two input terminals intended to be 
connected to a DC voltage source and with said input terminals connected 
together by means of a series arrangement of a load circuit comprising at 
least an induction coil and a parallel arrangement of the lamp and a 
capacitor, as well as a first semiconductor switching element. The load 
circuit is shunted by a circuit comprising a second semiconductor 
switching element, said semiconductor switching elements being rendered 
alternately conducting and non-conducting by means of control circuits. A 
converter of this type is known from the Netherlands Patent Application 
No. 8400923 (which corresponds to U.S. Pat. No. 4,647,820, issued 3/3/87). 
This Patent describes a high-frequency operated half-bridge converter with 
a discharge lamp incorporated in the load circuit. It has been found that 
the voltage across the lamp in the known circuit is not low enough during 
pre-heating of the electrodes. This is detrimental because it creates the 
risk of the lamp igniting on too cold electrodes, which adversely affects 
the life-time of the lamp. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a DC/AC converter for operating 
a discharge lamp which obviates the above-mentioned drawback. 
According to the invention, a DC/AC converter of the type described in the 
opening paragraph is therefore characterized in that a second capacitor is 
arranged in series with the induction coil and the lamp, which capacitor 
is shunted by a third switching element which is non-conductive during the 
period of pre-heating the electrodes and is conductive at least during 
ignition of the lamp. 
As compared with the known circuit, the arrangement of an extra capacitor 
in the load circuit of the lamp and the induction coil will result in a 
lower voltage across the lamp during pre-heating of the electrodes at the 
same electrode current. The arrangement of a larger capacitor in parallel 
across the lamp for the purpose of achieving this object is avoided. 
During operation, such a capacitor gives rise to large energy losses in 
the induction coil, the lamp electrodes and the semiconductor switching 
elements. After pre-heating, the switching element (consisting of, for 
example, a triac, a diode bridge with a switching transistor or a 
thyristor) is rendered conductive so that the said capacitor is 
short-circuited. Immediately after the short circuit a high voltage is 
produced across the lamp for ignition purposes. 
It is to be noted that a circuit for a system using two "rapid-start" 
discharge lamps is described in U.S. Pat. No. 4,339,690 in which a 
capacitor is arranged between the lamps in a circuit of a series 
arrangement of these lamps. This capacitor is shunted by a switching 
element and is short-circuited during ignition of the lamps by closing the 
switching element. Subsequently, the switching element is opened. The 
capacitor is used as a safeguard to limit the lamp current in lamps which 
have already ignited. During ignition the voltage is relatively high, so 
that there is a risk that the lamps ignite on too cold electrodes. In the 
circuit according to the invention a low lamp voltage yields a relatively 
large current during preheating of the electrodes. 
In a special embodiment of the converter according to the invention is 
characterized in that the second capacitor (parallel in across the lamp) 
has substantially the same impedance as the first capacitor. 
An advantage of this embodiment is that the induction coil, which is 
arranged in series with the lamp, has substantially the same value and 
dimension as compared with the coil in the known circuit. 
In another embodiment the third switching element is rendered 
non-conducting after ignition of the lamp by rendering the converter 
inoperative for a given period of time (for example, approximately 100 
.mu.s). Subsequently, the converter is started again and the element (for 
example, a triac) remains non-conducting. If the frequency remains equal, 
the apparent impedances of the coil and the capacitor arranged in series 
therewith jointly become smaller so that the lamp current increases. The 
light output of the lamp is then higher.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the drawing the reference numeral 1 denotes a tubular low-pressure 
mercury vapour discharge lamp. The lamp has two pre-heatable electrodes 2 
and 3. 
The terminals C and D are the input terminals of the DC/AC converter. They 
are intended to be connected to the DC voltage source which is constituted 
by a diode bridge 4, with a smoothing capacitor 5. The bridge 4 is 
connected via the coil 6 and the capacitor 7 to an AC voltage source 
between terminals A and B (220 V, 50 Hz). The coil 6 and the capacitor 7 
constitute an input filter. 
The terminals C and D are interconnected by means of a series arrangement 
comprising a capacitor 9, an induction coil 10, a capacitor 11, a parallel 
arrangement of the lamp 1 with the capacitor 12, as well as a first 
semiconductor switching element 13. The series arrangement of elements 9, 
10, 11 and 1 with 12 is shunted by a circuit comprising a second 
semiconductor switching element 14. The two semiconductor switching 
elements 13 and 14 are rendered alternately conducting by means of control 
circuits 13a and 14a. 
The capacitor 11 arranged in series with the induction coil and the lamp is 
shunted by a switching element 15 (triac) which is non-conducting during 
the period when the electrodes are pre-heated and is rendered conducting 
by means of a control circuit, at least during the subsequent ignition of 
the lamp. The capacitor 16 connects terminal D to the junction point of 
capacitor 9 and coil 10. The input terminal C is also connected to 
terminal D via the series arrangement of a resistor 17 and a capacitor 18. 
The junction point of 17 and 18 is connected to one end of a primary 
winding 21 of a transformer 22 via the breakdown element 19 (diac) and the 
resistor 20 arranged in series therewith. The other end of this winding is 
connected to terminal D. The secondary winding 23 of the transformer is 
arranged across the control electrode and an output of triac 15. The 
elements 17 to 23 constitute the control circuit of the triac 15. The two 
switching elements 13 and 14 are shunted by a series arrangement of a 
resistor 25 and a capacitor 26. The junction point of 25 and 26 is 
connected to an input of a logic `AND`-gate circuit 26a, the output of 
which is connected to a monostable multivibrator 27 which is connected to 
the base of switching transistor 28. This transistor is arranged between 
the gate of transistor 13 and terminal D. The other input of the logic 
gate circuit is connected to a voltage P which is optionally zero or which 
has a given fixed value. 
Point P is connected, for example, to a photosensitive cell. The output of 
gate circuit 26a is connected via resistor 29 to the base of switching 
transistor 30. Together with resistor 31, the transistor 30 is arranged in 
parallel across the capacitor 18. 
The converter operates as follows. If the terminals A and B are connected 
to the mains (220 V, 50 Hz), the capacitor 5 will be charged via the diode 
bridge 4. This causes the capacitors 9 and 16 to be charged. A starter 
circuit (not shown in the drawing) will also be activated, so that the 
switching elements 13 and 14 are rendered alternately conducting by means 
of the control circuits 13a and 14a. 
After a short time, which is required to pre-heat the electrodes 
(approximately 1 sec), the breakdown voltage of the element 19 is reached 
so that a control current is generated in the winding 21 of the 
transformer 22. The latter element is rendered conducting via the 
secondary winding 23 and the control electrode of triac 15. The voltage 
across capacitor 12 increases. The lamp can then ignite. If necessary, the 
parallel circuit across capacitor 11 is interrupted by means of a separate 
switch (not shown) after ignition of the lamp. 
The control of switching element 13 is short-circuited by means of the 
elements 25, 26, 26a, 27 and 28. The control of switching element 14 is 
then also interrupted. (The control circuits 13a and 14a are coupled, for 
example, via a transformer as described in U.S. Pat. No. 4,647,820; the 
coupling is denoted by a broken line.) Due to the short circuit the 
converter is inoperative for a short time (approximately 1 msec) so that 
triac 15 is turned off and capacitor 11 is again operative. After this 
short time the converter is started again to prevent the lamp from 
igniting on too cold electrodes. If triac 15 remains turned off during 
further lamp operation, the intensity of the current through the lamp is 
larger. The lamp then has a higher light output. The light output of the 
lamp can be controlled by means of the elements 26a, 27, 29, 30 and 31. If 
the voltage at terminal P is set to a relatively high value (for example, 
5 V), the voltage at the output of the logic gate circuit 26a is also high 
so that the switching element 30 becomes conductive. This switching 
element ensures that diac 19 remains non-conducting. Triac 15 is then also 
turned off and capacitor 11 remains operative. However, if a user sets the 
voltage at P to a low, fixed value (for example, 0 V), the voltage at the 
output of 26a is also relatively low and switching element 30 is 
non-conducting. 
Triac 15 then remains turned on and capacitor 11 is then short-circuited. 
The light output of the lamp is then lower than in the case where the 
capacitor 11 is arranged in series with the lamp. 
Thus, with the aid of the voltage at terminal P (which is connected, for 
example, to a photo-electric cell) a dimming effect is realized by means 
of the elements 26a, 27, 29, 30 and 31. 
Immediately after ignition of the lamp the short circuit of capacitor 11 
can be eliminated by means of the elements 26a, 27, 29, 30 and 31. This is 
effected by giving terminal P a high voltage whereafter element 28 is made 
conductive (about 1 msec) after several seconds (RC time of 25 and 26). 
Element 30 is then conductive permanently. The converter is then stopped 
for a short time (approx. 1 msec). When it is switched on again triac, 15 
remains turned off and capacitor 11 is constantly operative. 
The converter may also be rendered inoperative if a remote control system 
is used in which a command pulse is processed in the converter. For 
example, first the frequency of the converter is increased to 50 kHz (at 
which the lamp extinguishes). Upon switching on, the converter is switched 
off for a short time (in a manner as described hereinbefore) whereafter 
the converter is started again so as to proceed through the entire cycle. 
In one embodiment the most important circuit elements had the following 
values: 
capacitor 11: 10 nF 
capacitor 12: 10 nF 
coil 10: 3 mH 
capacitor 9: 470 nF 
capacitor 16: 470 nF 
capacitor 5: 47 .mu.F. 
The discharge lamp was a tubular low-pressure mercury vapour discharge lamp 
(approximately 1.20 m) having a power of 32 W. The two semiconductor 
switching elements 13 and 14 were of the MOS-FET type. The frequency was 
approximately 25 kHz. The triac 15 was of the Philips BT 136 type.