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

A DC/AC converter for igniting and supplying a gas discharge lamp (1) has two input terminals (C, D) intended to be connected to a DC voltage source. These input terminals are connected together by means of a series arrangement of a load circuit comprising at least the discharge lamp (1) and an induction coil (8), as well as a first semiconductor switching element (9). This load circuit being shunted by a circuit comprising a second semiconductor switching element (10). The switching elements (9, 10) are rendered alternately conductive and non conductive at a high frequency. The lamp is shunted by a third semiconductor switching element (12) which is conductive while the lamp electrodes (2, 3) are being pre-heated, whereafter the converter is rendered inoperative for a short period of time in order to ignite the lamp, which time is shorter than the time required to cool the lamp electrodes to below their emission temperature. The converter is thereafter rendered operative again.

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, said input terminals being connected 
together by means of a series arrangement which includes a load circuit, 
comprising at least the discharge lamp and an induction coil, and a first 
semiconductor switching element. The load circuit is shunted by a circuit 
comprising a second semiconductor switching element, said switching 
elements being rendered alternately conductive and non-conductive at a 
high frequency. A converter of this type is known from the U.S. PAT. NO. 
4,647,820 (3/3/87). 
This U.S. Patent describes a converter of the half-bridge type in which the 
lamp is shunted by a capacitor and a resistor having a positive 
temperature coefficient (PTC). A fairly large pre-heating current then 
flows through the electrodes, whereafter the lamp ignites readily. A 
current, though small, also flows continuously through the electrodes 
during the lamp operation in such a converter. This is detrimental to the 
efficiency of the converter. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a DC/AC converter having a high 
efficiency in which energy dissipation in the electrodes of the lamp is 
minimized. 
According to the invention a DC/AC converter of the type described in the 
opening paragraph is therefore characterized in that the lamp is shunted 
by a third semiconductor switching element which is conductive while the 
electrodes are being pre-heated, whereafter the converter is rendered 
inoperative for a short time in order to ignite the lamp, which time is 
shorter than the time required to cool the lamp electrodes to below their 
emission temperature, whereafter the converter is rendered operative 
again. 
The said third switching element is thus closed while the electrodes of the 
lamp are being pre-heated. Since the high-frequency converter is entirely 
rendered inoperative (for example, by short-circuiting the control of one 
of the (switching transistors), the third switching element is also 
rendered non-conducting and is subsequently not rendered conducting 
anymore during lamp operation. The continuous flow of a current through 
the electrodes during lamp operation is then avoided. The period of time 
during which the converter is switched off is limited by the period of 
time it takes for the temperature of the electrodes to drop below the 
electrode-emission temperature. If the converter is inoperative too long, 
the electrode temperature may drop to such a low value that it creates the 
risk of igniting the lamp on too cold electrodes. In a practical 
embodiment, with conventional low-pressure mercury vapour discharge lamps, 
this period of time is at most 10 ms. 
The switching element is integrated in a DC/AC converter operated at a high 
frequency. Unlike, for exampele, a circuit of an electronic starter, the 
electrodes are preheated with relatively few elements. 
In a preferred embodiment of the converter, the third semiconductor 
switching element is a triac, and the converter is rendered inoperative 
for a period of time which is longer than the recovery time of the triac. 
The triac has an opportunity to be turned off. In fact, a current having a 
frequency which is larger than approximately 20 kHz flows through the 
triac during the preheating stage. This is such a high frequency that the 
triac is not turned off. 
The period of time in which the current is interrupted is chosen so that 
the temperature of the electrodes is still sufficiently high upon ignition 
of the lamp, and that there are still sufficient ionized particles in the 
discharge space of the lamp. Dependent on the type of triac and the 
quality of the electrodes, the said period of time is between 10 /.mu.s 
and 10 ms. In a practical embodiment the inoperative period of the 
converter covers approximately 2 ms.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
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 high-frequency DC/AC 
converter. They are intended to be connected to a DC source which is 
constituted by the diode bridge 4, with capacitor 5. The bridge is 
connected via input filter 6 to terminals A and B across which an 
alternating voltage is present (220 V, 50 Hz). 
The terminals C and D are connected together by means of a series 
arrangement of a load circuit comprising a series-arranged capacitor 7, 
the lamp 1, an induction coil 8 and a first semiconductor switching 
element (transistor) 9. The circuit comprising the capacitor 7, the lamp 
1, and the coil 8 is shunted by a circuit comprising a second 
semiconductor switching element (transistor) 10. The free-wheeling diodes 
9b and 10b are arranged parallel across transistors 9 and 10. 
The two switching elements are rendered alternatately conducting and 
non-conducting at a high-frequency by means of control circuits 9a and 10a 
(shown diagrammatically). The lamp 1 is shunted by capacitor 11 and by a 
third semiconductor switching element 12 (triac) which is conductive 
during pre-heating of the electrodes 2 and 3. The control electrode of the 
triac 12 is connected to terminal D via a series arrangement of a diode 13 
and a capacitor 14. The junction point of the lamp 1 and the capacitor 7 
is also connected to terminal D via capacitor 15. 
The series arrangement of the two semiconductor switching elements 9 and 10 
is shunted by a series arrangement of a resistor 16 and a capacitor 17. 
The junction point of elements 16 and 17 is connected to a monostable 
multivibrator 18 which is connected to the base of a switching transistor 
19 arranged between the control electrode and the emitter of switching 
element 9. 
After the lamp electrodes have been pre-heated, the converter is rendered 
inoperative for a short period of time (approximately 2 ms) with the aid 
of the elements 13 ,14, 16, 17 and thereby 18 by turning on the transistor 
19 and short-circuiting the control of the switching element 9. Coupling 
of control circuit 9a with the control circuit 10a (for example, via a 
transformer, see U.S. Pat. No. 4,647,820) results in control circuit 10a 
also being turned off. This coupling is diagrammatically shown by means of 
a line between 9a and 10a in the drawing. The short-circuit time is 
shorter than the time which is required to cool the lamp electrodes to 
below the emission temperature. If the short-circuit of transistor 9 is 
eliminated again (and high-frequency switching of the converter is started 
again, for example, by means of a starting pulse with a diac, see also 
U.S. Pat. No. 4,647,820) the lamp does not ignite on too cold electrodes. 
The required time for the triac to be turned off (by rendering the 
converter inoperative) should be at least 10 /.mu.s, dependent on the 
type. 
The circuit operated as follows. After connecting the terminals A and B to 
the A.C. power supply, the capacitors 7 and 15 are charged via bridge 4. 
The converter is started via a starter circuit (not shown). The triac 12 
is rendered conducting via diode 13 and capacitor 14 and the electrodes 2 
and 3 are pre-heated. Since elements 9 and 10 switch at a high frequency, 
a high-frequency current flows through triac 12. The voltage across 
capacitor 17 increases above the threshold value of the monostable 
multivibrator 18, which in turn applies a pulse to the base of transistor 
19 which is then turned on and short-circuits the control of transistor 9. 
The converter is then inoperative for 2 ms. Then the pulse stops and the 
switches 9 and 10 are rendered alternately conducting and non-conducting 
via the starter circuit. Triac 12 is then no longer turned on because the 
capacitor 14 is charged. When the converter switches on again, no current 
flows through the gate of triac 12 due to the reverse bias provided by the 
voltage on capacitor 14. 
In one embodiment the most important circuit elements had the followign 
values: 
capacitor 5: 10 /.mu.F 
capacitor 7: 0.5 /.mu.F 
capacitor 15: 0.5 /.mu.F 
capacitor 11: 12 nF 
capacitor 14: 100 nF 
coil 8: 2 mH. 
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 9 and 10 are of the BUT11 type (Philips). The triac 12 
was a BT136 (Philips).