Discharge lamp device having a preheating electrode circuit

The invention relates to a circuit arrangement for operating a discharge lamp with a high frequency current comprising a power feedback circuit and a electrode preheater. The circuit arrangement comprises an antiboost switch for disabling the power feedback circuit before the lamp has ignited and enabling the power feedback circuit after the lamp has ignited- In accordance with the invention the antiboost switch is also used to enable the electrode preheater before the lamp has ignited and to disable the electrode preheater after the lamp has ignited.

BACKGROUND OF THE INVENTION 
This invention relates to a circuit arrangement for operating a discharge 
lamp with a high frequency current comprising 
input terminals for connection to a source of low frequency supply voltage, 
rectifier means coupled to said input terminals for rectifying said low 
frequency supply voltage, 
a first circuit comprising a series arrangement of first unidirectional 
means, second unidirectional means and first capacitive means, said first 
circuit being coupled to a first output terminal N3 of said rectifier 
means and a second output terminal N5 of said rectifier means, 
inverter means coupled to said first capacitive means for generating the 
high frequency current, 
a load circuit comprising inductive means, second capacitive means and 
terminals for lamp connection, said load circuit being coupled to said 
inverter means, 
power feedback means connecting a terminal N6 of said load circuit to a 
terminal N7 between the first unidirectional means and the second 
unidirectional means, 
a second circuit comprising an antiboost switching element S and shunting 
at least one of the first and second unidirectional means, a control 
electrode of said switching element being coupled to a control circuit for 
rendering the switching element conductive and non-conductive, and 
a third circuit for heating the electrodes of the discharge lamp comprising 
a first and a second secondary winding, said first and second secondary 
windings during operation each being part of a series arrangement shunting 
a lamp electrode. 
Such a circuit arrangement is known from WO 97/19578. The known circuit 
arrangement is very suitable to be powered from a regular mains supply 
generating, e.g. a supply voltage having an r.m.s. voltage of 230 Volt and 
a frequency of 50 Hz. Since the known circuit arrangement is equipped with 
power feedback means, it has a relatively high power factor that is 
realized with comparatively simple means. The circuit arrangement is so 
dimensioned that during stationary lamp operation there exists a balance 
between the amount of power fed back by the power feedback means and the 
amount of power consumed by the lamp. Before the lamp is ignited, however, 
the lamp does not consume any power which can lead to the power feedback 
means charging the first capacitive means to such a high voltage that part 
of the circuit arrangement, e.g. the inverter means, could be damaged. To 
prevent this, the circuit arrangement is equipped with the second circuit. 
In the known circuit arrangement the control circuit that is comprised in 
the second circuit monitors the voltage over the first capacitive means. 
If this voltage becomes higher than a first predetermined value, the 
control circuit renders the antiboost switching element S conductive, 
thereby disabling the power feedback means. After the lamp has ignited it 
starts to consume power so that the voltage across the first capacitive 
means drops below a second predetermined value, whereupon the control 
circuit renders the antiboost switching element S non-conductive thereby 
once more enabling the power feedback means. In the known circuit 
arrangement the secondary windings comprised in the third circuit are 
magnetically coupled to the inductive means comprised in the load circuit. 
Both secondary windings are arranged in series with a capacitor and the 
resulting series arrangements shunt respective electrodes of the lamp. 
Before the ignition of the lamp the inverter operates at a frequency at 
which the impedances of the capacitors comprised in the third circuit are 
relatively small. As a result a current with a relatively high amplitude 
flows through the lamp electrodes so that they are heated effectively. 
After ignition of the lamp the inverter operates at a much lower frequency 
so that the impedances of the capacitors are relatively high and the lamp 
electrodes carry a relatively small current. A disadvantage of the known 
circuit arrangement is that the current that flows through the lamp 
electrodes during stationary operation, though it is relatively small, 
continuously dissipates power in the electrodes thereby lowering the 
efficacy of the circuit arrangement. 
SUMMARY OF THE INVENTION 
The invention aims to provide a circuit arrangement for operating a 
discharge lamp that warms the electrodes of the discharge lamp effectively 
before lamp ignition and does not dissipate electrode heating power in the 
electrodes during stationary operation. 
A circuit arrangement as described in the opening paragraph is therefore 
according to the invention characterized in that the second circuit 
comprises a series arrangement of third unidirectional means and the 
antiboost switching element S and in that a fourth circuit comprising a 
primary winding that is magnetically coupled with the first and second 
secondary winding is coupled between a common terminal of the switching 
element and the third unidirectional means and a terminal of the load 
circuit. 
Before ignition of the lamp the control circuit renders the antiboost 
switching element S conductive. In a circuit arrangement according to the 
invention, this not only prevents an overvoltage over the first capacitive 
means by disabling the power feedback means, but also causes current to 
flow in the primary winding comprised in the fourth circuit. Since the 
primary winding is magnetically coupled to both secondary windings 
comprised in the third circuit, these secondary windings cause an 
electrode heating current to flow in both electrodes. When, after ignition 
of the lamp the control circuit renders the antiboost switching element S 
non-conductive, this does not only enable the power feedback means but 
also makes sure that the primary winding in the third circuit can no 
longer conduct current. As a result no electrode heating power is 
dissipated in the lamp electrodes after the ignition of the lamp, so that 
the circuit arrangement according to the invention has a relatively high 
efficacy during stationary operation. The relatively high efficacy of the 
circuit arrangement according to the invention is achieved using only 
relatively few additional components since the antiboost switching element 
S in a circuit arrangement according to the invention thus has two very 
different functions. 
Preferably, the series arrangement comprised in the fourth circuit 
comprises third capacitive means. These third capacitive means prevent the 
flow of a DC current in the series arrangement. 
Good results have been obtained for a circuit arrangement according to the 
invention, wherein said inverter means comprise a series arrangement of a 
first switching element, a terminal N1 and a second switching element, 
said terminal N1 being positioned between the first and second switching 
elements, and a drive circuit DC coupled to the switching elements for 
generating a drive signal for rendering the switching elements alternately 
conducting and non-conducting. Preferably the series arrangement of the 
first and second unidirectional means is shunted by a series arrangement 
of fourth and fifth unidirectional means and a common terminal N2 of the 
fourth and fifth unidirectional means is connected to terminal N1 by means 
of the load circuit. In this way the circuit arrangement incorporates an 
extra power feedback. Because of this extra power feedback the circuit 
arrangement causes relatively little harmonic distortion of the low 
frequency supply current, while the circuit arrangement is also capable of 
operating discharge lamps having a relatively high lamp voltage without 
the drawback of components comprised in the load circuit and the inverter 
having to conduct a relatively large current during lamp operation. It has 
been found that the functioning of the circuit arrangement improved where 
the circuit arrangement comprises a fifth circuit comprising fourth 
capacitive means for connecting terminal N2 to a terminal N4 between the 
first capacitive means and the fifth unidirectional means. This fifth 
circuit can comprise only the fourth capacitive means, but alternatively 
it is also possible that the fifth circuit comprises for instance a series 
arrangement of the first capacitive means and the fourth capacitive means. 
In a preferred embodiment of a circuit arrangement according to the 
invention the circuit arrangement comprises a series arrangement 
comprising the antiboost switching element S and the primary winding, said 
series arrangement connecting terminal N1 to terminal N4. 
The power feedback means preferably comprises capacitive means. In this way 
it is prevented that the power feedback means carry a DC current. 
A satisfactory functioning of the circuit arrangement has been found where 
the control circuit comprises means for rendering the antiboost switching 
element S conductive and non-conductive dependent upon of the voltage 
across said first capacitive means.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 K1 and K2 are input terminals for connection to a source of low 
frequency supply voltage. L2 and L2' are inductors that form an input 
filter together with capacitor C3. Diodes D1-D4 are rectifier means for 
rectifying said low frequency supply voltage. In this embodiment diodes D7 
and D8 form first and second unidirectional means respectively. Capacitor 
C4 is first capacitive means and forms together with diodes D7 and D8 a 
first circuit. Switching elements Q1 and Q2 together with drive circuit DC 
form inverter means. Drive circuit DC is a circuit part for generating 
drive signals for rendering switching elements Q1 and Q2 conducting and 
non-conducting. Inductor L1 capacitor C2 and terminals K3 and K4 for 
connection to a discharge lamp together form a load circuit. In the 
embodiment shown in FIG. 1 inductor L1 forms inductive means, capacitor C2 
forms second capacitive means and terminals K3 and K4 form terminals for 
lamp connection. Capacitor C1 forms a fifth circuit and fourth capacitive 
means. Diodes D5 and D6 form fourth and fifth unidirectional means 
respectively. Capacitor C5 forms fifth capacitive means and also power 
feedback means. Diode D9 and antiboost switching element S together with 
resistors R1 and R2 and circuit part ST form the second circuit. Resistors 
R1 and R2 and circuit part ST together form a control circuit for 
rendering the antiboost switching element conductive and non-conductive. 
Diode D9 forms third unidirectional means. Primary winding Lprim together 
with capacitor Cprim forms a fourth circuit. Cprim forms third capacitive 
means. First secondary winding L2, second secondary winding L3 and 
capacitors C6 and C7 together form a third circuit for heating the 
electrodes of the discharge lamp. 
Input terminals K1 and K2 are connected by means of a series arrangement of 
inductor L2, capacitor C3 and inductor L2' respectively. A first side of 
capacitor C3 is connected to a first input terminal of the rectifier 
bridge and a second side of capacitor C3 is connected to a second input 
terminal of the rectifier bridge. A first output terminal N3 of the 
rectifier bridge is connected to a second output terminal N5 of the 
rectifier bridge by means of a series arrangement of diode D5, diode D6 
and capacitor C4. N2 is a common terminal of diode D5 and diode D6. N4 is 
a common terminal of diode D6 and capacitor C4. Terminal N2 is connected 
to terminal N4 by means of capacitor C1. The series arrangement of diodes 
D5 and D6 is shunted by a series arrangement of diodes D7 and D8. Diode D8 
is shunted by a series arrangement of diode D9 and antiboost switching 
element S. N7 is a common terminal of diodes D7 and D8. Capacitor C4 is 
shunted by a series arrangement of switching elements Q1 and Q2. A control 
electrode of switching element Q1 is connected to a first output terminal 
of drive circuit DC. A control electrode of switching element Q2 is 
connected to a second output terminal of drive circuit DC. N1 is a common 
terminal of switching element Q1 and switching element Q2. Terminal N1 is 
connected to terminal N2 by means of a series arrangement of respectively 
inductor L1, capacitor C2, terminal K3, discharge lamp LA and terminal K4. 
N6 is a common terminal of capacitor C2 and terminal K3. Terminal N6 is 
connected to terminal N7 by means of capacitor C5. A first electrode of 
lamp LA is shunted by a series arrangement of first secondary winding L2 
and capacitor C6. A second electrode of lamp LA is shunted by a series 
arrangement of second secondary winding L3 and capacitor C7. A common 
terminal of diode D9 and antiboost switching element S is connected to 
terminal N1 by means of a series arrangement of Lprim and Cprim. Capacitor 
C4 is shunted by a series arrangement of resistors R1 and R2. A common 
terminal of resistor R1 and resistor R2 is connected to an input terminal 
of circuit part ST. An output terminal of circuit part ST is coupled to a 
control electrode of antiboost switching element S. This latter coupling 
is shown in FIG. 1 by means of a dotted line. 
The operation of the circuit arrangement shown in FIG. 1 is as follows. 
When input terminals K1 and K2 are connected to the poles of a source of a 
low frequency supply voltage, the rectifier bridge rectifies the low 
frequency supply voltage supplied by this source so that a DC-voltage is 
present over capacitor C4 serving as a buffer capacitor. Drive circuit DC 
renders the switching elements Q1 and Q2 alternately conducting and 
non-conducting and as a result a substantially square wave voltage having 
an amplitude approximately equal to the amplitude of the DC-voltage over 
capacitor C4 is present at terminal N1. Power feedback is effected both 
via capacitor C5 and diodes D7 and D8, as well as via the load circuit and 
diodes D5 and D6. Before the lamp LA has ignited, it does not consume 
power so that at this stage of the lamp operation there is an unbalance 
between the power fed back and the amount of power consumed by the lamp. 
As a result the voltage over capacitor C4 increases to a value that is 
larger than a first predetermined level so that the control circuit 
renders the antiboost switching element S conductive. The power feedback 
that takes place via capacitor C5 and diodes D7 and D8 is thereby disabled 
and an overvoltage over capacitor C4 is prevented. The fact that the 
antiboost switching element S is conductive also causes an alternating 
current to flow through the series arrangement of primary winding Lprim 
and capacitor Cprim. Since the primary winding Lprim is magnetically 
coupled to secondary windings L2 and L3, this alternating current causes 
alternating voltages to be present across secondary windings L2 and L3, 
which alternating voltages in turn cause electrode heating currents to 
flow through the electrodes of the lamp LA. After the lamp has ignited and 
starts to consume power the voltage over capacitor C4 drops beneath a 
second predetermined value and the control circuit renders the antiboost 
switching element S non-conductive thereby enabling the power feedback by 
means of capacitor C5 and diodes D7 and D8. After the antiboost switching 
element S has become non-conductive the primary winding Lprim no longer 
carries a current so that the electrode heating current becomes zero.