Circuit arrangement for operating at least one discharge lamp

Description

I. PRIOR ART

Such a circuit arrangement is disclosed, for example, in German Laid-Open Specification DE 196 19 580. This document describes a safety disconnection device for a half-bridge inverter, said safety disconnection device detecting the occurrence of a rectifying action in the discharge lamp and disconnecting the half-bridge inverter in this case. The safety disconnection device is designed such that it responds to a DC voltage across the half-bridge capacitor which is caused by a DC component of the lamp current. This is because the occurrence of a pronounced rectifying action in the discharge lamp marks the end of the useful life of the discharge lamp and causes a voltage drop across the half-bridge capacitor which is different from the nominal value—that is to say from half the intermediate circuit voltage. This comparatively complex safety disconnection device comprises a threshold switch and a bistable multivibrator which can be used to remove the drive signal from a half-bridge inverter transistor.

II. ILLUSTRATION OF THE INVENTION

The object of the invention is to provide, for a circuit arrangement of this generic type, an improved apparatus for detecting a rectifying action occurring in the at least one discharge lamp.

The invention achieves this object by means of the characterizing features of Patent claim

1

. Particularly advantageous embodiments of the invention are described in the dependent claims.

To monitor the voltage drop across the half-bridge capacitor, the circuit arrangement according to the invention has a first and a second RC element, which are connected to one another by a threshold switch, and means for detecting the switching state of the threshold switch. The first RC element is connected to the center tap of the half-bridge inverter, and the second RC element is connected to the half-bridge capacitor. This means that a DC voltage develops across the capacitor in the first RC element, said DC voltage being equivalent to half the intermediate circuit voltage, that is to say to half the supply voltage of the half-bridge inverter, while a DC voltage develops across the capacitor in the second RC element which is equivalent to the voltage drop across the half-bridge capacitor. The high-frequency voltage components are filtered out of the half-bridge capacitor voltage by the second RC element. During fault-free lamp operation, the aforementioned DC voltages are of approximately the same size, so that there is no significant potential difference between the two RC elements. If a rectifying action occurs in the at least one discharge lamp, then the voltage drop across the half-bridge capacitor changes, as does the DC voltage across the capacitor in the second RC element, accordingly. If the potential difference between the capacitors in the two RC elements reaches the threshold voltage of the threshold switch, then said threshold switch is switched to the conductive state and a current flows between the two aforementioned capacitors in order to equalize their different charge state. The switching state of the threshold switch or the flow of current between the capacitors in the two RC elements is detected by the drive apparatus in the half-bridge inverter using suitable means, so that the half-bridge inverter can be disconnected in a fault situation.

The circuit arrangement according to the invention advantageously has a nonreactive resistor which is arranged in parallel with the threshold switch and forms a voltage divider with the resistors in the first and second RC elements. Using this additional resistor, the switching threshold of the monitoring circuit according to the invention can, with suitable dimensioning of the aforementioned resistors, be set to any desired voltage value above the threshold voltage of the threshold switch. In addition, the circuit arrangement according to the invention advantageously has a capacitor which is arranged in parallel with the threshold switch and holds the threshold switch, once it has been switched on, in the conductive state sufficiently long to ensure that the charges in the capacitors in the first and second RC elements are equalized. The threshold switch used is advantageously a diac, since this reacts both to positive and to negative voltages. To detect the switching state of the threshold switch, the monitoring circuit according to the invention advantageously has a further nonreactive resistor, through which the current equalizing the different charges in the capacitors in the two RC elements flows. A rectifier is advantageously connected upstream of this resistor, so that the aforementioned equalization current between the capacitors in the RC elements produces unipolar voltage pulses across the resistor whose polarity is independent of the direction of flow of the equalization current. These unipolar voltage pulses can be used by the monitor input of an integrated circuit in order to disconnect the half-bridge inverter.

The monitoring circuit according to the invention may advantageously be used for separately controlled half-bridge inverters. It is also distinguished by the fact that it is not susceptible to faults, because, with the exception of the resistor through which the equalization current flows, all the components of the monitoring circuit according to the invention are arranged in the high-voltage part of the circuit arrangement, and the signal line from the rectifier to the resistor through which the equalization current flows is terminated at low resistance by said resistor.

III. DESCRIPTION OF THE PREFERRED ILLUSTRATIVE EMBODIMENT

The invention is explained in more detail below with the aid of a preferred illustrative embodiment. The figure shows a schematic illustration of the circuit arrangement according to the preferred illustrative embodiment.

The circuit arrangement according to the preferred illustrative embodiment is a separately controlled half-bridge inverter for high-frequency operation of a fluorescent lamp LP. The half-bridge inverter has two alternately switching field-effect transistors T

1

, T

2

and a drive apparatus A for these transistors T

1

, T

2

. The drive apparatus A is partly in the form of an integrated circuit IC, in particular a high-voltage IC. The DC voltage supply for the half-bridge inverter is obtained from the mains AC voltage by rectification, in the customary manner. Arranged between the center tap M of the half-bridge inverter and the tap V

1

on the half-bridge capacitor CK is a load circuit which is in the form of a series resonant circuit and is essentially formed by the lamp inductor LD and the ignition capacitor CR. In addition, the load circuit has two pairs of electrical connections j

1

, j

2

for the electrode coils E

1

, E

2

of a fluorescent lamp LP. The fluorescent lamp LP is connected in parallel with the ignition capacitor CR. In addition, the circuit arrangement can have a heating apparatus (not shown) for preheating the electrode coils E

1

, E

2

of the fluorescent lamp LP. The circuit arrangement is also equipped with a monitoring circuit which has a first RC element R

1

, C

1

and a second RC element R

2

, C

2

, and also a diac DC. The first RC element R

1

, C

1

is connected to the center tap M of the half-bridge inverter, while the second RC element R

2

, C

2

is connected to the tap V

2

of the half-bridge capacitor CK. The capacitors C

1

, C

2

in the two RC elements R

1

, C

1

, R

2

, C

2

are connected to one another by the diac DC. Connected in parallel with the diac DC is a nonreactive resistor R

3

which forms a voltage divider with the resistors R

1

, R

2

in the RC elements. In addition, the monitoring circuit has a capacitor C

3

in parallel with the diac DC and in parallel with the resistor R

3

, said capacitor C

3

holding the diac DC in the conductive state for longer after it has turned on. The ground-side connections of the capacitors C

1

, C

2

are connected to a rectifier comprising four diodes D

1

, D

2

, D

3

, D

4

. The DC voltage output of the rectifier D

1

, D

2

, D

3

, D

4

is connected to ground via a low-value resistor R

4

. The voltage drop across the resistor R

4

is monitored by the drive apparatus A in the half-bridge inverter via a connecting line between the resistor R

4

and a monitor input of the integrated circuit IC.

Once the circuit arrangement has been switched on, the half-bridge capacitor CK is charged, so that the tap V

1

is at an electrical potential which is equivalent to half the supply voltage of the half-bridge inverter. The alternately switching transistors T

1

, T

2

in the half-bridge inverter apply a high-frequency AC voltage between approximately 30-50 kHz to the load circuit. The capacitors C

1

and C

2

in the two RC elements R

1

, C

1

and R

2

, C

2

are charged, so that they have a DC voltage which is the same as half the intermediate circuit voltage, that is to say the same as half the supply voltage of the half-bridge inverter. In this case, the high-frequency AC components are filtered out by the capacitors C

1

, C

2

. A heating apparatus (not shown) can be used to preheat the electrode coils E

1

, E

2

in the fluorescent lamp. The resonance step-up method is used to provide the ignition capacitor CR with the ignition voltage required to ignite a gas discharge in the fluorescent lamp LP. Once a gas discharge has formed between the electrode coils E

1

, E

2

in the fluorescent lamp LP, the ignition capacitor CR is bridged by the discharge path. A high-frequency alternating current regulated to a constant level by the half-bridge inverter flows through the fluorescent lamp LP. Ideally, the lamp current is symmetrical with respect to the two half-periods of the high-frequency AC voltage, so that the potential at the tap V

1

continues to be equivalent to half the supply voltage of the half-bridge inverter. This means that, normally, no or at least no significant voltage drop can be measured across the resistor R

3

, the charge state of the capacitors C

1

, C

2

is approximately the same and the diac DC is turned off.

If a rectifying action occurs in the fluorescent lamp LP, which results in a preferential direction developing for the lamp current, then the voltage drop across the half-bridge capacitor CK is increased or reduced depending on which half-cycle of the lamp current is let through as a preference by the fluorescent lamp LP. The potential at the tap V

1

is accordingly increased or reduced, and the capacitor C

2

in the second RC element has its charge reversed as appropriate. As a result, an accordingly altered potential difference develops between the taps M and V

1

, divided between the resistors R

1

, R

2

, R

3

in the ratio of the resistance values. If the voltage drop across the resistor R

3

reaches the threshold voltage of the diac DC, which in the present case is ±32 V, then the diac DC turns on. A current equalizing the different charges in the capacitors C

1

, C

2

then flows either via the diode D

3

or via the diode D

2

and through the diac DC and also through the resistor R

4

. The diac DC switches to the off state again only when its holding current is undershot. The capacitor C

3

arranged in parallel with the diac DC is dimensioned such that it holds the diac DC in the conductive state sufficiently long to ensure complete equalization of the charges in the capacitors C

1

, C

2

. After charge equalization has taken place between the capacitors C

1

, C

2

and the diac DC has switched to the off state again, a voltage develops across the capacitor C

2

again which is equivalent to half the intermediate circuit voltage plus the shift in potential, caused by the rectifying action, on the tap V

1

. As soon as the threshold voltage of the diac DC has developed across the resistor R

3

again, the diac DC is switched to the conductive state again and charge equalization again takes place between the capacitors C

1

, C

2

. This process is repeated cyclically. The time interval for the capacitors' charge equalization described above, and hence also the time interval between the current pulses through the diac DC, depend on the intensity of the rectifying action. The larger this potential difference is, the shorter the time interval between the current pulses. Or in other words, the greater the difference between the voltage across the half-bridge capacitor CK and half the intermediate circuit voltage, the shorter the intervals between the current pulses. The low-value resistor R

4

and the bridge rectifier D

1

, D

2

, D

3

, D

4

convert these current pulses causing the charge equalization between the capacitors C

1

, C

2

into unipolar voltage pulses of positive polarity. These voltage pulses generated on the resistor R

4

are supplied to a monitor input of the integrated circuit IC in the drive apparatus A in the half-bridge inverter. The drive apparatus A is programmed such that it disconnects the half-bridge inverter after an input voltage has been detected at the aforementioned monitor input.

Table I indicates dimensions for the components in the monitoring apparatus of the circuit arrangement according to the invention, which is suitable for operating one or more series-connected fluorescent lamps and for a lamp current of 170 mA.

The invention is not restricted to the illustrative embodiment explained above in more detail. By way of example, the monitoring apparatus according to the invention may also be used in circuit arrangements used for operating a plurality of series-connected or parallel-connected discharge lamps. The circuit arrangement according to the invention is suitable not only for operating fluorescent lamps, but may also be used generally for operating low-pressure discharge lamps. In addition, the monitoring apparatus according to the invention can be used not only for separately controlled half-bridge inverters, but also for free-running half-bridge inverters. In addition, the drive apparatus A in the half-bridge inverter can also be designed such that it reacts suitably to the voltage pulses on the resistor R

4

. By way of example, the drive apparatus A can be designed such that it disconnects the half-bridge inverter only from a particular, prescribed pulse frequency of the voltage pulses detected on the resistor R

4

.

TABLE I

Dimensions of the components in the monitoring

apparatus according to the invention for a circuit

arrangement regulating the lamp current to a value of 170 mA

R1, R2

100 kΩ, 10%, 500 V

R3

390 kΩ, 5%

R4

150 Ω

C1, C2

10 nF, 250 V, RM 7.5

C3

47 nF, 50 V

D1, D2, D3, D4

Diode SMD, 75 V-150 mA

SOD80

DC

Diac, 32 V

Claims

1. Circuit arrangement for operating at least one discharge lamp, the circuit arrangement having the following features:a half-bridge inverter (T1, T2, A) having a drive apparatus (A) and having a downstream load circuit (LD, CR, j1, j2), at least one half-bridge capacitor (CK) connected to the load circuit and to the half-bridge inverter, the load circuit (LD, CR, j1, j2) having electrical connections (j1, j2) for at least one discharge lamp (LP), and means for monitoring the voltage drop across the at least one half-bridge capacitor (CK), characterized in that the means for monitoring the voltage drop across the half-bridge capacitor (CK):have a first RC element (R1, C1) and a second RC element (R2, C2), which are connected to one another by a threshold switch (DC), the first RC element (R1, C1) being connected to the center tap (M) of the half-bridge inverter (T1, T2, A), and the second RC element (R2, C2) being connected to the half-bridge capacitor (CK), and have means for detecting the switching state of the threshold switch (DC).
2. The circuit arrangement according to claim 1, characterized in that the means for detecting the switching state of the threshold switch (DC) has a nonreactive resistor (R4) which detects the flow of current through the threshold switch (DC).
3. The circuit arrangement according to claim 2, characterized in that a rectifier (D1-D4) is connected upstream of the nonreactive resistor (R4).
4. The circuit arrangement according to claim 3, characterized in that the drive apparatus (A) in the half-bridge inverter (T1, T2, A) has an integrated circuit (IC) with a monitor input, the monitor input being connected to the nonreactive resistor (R4) detecting the flow of current through the threshold switch (DC).
5. The circuit arrangement according to claim 1, characterized in that the threshold switch (DC) has a nonreactive resistor (R3) connected in parallel with it, said nonreactive resistor forming a voltage divider with the resistors (R1, R2) in the first RC element (R1, C1) and the second RC element (R2, C2).
6. The circuit arrangement according to claim 1, characterized in that the threshold switch (DC) has a capacitor (C3) connected in parallel with it.
7. The circuit arrangement according to claim 1, characterized in that the threshold switch (DC) is a diac.

Priority Claims (1)

Number	Date	Country	Kind
199 41 437	Aug 1999	DE

US Referenced Citations (6)

Number	Name	Date
5493182	Sowa et al.	Feb 1996
5677602	Paul et al.	Oct 1997
5770925	Konopka et al.	Jun 1998
5869937	Konopka	Feb 1999
5994847	Konopka	Nov 1999
6177768	Kamata et al.	Jan 2001

Foreign Referenced Citations (1)

Number	Date	Country
196 19 580	Nov 1997	DE

Circuit arrangement for operating at least one discharge lamp

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

Priority Claims (1)

US Referenced Citations (6)

Foreign Referenced Citations (1)