CN101755489A - Circuit arrangement and method for operation of a discharge lamp - Google Patents

Abstract

The invention relates to a circuit arrangement for operating a discharge lamp (EL) with a rectifier (GL) having a first input terminal and a second input terminal for coupling to an AC supply voltage and for a first output terminal and a second output terminal providing a DC drive voltage (UZW); a free-running half-bridge inverter comprising a series circuit of two voltage-controlled electronic switches (T1, T2) coupled in Between the first output terminal and the second output terminal of the rectifier (GL), where a half-bridge midpoint (HB) is formed between the first electronic switch (T1) and the second electronic switch (T2); for starting the inverter The starting circuit of the free oscillation mode of the device (T1, T2), wherein the starting circuit includes a driving circuit (AS) with a first output terminal and a second output terminal, wherein the first output of the driving circuit (AS) terminals coupled to the control electrodes of the first electronic switch (T1) and the second electronic switch (T2); and a startup capacitor (C5) having a first terminal and a second terminal, wherein the first terminal of the startup capacitor (C5) is a Coupled on the one hand to the first output terminal of the rectifier (GL) via a first ohmic resistor (R1) and on the other hand to the second output terminal of the drive circuit (AS), the second terminal of the boot capacitor to the half-bridge midpoint ( HB) coupled; and the half-bridge midpoint is coupled with the second output terminal of the rectifier (GL) through a pull-down resistor (R2). The invention also relates to a method for operating a discharge lamp with such a circuit arrangement.

Description

Circuit arrangement and method for operating a discharge lamp

technical field

The invention relates to a circuit arrangement for operating a discharge lamp, having a rectifier with a first input terminal and a second input terminal for coupling to an AC supply voltage and a second input terminal for supplying a DC operating voltage. an output terminal and a second output terminal; a free-running half-bridge inverter comprising a series circuit of two voltage-controlled electronic switches coupled between the first output terminal and the second output terminal of the rectifier, wherein a half-bridge midpoint is formed between the first electronic switch and the second electronic switch; a starting circuit for starting the oscillation of the free-running mode of the inverter, wherein the starting circuit comprises a first output terminal and a second output terminal A drive circuit for terminals, wherein a first output terminal of the drive circuit is coupled to the control electrodes of the first electronic switch and the second electronic switch; and a boot capacitor with the first terminal and the second terminal. Furthermore, the invention relates to a method for operating a discharge lamp by means of such a circuit arrangement.

Background technique

The invention generally relates to a circuit arrangement for independently starting a free-running half-bridge inverter when a supply voltage is applied by means of voltage-controlled transistors such as MOSFETs or IGBTs as switches.

From EP 0 917 412 B1 (which serves to express the generic concept of the independent claim) it is disclosed that the voltage required to initially switch on the half-bridge transistors is provided by charging a capacitor: The transistors of the drive circuit are connected in series. In order to avoid an undesired, stable state of charge of the capacitors inserted in series, and thus avoid "jamming" (Haengenbleiben) of the half-bridge inverter, it is proposed in the mentioned patent The capacitor is charged. This grid terminal enables repeated start-up attempts at half the grid frequency for a long period of time until the self-oscillating oscillation of the half-bridge inverter is started. In the mentioned patent, only embodiments are shown in which separate driver circuits are designed for the two half-bridge transistors.

To reduce costs or to simplify the circuit arrangement, half-bridge inverters can also be constructed from complementary transistors (n-channel and p-channel), both transistors being controlled with the same driver circuit. This principle is disclosed in EP 0 781 077 B1, where a diac is also required for starting the half-bridge inverter. The idea according to EP 0 917 412 B1 can be transferred to the circuit arrangement disclosed in EP 0 781 077 B1 in that the capacitor required for starting the circuit is connected in series with one of the drive circuits. Here, the "grid start" function is fully provided.

However, it is disadvantageous here that the terminals of the resistors required for charging the starting capacitor are practically at half-bridge intermediate potential during discharge lamp operation. However, since the other terminal of the starting resistor is connected to one of the two grid lines, the high-frequency rectangular voltage of this intermediate potential is thus applied to the grid via the starting resistor. This leads to high radio interference values for the entire system, since the current is passed through the starting resistor and thus the high-frequency, almost rectangular interference signal is conducted via the radio interference filter directly into the mains. If, in order to avoid this, the starting resistor is connected to the terminals of the rectifier (where the rectifier supplies the positive supply voltage, the so-called intermediate circuit voltage) instead of to one of the two mains lines, the disadvantage is that this The circuit arrangement is only able to carry out one start attempt, ie the circuit "blocks" if the start attempt fails.

Contents of the invention

It is therefore the object of the present invention to improve a circuit arrangement or a method of the type mentioned at the outset in such a way that on the one hand high radio interference values are avoided and on the other hand a reliable start-up of the circuit arrangement is ensured after application of the mains voltage.

The invention is based on the recognition that combining the teachings of EP 0 781 077 B1 and EP 0 917 412 B1 with the subsequent switching of the starting resistor to the positive rectifier output instead of the grid line does not solve the above-mentioned task, because in Stable voltage relationships occur across the start capacitors which prevent repeated start attempts. According to the invention, therefore, the startup capacitor is coupled to the midpoint of the half-bridge, and the midpoint of the half-bridge is coupled via a pull-down resistor to the second output terminal of the rectifier. This configuration makes it possible to pull the potential of the midpoint of the half bridge back to the negative reference potential provided by the output of the second rectifier after a failed start-up attempt via the pull-down resistor. As a result, the start capacitor is recharged and a new start attempt can be carried out. Since in this circuit arrangement the capacitor is no longer connected to the mains line via its resistance, which is connected to the positive potential, but to the positive rectifier output, the transmission of radio interference into the mains is prevented.

In a preferred embodiment, the circuit arrangement also has a lamp inductor with at least one primary winding, which is coupled in series with the terminals for the discharge lamp, the driver circuit having a first terminal and a second an inductor at a terminal, the inductor being a secondary winding of the lamp inductor; and a series circuit of a second ohmic resistor and a parallel tank circuit coupled between the first terminal and the second terminal of the inductor. It is particularly preferred here that a bridge circuit comprising a capacitor is connected in parallel to the second ohmic resistor. This particularly preferred embodiment is based on the recognition that in order to start the circuit arrangement, the control voltage of the transistor which is switched on first of the half-bridge inverter must remain sufficiently high after the mains voltage has been applied, even if the starting capacitor has been switched on by switching on this transistor. The discharge and thus the effective control voltage (ie the sum of the voltage across the boot capacitor and the output voltage of the driver circuit) tends to decrease. It is preferably ensured that a sufficient control voltage of the transistor is maintained when the boot capacitor starts to discharge in such a way that the driver circuit quickly provides a sufficient output voltage. For this purpose, the second ohmic resistance required for driving the electronic switches of the half-bridge inverter during continuous operation is bridged, since the capacitor has a very low resistance to the high-frequency spectral content of the switch-on pulse. As a result, the charge carriers flowing through the capacitor enable a much faster charging of the capacitor arranged in the parallel resonant circuit. This makes it possible to provide a sufficiently high control voltage to the first half-bridge transistor to be switched on.

As a result, despite the discharge of the startup capacitor, the risk of not having sufficient control voltage at the first switched-on switch of the half-bridge inverter is effectively overcome.

Preferably, the bridge circuit includes a third ohmic resistor coupled in series with the capacitor. This third ohmic resistance brings about two advantages: On the one hand, it attenuates the rectangular component of the voltage supplied by the secondary winding on the lamp inductor. This therefore leads to the fact that the lamp inductor is connected on the one hand to the midpoint of the half-bridge, at which a high-frequency rectangular signal is present, and on the other hand to the lamp, which applies a largely sinusoidal signal to the other terminal of the lamp inductor. The third ohmic resistor makes it possible to provide an essentially sinusoidal signal for controlling the two electronic switches of the half-bridge inverter during continuous operation. On the other hand, the third ohmic resistor extends the possible dead time (Totzeit) during the transition of the half-bridge inverter. Lossless switching of the switches of the half-bridge inverter can thus be achieved. It generally applies that the third ohmic resistor is designed to be much smaller than the second ohmic resistor.

Furthermore, it is preferred that the circuit arrangement according to the invention also includes a diode, which is connected in parallel to the first ohmic resistor and is oriented such that it enables the discharge of the startup capacitor via the first electronic switch when the first electronic switch is switched on. The diode thus discharges the startup capacitor after a successful startup, so that no further startup attempts then interfere with the operation of the circuit arrangement.

In a preferred embodiment, the first connection of the starting capacitor is coupled to the second output connection of the rectifier via a further ohmic resistor. In this embodiment, the provision of the diodes mentioned in the preceding paragraphs can then be dispensed with. In this case, the boot capacitor is discharged via the first ohmic resistor and the further ohmic resistor. The reason for this is that during operation the two terminals of the boot capacitor are on average at a potential corresponding to exactly half the half-bridge voltage, so that the boot capacitor is discharged during operation.

In this case, it is preferred that a further capacitor is provided, which is coupled between the first output connection of the driver circuit and the control electrode of the second electronic switch. Since the purely ohmic discharge of the boot capacitor lasts for some time until the voltage across the boot capacitor has subsided, the output voltage of the driver circuit has a positive offset component. This offset component opposes the switching on of the second transistor, since the control voltage available to the second transistor is reduced by this offset component. The proposed further capacitor acts as a coupling capacitor and absorbs the mentioned offset voltage. Via an optional additional ohmic resistor which is coupled between the working electrode and the control electrode of the second electronic switch, the purely alternating voltage component of the output voltage of the driver circuit is then reduced, whereby a sufficiently negative voltage can be achieved immediately Used to control the second transistor.

The two electronic switches of the half-bridge inverter can be transistors of complementary polarity, but also transistors of the same polarity. The use of transistors of complementary polarity offers the advantage that only one driver circuit has to be provided.

Further advantageous embodiments emerge from the subclaims.

The preferred embodiments and their advantages presented with reference to the circuit arrangement according to the invention also apply to the method according to the invention, as far as applicable.

Description of drawings

Embodiments of the circuit arrangement according to the invention will be further described below with reference to the drawings. in:

FIG. 1 shows a schematic diagram of a first embodiment of a circuit arrangement according to the invention;

FIG. 2 shows a schematic diagram of a second embodiment of a circuit arrangement according to the invention;

FIG. 3 shows a schematic diagram of a third embodiment of a circuit arrangement according to the invention;

4 shows the time profile of the voltage across the starting capacitor, the control voltages of the two half-bridge transistors, the half-bridge voltage and the load current flowing through the lamp inductor for the exemplary embodiment of FIG. 1;

5 shows the voltage at the starting capacitor, the control voltages of the two half-bridge transistors, the half-bridge voltage and the time of the load current flowing through the lamp inductor with a different time resolution than in FIG. 4 for the exemplary embodiment of FIG. 1 The change curve on , especially the successful start-up process achieved after unsuccessful attempts;

FIG. 6 shows the time course of the quantity in FIG. 5 in the range of failed start-up attempts with high time resolution; and

FIG. 7 shows, in a similar manner to FIG. 4 , the time course of corresponding quantities of the circuit arrangement according to FIG. 2 .

Detailed ways

The same reference numerals are used below for identical or identically acting components, which are only described once for reasons of clarity.

FIG. 1 shows a schematic diagram of a first exemplary embodiment of a circuit arrangement according to the invention for operating a discharge lamp, which is shown as load EL. In this case, the mains voltage is supplied via the safety device SI to the rectifier GL, which feeds the electrolytic capacitor C1 or conducts the voltage through the electrolytic capacitor. The two supply branches are tapped at the electrolytic capacitor C1, tapped by the filter formed by the coil L1 in one of the branches and the capacitor C2 connecting the two branches. The lower supply branch in FIG. 1 has a negative potential and defines the reference potential on the rectified side of the circuit arrangement. The upper part corresponds to the positive power supply branch. Between the two power supply branches, there is a half-bridge composed of N-channel transistor T1 and P-channel transistor T2. Between the midpoint HB of the half-bridge and the positive supply branch there is a load circuit comprising a lamp inductor L2 connected in series with the load, a discharge lamp EL and a coupling capacitor C7 connected in series with the load. Furthermore, a load-parallel circuit connection with cold conductor KL for lamp ignition and two resonant capacitors C8 and C9 is provided.

In order to reduce the switching load of the MOSFET transistors T1 and T2, a capacitor C6 is connected in parallel with the upper half-bridge transistor T1.

The drive circuit AS is located between the source terminals of the transistors T1 and T2 as a reference potential inside the transistors and the respective gate terminals. The driver circuit AS comprises a parallel circuit of a coil L3, a capacitor C3 and a series circuit formed by a secondary winding HW1 of a control transformer, the primary winding of which is the already mentioned lamp inductor L2, and a resistor R3. A series circuit of capacitor C4 and resistor R4 is connected in parallel with resistor R3.

Between the gate and source terminals of transistor T1, a start-up capacitor C5 is coupled in series with the drive circuit AS. The connection point between driver circuit AS and starting capacitor C5 is connected to the positive supply branch via ohmic resistor R1, while the half-bridge midpoint is connected to the negative supply branch via pull-down resistor R2.

A diode D1 is connected in parallel with this ohmic resistor R1.

To start the circuit, the startup capacitor C5 is charged through resistor R1 and pull-down resistor R2. After a successful start-up, ie when transistor T1 is initially fully turned on, start-up capacitor C5 is substantially discharged again via diode D1 and transistor T1.

In the event of a failed start attempt, the potential at the half-bridge midpoint HB is pulled back to the negative reference potential via the pull-down resistor R2 , whereby the start capacitor C5 is recharged and a new start attempt is carried out.

The period of this start-up attempt is determined by the configuration of resistors R1 and R2 and capacitor C5 and the intermediate circuit voltage U _ZW present at capacitor C1 . A resistor R3 is capacitively bridged, wherein the parallel resonant circuit comprising coil L3 and capacitor C3 is connected via this resistor R3 to the secondary winding of the lamp inductor L2. The capacitive bridging enables rapid charging of the capacitor C3 of the parallel resonant circuit by means of a differential behavior and thus makes it possible to quickly supply a sufficiently high control voltage to the half-bridge transistor T1 to be switched on.

However, the purely capacitive bridging of the resistor R3 has the disadvantage that in normal operation the control voltage quickly switches on the corresponding transistor to be switched on during the commutation phase of the half bridge, resulting in losses. In order to weaken this effect, resistor R4 can also be connected in series with capacitor C4. By properly designing the components R3, R4 and C4, optimal characteristics of the entire circuit in terms of start-up and drive can be defined.

FIG. 2 shows an embodiment of the circuit arrangement according to the invention, in which the discharge of the starting capacitor C5 takes place not via a diode but via ohmic resistors R1 and R5.

In operation, the two terminals of the boot capacitor C5 are on average at a potential corresponding to exactly half the half-bridge voltage. The starting capacitor C5 is thereby discharged. During the time when the half-bridge is not oscillating, the start-up capacitor C5 is charged, since the terminals on the half-bridge side are brought to a negative supply potential via the pull-down resistor R2.

FIG. 3 shows an improved embodiment of the circuit arrangement according to FIG. 2 . Since the purely ohmic discharge of boot capacitor C5 lasts for some time until the voltage across the boot capacitor has subsided, the output voltage of driver circuit AS temporarily has a positive offset component. This offset component opposes the switching on of the second transistor T2 because the control voltage available to the second transistor T2 is reduced by this offset component.

Therefore, an additional capacitor C10 is proposed, which absorbs the above-mentioned offset voltage as a coupling capacitor. The pure AC voltage component of the output voltage of the driver circuit AS then drops across the resistor R10 , so that a sufficiently negative voltage can immediately be achieved to activate the transistor T2 .

4 to 7 show the voltage at the starting capacitor C5, the control voltage U _GS of the two transistors, the half-bridge voltage U _HB and the voltage flowing through the lamp inductor L2 in different operating phases for different exemplary embodiments of the invention. Time-varying curve of the load current _IL .

FIG. 4 shows the voltage relationship and current relationship at the start of the oscillation. It can clearly be seen that the control voltage U _GS of the transistors T1 , T2 increases, although the voltage U _GS across the boot capacitor C5 drops starting at time t1 when the half-bridge voltage U _HB reaches the positive supply potential. This means that the boot capacitor C5 is discharged via the diode D1 and the transistor T1. The gradient of the rise of the control voltage U _GS is greater than the gradient of the rise of the load current _IL . By bridging resistor R3 by means of a series connection of capacitor C4 and resistor R4 , the step-like increase in the voltage at secondary winding HW1 is transmitted differentially to parallel resonant circuits C3 , L3 .

FIG. 5 shows, with additional time resolution, a successful start-up process, which was carried out after a failed attempt. It can be seen from this figure that even in the event of a failed start-up attempt, the voltage across start-up capacitor C5 drops because start-up capacitor C5 is discharged at least once via diode D1. Since the half-bridge voltage U _HB is pulled back toward the negative supply potential via the pull-down resistor R2 , the original voltage state that was present before the network voltage was applied is reached and a new start-up attempt is automatically carried out. This characteristic enables a high operational reliability of the circuit arrangement.

FIG. 6 shows the failed start-up attempt in FIG. 5 with high time resolution. It can clearly be seen that the control voltage U _GS is sufficient to switch on the transistor T1 , whereby the half-bridge voltage U _HB reaches the level of the positive supply potential and the boot capacitor C5 is initially discharged via the diode D1 . However, negative oscillations of control voltage U _GS are not sufficient to switch on transistor T2 . A gradual decay of the entire oscillation occurs. The still-used oscillation of the load current I _L via the lamp inductor L2 cyclically drives the half-bridge voltage U _HB to a positive supply potential, whereby multiple discharges of the starting capacitor C5 are achieved.

FIG. 7 shows the start-up of the circuit arrangement of FIG. 2 in a similar manner to FIG. 4 . It can clearly be seen that the boot capacitor C5 is no longer suddenly discharged when the half-bridge voltage U _HB reaches the level of the positive supply potential.

Although an embodiment with complementary transistors T1 , T2 is shown in FIGS. 1 to 3 , it is also possible to implement the invention for a half-bridge circuit with two separate driver circuits and transistors of the same polarity. However, this is associated with increased outlay compared to the circuit arrangement described.

Claims (11)

1. A circuit arrangement for driving a discharge lamp (EL), the circuit arrangement having: - a rectifier (GL) having a first input terminal and a second input terminal for coupling with an AC supply voltage and a first output terminal and a second output terminal for providing a DC drive voltage (U _ZW ); - a free-oscillating half-bridge inverter comprising a series circuit of two voltage-controlled electronic switches (T1, T2) coupled between a first output terminal and a second output terminal of a rectifier (GL), wherein a half-bridge midpoint (HB) is formed between the first electronic switch (T1) and the second electronic switch (T2); - a start-up circuit for starting an oscillation in a free-running mode of an inverter (T1, T2), wherein the start-up circuit comprises a drive circuit (AS) with a first output terminal and a second output terminal, wherein the drive circuit ( the first output terminal of AS) is coupled to the control electrodes of the first electronic switch (T1) and the second electronic switch (T2); and a boot capacitor (C5) with a first terminal and a second terminal, It is characterized in that, The first terminal of the starting capacitor (C5) is coupled on the one hand to the first output terminal of the rectifier (GL) via a first ohmic resistor (R1) and on the other hand to the second output terminal of the drive circuit (AS), A second terminal of the boot capacitor is coupled to the half-bridge midpoint (HB); and the half-bridge midpoint is coupled to the second output terminal of the rectifier (GL) through a pull-down resistor (R2). 2. The circuit arrangement as claimed in claim 1, characterized in that The circuit arrangement also has a lamp inductor (L2) with at least one primary winding coupled in series with the terminals of the discharge lamp (LA), wherein the driver circuit (AS) has a circuit with a first terminal and a second terminal inductor (HW1), which is the secondary winding of the lamp inductor; and a series circuit consisting of a second ohmic resistor (R3) and a parallel oscillating circuit (C3, L3) coupled to the inductor (HW1) between the first terminal and the second terminal. 3. The circuit arrangement according to claim 2, characterized in that, A bridge circuit (C4, R4) comprising a capacitor (C4) is connected in parallel with the second ohmic resistor (R2). 4. The circuit arrangement according to claim 3, characterized in that, The bridge circuit includes a third ohmic resistor (R4) coupled in series with the capacitor (C4). 5. The circuit arrangement as claimed in claim 1, characterized in that The circuit arrangement also comprises a diode (D1) connected in parallel with the first ohmic resistor (R1) and oriented such that when the first electronic switch (T1) is turned on, the diode enables the start-up capacitor (C5) to pass through the first Electronic switch (T1) discharges. 6. The circuit arrangement as claimed in one of claims 1 to 4, characterized in that The first terminal of the boot capacitor (C5) is coupled to the second output terminal of the rectifier (GL) via a further ohmic resistor (R5). 7. The circuit arrangement according to claim 6, characterized in that The circuit arrangement comprises a further capacitor (C10) coupled between the first output terminal of the driver circuit (AS) and the control electrode of the second electronic switch (T2). 8. The circuit arrangement according to claim 7, characterized in that The circuit arrangement comprises a further ohmic resistor ( R10 ), which is coupled between the control electrode and the working electrode of the second electronic switch ( T2 ). 9. The circuit arrangement as claimed in claim 1, characterized in that The two electronic switches (T1, T2) of the half-bridge inverter are transistors of complementary polarity. 10. The circuit arrangement as claimed in claim 1, characterized in that The two electronic switches (T1, T2) of this half-bridge inverter are transistors of the same polarity. 11. A method for driving a discharge lamp (EL) by means of a circuit arrangement, having - a rectifier (GL) having a first input terminal and a second input terminal for coupling with an AC supply voltage and a first output terminal and a second output terminal for providing a DC drive voltage (U _ZW ); - a free-oscillating half-bridge inverter comprising a series circuit of two voltage-controlled electronic switches (T1, T2) coupled between a first output terminal and a second output terminal of a rectifier (GL), wherein a half-bridge midpoint (HB) is formed between the first electronic switch (T1) and the second electronic switch (T2); - a start-up circuit for starting an oscillation in a free-running mode of an inverter (T1, T2), wherein the start-up circuit comprises a drive circuit (AS) with a first output terminal and a second output terminal, wherein the drive circuit ( the first output terminal of AS) is coupled to the control electrodes of the first electronic switch (T1) and the second electronic switch (T2); and a boot capacitor (C5) with a first terminal and a second terminal, It is characterized by the following steps: a) Charging the startup capacitor (C5) via the first ohmic resistor (R1) and the pull-down resistor (R2) until the switching process of the first electronic switch (T1) is carried out and thus the startup capacitor (C5) is at least partially Ground discharge, wherein the first terminal of the startup capacitor (C5) is coupled to the second output terminal of the drive circuit (AS), the second terminal of the startup capacitor (C5) is coupled to the half-bridge midpoint (HB), the first a one-ohm resistor (R1) coupled to the DC drive voltage, said pull-down resistor (R2) coupled between the half-bridge midpoint (HB) and the second output terminal of the rectifier (GL), b) Step a) is repeated until the oscillation of the inverter (T1, T2) is started.

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