EP0871347B1 - Ballast with automatic restart - Google Patents

Abstract

In an externally controlled electronic ballast, a driving circuit which requires a dedicated supply voltage is provided for the purpose of driving an inventor half bridge. The driving circuit is constructed such that the inventor half bridge blocks when the supply voltage Vcc falls below a threshold value UVLO. Via a self-holding controllable electronic switch and a Z diode DZ2, a voltage monitoring circuit draws the supply voltage Vcc below the threshold value UVLO when an excessively high lamp voltage is detected. The ballast is thereby inactive. A current sensing path which leads via at least one filament of the low-pressure gas discharge lamp serves to detect a change of lamp. The current sensing path controls a further electronic switch, which is likewise connected to the supply voltage Vcc of the driving circuit and can, when it conducts, draw said voltage further to frame than the first switch. The second electronic switch connected to the current sensing path thus permits automatic restarting of the ballast after the change of lamp by bringing the first switch into a non-conductive state again by firstly further lowering the supply voltage Vcc below the value prescribed by said switch.

Description

The invention relates to an electronic ballast with the features of the preamble of the claim 1. In particular, the invention relates to an externally controlled Ballast, its inverter frequency is set by a controlled oscillator.

For the operation of low pressure gas discharge lamps ballasts increasingly used that the concerned Do not just ignite and discharge the gas discharge lamp supply the required voltage and current, but also monitor the operation of the lamp. E.g. is an electronic from DE 44 10 492 A1 Ballast with a free-swinging inverter became known who when a maximum lamp voltage is stopped. This is on the connection of the discharge lamp to which AC voltage is applied a voltage divider connected its Output via a four-layer diode to the gate of a thyristor is laid. The thyristor is on a base connection the inverter half bridge connected and locks the inverter when it is ignited. Exceeds the voltage present at the gas discharge lamp a threshold value, the thyristor thus switches the Inverter. The thyristor is connected through a resistor supplied with current from the DC link voltage and thus keeps in the conductive state. For restarting of the electronic ballast is after changing the lamp an at least brief separation of the ballast required by the network. However, often an automatic restart is required, i.e. after changing the lamp should the electronic ballast without further measures to be ready for operation again and the low pressure gas discharge lamp supply with current or voltage.

For this purpose, the circuit is, for example, from EP 0 239 793 B1 of a free-swinging electronic ballast became known in the case of the faulty, too high lamp voltages leading behavior of a gas discharge lamp about the increased voltage drop across a resonance inductor is captured with the lamp in series lies. The resonance inductance couples with a secondary winding, via a trigger circuit to the control electrode a thyristor is connected. This grounds the base of an inverter transistor in the event of a fault, to make the inverter circuit inactive switch. The thyristor is connected via a resistor combination from the DC link voltage with a holding current provided. After changing the lamp, it is briefly replaced by a commutation capacitor taken over when the Lamp with a connection via a resistor is charged to the DC link voltage. When a lamp is inserted, its filament switches the charged one Connection to ground, the capacitor with its other connection that through the thyristor flowing current takes over briefly, so this goes out and the generator circuit starts again can.

It is a free-floating control circuit.

In order to set the Lamp implemented power or the applied voltage and to be able to make the flowing currents and around Effects of properties of the gas discharge lamp to avoid the operation of the ballast the trend towards externally controlled ballasts with work at a predetermined frequency. Such a ballast is known, for example, from EP 0 727 921 A2. The electronic Ballast contains a generator circuit to generate an AC lamp voltage, the generator circuit Can be stopped via a control input is. A voltage monitor is connected to this, which turns off the generator circuit when a maximum voltage on the lamp is exceeded. More specifically is a corresponding generator circuit from the application notes for the L 6569 from SGS Thomson Microelektronics known. The generator described there has control connections for controlling an inverter half bridge on, and works at an operating voltage of, for example, 15 volts. If this operating voltage is below lowered a predetermined threshold UVLO, the Circuit the connected inverter half bridge. The operating voltage can be reduced via a thyristor take place, then from the DC link voltage continues to be supplied with a holding current.

In order to restart the circuit, this must be disconnected from the mains until the holding current subsided and the thyristor has become free.

Based on this, it is an object of the invention to create an electronically controlled ballast, that switches off when the lamp is defective and after changing the lamp automatically starts again.

This task is accomplished by an electronic ballast solved with the features of claim 1.

The electronic ballast has one of one Control circuit controlled half-bridge on the Output of one or more low pressure gas discharge lamps are connected. If there are several gas discharge lamps these connected in series. The control circuit sets the Frequency at which the inverter half bridge works firmly, so that interference due to tolerances of Lamp parameters on the operating frequency largely excluded or reduced.

The control circuit comes with a supply voltage powered by a controlled switch be reduced to a value with self-holding characteristics which has a UVLO (Undervoltagelockout) threshold falls below. The controlled switch is from a monitoring circuit for the lamp voltage controlled. If the lamp voltage exceeds a permissible one Dimension, this is seen as a sign that the Lamp is defective and the controlled switch lowers the Operating voltage of the control circuit under UVLO.

The second controlled switch, which is also connected to the Supply voltage of the control circuit connected can, if it is activated, lower it even further. To ensure this, the first is controlled Switch a fixed potential offset generating circuit connected in series. This circuit can be, for example, a Z diode, so that when the first switch the supply voltage approximately to the Z voltage collapses. The second controlled switch now lowers the supply voltage when activated below this Z voltage, causing the self-latching switch is de-energized and therefore blocked. The second controlled Switch is activated by a monitoring circuit, which has a sensor circuit that at least leads a lamp filament. Is the spiral broken or the lamp is pulled (i.e. from the electronic ballast disconnected), the second switch is activated the electronic ballast still is stopped. The first self-holding switch, the has overvoltage shutdown, however locked again. If now an intact lamp with the ballast is connected via the heating coil leading current path the second switch locked, and the control circuit receives its full supply voltage again. It now controls the inverter half bridge so that the lamp used is ignited and burns.

The lowering of the supply voltage of the control circuit to a relatively large nonzero Value below the threshold voltage UVLO for inactivation enables the self-holding first switch again to lock without the second controlled switch completely to switch through (very low impedance). It it is sufficient if this only extends the potential somewhat lowers. The corresponding monitoring circuit can therefore relatively high impedance, which has repercussions on the low pressure gas discharge lamp and the required Power loss reduced. Is the first switch controlled by one by two Transistors of different conductivity types are formed Circuit formed as mentioned in claim 2 very low holding currents are made possible, which is the Power loss in one to the DC link voltage leading supply resistance minimized. Also results yourself an inexpensive solution. Can if necessary however, an appropriately selected thyristor is also used become.

The one essentially fixed potential offset generating circuit can be by a zener diode or a other component are formed, which is a comparable Has characteristic. It is sufficient if the generated potential offset is approximately constant, like it is the case, for example, if the dynamic resistance of the Component is not zero but is relatively low.

The second controlled switch is preferably a pnp transistor connected as an emitter follower whose base is connected to ground with high resistance. Besides, is the base is connected to a sensing circuit with high impedance Series resistors over at least one coil of Gas discharge lamp leads. If this sensing circuit is interrupted, the small one above the basic series resistor of the Current flowing to transistor to ground this out to convert its locked state into a state in which he conducts and takes over the holding current of the thyristor. The transistor does not need to be fully switched on or even get saturated. That makes one very low power circuit.

This is particularly the case for two-flame operation the time constant according to claim 17 advantageous. This ensures that in the event of errors, in which one or both gas discharge lamps in rectifier mode pass over, the first switch closes securely without being deleted by the second switch.

Figur 1 ein erfindungsgemäßes elektronisches Vorschaltgerät für Einlampenbetrieb in einer schematischen Darstellung seiner Schaltung, unter Weglassung nicht erfindungswesentlicher Schaltungsteile, und

Figur 2 ein erfindungsgemäßes elektronisches Vorschaltgerät zum Betrieb zweier in Reihe geschalteter Niederdruck-Gasentladungslampen im vereinfachten Prinzipschaltbild ebenfalls unter Weglassung nicht erfindungswesentlicher Schaltungsteile.

Further details of advantageous embodiments of the invention are the subject of subclaims and result from the description of exemplary embodiments and associated drawings. Exemplary embodiments of the invention are shown in the drawing. Show it:

Figure 1 shows an electronic ballast according to the invention for single lamp operation in a schematic representation of its circuit, omitting circuit parts not essential to the invention, and

FIG. 2 shows an electronic ballast according to the invention for operating two low-pressure gas discharge lamps connected in series in a simplified basic circuit diagram, likewise omitting circuit parts that are not essential to the invention.

description

An electronic ballast 1 is shown in FIG shown in the block diagram that the operation of a Low pressure gas discharge lamp 2 is used. The electronic Ballast 1 has a rectifier and wall. circuit 3, which has an intermediate circuit voltage of outputs about 400 volts to ground 4. To generate the required to operate the low-pressure gas discharge lamp 2 symmetrical AC voltage from the DC link voltage, serves an inverter half bridge 6, the in the present example formed by two MOSFETs 7, 8 is. The swap body half bridge 6 is between the intermediate circuit voltage and ground 4 switched.

Serves to control the inverter half bridge 6 a preferably an integrated circuit, such as For example, the drive circuit containing the L 6569 from SGS-Thomson 11, the two with the gates of the MOSFETs 7, 8 connected output terminals 12, 13. The integrated Circuit of the control circuit 11 is with a provided external circuit, not shown, the sets a certain working frequency. This means, that control signals at the output connections 12, 13 in push-pull for the MOSFETs 7, 8 with a given Frequency so that the MOSFETs 7, 8 alternately, however, do not open overlapping or become conductive.

The control circuit 11 has a supply voltage connection V _CC , via which it is provided with supply voltage and at the same time with information as to whether it should control or block the MOSFETs 7, 8: If the supply voltage V _CC controls a defined threshold value UVLI (undervoltage lockin) the control circuit 11 alternately opens and closes the MOSFETs 7, 8 at a frequency which is predetermined by the external circuitry. If the supply voltage V _CC falls below the threshold value UVLO, both MOSFETs 7, 8 are blocked.

The supply voltage is generated when the electronic ballast 1 is running, i.e. the low pressure gas discharge lamp 2 lights up from the of the Inverter half bridge 6 generated square wave voltage. Two capacitors C1 and C2 are used, both with one connection each with a connection point 16 connected to the output of the inverter half bridge 6 forms. The connection point 16 is through the Connection of source and drain formed by the MOSFETs 7, 8. Over with capacitors C1 and C2 in series switched diodes D1, D2 are at the inverter frequency from about 30 kHz charge packets to one against Ground 4 switched smoothing or buffer capacitor C3 pumped from which the supply voltage to the corresponding supply voltage connection of the control circuit 11 is performed. A voltage surge will prevented by a Zener diode DZ1 which is connected to the anode of D1 connected and connected to ground with its own anode is.

To generate the supply voltage for the Enable control circuit 11 even before the inverter half bridge 6 is controlled and inverts, a resistor R1 is provided, which has one end with the DC link voltage and with its other end is connected to the capacitor C3. About the resistance R1, capacitor C3 is charged with a low current, until the voltage on the capacitor C3 the Threshold voltage exceeds UVLI and the control circuit 11 starts.

The one to be operated by the electronic ballast 1 Gas discharge lamp 2 is indirect via a Resonance choke L1 and a coupling capacitor C4 to the Connection point 16 connected to the output of the Inverter half bridge 6 forms and with that of the control circuit 11 predetermined frequency between the DC link voltage and ground switched back and forth becomes. The series connection of the resonance choke L1 and the Coupling capacitor C4 is via a not shown Lamp holder with a connection 21 of the gas discharge lamp 2 connected. The connection leads via a filament 22 lying in the gas discharge lamp 2 into one Connection 23 to the outside, via a resonance capacitor C5 at a further connection 24 of the gas discharge lamp 2 is connected to a coil 25 and via this to a connection 26 which is connected to the DC link voltage is connected.

While the resonance inductor L1 and the resonance capacitor C5 form a series resonance circuit, which is in the undamped Resonance case at the gas discharge lamp 2 a Voltage drops, which the DC link voltage can exceed that the coupling capacitor C4 only the direct current separation of the gas discharge lamp 2 from the inverter half bridge 6, so that the Lamp current does not receive a DC component.

To monitor those falling on the gas discharge lamp 2 Voltage is used by a voltage monitoring circuit 27, which has a high-resistance resistor R2 with the lamp-side end of the resonance choke L1 is connected. The voltage monitoring circuit 27 contains on the input side another input resistor R3, which with the Resistor R2 forms a voltage divider, and the against Ground 4 is switched. The input resistance is R3 followed by a voltage doubler circuit 28 which that outputs a DC voltage signal at its output 29 corresponds to the lamp voltage. The output 29 is with a control input 31 of a first controllable switch 32 connected, connected at one end to ground 4 is. Its other end is via a voltage offset circuit 33 with the supply voltage of the control circuit 11 connected. The switch 32 is replaced by a npn transistor T1 and a pnp transistor T2 formed. The T1 emitter is connected to ground 4 and its collector is connected to the base of T2. The collector of T2 is connected to the base of T1, which is also via a resistor R4 and a capacitor C5 to ground 4 is switched. The base of T2 is through a resistor R5 and a capacitor C6 connected to its emitter. The transistors T1 and T2 form a bistable Circuit that is either a non-conductive state occupies the distance from the emitter of the Transistor T2 blocked to the emitter of transistor T1 is (locked state) or conducts (leading state). Through a The voltage signal at the control input 31 becomes the switch 32 via a Zener diode DZ3 from its blocking state to its Lead state transferred that maintained for so long until an adjustable by the resistors R4, R5 below the holding current. In leading condition the emitter of transistor T2 is almost at ground 4.

The voltage offset circuit 33, which in the simplest case is formed by a Zener diode DZ2, has a voltage drop that is less than the threshold voltage UVLO. The control circuit 11 is thus deactivated when the switch 32 conducts. If the voltage monitoring circuit 27 detects an excessively high voltage on the gas discharge lamp 2, it switches the switch 32 to its conductive state, as a result of which the latter blocks the control circuit 11 by lowering the supply voltage V _CC under UVLO.

In order to enable a restart after changing the lamp, the supply voltage V _{CC is} additionally connected via an optional resistor R7 to a controllable switch 34 which is connected to ground 4. The switch 34 is not a switch in the binary sense, but has a non-conductive state in which the current path from the resistor R7 to ground 4 is blocked, and a further state in which a certain current flow is permitted, the internal resistance of the switch 34 here can still have a relatively large value.

The switch 34 is formed by a circuit, the main part of which is a pnp transistor T3. Its emitter is connected to resistor R7 and its collector is connected to ground 4. Its base is connected to ground via a resistor R8 and a capacitor C7. Resistor R8 forms a base series resistor that sets a base current that is dimensioned such that the resulting emitter current is greater than the current supplied by resistor R1 and received by switch 32. The base of transistor T3 is connected via a protective diode D3 to a current sensing path 35, which contains a resistor R9 and leads to terminal 24. The current sensing path goes from this via the coil 25 to the intermediate circuit voltage. If the current sensing path 35 is interrupted at some point, for example by removing the gas discharge lamp 2 from its socket and thus interrupting the path from the connection 26 to the connection 24, the transistor T3 receives base current via the resistor R8. The transistor T3 becomes conductive to such an extent that it can take over the current supplied by R1 via the resistor R7. If, on the other hand, a gas discharge lamp 2 is inserted into the socket, the potential at the base of transistor T3 increases to such an extent that it supplies a voltage at its emitter that is greater than the supply voltage V _CC , with which switch 34 does not conduct, ie is open .

The electronic ballast described so far 1 works, especially with regard to the restart when changing lamps, as follows:

When the guest discharge lamp 2 is operating correctly, the supply voltage V _CC for the control circuit 11 is at a voltage which exceeds the threshold value UVLO. The inverter half bridge 6 provides an AC voltage with which the low-pressure gas discharge lamp 2 is ignited and operated. Via the resistor R2, the voltage monitoring circuit 27 detects a voltage that is lower than a predetermined maximum value. Consequently, the voltage present at the control input 31 of the switch 32 does not exceed an ignition voltage which is required to switch the switch 32 with a low resistance. However, if the low-pressure gas discharge lamp 2 shows a fault which causes the operating voltage to rise inadmissibly, this is detected by the voltage monitoring circuit 27 and the switch 32 is ignited by a signal at its control input 31. It thus becomes low-resistance and connects the anode of the Zener diode DZ2 to ground 4. The supply voltage V _CC thus drops to a value below the threshold voltage UVLO, with which the control circuit 11 completely blocks the inverter half bridge 6. This state is maintained by the switch 32 being held in place. A corresponding self-holding current is supplied from the DC link voltage via resistor R1.

As long as the defective low-pressure gas discharge lamp 2 is inserted in the socket and thus remains connected to the ballast 1, a small current flows via the filament 25 and the current sensing path 35 to the base of the transistor T3, as a result of which it blocks. However, if the filament 25 has a filament break or the gas discharge lamp 2 is removed from the socket, the current sensing path is interrupted, as a result of which the transistor T3 conducts more or less and further lowers the potential at the supply voltage input of the control circuit 11 below the potential specified by the Zener diode DZ2 . This blocks the switch 32, which no longer receives a holding current. However, the control circuit 11 remains inactive. Only when a gas discharge lamp 2 is connected to the ballast 1 again, that is to say is inserted into the lamp socket in question, is the current sensing path 35 closed and the transistor T3 becomes non-conductive, that is to say the switch 34 opens. The supply voltage V _CC can thus be built up again via the resistor R1, as a result of which the ballast 1 comes into normal operation.

Another embodiment of the invention is illustrated in Figure 2. Without a new description and As far as there is agreement, references become the same Reference numerals are used, the description corresponding applies. The difference from that described above Ballast 1 is only that instead of the low pressure gas discharge lamp 2, two in Series of gas discharge lamps 2a, 2b provided are. Their interconnected coils 22a, 25b via a transformer or transformer M, heated, which is connected in series with the resonance capacitor C5 is. In addition, the current sensing path 35 leads over the winding of the transformer M with the resonance capacitor C5 is connected in series. Around the high-resistance current sensing path to close, the capacitor C5 is a resistor R10 connected in parallel, like all resistors, the higher Voltage differences are exposed in practice formed by a series connection of individual resistors is.

In the case of an externally controlled electronic ballast 1, a control circuit 11, which requires its own supply voltage, is provided to control an inverter half bridge 6. The control circuit 11 is designed such that it blocks the inverter half bridge 6 when the supply voltage V _CC falls below a threshold value UVLO. A voltage monitoring circuit 27 pulls the supply voltage V _CC below the threshold value UVLO via a self-holding controllable electronic switch 32 and a Zener diode DZ2 if an excessively high lamp voltage is detected. The ballast is therefore inactive. A current sensing path 35, which leads over at least one filament of the low-pressure gas discharge lamp 2, is used to detect a lamp change. The current sensing path controls a further electronic switch 34, which is also connected to the supply voltage V _{CC of} the control circuit 11 and, if it conducts, can draw further to ground than the first switch 32. The second electronic switch 34 connected to the current sensing path 35 thus enables the ballast 1 to restart automatically after the lamp has been changed, by initially lowering the supply voltage V _CC below the value specified by the first switch 32 to bring it back into a non-conductive state.

Claims (17)

1. Electronic ballast (1), particularly for operation of low-pressure gas discharge lamps (2),

   with a direct current voltage source (3) which serves for current supply to at least one gas discharge lamp (2) which has two coils (22, 25) as electrodes,

   with at least one half-bridge (6) which is connected to the direct current voltage source (3) and delivers an alternating current voltage at an output terminal (16), its output terminal (16) being connected by way of coupling means (L1, C4) to the at least one gas discharge lamp (2),

   with a control circuit (11) which is provided for the half-bridge (6), is connected by way of control terminals (12, 13) to the half-bridge (6) and controls the latter with a determinable frequency and has a supply voltage input connected to a supply voltage (V_cc),

   wherein when the supply voltage exceeds a threshold value (UVLO) the control circuit (11) goes into an active mode in which it controls the half-bridge (6) with a given frequency, and

   wherein when the supply voltage (V_cc) falls below the threshold value (UVLI) the control circuit (11) goes into a passive mode in which the half-bridge (6) locks,

   with a first controlled switch (32) with self-holding characteristic which is connected to the supply voltage (V_cc) towards earth (4) in order to reduce the supply voltage below the threshold value (UVLO) when it is closed,

   characterised in that

   the first controlled switch (32) is connected in series to a circuit (33) which when the switch (32) is closed produces a substantially fixed potential displacement and is closed by a first monitoring circuit (27) which detects an inadmissible state on the at least one gas discharge lamp (2), so that the supply voltage (V_cc) is lowered below the threshold value (UVLO) but not to zero, and

   that a second controlled switch (34) is connected to the supply voltage (V_cc) towards earth (4) and has a non-conductive state and a state in which it offers at least limited conductivity, the said switch being connected to a second monitoring circuit (35) which detects a current flow through at least one coil (25) of each gas discharge lamp (2) in such a way that the second control switch (34) lowers the supply voltage (V_cc) of the control circuit (11) further than the first switch (32) when no current flow through the coil (25) is detected.
2. Ballast as claimed in Claim 1, characterised in that the first controlled switch (32) is formed by a p-n-p transistor (T2) and a n-p-n transistor (T1), of which the base and collector are connected to one another alternately and the emitters form the outer terminals of the switching path of the switch (32), wherein a base forms a control input (31).
3. Ballast as claimed in Claim 1, characterised in that the circuit (33) which produces a substantially fixed potential displacement is formed by a Z-diode (DZ1).
4. Ballast as claimed in Claim 3, characterised in that the Z-diode (DZ1) has a breakdown voltage which is only slightly lower than the threshold value (UVLO).
5. Ballast as claimed in Claim 1, characterised in that the first controlled switch (32) is a thyristor.
6. Ballast as claimed in Claim 1, characterised in that the second switch (34) is a transistor (T3).
7. Ballast as claimed in Claim 6, characterised in that the second controlled switch (34) is a p-n-p transistor (T3) which is connected as emitter follower, the emitter thereof being connected to the supply voltage (V_cc) and the collector thereof being connected to earth (4).
8. Ballast as claimed in Claim 7, characterised in that the base of the emitter follower (T3) is connected to a high-resistance resistor (R8) towards earth (4).
9. Ballast as claimed in Claim 1, characterised in that for detection of the current flow through at least one coil (25) of the at least one gas discharge lamp (2) the second monitoring circuit (35) is formed by a current path (35) which leads from the direct current voltage source (3) by way of the at least one coil (25) and at least one high-resistance resistor (R9) to the control input of the second switch (34).
10. Ballast as claimed in Claim 8 and 9, characterised in that the at least one high-resistance resistor (R9) is connected to the base of the transistor (T3).
11. Ballast as claimed in Claim 1, characterised in that two gas discharge lamps (2a, 2b) are connected in series between the direct current voltage source (3) and the half-bridge (6).
12. Ballast as claimed in Claim 1, characterised in that the gas discharge lamp (2) is connected to a voltage-magnifying series-resonant circuit (L1, C5).
13. Ballast as claimed in Claim 1, characterised in that the coupling element for connection of the gas discharge lamp to the half-bridge is a coupling capacitor (C4) for suppression of direct current components.
14. Ballast as claimed in Claim 1, characterised in that for monitoring the voltage on the gas discharge lamp (2) the first monitoring circuit (27) has a high-resistance current path (R2) which, starting from one end of a resonant choke (L1) connected with its other end to the half-bridge (6), leads to a resistor (R3) connected towards earth (4) and forms a voltage divider (R2, R3) therewith.
15. Ballast as claimed in Claim 14, characterised in that a rectifying circuit (28) is connected to the voltage divider (R2, R3).
16. Ballast as claimed in Claim 15, characterised in that the rectifying circuit (28) has an output (29) which is connected to the control terminal (31) of the first controllable switch (32).
17. Ballast as claimed in Claim 1, characterised in that the second switch (34) has a switch-on time constant (τ1) which is greater than a switch-on time constant (τ2) of the first switch (27).

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