EP0797377A1 - Improved half bridge drive for discharge lamps - Google Patents

Abstract

The driver utilises a method of half-bridge control stabilisation and a safety switch-off circuit for driving a fluorescent lamp with protection against externally coupled noise. High frequency noise voltages coupled into the circuit are tapped by at least a secondary winding (RKc) of an inductance with a primary winding (RKa) in the lamp's current path (La) and are rectified. The rectified voltage charges a capacitance (C5) and optimally ensures switch-off element (Q7) operation via the safety switch-off circuit (SAK).

Description

The present invention relates to a method and a circuit for stabilizing the known half-bridge control in electronic circuits for operating fluorescent lamps. These fluorescent lamps are operated by so-called electronic ballasts, which are electronic ballasts, which are arranged between the mains and the lamp. It is a circuit with active and passive components, which take care of the ignition, the current limitation, the operation, the safety shutdown and the shutdown in general etc. and work in the high frequency range.

In slim luminaires, made up of several fluorescent lamps arranged in the form of a rod, these electronic circuits are connected to the lamps to be supplied via long supply lines. Insofar as simple unshielded cables for this purpose, usually also bundled past several lamps for the operation of several lamps, the high-frequency operation causes very disadvantageous coupling-in of undesired high-frequency fields with one another. Particularly large voltage amplitudes from undesirably coupled high-frequency interference occur when igniting or switching off adjacent lamp rods. However, the step-by-step switching on and off of fluorescent lamps combined in a lighting system is often desirable in order to adapt the lighting requirements. The high-frequency energy interspersed by adjacent switching operations is so large that the correct function of circuit parts for monitoring the operation can be disturbed in the worst case.

The compact arrangement of electronic ballast circuits on small circuit boards with the smallest components in narrow luminaire housings leads to the advantageous tight mounting of the circuit boards, for example in a stack, provided that sufficient cooling is available. The disadvantage of the tight and compact arrangement of the circuits lies in the risk of mutual interference by coupling processes. These are mainly caused by the strong stray fields emanating from the lamp chokes in the individual circuits for each lamp. The field strength of the stray field that occurs outside a series circuit board, in particular in the case of lamp chokes with a continuous external air gap, is at a maximum during an ignition process in accordance with the current flow through the choke. This can lead to the fact that an individual lamp of the luminaire that is not switched on, but that is even removed from the mains by a safety shutdown, begins to ignite unintentionally.

In order to overcome the problem, attempts have been made to use lamp chokes with an internal air gap on a specifically constructed circuit. In fact, this leads to a two to four times smaller external stray field. However, this is eliminated with the great disadvantage of greater heating up to overheating of the internal turns of the chokes in the same design.

It would also be possible to shield individual lines, in particular feed lines to individual lamps. However, all traces of the electronic circuit cannot easily be shielded and in any case the assembly effort becomes greater, energy losses occur and the larger cable diameters of shielded lines lead to higher weights and lead to space problems.

There is therefore the task of finding a circuit solution for stabilizing the half-bridge control. According to the invention the problem is solved in that of the anyway existing half-bridge control circuits and safety shutdown circuits in the electronic ballast circuits. The half-bridge circuits are connected to one another by an induction element with a common core, of which the primary winding in the lamp current path and two secondary windings each lie in a control circuit for the switching elements of the respective half-bridge. The induced interference voltage is derived from at least one of the secondary windings of an inductor located in the lamp current path and rectified via a diode. This high rectified current, which depends on the captured disturbance in intensity, is both supplied to a capacitance and also used to supply a safety shutdown circuit, which thereby controls a shutdown element. This ensures reliable operation of the circuit arrangement affected by the interference, which is dependent on the intensity of the interference radiation.

Further embodiments of the invention result from the subclaims.

Further details, features and advantages of the invention will become apparent from the following description of an embodiment shown in the accompanying drawings.

Figur 1

eine schematische Schaltung des neuen elektronischen Vorschaltgerätes,

Figuren 2 und 3

verschiedene gemessene Kurven der Störspannungswirkungen unter verschiedenen Betriebsbedingungen.

Show it:

Figure 1

a schematic circuit of the new electronic ballast,

Figures 2 and 3

different measured curves of the effects of interference voltage under different operating conditions.

The circuit shown only schematically in FIG. 1 shows partly as a block diagram, partly in one detailed linking of components that can actually be used as an example, the part of the present invention that is essential according to the invention. Here, a power supply for supplying the circuit and the lamp L is omitted and the conventional circuits of a safety shutdown SAK and an electronic control circuit VS for preheating the filament are only shown as a block.

The voltages + UB1 and + UB2 are available from the power supply (not shown). The lamp L receives an ignition and operating voltage, which is obtained from + UB2 against ground. A lamp current path L _a leads from a half-bridge center M to the hot end H of the lamp L and this to the circuit ground point Mp.

The high-frequency supply of the lamp L takes place via two half-bridge branches. One half-bridge, here called "upper" half-bridge, receives its supply voltage + UB2 from the power supply unit, not shown, and is constructed in a conventional manner with a bipolar npn transistor Q1. The series resistors R ₄ , R ₇ and R ₈ are low-resistance ME sheet resistors.

In the lamp current path L _{a there} is a primary winding RK _{a of} a toroidal core choke starting from the half-bridge center M in series with a lamp choke LD, followed by a capacitor C ₉ and the lamp L. In the base circuit of the transistor Q1 there is a first secondary winding RK _b . An identical second secondary winding RK _a is described in more detail below, these windings having the ratio a: b: c = 14: 2: 2 in the present example.

The second half-bridge, here called "lower" half-bridge, has a transistor Q2 of the same type as transistor Q1 and is fed via the half-bridge or switching center M. The safety shutdown circuit with a transistor Q5 in the present example is from + UB1 via a Resistor R ₁ (for example 1MΩ) and a capacitance C ₅ essential to the invention (for example with 680 nF) are supplied. The resistor R _1, a Zener diode Z ₄ and a diode D ₉ lie one after the other in the path between the supply UB1 and the half-bridge center.

To initiate the oscillation process, the lower half-bridge branch is triggered via a diode with a symmetrical, non-controllable breakdown behavior, a DIAC DI. Here, the base circuit is supplied by transistor Q ₂ via the DIAC DI in series with Zener diode Z ₄ from capacitance C ₅ and voltage source UB ₁ in series with resistor R ₁ . At a point P of a base control circuit for the transistor Q ₂ , which consists of the winding RK _{c of} the toroidal choke and resistors R ₅ R ₆ and R ₉ , a diode D ₁₂ connects as a component essential to the invention, which is connected to the safety shutdown circuit SAK and the capacitance C. _{5 is} connected.

The two half-bridge transistors Q1 and Q2 each receive a diode D ₄ or D _{5 connected in} antiparallel to enable the choke circuit to run freely in the lamp circuit La. The safety shutdown circuit SAK is connected via a resistor R ₂₃ and a shutdown transistor Q7 to the base circuit of the transistor Q2. Finally, there is a connection between the hot end H of the lamp L and the safety shutdown circuit. A resonance capacitance C _{6 is} arranged parallel to the lamp L and on the inside thereof.

The essential for the operation of the circuit described above is that the lower half-bridge transistor Q2 is controlled via the winding RK _{c of} the toroidal choke. Interference from the primary winding a in the lamp circuit causes an interference voltage to occur on the winding c on the secondary side. This voltage affects the circuit components that drive the turn-off transistor Q7.

The additionally inserted into the circuit according to the invention In its scope of operation, diode D ₁₂ rectifies the voltage induced on the winding RK _c on the secondary side and feeds an additional current into the capacitance C ₅ and thus contributes to the additional supply to the circuit components of the safety shutdown.

In normal operating mode, with the lamp switched on, the voltage set by the zener diode Z ₄ and diode D ₉ at the capacitance C _{5 is} greater than that of the rectified induced voltage in the winding RK _c for controlling the transistor Q2. Thus diode D ₁₂ blocks and the control of the "lower" half-bridge is not affected.

High-frequency interference voltages coupled in from the outside are strongly attenuated due to the low lamp internal resistance in normal operation in the system and therefore have only a minor influence on the system of electronic circuit and lamp, which could interfere with operation.

In the event of a safety shutdown, the shutdown transistor Q7 is controlled by the safety shutdown circuit SAK. The result is a low resistance connection between the base of transistor Q2 and the ground. The thereby removed control of the "lower" half-bridge transistor Q2 allows the oscillation process to end. In this process, the capacitance C ₅ discharges to about 1.5 volts.

The high-frequency interference coupled in from the outside via supply lines or adjacent lamp chokes cause an interference voltage induced on the winding RK _c . This interference voltage is present on the diode D ₁₂ . In the event of significant disturbances, the voltage rectified by diode D _{12 will be} greater than the voltage normally present at capacitor D ₅ . The diode D ₁₂ conducts and supplies an additional current to supply the components of the safety shutdown circuit SAK. As a result, the shutdown transistor Q7 receives a stronger base current control from the safety shutdown circuit SAK and thus has a stronger effect damping the control of transistor Q2. The greater the energy coupling due to a fault, the more energy is fed into the shutdown, the more the transistor Q7 is driven. The high-frequency energy coupled into the lamp circuit from outside is diverted by diode D ₁₂ regardless of the type and strength of the coupling and regardless of whether it comes from one or more neighboring sources of interference and is used to supply the circuit components of the safety shutdown circuit SAK.

FIGS. 2 and 3 show the voltage profiles measured at the capacitance C ₅ on a test setup of two adjacent electronic ballasts. The voltage curve according to FIG. 2a shows the high-frequency coupling via the lamp choke LD into a switched-off second electronic ballast EVG2 not constructed according to the invention by an adjacent electronic ballast EVG1 which is in the “preheating / ignition” operating state. The upper curve is the measured ignition voltage curve U _v in the device EVG1 that is just switched on. The middle curve shows the corresponding preheating current I _v in the EVG1 device. The lower curve shows that the disturbance resulting from this process without the circuit according to the invention in the device EVG2 has a peak at the capacitance C ₅ which is greater than the above-mentioned voltage of U _c5 = 1.5 volts with the result that the Safety shutdown is not effective and the lamp on the EVG2 device has started up again unintentionally.

The voltage curve according to FIG. 2b shows the high-frequency coupling via the lamp choke LD into a switched-off second electronic ballast EVG2 constructed according to the invention by an adjacent electronic ballast EVG1 which is in the “preheating / ignition” operating state. Here the safety shutdown according to the invention remains effective with the same interference voltage. The voltage at C ₅ remains lower.

The voltage curve according to FIG. 3a shows the high-frequency coupling via the lamp choke LD into a switched-off second electronic precharger EVG2 not constructed according to the invention by an adjacent electronic precharger EVG1, which is in the “ignition without preheating” operating state. In this example, the preheating cannot take place, for example, because a heating coil is broken. Without the circuit according to the invention, this malfunction also triggers an excessive rise in the voltage across the capacitance C ₅ due to the safety shutdown becoming ineffective.

The voltage curve according to FIG. 3b shows the high-frequency coupling via the lamp choke LD into a switched-off second electronic ballast EVG2 constructed according to the invention by an adjacent electronic ballast EVG1 which, as before, is in the “ignition without lamp” operating state. Here, the circuit according to the invention has no influence on the operating state of the safety shutdown without consequences and the excessive voltage increase in the capacitance C ₅ .

Claims (8)

Method for stabilizing a half-bridge control and a safety shutdown circuit (SAK) for the operation of fluorescent lamps against external coupling, characterized in that the coupled-in high-frequency interference voltages are tapped off at least by a secondary winding (RK _c ) of an inductance with its primary winding (RK _a ) in the lamp current path (L _a ) and rectified, this rectified voltage simultaneously filling up a capacitance (C ₅ ) and optimally ensuring the actuation of a shutdown element (Q7) via the safety shutdown circuit (SAK). Electronic circuit arrangement (EVG) for operating fluorescent lamps (L) from the network with a safety shutdown circuit (SAK) and control via two half-bridge branches, which are coupled via an inductance to a lamp current path (L _a ), of which a primary winding (RK _a ) is arranged in the lamp current path (L _a ) and one secondary winding (RK _{a, b} ) in the corresponding half-bridge branch, characterized in that at least one secondary winding (RK _c ) via a diode (D ₁₂ ) simultaneously with a capacitance (C ₅ ) and over the safety shutdown circuit is connected to a shutdown element (Q7). Circuit arrangement according to Claim 1 or 2, characterized in that the inductance lying in the lamp current path (La) is a lamp inductor (LD) with two auxiliary windings (b, c). Circuit arrangement according to Claims 1 or 2, characterized in that the inductance in the lamp current path (La) is a current transformer (Rk _a ) and a lamp choke (LD) connected in series. Circuit arrangement according to claim 4, characterized in that the current transformer (RK) has a ring core with three windings (a, b, c). Circuit arrangement according to Claims 1 or 2, characterized in that the switch-off element (Q ₇ ) is a transistor. Circuit arrangement according to Claims 1 or 2, characterized in that the switch-off element (Q ₇ ) is an integrated semiconductor switch. Circuit according to one or more of the preceding claims, characterized in that the secondary windings (SK _{b, c} ) are arranged in the respective control circuit of the transistors in the half-bridge branch and the diode (D ₁₂ ) for rectifying the interfering voltage at the end of a secondary winding (RK _c ) , which faces the control electrode of the associated transistor (Q2), connects to the safety shutdown circuit (SAK) and the capacitance (C5).

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