DE9114039U1 - Circuit arrangement for operating one or more low-pressure discharge lamps - Google Patents

Description

Patent Trust Company

for electric light bulbs mbH., Munich

Circuit arrangement for operating one or more low -pressure discharge lamps&eegr;

The invention relates to a circuit arrangement for operating one or more low-pressure discharge lamps according to the preamble of claim 1.

Such a circuit arrangement is disclosed, for example, in US-PS 4,438,372 and in EP-PS 276,460. These circuits have a self-controlled push-pull frequency generator consisting of two switching transistors arranged in a half-bridge and connected to a series resonant circuit. The switching transistors are controlled via the secondary windings of a saturated control transformer, the primary winding of which is arranged in the series resonant circuit and which, in the simplest case, is only connected to a base and an emitter resistor.

With this type of control, a favorable compromise has been found between good switching behavior of the transistors during lamp operation and sufficiently high ignition voltage.

However, the ignition voltage requirement of low-pressure discharge lamps increases at low temperatures, such as those that occur in outdoor applications, so that in the circuits mentioned above, the open circuit voltage of the push-pull frequency generator must be increased in order to ensure reliable ignition of the low-pressure discharge lamps even at low temperatures.

- 2 ensure.

On the other hand, an increase in the no-load voltage of the push-pull frequency generator with the simple control of the transistors in the above circuits also leads to an increase in the power losses in the transistors, so that this simple control has its limits.

It is the object of the invention to provide a circuit arrangement for operating one or more low-pressure discharge lamps, which ensures reliable ignition of the low-pressure discharge lamps with little effort, even at low temperatures, without affecting the burning operation of the low-pressure discharge lamps - that is, the operation after the ignition phase.

This object is achieved according to the invention by the characterizing features of claim 1. Particularly advantageous embodiments of the invention are specified in the dependent claims.

By means of a simple extension of the control for the switching transistors of the push-pull frequency generator, the ignition of low-pressure discharge lamps at low temperatures can be improved without increasing the losses in the transistors during the operation of the lamps. In the preferred embodiment, the bases of the two switching transistors each have only one base series resistor, to which a threshold switch and an ohmic resistor are connected in parallel, so that during the ignition phase of the low-pressure discharge lamps,

these parallel branches can provide an additional base current for the transistors. Due to the additional positive base current for the transistors, the oscillation frequency of the push-pull frequency generator is reduced and comes closer to the resonance frequency of the series resonance circuit. This increases the open circuit voltage on the resonance capacitors and thus the possible ignition voltage on the lamps.

Zener diodes are advantageously used as threshold switches, the breakdown voltage of which is only reached during the ignition phase of the low-pressure discharge lamps, so that the additional base current for the transistors can only flow via the parallel branches to the base series resistors during the ignition of the lamps, but not during burning operation, i.e. after ignition has taken place.

In the preferred embodiment, the parallel branches to the base series resistors contain, in addition to the threshold switch, a diode which prevents a negative base current for the switching transistors from flowing through these branches.

The invention is explained in more detail below using a preferred embodiment. Shown are:

Figure 1 shows the principle of the circuit arrangement according to the invention according to a first embodiment

Example

Figure 2 shows a section of a circuit arrangement according to a second embodiment, which

differs from the first by the part outlined in dashed lines in Figure 1.

Figure 1 illustrates the principle of the circuit arrangement according to the invention. Only the electronic components essential for understanding the invention are shown. The circuit diagram in Figure 1 shows two bipolar transistors T1, T2, each with an emitter resistor R5, R6 and a base series resistor R3, R4. The two alternately switching transistors T1, T2 are connected in a half-bridge and form the push-pull frequency generator. The series resonance circuit, consisting of the primary winding RK1a of a toroidal transformer RK1, the coupling capacitor C10, the resonance inductance LD1, the resonance capacitor C17 and the low-pressure discharge lamp LP1, is connected to the center tap M1 between the transistors T1, T2. The alternately switching transistors T1, T2 are controlled via the secondary windings RK1b, RK1c of the toroidal transformer RK1, which are each integrated in a base circuit. In addition, the base circuit of the transistor T2 is connected to a start circuit Z, as described, for example, in the book "Electronic Circuits" by W. Hirschmann (Siemens AG), p. 147 148.

The center tap M1 is alternately at high and low potential, depending on which switching transistor is currently conducting. This creates a high-frequency alternating current in the series resonant circuit. The size of the open circuit voltage available at the resonant capacitor C17 and thus also the level of the ignition voltage for the lamp LP1 depends on the detuning

the switching frequency of the transistors T1, T2 against the resonance frequency of the series resonance circuit. The circuit arrangement according to the invention is dimensioned so that a sufficient no-load voltage is available at the resonance capacitor C17 in order to ensure, on the one hand, reliable ignition of the lamp LP1 at room temperature and, on the other hand, to keep the losses in the switching transistors T1, T2 as low as possible.
10

At low temperatures, the ignition voltage of the lamp LP1 increases, so that a higher no-load voltage must be provided at the resonance capacitor C17 during the ignition phase. This is achieved by means of the parallel branches to the base series resistors R3 and R4, which here each consist of a Zener diode D17 or D19 and an ohmic resistor Rl1 or R12.

In idle mode, i.e. when the lamp LP1 has not yet been ignited, the toroidal transformer RK1 provides an increased control voltage for the transistors T1, T2, which is sufficient to make the Zener diodes D17 and D19 conductive. This causes an additional (positive) base current to flow for the transistors T1, T2, which increases their storage time and thus reduces the switching frequency of the push-pull frequency generator or the half-bridge, so that the switching frequency and the resonance frequency are closer together, which results in an increase in the idle voltage at the resonance capacitor C17.

When the lamp LP1 is burning, i.e. after ignition, the parallel branches remain connected to the Zener

diodes D17 and D19 are ineffective because the base circuits are dimensioned in such a way that the control voltage from the toroidal transformer RK1 is not sufficient to make the Zener diodes D17 and D18 conductive when the lamp LP1 is burning.

After the lamp has been ignited, the operating state optimized with regard to the power losses in the switching transistors T1, T2 is restored.

A rectifier diode D18 or D20 is also integrated in both parallel branches, which prevent a negative base current for the transistors T1, T2 through these parallel branches.

This circuit arrangement is also suitable for operating two or more low-pressure discharge lamps connected in parallel. Each additional low-pressure discharge lamp is then in its own series resonance circuit, which is connected to a center tap between the coupling capacitor C10 and the resonance inductance LD1 and is connected in parallel to the lamp LP1 and the inductance LD1. Each additional series resonance circuit contains a low-pressure discharge lamp, a resonance inductance and a resonance capacitance (not shown).

Figure 2 shows a section of the principle of the circuit arrangement according to the invention in accordance with a second embodiment. This differs from the first embodiment shown in Figure 1 only by the part of the circuit framed with dashed lines, and only this part

- 7 circuit part is therefore shown in Figure 2.

In the second embodiment, both transistors T1, T2 each have two base resistors R3 ¹ , RII* or R4', R12' connected in series, with a Zener diode D17' or D19 ¹ being arranged in parallel to the resistor R11' or R12 ¹ . The functional principle of both embodiments is the same.

The invention is not limited to the embodiments explained above. It is, for example, possible to expand the circuit arrangements of the two embodiments to include a high-frequency or harmonic filter etc. without thereby affecting the circuit part essential to the invention.

Claims (6)

Protection claims . Circuit arrangement for operating one or more low-pressure discharge lamps, consisting of - a self-controlled push-pull frequency generator containing at least two controllable electronic switches (T1, T2), - at least one series resistor (R3, R4, R3 ¹ , R4') for the control electrode of each electronic switch (T1, T2) of the push-pull frequency generator, - a series resonance circuit, - a control circuit with several inductances (RK1b, RK1c) that enable the self-control of the push-pull frequency generator by means of feedback to the series resonance circuit, - connections for at least one low-pressure discharge lamp (LP1), characterized in that for each controllable electronic switch (T1, T2) at least one threshold switch (D17, D19, D17', D19') is connected in parallel to a series resistor (R3, R4, RTI', R12 ¹ ) of its control electrode, so that an additional control current can be provided for the electronic switches (T1, T2). 2. Circuit arrangement according to claim 1, characterized in that each controllable electronic switch (T1, T2) of the push-pull frequency generator has exactly one series resistor (R3, R4) for its control electrode, to which a threshold switch (D17, D19) and an ohmic resistor (R11, R12) are connected in parallel. 3. Circuit arrangement according to claims 1 or 2, characterized in that a rectifier diode (D18, D20, D18 ¹ , D20 ¹ ) is connected in series with the threshold switches (D17, D19, D17 ^! , D19 ¹ ). 4. Circuit arrangement according to claims 1 or 2, characterized in that the threshold switches (D17, D19, D17 ¹ , D19 ¹ ) are Zener diodes. 5. Circuit arrangement according to claim 1, characterized in that the push-pull frequency generator contains two controllable electronic switches (T1, T2) which are arranged in a half-bridge circuit. 6. Circuit arrangement according to claim 5, characterized in that the controllable electronic switches (T1, T2) of the push-pull frequency generator are bipolar transistors.