WO2000065886A1 - Electronic ballast - Google Patents

Abstract

The invention relates to an electronic ballast with pre-heating function for operating low pressure discharge lamps. Known electronic ballasts have a current inverter that provides at least one load circuit. Said load circuit comprises a discharge lamp and serial resonant circuit whose capacitor is connected parallel to the discharge lamp. The discharge lamp has heatable electrodes and a transformer whose primary winding is connected in series to the capacitor and whose two secondary windings are both connected in parallel to the electrodes. Said transformer is designed in such a manner that it quickly reaches the state of saturation once it is switched on.

Description

Electronic ballast

The invention relates to an electronic ballast according to the preamble of the main claim.

Electronic ballasts for operating low-pressure discharge lamps usually have a preheating of the electrodes, a current-controlled preheating and a voltage-controlled preheating being known. In the case of current-controlled preheating, the heating filaments of the discharge lamp are operated in series with the lamp parallel capacitor. A preheating is possible due to a frequency shift, the preheating frequency being higher than the operating frequency and the current falling with increasing frequency. The level of preheating, ie the preheating current, is determined by the size of the capacitor, the preheating frequency and the maximum preheating voltage on the lamp. Depending on the type of lamp, however, large operating and a very low starting voltage. In addition, a capacitor assigned to each lamp type must be used, so that the ballast must always be adapted to the lamp type used.

In the case of voltage-controlled preheating, a voltage is applied to the heating coils, for example, by means of extra heating transformers or heating windings on the output choke. Preheating by frequency shift is also possible here. An advantage of this circuit is that the lamp parallel capacitor can be chosen relatively freely, but a decisive disadvantage is that at first it is a relatively large one with cold heating coils

Heating current flows and therefore further measures for better heating current setting are necessary. An ECG with such a preheating is known from EP 0 535 911 AI and EP 0 769 889 AI, the heating current being limited by a capacitive load or a capacitive and ohmic load.

The invention is therefore based on the object of providing an electronic ballast with preheating in which the operating losses due to the preheating are kept low and in which an excessive inrush current is avoided.

According to the invention, this object is achieved by the characterizing features of the main claim. Due to the use of a transformer, which quickly saturates when switched on and in which the primary winding is connected in series to the lamp parallel capacitor and in which a secondary winding is parallel to the respective heating coil, practically both previously known preheating t linked together. In the first switch-on moment, the current-controlled preheating prevails in order to switch to a voltage-controlled preheating within a very short time. As a result, there is no first greatly increased inrush current, so that even sensitive heating coils can be controlled without much additional effort. In addition, a so-called universal electronic ballast can be provided, since the lamp parallel capacitor can be chosen relatively freely and thus only one capacitor can be used for all or more types of lamps.

Since only one capacitor is required, which takes on both the function of supplying the ignition voltage and the function of supplying the preheating energy limited by the toroidal transformer, components can be saved, i.e. the amount of components compared to the prior art is much lower.

An embodiment of the invention is shown in the drawing and is explained in more detail in the following description. The single figure shows the circuit configuration of a load circuit supplied by an inverter.

The circuit for electronic ballasts shown in the figure has a standard half-bridge circuit as the output circuit of the ballast, which is represented by the electronic switches T1 and T2 and by the two half-bridge capacitors C _H ι and C _H2 . The half-bridge circuit is connected to the supply voltage + Ut _> . A choke L1 is connected to the connection point between the two electronic switches T1, T2, which connects one heating filament 2 of the discharge lamp 1 to one connection connected is. The other heating coil 3 is connected with a connection to the connection point of the two half-bridge capacitors C _H ι, C _H2 . Usually, however, one or the other capacitor of the half-bridge can also be omitted.

The capacitor which is usually parallel to the lamp 1 is denoted by Ci, the capacitor being connected to the primary winding 4 of a transformer 2 in such a way that the series circuit comprising the capacitor CI and the primary winding 4 is connected in parallel to the discharge lamp 1. The capacitor CI is dimensioned so that it does not reach the ignition voltage at the preheating frequency, which is, however, supplied by the frequency shift. In addition to the primary winding 4, the transformer 2 has two secondary windings 5, 6, all windings being arranged on a toroidal core, so that the transformer is designed as a toroidal core transformer. In series with the respective secondary winding 5, 6 is a coupling capacitor Ci and C _k2 , the coupling capacitor and secondary winding being arranged in parallel with the respective heating coil ₂ , 3. The coupling capacitors are used for DC decoupling. The toroidal transformer 2 is characterized in that it immediately saturates when there is a surge above a limit, since there is no air gap.

In exemplary embodiments not shown, the lamp parallel circuit, that is to say a capacitor with a heating transformer, can be placed directly against + U _b or ground (GN). A connection to the capacitive half bridge is not absolutely necessary.

At the moment of switching on, after a very short time (a few microseconds) generates a high-frequency AC voltage, and at a higher preheating frequency, a current flows through the output resonant circuit consisting of reactor L1 and capacitor CI. The voltage drop across the capacitor determines the preheating voltage. The current flowing in the primary winding 4 induces, via the toroid in the secondary windings, the preheating currents for the heating coils 2, 3, which are limited by the saturation phenomenon in the toroid, since the toroid when the ballast starts up, at which a high current flows , immediately went into saturation. At the transition to normal frequency, the voltage across the capacitor CI increases due to the onset of resonance. The current is also increasing, but the current is limited

Toroidal core here also the heating currents. As soon as the ignition voltage of the illuminant or the discharge lamp 1 is reached, the lamp ignites and the voltage breaks down to the operating voltage of the lamp. At the operating frequency, the reactive current through the capacitor Ci is much smaller, as a result of which the heating current also becomes smaller and the losses in the heating coils 2, 3 are reduced.

The circuit provides a preheating combined from a current-controlled preheating and a voltage-controlled preheating, with the current-controlled preheating predominating in the first switch-on moment, which merges into a voltage-controlled preheating within a very short time.

Claims

claims 1. Electronic ballast for operating low-pressure discharge lamps with an inverter which supplies at least one load circuit, the load circuit having a discharge lamp, a choke connected in series therewith, and a capacitor lying parallel to the discharge lamp and supplying the ignition voltage for the discharge lamp, and wherein the discharge lamp has electrodes which can be heated via a heating transformer, characterized in that the primary winding (4) of the transformer (1) is in series with the capacitor (Ci) and this series connection is parallel to the discharge lamp, and a secondary winding in parallel with the electrodes ( 5, 6) are connected, the transformer (2) being designed such that it quickly saturates when switched on. 2. Electronic ballast according to claim 1, characterized in that the secondary windings (5, 6) are each in series with a coupling capacitor (C _k ι, C _k2 ). 3. Electronic ballast according to claim 1 or claim 2, characterized in that the transformer (2) is designed as a toroidal transformer.