EP0279073A2 - Circuit arrangement for the high-frequency operation of fluorescent lamps having pre-heating electrodes - Google Patents

Abstract

An intermediate piece connected both to the supply and lamp sides is mechanically compatible with both. An adaptor or ballast device mechanically and electrically connects at least one lamp to a supply in this way. At least connection points comprising connection base and/or connection holder are provided for different holders and bases respectively.

Description

The invention relates to a circuit arrangement for the high-frequency operation of fluorescent lamps with electrodes to be preheated.

It is known that the luminous efficiency of a fluorescent lamp increases with an increasing operating frequency. The higher the frequency, the more the fluorescent lamp tends to be cold, i.e. to ignite if the electrodes have not been preheated or are not sufficiently preheated. Such cold starts significantly reduce the life of the lamp.

Another advantage of higher operating frequencies is that the lower limit of the ambient temperature at which the lamp can still be operated reliably is lower than at a lower operating frequency, for example at 50 Hz. This is for the use of fluorescent lamps in cold rooms and outdoors essential. This advantage is also offset by the greater tendency to cold start at higher frequencies, even when used of special lamps for low temperatures.

The invention seeks to remedy this. The invention, as characterized in claim 1, solves the problem of creating a circuit arrangement for the high-frequency operation of fluorescent lamps with preheated electrodes, with which the tendency to cold starts can be avoided without impairing the luminous efficacy and operability at low ambient temperatures. In the claims 2 to 9 embodiments of the invention are given.

The advantages achieved by the invention are essentially to be seen in the fact that all the advantages of high-frequency operation are achieved without the cold start tendency which shortens the service life.

In the following, the invention is explained in more detail with reference to a drawing which shows only one embodiment. The single figure shows the diagram of a circuit arrangement according to the invention for the high-frequency operation of a fluorescent lamp with electrodes to be preheated.

In the drawing, the circuit of a high-frequency ballast for a fluorescent lamp 1 with electrodes 2 and 3 to be preheated is shown, which is designed for connection to an AC network (220 V). The basic structure of this ballast consists of a rectifier 5 with alternating current connections 6 and 7 for the network and a smoothing capacitor 8 for the rectified alternating voltage which feeds a high-frequency inverter 10. The inverter 10 supplies at two connections 11 and 12 the high-frequency combustion current for the lamp 1, which is in series with the self-induction required for its operation coil 13 is; it contains a current path 15, 16, 17 with connections 18 and 19, which is connected in series with the electrode coils (electrodes 2) instead of the starter, for example glow starter, which is usual when operating at mains frequency. The coil 13 has a magnetic core (ferrite) with an air gap.

The basic principle of the inverter 10 is a "half-bridge current feedback inverter" with a transistor 22 for the half-waves in one direction and a transistor 23 for the half-waves in the other direction of the high-frequency current. It works according to this principle with current feedback. A current transformer 27 with a magnetic core (ferrite) is used for this. The core and the number of turns of the primary winding 24 are dimensioned such that the core is magnetically saturated during part of each high-frequency half-wave. The current transformer 27 has two secondary windings 25 and 26, each of which is connected to a resistor 28 or 29 in the control circuit (base-emitter circuit) of one of the transistors 22 and 23. The windings 25 and 26 are connected so that the control currents are in phase opposition. Inverters of this principle require a surge to start up. Such is generated when the inverter 10 is switched on by the circuit elements 32, 33 and 34. The function of these and the other circuit elements can be seen from the following description of the mode of operation of the inverter 10. For example, data of the circuit elements are given in parentheses.

After the inverter 10 has been switched on, the capacitor 33 (100 nF) is charged via the resistor 32 (470 kohm) and, when the breakdown voltage of the zener diode 34 (30-36 V) is exceeded, partially discharged by a current surge which flows via the base emitter Route of Transistor 23 flows and ends when the reference voltage of the Zener diode 34 is undershot. This current surge produces one of the operational states of the inverter 10, in which one of the transistors 22 and 23 is conductive and the other is non-conductive. The inverter 10 can swing.

When the transistor 22 is conducting, a current charging the capacitor 37 (68 nF) flows through the lamp 1 connected to the connections 11/12 and 18/19, the primary winding 24, the coil 13 (5.5 mH), the transistor 23 and the Resistor 35 (3.9 ohms). When the lamp 1 is lit, this current only flows between the connections 11 and 12, while the lamp electrodes 2 and 3 are preheated, this current flows from the connection 1 through the electrode 2 (heating coil) to the connection 18, through the current path 15, 16, 17 to the connection 19 and from this through the electrode 3 to the connection 12. This will initially be ignored in the following. The mode of operation of the inverter 10 is first described, regardless of whether its load current is the discharge current or the preheating current of the lamp, and only then does the process of preheating the electrodes 2 and 3 and burning the lamp 1 be discussed. The current charging the capacitor 37, flowing through 13, 24, 23 and 35, generates, as it rises, a control current in the secondary winding 26 which keeps the transistor 23 conductive after the end of the current surge triggered to oscillate. As soon as the primary current begins to saturate the magnetic core of the current transformer 27, this control current decreases. Finally, the transistor 23 blocks and interrupts the charging current and thus the primary current of the current converter 27.

During this primary current interruption, a current is induced in the other secondary winding 25, which is caused by The base-emitter path of the transistor 22 flows so that it becomes conductive. Now a current discharging the capacitor 37 flows through the transistor 22, the resistor 40 (3.9 ohms), the primary winding 24, the lamp 1 and the coil 13. The increase in the discharge current now opposite to the previous charging current in the primary winding 24 induces in the secondary winding 25 has a current in the same direction as the current that was previously generated in the same secondary winding 22 when the charging current decreased. Thus, the control current now induced in the secondary winding 25 and flowing through the base-emitter path of the transistor 22 keeps the transistor 22 conductive. As soon as the current discharging the capacitor 37 via 22, 40, 24, 13 and 1 begins to saturate the core of the current transformer 27, the control current induced in the secondary winding 25 which maintains it decreases. Finally, the transistor 22 blocks. The current drop in the primary winding 24 induces a current in the secondary winding 26 through which the transistor 23 becomes conductive. The cycle described begins again.

When transistor 23 or 22 turns off, capacitor 38 causes the current flowing through primary winding 24 in either direction to be maintained for a short time by different charging voltages of capacitors 37 and 38 (3.3 nF) , so that the other transistor 22 or 23 only becomes conductive after this time has elapsed. This prevents both transistors 22 and 23 from conducting simultaneously as a result of different response times, as a result of which the DC supply voltage of the inverter 10, which is connected to the capacitor 8, is short-circuited and the transistors are destroyed. The difference in the charging voltages across the capacitors 37 and 38 is due to this that the capacitor 37 has a much larger capacitance than the capacitor 38, and that the lamp 1, the coil 13 and the primary winding 24 are in its charge or discharge circuit, while the capacitor 38 has a much smaller capacitance through the small resistor 35 and the collector-emitter path of transistor 23, the resistance of which is low in the conductive state, is charged and discharged via the collector-emitter path of transistor 22, the resistance of which is small in the conductive state, and the likewise low resistance 40 is discharged. The time constant of the capacitor 38 and resistor 39 (220 kiloohms) is much larger than the inverter half-period.

The diodes 42 and 43 promote the ignition process, in which - as described below - the ignition voltage is generated at the coil 13, one end of which is connected directly to the lamp electrode 3. The ignition current flows from the other end of the coil through the primary winding 24 and, depending on its direction, through the diode 42 and the capacitor 37 or through the diode 43, the capacitor 8 and the capacitor 37 to the other electrode 2. The diodes 42 and 43 protect the transistors Reverse voltages that also occur when the primary current is interrupted in current transformer 27. The diode 45 prevents the inverter 10 from being triggered again in the vibrating state via the circuit elements 32, 33, 34.

As can be seen, the frequency generated by the inverter 10 (without the lamp 1, 2, 3 and without the current path 15, 16 17) is essentially determined by the saturation behavior of the current transformer 27. The inverter 10 also swings without the lamp 1 and the coil 13 when on a resistor (e.g. 1170 Ohm) is connected to it, with a frequency that is higher (50 kHz) than the frequency when the lamp 1 is operated. As explained below, this frequency is lower when the lamp electrodes 2 and 3 are preheated ( approx. 25 kHz) than when the lamp 1 is on (approx. 35 kHz), so that the cold start tendency of the lamp 1 during the preheating of the electrodes 2 and 3 is eliminated by the lower frequency, and when the lamp burns, the greater light output of the higher frequency comes into its own.

In addition to the coil 13, which is in the usual way in series with the discharge path of the lamp 1, the series connection of the capacitor 15 (3300 pF), the resistor 16 (220 ohm) and the SIDAC 17 in the heating circuit of the electrodes 2 and 3 (130 V) is provided, whose impedance (without the SIDAC 17) is essentially determined by the capacitance of the capacitor 15, which is significantly smaller than that of the capacitor 37. The breakdown voltage of the SIDAC 17 is higher than the operating voltage of the lamp 1 at higher frequency (35 kHz). The capacitance of the capacitor 15 and the inductance of the coil 13 are dimensioned such that an appropriate preheating current flows through the electrodes 2 and 3 at the lower frequency (25 kHz), i.e. before the lamp 1 is ignited, and that the voltage drop across the series circuit 15, 16, 17 is sufficiently below the voltage at which the lamp 1 burns at the higher frequency (35 kHz), and that an optimal discharge current is achieved at the higher frequency with the inductance of the coil 13, the series circuit 15, 16 , 17 is de-energized because the operating voltage of the lamp 1 is less than the breakdown voltage of the SIDAC 17.

When the inverter 10 is switched on with the lamp 1, a high-frequency current flows through 37, 15, 16, 17 and 13, the frequency of which, under the action of the capacitor 15 and the coil 13, is lower (25 kHz) than the frequency (50 kHz) with which the inverter 10 would vibrate without these circuit elements. The high-frequency current consists of successive half-waves, each of which increases very steeply when the breakdown voltage of the SIDAC 17 is reached. These very sudden current increases induce 13 voltage pulses on the coil, the voltage of which leads to the ignition of the lamp 1 as soon as the electrodes 2 and 3 are preheated. When the lamp 1 is then ignited, the voltage across the series circuit 15, 16, 17 drops to the operating voltage of the lamp 1, and thus the voltage across the SIDAC 17 falls below its breakdown voltage. The series circuit 15, 16, 17 is de-energized. Now there is no longer the capacitance of the capacitor 15 but the much smaller capacitance (50 to 100 pF), which the SIDAC 17 represents in the blocking state, in series with the self-induction of the coil 13, because in the series connection of the capacitances of the capacitor 15 and the SIDAC 17 is not considered as the much larger one. Parallel to the capacity of the SIDAC 17 and lamp 1 is the capacity of the lines connected to them. The total capacitance is considerably smaller than that of the capacitor 15. Therefore, the inverter 10 now vibrates at the higher frequency (35 kHz), and the higher luminous efficiency is achieved at the higher frequency.

With the inverter 10 described, several fluorescent lamps can also be fed in parallel, with each lamp being assigned a coil 13 and a heating current path 15, 16, 17.

For spark interference, an interference protection filter for the harmonics of the inverter frequency, for example a so-called “boost converter”, can be connected in front of the inverter 10 or the rectifier 5.

To operate the fluorescent lamp 1 instead of an incandescent lamp, the inverter 10 can be installed together with the rectifier 5, 8 (and, if desired, a noise protection filter) in an intermediate piece which has a base corresponding to the incandescent lamp base, with the connections 6 and 7 to be received by an incandescent lamp holder, and one the fluorescent lamp 1 has an interchangeable socket with contacts for the connections 11, 12, 18 and 19.

Experiments have shown that the life span of the lamp, measured by the number of switch-on processes (starts, ignitions), is approximately 60,000 switch-ons with the circuit arrangement described, whereas it was only approx. 35,000 to 40,000 switch-ons with previous ballasts.

Claims (9)

1. Circuit arrangement with a high-frequency generator (10) for the operation of a fluorescent lamp (1) with electrodes to be preheated (2, 3), characterized in that the frequency of the high-frequency generator (10) during the preheating of the electrodes (2, 3) of the fluorescent lamp ( 1) is lower than when the lamp is on. 2. Arrangement according to claim 1, characterized in that the self-induction of a coil (13) which is in series with the lamp (1) and which limits the preheating current and the combustion current of the lamp also determines the frequency of the current generated by the high-frequency generator (10), and that in the electrode preheating circuit the coil (13) is connected in series with a capacitor (15) and a symmetrical circuit element (17) which becomes current-permeable when a voltage which is higher than the operating voltage of the lamp (1) is exceeded. 3. Arrangement according to claim 2 or 3, characterized in that the coil (13) has a magnetic core and this core has an air gap. 4. Arrangement according to claim 2 or 3, characterized in that the symmetrical circuit element (17) is a SIDAC. 5. Arrangement according to one of claims 2 to 4, characterized in that a resistor (16) is connected in series with the capacitor (15) and the symmetrical circuit element (17). 6. Arrangement according to one of claims 1 to 5, characterized in that the alternating current generator is an inverter (10) with current feedback through a current converter (27) operated up to the magnetic saturation range of its core, the primary winding (24) of which in the lamp (1 ) when burning and when preheating the electrodes (2, 3) supply circuit, and (27) has two secondary windings (25, 26), each of which is in the control circuit one of two controllable semiconductor elements (22, 23), which alternately Conduct current of one and the current of the opposite half-wave of the generated high-frequency current. 7. Arrangement according to one of claims 2 to 6, characterized in that in the fluorescent lamp (1) when preheating its electrodes (2, 3) and when burning high-frequency circuit (15, 16, 17; 1) is a block capacitor (37) whose capacitance is a multiple of the capacitance of the capacitor (15) which is connected in series with the symmetrical circuit element (17). 8. Arrangement according to claim 6 or 7, characterized in that each of the two controllable semiconductor elements (22, 23) is bridged by a diode (42, 43) with opposite polarity to the semiconductor element. 9. Arrangement according to one of claims 1 to 8, characterized in that the high-frequency generator (5, 8, 10) is installed in an intermediate piece which with a instead of an incandescent lamp to be received by a bulb holder, with the power supply connections (6, 7) of High-frequency generator connected base and verse with a lamp (1) interchangeable socket hen, which has a pair of contacts (11, 18; 12, 19) for each of the heatable lamp electrodes (2, 3).

Images