FI63148C - ELEKTRONISKT FOERKOPPLINGSDON FOER URLADDNINGSLAMPA - Google Patents

Description

1 63148

Electronic ballast for ballast lamp. - Electronic communications for the purpose of the law.

The invention relates to an electronic ballast for a discharge lamp, comprising an oscillator to be connected to a direct current source, having two transistors connected in series between the terminals of a direct current voltage source, a transformer having a primary winding and two secondary windings, one terminal of the primary winding being connected to provide their opposite phase control voltages, an induction coil connected in series with the primary winding and a lamp, and one or more capacitors in the latter series circuit between the power supply and the lamp, the inductance of said primary winding

One characteristic of transistors is that when the transistor is switched on, the rise time of the current is faster than the falling time when the transistor goes into a non-conducting state. This phenomenon is hereinafter referred to as the switching delay difference.

It is due to this switching delay difference that in the above-mentioned connection, in which the transistors have control voltages in opposite phases and at the same time changing direction, simultaneous operation of both transistors takes place. That is, a transistor whose base control is cut is still conducting when another transistor is controlled to conduct.

A ballast of the above-mentioned type is known from U.S. Pat. No. 4,075,476, in which the problem caused by the switching delay difference is avoided by a switching arrangement in which sufficient filter circuit provides sufficient compensation for the switching delay of the transistors.

The object of the invention is to provide a sufficient limitation of said switching delay difference with a considerably simpler and cheaper circuit solution than in the above-mentioned prior art.

2 63148 In order to achieve this object, the connection device according to the invention is characterized in that diodes are connected in series with the switching transistors, in order to reduce the time during which both transistors conduct simultaneously.

Thanks to the switching arrangement according to the present invention, the rise current of the transistor starts later, only when the forward voltage of the diode and the counter-voltage of the base-emitter junction are exceeded.

In the following, an embodiment of the invention will be described with reference to the accompanying drawing, which shows a circuit diagram of a ballast according to the invention, which has been simplified for the sake of clarity to include only the most essential components.

Two transistors 1, 2 connected in series are connected between the terminals of a rectifier current source, which may be formed by a rectifier connected to the AC power supply. the second pole is connected to the induction coil 7, which in turn is connected to the second electrode of the lamp 8. The secondary windings 5 and 6 of the transformer 3 are connected to the bases of the transistors 1 and 2 so as to receive control phases in opposite phases. Then, when the second transistor is conductive, the second transistor is non-conductive and vice versa.

The second terminal of the lamp 8 is connected by capacitors 10 and 11 to opposite terminals of the power supply. In addition, alongside the lamp 8, in series with the filaments of the electrodes, there is a capacitor 9 which is considerably smaller in size than the capacitors 10 and 11 and which, at a high oscillation frequency, then produces a sufficiently high voltage to light the lamp 8. When the lamp is lit, current flows through it at a frequency determined by the inductance formed by the primary winding 4 and the induction coil 7 on the one hand and the capacitance of the capacitor 10/11 on the other hand. When the transistors 1 and 2 are alternately conductive, it is observed that the current flows alternately in opposite directions through the lamp 8 at a frequency determined by said components.

An essential problem with the operation of the circuit described above is that when the transistor is switched on, the current rise time is faster than the fall time when the transistor is driven to a non-conducting state. This switching delay difference, which still increases as a function of temperature, causes the transistors to be conducted simultaneously. In the invention, the pass voltage of the diodes 12 and 13 connected in series with the transistors causes that when the base control voltage in the transformer repeats changes direction at a limited speed determined by the stray variables of the transformer, the voltage change at the secondary winding terminals becomes greater than the diode output voltages. At the same time, this larger voltage change means a longer time before current begins to flow in the base circuit of the transistor driven into the conductive state. As a result, the common conduction time and current of the transistors is substantially reduced. It should be noted, however, that in practice most of the switching delay difference is controlled by the phase shift between the base and collector current generated in the transformer. The loss power due to the switching delay difference can thus be avoided and practically completely eliminated with a substantially simpler circuit solution.

Claims (1)

An electronic ballast for a discharge lamp, comprising an oscillator for connection to a direct current source and two transistors (1, 2) connected in series between the terminals of a direct current supply, a transformer (3) with a primary winding (4) and two secondary windings (5, 6) , wherein the second pole of the primary winding (4) is connected between the transistors (1, 2) and the secondary windings (5, 6) are connected to the bases of the transistors (1, 2) to provide their opposite phase control voltages, the induction winding (7) connected in series with the primary winding (4) and a lamp (8), and one or more capacitors (10, 11) in the latter series circuit (4, 7, 8) between the power supply and the lamp (8), wherein when the lamp (8) burns said primary winding (4) and induction winding (7) ) inductance on the one hand and the capacitance of said capacitor (10, 11) on the other hand determine the oscillation frequency of the oscillator, characterized in that diodes (12, 13) are connected in series with the switching transistors (1, 2), reduces the time at which both transistors conduct simultaneously.