FI91120B - Electronic connection device for discharge lamp - Google Patents

Abstract

The invention concerns an electronic connection device for a discharge lamp, comprising a drive circuit supplying a high-frequency current, a choke device Lp/Ls which has its primary winding Lp between the drive circuit and the lamp, and a condenser Cs in parallel with the lamp. The peak value of the drive circuit current is limited to a certain threshold value Ik which is essentially independent of the frequency of the current. The secondary winding Ls of the choke device is connected to the current circuit which has a connection or a connection device S which after the predetermined time T required by the incandescence of the lamp electrodes is put into an open or a current-restricting condition. <IMAGE>

Description

91120

Electronic ballast for ballast lamp. - Electronic copying arrangements for the transfer of securities.

The invention relates to an electronic ballast for a discharge lamp, comprising a driving circuit supplying high frequency current, an impedance next to the lamp, an inductance between the driving circuit and the lamp, and circuit elements controlled by a control circuit, which can influence the frequency of the lamp current.

FI patent publication 63146 discloses an electronic ballast for a discharge lamp, in which the control transformer of the end transistors has a secondary winding, by means of which the operation of the end transistors is influenced so that the current flowing through the lamp can be adjusted. In this case, the control transformer thus has nothing to do with the annealing current passing through the lamp electrodes and the lamp ignition voltage. This known ballast uses a PTC component connected in series with a capacitor adjacent to the lamp, which initially allows the annealing current to flow and then, when heated, limits the current so that the voltage across the lamp terminals increases enough for ignition. However, there are several drawbacks to this arrangement. The time constant of a PTC resistor is vague because it is based on a thermal time constant. It is an aging component with poor reliability compared to other typical ballast components. In lamp re-igniters, the hot PTC resistor does not work, but prevents the ignition current from flowing.

The object of the invention is to provide an improved connection device which ensures the flow of the annealing current and the generation of the ignition voltage without the above-mentioned drawbacks.

This object is achieved on the basis of the features of the invention set out in the appended claim 1.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which Figure 2 shows a lamp circuit according to the invention and Figure 2 shows an example of the voltages (output voltage and ampere voltage) and current of the lamp circuit of Figure 1.

The structure and operation of the lamp circuit is as follows.

The driving circuit supplies a rectangular voltage Uuhta from its 1 output pin, which varies between V + and V-. The primary inductance si Lp of the choke limits the rate of rise of the current li. By short-circuiting the secondary winding Ls, the inductance seen by the current li is reduced and thus the rise rate of Ii is increased.

The output of the driving circuit changes state when li has risen to the threshold value Ik. This threshold does not depend on the rate of rise or frequency of Ii.

When the supply voltage is switched on, the driving circuit starts and the monostable of the control circuit triggers, at the same time closing the switch S. The inductance seen by the current li is small and the rise rate is high. When the driving circuit sets the peak value of the current li to a certain threshold value Ik, the frequency of the current li is high; is marked fi. In this case, a current In / proportional to the current threshold value Ik flows through the capacitor Cs and the voltage U1 across the lamp is small. The current threshold value Ik is selected so that the current In becomes the annealing current required by the lamp.

After a predetermined annealing time T, the monostable opens the switch s, whereby the current li sees the full inductance Lp. The rate of rise of the current l1 decreases and thus the frequency decreases to a value f2 which is substantially lower than the frequency f1, as can be seen from the current curve in Fig. 2. The capacitance of the capacitor C »is selected so that at the frequency f2 the current li raises the voltage Ut over the ignition voltage Ue required by the lamp and the lamp lights up.

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Depending on the type of lamp and the amount of annealing current used, the annealing time T can typically vary between 400 ms and 2 s.

As the switch S, it is possible to use, for example, a triac or two thyristors, which are connected in parallel so that when the thyristors are controlled to the conductive state, the current flows in both directions. The switch S can either completely open or close the circuit associated with the secondary windings Ls, or it can also be a current-limiting switch element which is controllable between the low-ohm and high-resistance modes. In any case, the control of the switching element S causes a rapid step change to the current li frequency and the voltage Un-

The capacitors C1 and C2 in the circuit prevent the passage of direct current in the shooting circuit 1 and keep the cathode connected to its common point at the desired potential.

In normal operation, the voltages Uci and UC2 are equal. Depending on the implementation of the driving circuit, some external means may be required to keep the voltages equal. Several possibilities have been presented in the literature (e.g. K.H.

Billings, Handbook Of Switchmode Power Supplies, 1989 pp. 3.156-3.157). In the circuit, voltage information is applied from the common point of Ci and C2 to the pin of the driving circuit comp. In this case, the driving circuit changes its function so that Uci and UC2 remain equal. This is accomplished by a few positive and negative half-cycle threshold currents Ik. values to slightly different magnitudes so that the potential difference between Uci and UC2 is reduced.

If a switching element S is used which does not need to be separated from the lamp circuit, Lp and Ls can be connected to each other: one can be a part of the other or they can be one and the same winding.

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An effect similar to that of switch S can be obtained with a DC or AC supply member.

The lamp circuit can also be implemented in such a way that the annealing is effected by opening the switch S and the ignition by closing it. In this case, the capacitor Cs must be replaced, for example, by an inductance or a transformer in order to achieve the desired function. It is essential in the invention that by influencing the inductance LP, the frequency of the high-frequency current of the lamp circuit is changed so that the effect of the impedance Cs on the current flowing through it becomes more restrictive of the current flow after the annealing time before ignition has elapsed. The invention thus provides a simple and efficient way to provide an efficient annealing current and lamp ignition, while ensuring that when the lamp burns, only a very small current flows through the impedance next to it, which does not cause significant power losses.

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Claims (7)

An electronic ballast for a discharge lamp, comprising a high-frequency supply circuit, an impedance (Ca) alongside the lamp, an inductance (Lp) between the driving circuit and the lamp, and control circuit-controlled circuit elements (La, S) capable of influencing the lamp current frequency, characterized in (La, S) change the value of the inductance (Lp) at a time determined by the control circuit, when it is possible to change the current through the lamp cathodes and the voltage across the lamp and impedance (C «) and said time of change of inductance (Lp) does not depend on said high frequency current grandeur. Ballast according to claim 1, characterized in that said inductance (Lp) is influenced such that the frequency of said high-frequency current during the pre-ignition annealing time (T) is substantially higher than after ignition when the lamp is lit and said impedance (C «) includes at least one capacitor. Connecting device according to Claim 1 or 2, characterized in that the secondary winding (La) of the inductance (Lp) is connected to a circuit with a controllable switching element (S). Ballast according to Claim 3, characterized in that the switching element (S) is closed or conductive for a predetermined time (T) required for annealing the lamp electrodes, and after said predetermined time it is opened or controlled to a current-limiting state. Ballast according to claim 4, characterized in that said switching element (S) is controlled by a control circuit which controls the current of the switching element to a conductive state when the operating voltage is connected to the lamp circuit, and that the control circuit is mono-stable after a predetermined time (T). to the current limiting state of the switch member. 5,91120 Connecting device according to Claim 1 or 2, characterized in that the inductance (Lp) has a secondary winding (L «) or a part of the primary winding, which is connected to a current-limiting element (S) or a current-supplying element. Ballast according to one of Claims 1 to 6, characterized in that the peak value of the current (Ix) of the driving circuit is limited to a certain threshold value (Ik) which is substantially independent of the frequency of the current. Il 7 91120