Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Periodic switch in the primary circuit of the supply...
Reexamination Certificate
2000-01-14
2001-11-06
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Periodic switch in the primary circuit of the supply...
C315S307000
Reexamination Certificate
active
06313585
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The invention relates to a method for dimming a fluorescent lamp arranged in the secondary circuit of a transformer, in which the transformer is driven in a push-pull manner with pulse trains of adjustable duration on the primary side, and also to an arrangement for carrying out the method.
For many applications it is necessary for the brightness of fluorescent lamps to be varied over a large range of adjustment. That is important for example in the context of illuminating display devices in motor vehicles. Such a large range of adjustment is necessary in this case owing to the great fluctuations in the ambient light, in particular between day and night.
Arrangements for operating a fluorescent lamp with controllable luminance are known in which the discharge path of the fluorescent lamp is connected to a controllable AC source via a transformer. The current of the current source is fed to the primary winding of the transformer via two switches operating in a push-pull manner. The switches are in this case driven by a push-pull generator.
Cold-cathode fluorescent lamps have an ignition delay time, that is to say that after the application of an AC voltage to the lamp, a certain period of time lapses before the cold-cathode fluorescent lamp emits light. This ignition delay time is not constant and, moreover, is temperature-dependent. With sine-beat driving of the cold-cathode fluorescent lamp and a lamp that is dimmed to great extent, the fluctuation in the ignition delay time is greater than the period of time in which the lamp emits light. Flickering of the fluorescent lamp occurs as a consequential phenomenon.
In accordance with WO 97/03541, the period of time in which the fluorescent lamp is lit is controlled in order, in this way, to minimize the flickering of the fluorescent lamp.
U.S. Pat. No. 5,311,104 proposes applying an additional ignition pulse, generated by a separate ignition device, to the fluorescent lamp in each case before the beginning of the individual pulse trains.
In accordance with DE 42 22 634 A1, a method for operating a fluorescent lamp is known in which there are generated an AC voltage having a relatively high frequency as operating voltage and a square-wave voltage having a relatively low frequency with an adjustable pulse width for periodically inhibiting and enabling the AC voltage at the fluorescent lamp. In this case, the effect achieved with the aid of a delay device is that the undelayed pulses trigger the ignition pulses, while the delayed pulses lead to the interruption of the electric circuit.
SUMMARY OF THE INVENTION
Consequently, the invention is based on the object of specifying the method which ensures flicker-free light emission over a wide temperature range at the highest possible dimming rate without additional outlay on equipment.
The advantage of the invention is that after the momentary activation of the resonant circuit, the gas in the cold-cathode fluorescent lamp can be ionized and organized and the cold-cathode fluorescent lamp can immediately emit light in a flicker-free manner during subsequent normal operation.
In a refinement, the transformer is driven by individual pulses which are variable in respect of number and/or frequency and/or waveform and also the DC voltage component.
In a development of the method, a drive intermission for the ionization of the fluorescent lamp takes place between the ionization phase and the positive control of the fluorescent lamp.
In an advantageous manner, after the conclusion of the positive control of the fluorescent lamp, energy is drawn from the resonant circuit by the two switches being driven simultaneously. The light emission of the cold-cathode fluorescent lamp terminates immediately.
In one realization of the method according to the invention, an arrangement has a fluorescent lamp in the secondary circuit of the transformer, which fluorescent lamp can be driven via an impedance. The primary circuit of the transformer is connected to a first and a second switch, which switch in a push-pull manner and are driven by a control logic unit.
The switches are advantageously designed as MOS field-effect transistors.
In order to ensure reliable ignition of the cold-cathode fluorescent lamp under all operating conditions, the highest possible ignition voltage is worked with. In order to limit the maximum permissible secondary voltage of the transformer, two reverse-connected series zener diodes are arranged on the primary side.
In a development, a shunt resistor is arranged at the reference-ground point of the MOS field-effect transistors and is connected via a comparator to the control logic unit. Current regulation is thereby realized by superposed pulsing of the field-effect transistors.
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Dinh Trinh Vo
Farber Martin A.
Mannesmann VDO AG
Wong Don
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