Discharge-lamp lighting circuit

Details Discharge-lamp lighting circuit Discharge-lamp lighting circuit

2000-03-09

2001-03-27

Vu, David (Department: 2821)

Electric lamp and discharge devices: systems

Current and/or voltage regulation

Automatic regulation

C315S219000, C315S224000

Reexamination Certificate

active

06208089

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a technique for controlling ON/OFF of switching devices constituting a DC—DC converter of a discharge-lamp lighting circuit.
BACKGROUND OF THE INVENTION
A lighting circuit for a discharge lamp, such as a metal halide lamp, is known which incorporates a DC power source, a switching power source circuit, a DC-AC converting circuit and a start circuit. As a control method for a switching power source circuit formed into a DC—DC converter circuit, a PWM (Pulse Width Modulation) method is known. The method is structured such that the frequency (a so-called switching frequency) of a control signal for a switching device in a converter circuit is made to be constant. Moreover, the ON/OFF ratio (duty ratio or duty cycle) is varied so that the output voltage is changed. For example, control is performed such that when the ON period of the switching device is elongated, the OFF period can be shortened.
When the conventional lighting circuit encounters a nonuniform OFF period of the switching device with respect to change in the switching frequency, there arises a problem in that the efficiency of using the power source deteriorates owing to change in the power supply voltage of the lighting circuit. Another problem arises in that the stability of lighting of the discharge lamp cannot easily be maintained with respect to change in the status (lowering of the tube voltage) of the discharge lamp (for example, the discharge lamp is undesirably turned off).
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to realize stable lighting of a discharge lamp and improve lighting efficiency by realizing a lighting circuit of the discharge lamp on which an influence of change in the input/output condition (change in the power source or a state of a load) is not easily exerted.
To solve the foregoing problems, according to one aspect of the present invention, there is provided a discharge-lamp lighting circuit comprising: a switching power source portion having a DC—DC converter structure to convert DC voltage; and, a control circuit for controlling output voltage by controlling ON/OFF of switching devices constituting the switching power source portion, wherein the control circuit controls the ratio of an ON period and an OFF period of the switching device by changing the length of the ON period such that the length of the OFF period is made to be constant.
According to the present invention, control of switching of the switching device is performed such that the length of the ON period is changed on the assumption that the length of the OFF period is made to be constant. Therefore, an influence on the lighting characteristic and efficiency of the discharge lamp, caused from change in the OFF period, can be eliminated.


REFERENCES:
patent: 5142203 (1992-08-01), Oda et al.
patent: 5449973 (1995-09-01), Yamashita et al.
patent: 5608294 (1997-03-01), Derra et al.
patent: 4-141988 (1992-05-01), None

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