Lighting circuit for discharge lamp

Details Lighting circuit for discharge lamp Lighting circuit for discharge lamp

2000-03-16

2002-01-22

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Current and/or voltage regulation

Automatic regulation

C315S224000, C315S225000

Reexamination Certificate

active

06340870

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a circuit for preventing rupture of a device caused by overcurrent, in a lighting circuit for a discharge lamp incorporating a DC-DC converter. The circuit limits an electric current of a switching device of the converter.
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. The switching power source circuit configured as a DC-DC converter circuit is controlled by, for example, a known PWM (Pulse Width Modulation) method.
The conventional method is arranged to control change in the ON/OFF ratio or the duty ratio (the duty cycle) with respect to a semiconductor switching device constituting the converter circuit so that change in the output voltage is permitted. As a reliable countermeasure to prevent rupture because of overcurrent in the switching device in the converter circuit, a so-called pulse-by-pulse current limiting method is known.
FIG. 11
shows an essential portion of an example
71
of a circuit structure incorporating a DC power source
72
, a capacitor
73
, a transformer T (only a primary winding
74
is illustrated), a FET (Field Effect (type) Transistor)
75
serving as a switching device, and a current detecting resistor
76
. That is, the capacitor
73
is disposed in parallel with the DC power source
72
. An end of the primary winding
74
of the transformer T is connected to the positive terminal of the DC power source
72
. Another end of the transformer T is connected to the drain of the FET
75
. The source of the FET
75
is connected to a negative terminal of the DC power source
72
through the current detecting resistor
76
. The gate of the FET
75
is supplied with a control signal from a control circuit (for example, a circuit incorporating a PWM control IC)(not shown). In response to the signal, the ON/OFF operation of the FET
75
is controlled.
FIG. 12
schematically shows waveforms of electric current “I” which flows in the FET
75
and a control signal “SD” for the FET (period “Ton” indicates ON-period of the device and “Toff” indicates OFF-period of the device). The voltage of the current I is converted and detected by the current detecting resistor
76
(that is, when the resistance value of the current detecting resistor
76
is “R”, detected voltage value is “I•R”). The detection signal is transmitted to the control circuit.
The level “Vlim” indicated with a dashed line shown in
FIG. 12
indicates a level (an upper limit) to which the electric current is limited. When the current I reaches this level, the FET is brought to the OFF-state. That is, as the level Vlim is set to a higher level, the length of the OFF-period Toff is shortened. When the level Vlim is set to a lower level, the length of the OFF-period is elongated. As a result of this control, the current value is limited to prevent flow of overcurrent in the switching device (FET). As a result, heating and rupture of the device are prevented.
The circuit having the foregoing current limiting function, however, requires the current detecting resistor
76
to have a relatively large capacitance (for example, a resistor device of a several-W class). Thus, there arises a problem in that a reduction in size and cost of the apparatus cannot be achieved. A similar problem arises when a current transformer for current/voltage converting is employed without use of the current detecting resistor
76
.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to easily realize current limitation of a switching device, in a DC-DC converter of a lighting circuit for a discharge lamp, and to reduce the size and cost of the associated circuit apparatus.
According to the present invention, there is provided a lighting circuit for a discharge lamp comprising: a DC-DC converter for converting DC voltage; and, a control circuit for controlling output voltage from the converter by controlling ON/OFF of a field effect transistor that serves as a switching device in the converter, wherein an electric current which flows in the field effect transistor is monitored by detecting a voltage between a drain and a source of the transistor, the control circuit turning the field effect transistor off when the voltage value is not smaller than a reference value.
The present invention has a structure that provides for monitoring the electric current which flows in the field effect transistor by detecting voltage between the drain and the source. When the voltage value is not smaller than a reference value, the field effect transistor is turned off to limit the ON-period. Thus, rupture of the device because of overcurrent can be prevented. Moreover, a current detecting resistor or the like having a large capacitance, for detecting the electric current w flows in the field effect transistor, is not required.


REFERENCES:
patent: 5142203 (1992-08-01), Oda et al.
patent: 5365152 (1994-11-01), Ozawa et al.
patent: 5434474 (1995-07-01), Ukita et al.
patent: 5449973 (1995-09-01), Yamashita et al.
patent: 5463287 (1995-10-01), Kurihara et al.
patent: 5706185 (1998-01-01), Toyama et al.
patent: 5907224 (1999-05-01), Yamashita et al.
patent: 4-141988 (1992-05-01), None

Affiliated with

Also associated with

Rating

Lighting circuit for discharge lamp does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Lighting circuit for discharge lamp, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Lighting circuit for discharge lamp will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2850325