Discharge-lamp lighting circuit

Details Discharge-lamp lighting circuit Discharge-lamp lighting circuit

2000-03-09

2001-01-16

Vu, David (Department: 2821)

Electric lamp and discharge devices: systems

With automatic shunt and/or cutout

Supply circuit current and/or potential actuated switch

C315S224000, C315S308000, C315S360000

Reexamination Certificate

active

06175190

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a technique for preventing occurrence of an anomalous state of a discharge-lamp lighting circuit that incorporates DC-AC converting means having a full bridge type circuit structure composed of a plurality of pairs of switching devices. The anomalous state is caused from a passing current which flows when a switching device positioned in a higher stage has been ruptured because of a short 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. The switching power source circuit is formed into a DC-DC converter circuit using a DC chopper type or a flyback type circuit.
The DC-AC converting circuit for converting DC voltage into AC voltage (rectangular waves or the like) uses a known structure of a full bridge circuit having a plurality of pairs of switching devices (or switch devices). The structure causes two pairs of switching devices to be operated contradictorily to obtain alternating output.
FIG. 5
shows an equivalent circuit of an example of a conventional circuit structure. A lighting circuit
31
comprises a switching power source portion including a DC power source
32
, an inductor
33
, a switching device
34
(for example, a FET (Field Effect Transistor) or the like), and a full bridge circuit
36
(operating circuits for the devices are omitted from illustration) including four switching devices
35
-
1
to
35
-
4
(each of which is indicated with the symbol of a switch).
That is, the positive electrode of the DC power source
32
is connected to an end of the inductor
33
. Another end of the inductor
33
is connected to the drain of an N-channel FET which is the switching device
34
. The output voltage from the switching power source portion is supplied to the full bridge circuit
36
. Note that the source of the FET is connected to a negative electrode of the DC power source
32
. A signal transmitted from a control circuit (not shown) is supplied to the gate of the FET.
The switching devices constituting the full bridge circuit
36
are disposed in the higher and lower stages. The switching devices
35
-
1
and
35
-
3
are disposed in the upper stage, while the switching devices
35
-
2
and
35
-
4
are disposed in the lower stage. An end of the switching device
35
-
1
is connected to the inductor
33
, while another end of the switching device
35
-
1
is connected to the switching device
35
-
2
. An end of the switching device
35
-
3
is connected to the inductor
33
, while another end of the switching device
35
-
3
is connected to the switching device
35
-
4
. Note that a discharge lamp g has an end connected to a connection point A between the switching devices
35
-
1
and
35
-
2
. Another end of the discharge lamp
37
is connected to a connection point B between the switching devices
35
-
3
and
35
-
4
.
A portion of the terminals of the switching devices
35
-
2
and
35
-
4
disposed opposite to the connection point of the discharge lamp
37
are connected to the source of the FET through a shunt resistance
38
provided to detect an electric current of the discharge lamp
37
.
In the lighting circuit
31
, the output voltage from the switching power supply portion is controlled in response to a control signal supplied to the switching device
34
so as to be supplied to the full bridge circuit
36
. In the full bridge circuit
36
, combination of the switching devices
35
-
1
and
35
-
4
and that of switching devices
35
-
2
and
35
-
3
are constituted so as to contradictorily be switched. Thus, a rectangular AC output is supplied from each of the connection points A and B to the discharge lamp
37
.
There is a great danger of the conventional circuit producing anomalous heat, smoke or fire if breakdown of the switching device occurs due to a short circuit owing to a passing current generated when either electrode of the discharge lamp encounters a ground fault.
A case will now be described in which a lighting circuit of a discharge lamp for a vehicle encounters a short circuit between either electrode of the discharge lamp and the car body. If short circuit occurs in an output stage of the full bridge circuit
36
as indicated with a two-dot chain line shown in
FIG. 5
, a passing current I undesirably flows through a route as indicated with a dashed line shown in
FIG. 5
in a state where the switching devices
35
-
1
and
35
-
3
are switched on and the switching devices
35
-
2
and
35
-
4
are switched off.
The passing current I causes wasteful consumption of electric currents of the DC power source
32
. What is worse, heat and fire can be produced by the switching devices
35
-
1
to
35
-
4
of the full bridge circuit
36
. Therefore, the passing current I must be prevented.
To prevent generation of the passing current I, the passage for the electric current must be obstructed at the switching devices
35
-
1
and
35
-
3
in the higher stage. Therefore, it might easily be considered to employ a method of switching the switching devices
35
-
1
and
35
-
3
off.
In a state shown in
FIG. 6
, an occurrence of the passing current I can be prevented if the switching devices
35
-
1
and
35
-
3
in the higher stage are switched off and the switching devices
35
-
2
and
35
-
4
in the lower stage are switched on, and the operations of the switching devices
35
-
1
and
35
-
3
are always normal.
However, if the switching device
35
-
3
is ruptured because of a short circuit generated due to some reason, passing current I′ undesirably flows through a route (the positive electrode of the DC power source
32
to the switching device
35
-
3
to the switching device
35
-
4
to the shunt resistance
36
to the negative electrode of the DC power source
32
) indicated with a dashed line shown in FIG.
6
.
To prevent generation of the passing currents I and I′, a method must be employed to switch all of the switching devices
35
-
1
to
35
-
4
off. The conventional structure and control of such an operating circuit (a so-called “bridge driver) for the switching devices required to realize this state become too complicated. It leads to a fact that the cost is raised excessively. Also in the foregoing case, all of the switching devices
35
-
1
to
35
-
4
can be switched off only when each of the switching device is free of any anomaly.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to protect a circuit by preventing occurrence of a passing current caused from anomaly of a switching device of a plurality of pairs of switching devices configured as a full bridge circuit and disposed in a higher stage.
To solve the foregoing problems, according to one aspect of the present invention, there is provided a discharge-lamp lighting circuit incorporating a full bridge circuit having DC-AC converting means arranged to convert DC voltage into AC voltage and having a plurality of pairs of switching devices, the discharge-lamp lighting circuit comprising: anomaly detecting means for detecting an anomalous state of the lighting circuit; current detecting means for detecting whether or not an electric current is flowing in the full bridge circuit after the operation of the lighting circuit has been stopped because of detection of an anomalous state of the lighting circuit by the anomaly detecting means; and, device-state determining means for switching on switching devices of the plural pairs of the switching devices, configured as the full bridge circuit and positioned in a higher stage, and for switching off switching devices which are positioned in a lower stage.
The present invention is structured such that when an electric current which flows in the full bridge circuit is detected as a sign of a passing current, switching devices of the plurality of pairs of switching devices, configured as the full bridge circuit and disposed in the higher stage, are switched

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