Discharge lamp lighting circuit

1. Field of the Invention

The present invention relates to a lighting circuit for supplying an AC voltage to a discharge lamp by converting a DC voltage from a DC power source circuit. The circuit has a current detection means to detect a current flowing to the discharge lamp in the DC power source circuit.

2. Related Art

A lighting circuit for a discharge lamp, having a DC power source circuit, a DC-AC conversion circuit, and a starter circuit, is known.

For example, a lighting circuit has a DC-DC converter in the DC power source circuit, and a DC-AC converter containing a driver circuit and a full bridge circuit having two pairs of semiconductor switch elements for conducting switching control. The output voltage of the DC-DC converter is converted into a rectangular wave-shaped voltage by the full bridge circuit and supplied to the discharge lamp.

To detect a current flowing to the discharge lamp, a shunt resistance is inserted between the discharge lamp and the ground, and a signal after voltage conversion is detected.

However, the above configuration is subject to a surge voltage and may face safety problems.

When a discharge lamp is lighted, a surge voltage may be generated by a distributed or stray capacitor. For a mobile lighting circuit, a high voltage pulse of several kV's to tens of kV's generated by a starter circuit is sent to the discharge lamp through a high voltage cord. A shield is wrapped around the cord to suppress electromagnetic interferences brought on by the generation of radio frequency noise. The stray or distributed capacity are also charged between the high voltage cord and the car body or the shield, both at ground. Thus, when the discharge lamp breaks down and conducts after receiving the high voltage pulse, the electrical charge accumulated in the stray capacity or the distributed capacity discharges through the shunt resistance. Consequently, the shunt resistance or elements of the current detection circuit provided downstream to the resistance can be damaged.

Further, if the wiring between the shunt resistance and the discharge lamp short circuits by contacting, for example, the car body, the current stops flowing through the shunt resistance and flows out through the car body (ground potential). Since no current is flowing to the discharge lamp, the control circuit ramps the power supply to provide more current to the discharge lamp. A fail-safe circuit then detects the above condition to stop the operation. Circuits such as a fail-safe circuit increase costs, size, and the number of parts of the lighting circuit.

According to the present invention, a discharge lamp lighting circuit comprises a DC power source circuit to output a DC voltage, a DC-AC conversion circuit for converting the DC voltage into an AC voltage to be supplied to a discharge lamp, a current detection circuit to detect a current flowing to the discharge lamp, and a control circuit to control the output voltage of the DC power source circuit based on a status detection signal of the discharge lamp including a detection signal from the current detection circuit. The present invention can have the following structures, (A) to (C).

(A) A positive polarity voltage and a negative polarity voltage respectively from two output terminals of the DC power source circuit are sent to the DC-AC conversion circuit. A plurality of switch elements provided in the DC-AC conversion circuit for switching these output voltages are alternately operated by their driving circuits to generate AC voltage, which is supplied to the discharge lamp.

(B) One electrode of the discharge lamp is connected to one of the output terminals of the DC-AC conversion circuit through an inductive load, and the other electrode is connected to the ground.

(C) A current detection element to detect a current flowing to the discharge lamp is provided in the DC power source circuit, and a signal from the current detection element is received by the current detection circuit.

According to the present invention, the current detection element for detecting a current flowing to the discharge lamp is provided in the DC power source circuit and not connected to the discharge lamp. For example, the element may be inserted between the output terminal and the ground. Thus, problems associated with the surge voltage generated when the discharge lamp is lighted can be averted. Also, a protection circuit for protecting circuits is not needed because the current can be detected by the current detection element even if the current leaks to the car body.

FIG. 1 is an exemplary circuit block diagram of a discharge lamp lighting circuit according to the present invention.

FIG. 2 is an exemplary circuit diagram of a DC power source circuit according to the present invention.

FIG. 3 is an exemplary circuit diagram of a current detection circuit according to the present invention.

FIG. 4 is an exemplary lighting circuit for two discharge lamps according to the present invention.

FIG. 5 is another exemplary circuit diagram of a DC power source circuit according to the present invention.

FIG. 6 is another exemplary circuit diagram of a current detection circuit according to the present invention.

FIG. 1 shows an exemplary discharge lamp lighting circuit for one discharge lamp according to the present invention.

The discharge lamp lighting circuit 1 has a power source 2 , a DC power source circuit 3 , a DC-AC conversion circuit 4 , and a starter circuit 5 , and controls the lighting of a discharge lamp 6 (metal halide lamp, or the like).

The DC power source circuit 3 receives a DC input voltage Vin from the power source 2 and generates a desired DC output voltage, which is variably controlled corresponding to a control signal from a control circuit 7 . A switching regulator is used in a DC-DC converter in the DC power source circuit 3 . The DC-DC converter can be a chopper or fly back type. A DC-DC converter 3 A generates a positive polarity output (positive potential to the ground), and a DC-DC converter 3 B generates a negative polarity output (negative potential to the ground). That is, the present circuit generates a bipolar output voltage of positive and negative polarity. The positive polarity and negative polarity voltages separately generated from two output terminals are sent to the DC-AC conversion circuit 4 .

FIG. 2 shows an exemplary DC power source circuit 3 . One end of a primary coil Tp of a transformer T is connected to a DC input terminal ta, where the voltage Vin is applied. The other end of the primary coil Tp is grounded through a semiconductor switch element SW and a current detection resistance Rs. In FIG. 2, the switch SW shown by a switch sign can be a field effect transistor (FET). A signal Sc from a control circuit 7 is supplied to a control terminal of the semiconductor switch element SW (if an FET, to a gate) to control the element SW.

One end of a secondary coil Ts of the transformer T is connected to the anode of a diode D 1 . The cathode of the diode D 1 is connected to one end of a capacitor C 1 . The other end of the capacitor C 1 is connected to an intermediate tap of the secondary coil Ts, and is grounded through a current detection element Ri to obtain a detection signal for the current flowing to the discharge lamp. A positive polarity output voltage Vdcp is taken out from a terminal to 1 at the node of the capacitor C 1 and the diode D 1 .

Further, the other end of the secondary coil Ts is connected to the cathode of a diode D 2 . The anode of the diode D 2 is connected to one end of a capacitor C 2 , and connected to the terminal to 2 . The negative polarity output voltage Vdcn is obtained through the terminal to 2 . The other end of the capacitor C 2 is grounded through the current detection element Ri.

As stated before, the DC power source circuit 3 has a converter 3 A including the transformer T, the diode D 1 , and the capacitor C 1 , and the converter 3 B including the transformer T, the diode D 2 , and capacitor C 2 , and the positive polarity and the negative polarity voltage Vdcp, Vdcn are respectively generated from two output terminals to 1 and to 2 . The current corresponding to the current flowing to the discharge lamp can be detected by the voltage conversion from the current flowing through the current detection element Ri, and from the detection terminal toi connected to the node of the capacitors C 1 and C 2 . A detection signal Vi is thus obtained.

A mark “.” on the transformer T indicates a start point of the coil. For example, in the secondary coil Ts, the mark “.” indicates the start point of the coil at the end close to the diode D 2 . Another mark indicate the start point at the intermediate tap.

In FIG. 2, as the current detection element, the resistive element Ri is inserted between the output terminal of the DC power source circuit 3 and the ground The capacitors C 1 and C 2 are also inserted. However, the present invention is not limited to this configuration. A Hall element or a MR (magnetic resistance) can be used to detect the current flowing to the discharge lamp. However, in terms of costs and size, a resistive element may be most practical.

The positive and negative polarity voltages from the DC power source circuit 3 are sent to the DC-AC conversion circuit 4 . As shown in FIG. 1, a pair of semiconductor switch elements sw 1 and sw 2 , which can be FETs, switches the output voltages Vdcp and Vdcn of the DC power source circuit 3 . An AC voltage generated when each element is alternately operated by the drive circuit DVR, is supplied to the discharge lamp 6 through the inductive load of the starter circuit 5 .

That is, sw 1 is connected to the output terminal of the converter 3 A and connected to the output terminal of the converter 3 B through sw 2 . For the drive circuit DRV to reciprocally control these switching elements, an integrated circuit (IC) is used as a half bridge driver. The alternate operation of the half bridge is conducted so that, based on the signal respectively supplied from the drive circuit DRV to the control terminal of each switch element, when the element sw 1 is on, the element sw 2 is turned off, and reversibly, when element sw 1 is off, the element sw 2 is turned on. In this way, a DC voltage is converted into an AC voltage. The drive circuit DRV is operated according to the negative polarity voltage Vdcn. Accordingly, the power source voltage for the drive circuit DRV is necessary. Further, also for the control signal sent from the control circuit 7 to the drive circuit DRV, the same consideration is necessary.

The starter circuit 5 generates a high voltage signal for initiating the lighting of the discharge lamp. The high voltage signal is superimposed on the AC voltage Vout generated from the DC-AC conversion circuit 4 and applied to the discharge lamp 6 . For example, consider a circuit with a capacitor or a switch element such as a self-yielding switch element or a thyristor controlled by external signals in the primary circuit of the transformer, which includes a magnetic substance (core) and a primary coil and a secondary coil. If the discharge lamp 6 is started and when the voltage across the both terminals of the capacitor exceeds its threshold capacitive charge, or subsequently, when a trigger signal is received, the switch element conducts and the current flows to the primary coil of the transformer. Thus, the high voltage signal is applied to the discharge lamp through the secondary coil.

In FIG. 1, the discharge lamp 6 is connected to a node A of the switch elements sw 1 and sw 2 of the DC-AC conversion circuit 4 through the inductive load, which is the secondary coil of the trigger transformer or the inductor in the starter circuit 5 . The other side of the discharge lamp 6 is connected to the ground.

The current detection circuit 8 is provided to obtain the detection signal for the current flowing to the discharge lamp 6 . A signal Vi from the current detection element Ri is received.

FIG. 3 shows a non-reversal amplifier circuit and a reversal amplifier circuit provided in parallel with each other. The voltage drop is due to the current detection resistance Ri. The output voltage is selected from one or the other circuit.

An operational amplifier OP 1 is in the non-reversal amplifier circuit. The non-reversal input terminal is connected to the detection terminal toi (a node between the current detection resistance Ri and the smoothing capacitors C 1 and C 2 ) through the resistance R 1 a . The cathode of a diode D 1 a is connected to the non-reversal input terminal of the operational amplifier OP 1 , and its anode is grounded. The diode D 1 a and the diode D 2 a protect the operational amplifier when the input voltage to the operational amplifier is reversed to a negative value.

The output terminal of the operational amplifier OP 1 is connected to the anode of the diode D 1 b , and the cathode of the diode D 1 b is connected to the current detection output terminal tDET and grounded through the resistance R 2 c . Then, the reversal input terminal of the operational amplifier OP 1 is grounded through the resistance R 1 b and connected to the cathode of the diode D 1 b through the resistance R 1 c . The resistance values of the resistances R 1 a , R 1 b , and R 1 c are the same.

The operational amplifier OP 2 is in the reversal amplifier circuit. The reversal input terminal is connected to the detection terminal toi through the resistance R 2 a . The cathode of a diode D 2 a is connected to the reversal input terminal of the operational amplifier OP 2 , and its anode is grounded.

The output terminal of the operational amplifier OP 2 is connected to the anode of the diode D 2 b , and the cathode of the diode D 2 b is connected to the current detection output terminal tDET and grounded through the resistance R 2 c . The reversal input terminal of the operational amplifier OP 2 is connected to the cathode of the diode D 2 b through the resistance R 2 b , and non-reversal input terminal of the operational amplifier OP 2 is grounded. The value of R 2 is set to two times the value of the resistance R 2 a.

The value of voltage drop in the current detection resistance Ri is amplified by a factor of two by the operational amplifier OP 1 in the non-reversal amplifier circuit. On the other hand, it is negatively amplified by a factor of two by the operational amplifier OP 2 in the reversal amplifier circuit. The higher voltage of the two is selected by the diodes D 1 b and D 2 b provided at the output terminal of each operational amplifier, and the selected voltage is taken out from the current detection output terminal tDET. That is, for a positive supply voltage to the discharge lamp, an output voltage is obtained from the operational amplifier OP 1 at the output terminal tDET. For a negative supply voltage to the discharge lamp, an output voltage is obtained from the operational amplifier OP 2 at the output terminal tDET. The detection voltage obtained as described above is used to determine whether the discharge lamp 6 is turned on or to discriminate the status of the discharge lamp 6 and to regulate the electric power supply.

The control circuit 7 in FIG. 1 controls the output voltage of the DC power source circuit 3 in correspondence with the status detection signal of the discharge lamp 6 , including the detection signal from the current detection circuit 8 . For example, the detection signal according to the tube voltage of the discharge lamp or its corresponding signal Vdcp or Vdcn, other than the detection signal according to the tube current of the discharge lamp, are entered into the control circuit 7 as a status detection signal according to the lighting status of the discharge lamp 6 . For example, after the electric power exceeding the rated electric power is supplied for accelerating the emission during the initial lighting of the discharge lamp, the electric power supply is controlled based on the detection signal to decrease gradually. The system is then controlled to reach smoothly a constant rated electric power. A known circuit structure adapted for the pulse width modulation (PWM) control method can be used for the present circuit.

The current detection element, which can be a resistive element, to detect the current flowing to the discharge lamp can be inserted between the output terminal of the DC power source circuit 3 and the ground. Because the current flowing through the current detection element is detected by the current detection circuit 8 , the electrical charge accumulated in the stray or distributed capacity does not flow through the current detection element even if the discharge lamp breaks down. That is, the accumulated electrical charge directly flows from the discharge lamp to the ground. Further, even if current leaks to the car body, the lighting circuit is not subject to an erroneous signal by the current detection. Further, because no special protection circuit is needed, the size of the circuit can be reduced.

FIG. 4 shows an exemplary circuit for two discharge lamps 6 - 1 , and 6 - 2 turned on by a common lighting circuit 1 A.

A DC power source circuit 3 ′ comprises a DC-DC converter 3 ′A for a positive polarity output and a DC-DC converter 3 ′B for a negative polarity output.

FIG. 5 illustrates an exemplary DC power source circuit 3 ′, which has two transformers T 1 and T 2 . The transformer T 1 has a primary coil T 1 p and a secondary coil T 1 s . The transformer T 2 has a primary coil T 2 p and a secondary coil T 2 s.

Each terminal of the primary coils T 1 p and T 2 p is connected to the DC input terminal ta, and each opposite terminal of T 1 p and T 2 p is grounded through semiconductor switch elements SW 1 and SW 2 , respectively. The switch elements SW 1 and SW 2 , which can be separately turned on or off, are controlled by the control signals Sc 1 and Sc 2 . Thus, each of the secondary outputs can be independently and variably controlled.

A capacitor C 0 provided in parallel with the primary coils T 1 p and T 2 p is connected to the DC input terminal ta, and the opposite end of the capacitor C 0 is grounded.

The DC-DC converter 3 ′A includes the transformer T 1 and switch element SW 1 , the rectifying diode D 1 connected to the secondary coil T 1 s and smoothing capacitor C 1 , and a current detection resistance Ri 1 . One end of the secondary coil T 1 s is connected to the anode of the diode D 1 . The cathode of the diode is connected to the output terminal to 1 and connected to one end of the capacitor C 1 . The other end of the capacitor C 1 is connected to the secondary coil T 1 s and is grounded through the current detection resistance Ri 1 .

The current flowing to the primary coil T 1 p of the transformer T 1 is controlled by the on/off control of the switch element SW 1 according to the control signal Sc 1 , and the positive polarity voltage Vdcp is obtained at the output terminal to 1 from the secondary coil T 1 s through the diode D 1 and capacitor C 1 . A terminal toil is a current detection terminal connected to a node of the capacitor C 1 and the current detection resistance Ri 1 .

On the other hand, the DC-DC converter 3 ′B includes the transformer T 2 and switch element SW 2 , and rectifying diode D 2 connected to the secondary coil T 2 s and smoothing capacitor C 2 , and the current detection resistance Ri 2 . That is, one end of the secondary coil T 2 s is connected to the cathode of the diode D 2 . The anode of the diode D 2 is connected to the output terminal to 2 and connected to one end of the capacitor C 2 . The other end of the capacitor C 2 is connected to the secondary coil T 2 s and is grounded through the current detection resistance Ri 2 .

The current flowing to the primary coil T 2 p of the transformer T 2 is controlled by the on/off control of the switch element SW 2 according to the control signal Sc 2 , and the voltage Vdcn is obtained at the output terminal to 2 from the secondary coil T 2 s through the diode D 2 and capacitor C 2 . A terminal toi 2 is a current detection terminal connected to a node of the capacitor C 2 and the current detection resistance Ri 2 .

As shown in FIG. 4, the DC-AC conversion circuit 4 A with a full bridge type circuit comprises four semiconductor switch elements sw 1 , sw 2 , sw 3 , and sw 4 .

The switch elements sw 1 and sw 2 are serially connected to each other. One end of sw 1 is connected to the output terminal of the DC-DC converter 3 ′A, and the other end is connected to the output terminal of the DC-DC converter 3 ′B through the switch element sw 2 . The first discharge lamp 6 - 1 is connected to a node α of both switch elements through the starter circuit 5 - 1 , that is through the inductive load in the circuit 5 - 1 .

The switch elements sw 3 and sw 4 are serially connected to each other. One end of sw 3 is connected to the output terminal of the DC-DC converter 3 ′A, and the other end is connected to the output terminal of the DC-DC converter 3 ′B through the switch element sw 4 . The second discharge lamp 6 - 2 is connected to a node β of both switch elements through the starter circuit 5 - 2 , that is through the inductive load of the circuit 5 - 2 .

Each of the electrode terminals of discharge lamps 6 - 1 and 6 - 2 that is not connected to either the node α or β is grounded.

An integrated circuit (IC) for a half bridge driver is used for drive circuits DRV 1 and DRV 2 . The polarity of the bridge is regulated by receiving the signal CK from the control circuit 7 A and a logical NOT signal CK-bar, respectively. These signals can be easily obtained by dividing the rectangular wave-shaped clock signal generated from the signal generation circuit, which generates a reference signal, by using a flip-flop or counter circuit.

The drive circuit DRV 1 in the DC-AC conversion circuit 4 A controls the on/off control of the switch elements sw 1 and sw 2 . The drive circuit DRV 2 controls the on/off control of the switch elements sw 3 and sw 4 . When the switch element sw 1 is turned on and the switch element sw 2 is turned off by the drive circuit DRV 1 , the switch element sw 3 is turned off and the switch element sw 4 is turned on by the drive circuit DRV 2 . Further, when the switch element sw 1 is turned off and the switch element sw 2 is turned on status by the drive circuit DRV 1 , the switch element sw 3 is turned on and the switch element sw 4 is turned off by the drive circuit DRV 2 . In this manner, the switch elements sw 1 and sw 4 are in the same status and the switch elements sw 2 and sw 3 are in the same status. These switches are reciprocally and alternately operated.

Accordingly, by the on/off operation of two pairs of the switch elements, while the positive polarity voltage is supplied to the first discharge lamp 6 - 1 , the negative polarity voltage is supplied to the second discharge lamp 6 - 2 . Conversely, while the negative polarity voltage is supplied to the first discharge lamp 6 - 1 , the positive polarity voltage is supplied to the second discharge lamp 6 - 2 .

FIG. 6 shows an exemplary current detection circuit 8 A comprising a detection circuit 8 A 1 , which receives a signal Vi 1 , and a detection circuit 8 A 2 which receives a signal Vi 2 . The output of each circuit is switched corresponding to the signal CK and its inverse signal. Compared to having a pair of the detection circuit shown in FIG. 3, the number of elements would be less.

In the detection circuit 8 A 2 , the detection voltage Vi 2 is obtained from the detection terminal toi 2 as a terminal voltage of the current detection resistance Ri 2 provided in the DC-DC converter 3 ′B. The detection voltage Vi 2 is supplied to the non-inverse input terminal of the operational amplifier OP 3 . The inverse input terminal of the operational amplifier OP 3 is grounded through the resistance R 3 a , which is inserted between the output terminal and the inverse input terminal.

Two analog switches A-SW 1 and A-SW 3 , which can be FETs are provided in the output voltage of the operational amplifier OP 3 . When the switch A-SW 1 receives the signal CK and turns on, the output voltage is drawn from the current detection terminal t 1 for the discharge lamp 6 - 1 . When the switch A-SW 3 is turned on by the inverse signal CK-bar, the output voltage is drawn from the current detection terminal t 2 for the discharge lamp 6 - 2 .

For the detection circuit 8 A 1 , the detection voltage is obtained from a detection terminal toil as a terminal voltage of the current detection resistance Ri 1 provided in the DC-DC converter 3 ′A. The detection voltage is supplied to the inverse input terminal of the operational amplifier OP 4 through the resistance R 4 a . The non-inverse input terminal of the operational amplifier OP 4 is grounded. The resistance R 4 b is inserted between the output terminal and the inverse input terminal of the operational amplifier OP 4 .

Two analog switches A-SW 2 , and A-SW 4 are provided for the output voltage of the operational amplifier OP 4 . When the analog switch A-SW 2 receives the inverse signal of the signal CK and turns on, the output voltage is drawn from the current detection terminal t 1 for the discharge lamp 6 - 1 . When the analog switch A-SW 4 turns on by receiving the signal CK, the output voltage is drawn from the current detection terminal t 2 for the discharge lamp 6 - 2 .

As described above, the reason why the detection terminals can be separated for each discharge lamp is that a positive polarity voltage can be supplied to one discharge lamp while a negative polarity voltage can be supplied to another discharge lamp. For example, in FIG. 4, when the signal CK is at a low level, the switch element sw 1 is on and the switch sw 2 is off, and the positive polarity voltage is supplied to the discharge lamp 6 - 1 . In this case, the switch element sw 3 is off and the switch element sw 4 is on. Because the analog switch A-SW 2 is on and the analog switch A-SW 1 is off, the detection voltage (positive voltage) detected from the current detection resistance Ri 1 is drawn from the operational amplifier OP 4 to the detection terminal t 1 . And because the analog switch A-SW 3 is on and the analog switch A-SW 4 is off, the output voltage (positive voltage) amplified by the operational amplifier OP 3 to the detection voltage by the current detection resistance Ri 2 is drawn from the detection terminal t 2 .

The signal from the detection terminal is, for example, sent to the control circuit 7 A. Control signals Sc 1 and Sc 2 are generated to control the output voltage for each of DC-DC converters 3 ′A and 3 ′B. That is, the output voltage of each converter in FIG. 5 is controlled when the switch element respectively connected to the primary coil of each transformer receives the control signal for on/off-control.

For the control circuit 7 A, for part other than using the above detection signal according to a tube current of the discharge lamp, a circuit comprising operational amplifiers such as in JP-A-4-141988, incorporated herein by reference, may be used to control the electric power supply to the discharge lamp according to the detection signal of a tube voltage of the discharge lamp or according to the detection signal obtained by dividing the output voltage Vdcp or Vdcn of each DC-DC converter by a resistance.

After an execessive power is supplied to the discharge lamp during the initial lighting period, exceeding the rating according to the control curve of the tube voltage—tube current characteristic diagram of the discharge lamp, the supplied power is controlled by the circuit 7 A to decrease gradually and to reach a constant rated electric power.

According to the present invention, the current detection element for detecting the current flowing to the discharge lamp is not connected to the discharge lamp but provided in the DC power source circuit. Thus, break-down of circuit elements because of a surge voltage generated during the discharge lamp lighting can be prevented. Also, the property of filter elements provided for reducing the effect of the surge can be downgraded. Further, since current does not leak through the detection element to give incorrect results, a protection circuit not necessary. Thus, the number of parts or costs are reduced. Further, the size of the apparatus can be reduced.

A less complicated detection circuit can be provided if a resistive element is used as a current detection element and is inserted between the output terminal of a DC power source circuit and the ground.

The lighting circuit structure can be simplified by having a detection circuit to detect a current flowing to a first current detection element and a detection circuit to detect a current flowing to a second current detection element to detect currents for each of discharge lamps separately.