JP5580677B2 - Discharge lamp lighting circuit - Google Patents

Description

  The present invention relates to a discharge lamp lighting circuit for driving a discharge lamp.

  In recent years, metal halide lamps (hereinafter referred to as discharge lamps) have been used as vehicle lamps such as headlamps in place of conventional halogen lamps having a filament. Although a discharge lamp can achieve luminous efficiency and a long life compared to a halogen lamp, it requires several tens to several hundreds V as a driving voltage. An electric lighting circuit (also called ballast) is required.

  The discharge lamp lighting circuit boosts the battery voltage and supplies it to the discharge lamp. Patent Document 1 discloses a DC / DC converter that boosts a battery voltage, an inverter circuit that converts an output voltage of the DC / DC converter into an alternating current, a control circuit that controls these circuits, and a high-voltage pulse for starting a discharge lamp. An igniter circuit for generating a discharge lamp lighting device is disclosed.

JP 2003-31385 A

  A starter circuit such as an igniter circuit described in Patent Document 1 uses a step-up operation of a DC / DC converter to charge a starter capacitor, and discharges energy charged in the starter capacitor to a high voltage transformer at a time. Generate a voltage pulse. Here, the length of the period from the start of charging the starter capacitor to the generation of the high voltage pulse is called a pulse interval.

  When the discharge lamp is not lit by the first high voltage pulse, the starter capacitor is recharged in the starter circuit, and a high voltage pulse is generated again. On the other hand, since the DC / DC converter is driven with substantially no load before the discharge lamp is turned on, the output voltage immediately reaches the upper limit value and is stabilized near the upper limit value. Therefore, there is a possibility that a difference occurs between the pulse interval when the output voltage of the DC / DC converter is in the rising process and the pulse interval after being stabilized. In particular, the pulse interval corresponding to the second and subsequent high voltage pulses may be longer than the pulse interval corresponding to the first high voltage pulse. If the pulse interval becomes long in this way, the lighting performance may be adversely affected.

  The present invention has been made in view of such circumstances, and an object thereof is to provide a discharge lamp lighting circuit having improved lighting performance.

  One embodiment of the present invention relates to a discharge lamp lighting circuit. In this discharge lamp lighting circuit, when the DC / DC converter that generates a driving voltage to be applied to the discharge lamp to be driven and the lighting state monitoring signal indicates that the discharge lamp is extinguished, the driving voltage does not exceed a predetermined upper limit value. And a control circuit for controlling the DC / DC converter. The DC / DC converter includes an input transformer and a switching element connected in series with the primary winding of the input transformer. When the lighting state monitoring signal indicates that the discharge lamp is extinguished, the control circuit is configured such that the energy stored in the primary winding of the input transformer during the ON period of the switching element indicates that the lighting state monitoring signal indicates that the discharge lamp is turned on. The on / off state of the switching element is controlled to be smaller than the corresponding energy.

  According to this aspect, the energy stored in the primary winding of the input transformer during the ON period of the switching element when the lighting state monitoring signal indicates that the discharge lamp is extinguished can be reduced.

  It should be noted that any combination of the above-described constituent elements and those obtained by replacing the constituent elements and expressions of the present invention with each other among apparatuses, methods, systems, etc. are also effective as an aspect of the present invention.

  According to the present invention, lighting performance can be improved.

It is a circuit diagram which shows the structure of the discharge lamp lighting circuit which concerns on 1st Embodiment, and the member connected to it. It is a time chart which shows the operation state of the discharge lamp lighting circuit of FIG. 3A and 3B are time charts showing an operation state after the DC period of the discharge lamp lighting circuit. It is a time chart which shows the change of the drive voltage from power-on to breakdown, and the both-ends voltage of the starter capacitor of a starter circuit. It is a time chart which shows the operation state of the discharge lamp lighting circuit before a drive voltage reaches | attains near upper limit voltage. It is a time chart which shows the operation state of the discharge lamp lighting circuit after a drive voltage reaches | attains near upper limit voltage. It is a time chart which shows the operation state of a discharge lamp lighting circuit when using the same current limiting value regardless of whether the discharge lamp is turned on or off in the current limiting circuit. It is a circuit diagram which shows the structure of the discharge lamp lighting circuit which concerns on 2nd Embodiment, and the member connected to it. It is a wave form diagram which shows the waveform of a drive voltage and a 3rd node voltage. It is explanatory drawing which shows the time change of the transistor current which flows through a drive voltage and a lighting auxiliary switch when not giving an offset to a 2nd node voltage. It is explanatory drawing which shows the time change of the transistor current which flows through a drive voltage and a lighting auxiliary switch, when providing the offset based on a 3rd node voltage to a 2nd node voltage.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and signals shown in the drawings are denoted by the same reference numerals, and repeated descriptions are appropriately omitted. In addition, in the drawings, some of the members that are not important for explanation are omitted.

  In this specification, “the state in which the member A is connected to the member B” means that the member A and the member B are electrically connected in addition to the case where the member A and the member B are physically directly connected. It includes the case of being indirectly connected through another member that does not affect the connection state. Similarly, “the state in which the member C is provided between the member A and the member B” refers to the case where the member A and the member C or the member B and the member C are directly connected, as well as an electrical condition. This includes the case of being indirectly connected through another member that does not affect the state of connection.

(First embodiment)
The discharge lamp lighting circuit according to the first embodiment transforms an input battery voltage using a switching converter, converts it into an AC voltage using an inverter, and supplies the AC voltage to a discharge lamp to be driven. In this discharge lamp lighting circuit, when it is determined that the discharge lamp is not lit, the energy that is phase-shifted from the input side to the output side of the switching converter for each cycle of ON / OFF of the switching element of the switching converter is calculated. Make it smaller than if it is determined to be after lighting. Accordingly, it is possible to suppress a decrease in the driving frequency of the switching element when the output voltage of the switching converter increases and approaches the upper limit value before the discharge lamp is turned on.

  FIG. 1 is a circuit diagram showing a configuration of a discharge lamp lighting circuit 100 and members connected thereto according to the first embodiment. The discharge lamp lighting circuit 100 drives the discharge lamp 4 which is an in-vehicle metal halide lamp. The discharge lamp lighting circuit 100 and the discharge lamp 4 are mounted on a vehicle lamp. The discharge lamp lighting circuit 100 is connected to an in-vehicle battery (hereinafter simply referred to as a battery) 6 and a power switch 8.

  The battery 6 generates a DC battery voltage (power supply voltage) Vbat of 12V (or 24V). The power switch 8 is a relay switch provided for controlling on / off of the discharge lamp 4 and is provided in series with the battery 6. When the power switch 8 is turned on, the battery voltage Vbat is supplied from the battery 6 to the discharge lamp lighting circuit 100.

  The discharge lamp lighting circuit 100 boosts the smoothed battery voltage Vbat, converts it into an alternating current, and supplies it to the discharge lamp 4. Hereinafter, a detailed configuration of the discharge lamp lighting circuit 100 will be described.

  The discharge lamp lighting circuit 100 includes a DC / DC converter CONV, a control circuit 10, a starter circuit 20, an inverter circuit 30, an input capacitor Cin, a current detection resistor Rd, a first current limiting resistor R1, and a second current limiting resistor R2. .

  Input capacitor Cin is provided in parallel with battery 6 and smoothes battery voltage Vbat. More specifically, the input capacitor Cin is provided in the vicinity of the input transformer 14 and fulfills a voltage smoothing function for the switching operation of the DC / DC converter CONV.

  The DC / DC converter CONV boosts the battery voltage Vbat. The DC / DC converter CONV is an insulating switching regulator, and includes an input transformer 14, an output diode D1, an output capacitor Co, and a switching element M1.

  The primary winding L1 of the input transformer 14 and the switching element M1 are provided in series between the input terminal Pin of the DC / DC converter CONV and the ground terminal (GND) in parallel with the input capacitor Cin. For example, the switching element M1 is composed of an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor). One end of the secondary winding L2 of the input transformer 14 is grounded, and the other end is connected to the anode of the output diode D1. The output capacitor Co is provided between the cathode of the output diode D1 and the ground terminal.

A control pulse signal S1 having a driving frequency f1 is applied to a control terminal (gate) of the switching element M1. For example, the drive frequency f1 is 400 kHz in the steady lighting state. The switching element M1 is turned on when the control pulse signal S1 is at a high level and turned off when it is at a low level. When the switching element M1 is turned on, a current starts to flow through the primary winding L1, and at the end of the on period, energy corresponding to 1/2 · Lp · Ip ² is stored in the primary winding L1. Here, Lp is the inductance of the primary winding L1, and Ip is the current value flowing through the primary winding L1 at the end of the ON period. Lp is typically about 1 μH. When the switching element M1 is turned off in this state, the energy stored in the primary winding L1 is transferred to the secondary winding L2. The transferred energy is stored in the output capacitor Co. By repeating the above energy transfer, the output capacitor Co is charged.

The inverter circuit 30 converts the DC drive voltage Vo boosted by the DC / DC converter CONV into an AC voltage having a lighting frequency f2, and supplies the AC voltage to the discharge lamp 4. As the inverter circuit 30, for example, a known inverter circuit such as an H bridge circuit is used.
The lighting frequency f2 is set lower than the driving frequency f1. The lighting frequency f2 is preferably set to 10 kHz or less, more preferably about 250 Hz to 750 Hz, and is set to 312.5 Hz in the present embodiment. The reciprocal of the lighting frequency f2 is referred to as a lighting cycle T2 (= 1 / f2 = 3.2 ms).

  The current detection resistor Rd is provided on the path of the lamp current IL flowing through the discharge lamp 4. In the circuit of FIG. 1, the current detection resistor Rd is provided on the ground wiring that connects the DC / DC converter CONV and the inverter circuit 30. A voltage drop Vd proportional to the lamp current IL is generated in the current detection resistor Rd.

  The starter circuit 20 generates a high voltage pulse and applies it to one end of the discharge lamp 4 in order to break down the discharge lamp 4. The starter circuit 20 generates a high voltage pulse by using energy (hereinafter referred to as primary energy) stored in the primary winding L1 of the input transformer 14 during the ON period of the switching element M1. The starter circuit 20 includes a charging circuit 22, a primary circuit 24, and a high voltage transformer 26.

  The secondary winding L4 of the high voltage transformer 26 is connected between one end of the discharge lamp 4 and the output terminal of the inverter circuit 30. The primary winding L 3 of the high voltage transformer 26 is connected to the primary circuit 24.

  The charging circuit 22 is connected to the DC / DC converter CONV via the first current limiting resistor R1 and the second current limiting resistor R2, and generates a charging voltage higher than the driving voltage Vo and supplies it to the primary circuit 24. In particular, the charging circuit 22 receives the voltage across the output diode D1, and multiplies the drive voltage Vo by n (n is an integer of 2 or more). For the point of multiplying the voltage by n, a known voltage amplification technique represented by the Cockcroft-Walton circuit may be used.

  The primary circuit 24 includes a starter capacitor (not shown) that is charged using a charging voltage, and a discharge gap type switch such as a spark gap that controls conduction between the starter capacitor and the primary winding L3 of the high voltage transformer 26. (Not shown). In the primary circuit 24, when the voltage Vst across the starter capacitor exceeds a predetermined dielectric breakdown voltage Vb, the discharge gap type switch becomes conductive. As a result, a large pulse current flows through the primary winding L3 of the high voltage transformer 26. A high voltage pulse generated in the secondary winding L4 in response to this current is applied to one end of the discharge lamp 4. The breakdown voltage Vb is typically set to about 800V.

  Even if a high voltage pulse is applied to the discharge lamp 4, the discharge lamp 4 does not break down in some cases. In this case, the starter circuit 20 charges the starter capacitor again to generate a high voltage pulse. In this way, the starter circuit 20 continues to generate high voltage pulses until the discharge lamp 4 breaks down.

  The first current limiting resistor R1 is provided between the anode of the output diode D1 and the first input terminal P1 of the starter circuit 20. The second current limiting resistor R2 is provided between the cathode of the output diode D1 and the second input terminal P2 of the starter circuit 20.

  The first current limiting resistor R1 and the second current limiting resistor R2 limit the amount of current supplied to the starter circuit 20. Therefore, it is possible to prevent a large current from flowing in a pulse manner to each element of the starter circuit 20 in the process of voltage amplification in the charging circuit 22. As a result, smaller and cheaper elements can be used, and the starter circuit 20 can be miniaturized and the cost can be reduced.

The control circuit 10 includes a function IC (Integrated Circuit) that controls the discharge lamp lighting circuit 100, and controls the discharge lamp lighting circuit 100 according to a predetermined drive sequence. The control circuit 10 executes the following driving sequence in the order of 1 to 5 to turn on the discharge lamp 4 and stabilize its light output.
1. Power on Breakdown 2. DC period 4. Run-up 5. Further details of the steady lighting drive sequence will be described later.

  The control circuit 10 controls the DC / DC converter CONV so that the drive voltage Vo does not exceed a predetermined upper limit voltage Vth2 from the viewpoint of the breakdown voltage of the element and safety. Further, when it is determined that the discharge lamp 4 is turned off, the control circuit 10 causes the primary energy to be smaller and constant than the primary energy when it is determined that the discharge lamp 4 is turned on. The switching element M1 is turned on and off.

The control circuit 10 includes a detection circuit 102 and a drive circuit 104.
The detection circuit 102 detects the state of the discharge lamp 4 such as whether the discharge lamp 4 is turned on / off or the power supplied to the discharge lamp 4. The detection circuit 102 includes a lighting determination circuit 106 and a lamp power calculation circuit 108.

The lighting determination circuit 106 monitors the drive voltage Vo. If the drive voltage Vo is higher than the predetermined lighting determination voltage Vth1, it is determined that the discharge lamp 4 is turned off, and the driving voltage Vo is lower than the lighting determination voltage Vth1. Determines that the discharge lamp 4 is lit. Here, “the discharge lamp 4 is turned off” can be restated as “the discharge lamp 4 is not turned on” or “before the discharge lamp 4 is turned on”.
The lighting determination circuit 106 outputs a lighting state monitoring signal S2 that is asserted when it is determined that the discharge lamp 4 is turned off and negated when it is determined that the discharge lamp 4 is turned on.
The lighting determination circuit 106 may monitor a lamp voltage VL that is a voltage applied to the discharge lamp 4.

  The lamp power calculation circuit 108 measures the voltage drop Vd generated in the current detection resistor Rd and calculates the lamp current IL. The lamp power calculation circuit 108 calculates lamp power WL, which is the power consumed by the discharge lamp 4, based on the calculated lamp current IL information and taking into account the drive voltage Vo and the lamp voltage VL as required. Calculate. The lamp power calculation circuit 108 outputs a power signal S3 indicating the calculated lamp power WL.

  The drive circuit 104 drives the switching element M1 of the DC / DC converter CONV based on the information detected by the detection circuit 102 by PWM (Pulse Width Modulation). The drive circuit 104 includes a duty ratio limiting circuit 110, a power control circuit 112, a sawtooth wave generation circuit 114, a voltage limiting circuit 116, a current limiting circuit 118, and a PWM comparator 120.

  The power control circuit 112 receives the power signal S3 and generates an error signal S4 having a voltage corresponding to the difference between the lamp power WL indicated by the power signal S3 and the target power. The target power is set so as to realize desired power control.

The sawtooth wave generation circuit 114 generates a triangular wave or sawtooth wave periodic signal S5 having a drive frequency f1.
The PWM comparator 120 generates an unrestricted PWM signal S6 by comparing the error signal S4 with the periodic signal S5.

The voltage limiting circuit 116 monitors the drive voltage Vo when the lighting state monitoring signal S2 is asserted. The voltage limiting circuit 116 outputs a voltage limiting signal S7 that is asserted while the monitored driving voltage Vo exceeds the upper limit voltage Vth2. In a duty ratio limiting circuit 110 to be described later, while the voltage limiting signal S7 is asserted, the control pulse signal S1 is at a low level and the switching element M1 is turned off. Therefore, it can be said that the voltage limiting circuit 116 turns off the switching element M1 while the drive voltage Vo exceeds the upper limit voltage Vth2.
When the upper limit voltage Vth2 is too high, the circuit element becomes large, and when it is too low, the lighting startability of the discharge lamp may be lowered. Therefore, the upper limit voltage Vth2 is set to 300 to 400 V particularly in the discharge lamp lighting circuit for automobiles.

  In the above description, the voltage limiting circuit 116 monitors the driving voltage Vo when the lighting state monitoring signal S2 is asserted. However, the driving voltage Vo in each of the DC period, the run-up period, and the steady lighting period is before the discharge lamp 4 is lit. Considering that the voltage is lower than the drive voltage Vo, the voltage limiting circuit 116 may monitor the drive voltage Vo without using the lighting state monitoring signal S2. That is, the voltage limiting circuit 116 may monitor the drive voltage Vo at least when the lighting state monitoring signal S2 is asserted.

  The current limiting circuit 118 outputs a current limiting signal S8. The current limit circuit 118 monitors the magnitude of the current flowing through the switching element M1, and generates a pulse in the current limit signal S8 when the magnitude of the current exceeds a predetermined current limit value. In the current limit circuit 118, the first current limit value Ith1 when the lighting state monitoring signal S2 is asserted is set to a value smaller than the second current limit value Ith2 when the lighting state monitoring signal S2 is negated. Is done. The first current limit value Ith1 is typically set to about 1A.

  In a duty ratio limiting circuit 110 described later, when a pulse is detected in the current limiting signal S8 during the ON period of the switching element M1, the control pulse signal S1 is set to the low level and the switching element M1 is turned off. Therefore, it can be said that the current limiting circuit 118 turns off the switching element M1 when the magnitude of the current flowing through the switching element M1 exceeds the current limit value.

The current limit circuit 118 includes a reference voltage generation circuit 122 and a limit pulse generation circuit 124.
When the lighting state monitoring signal S2 is asserted, the reference voltage generation circuit 122 generates the first reference voltage Vr1 corresponding to the first current limit value Ith1, and supplies the first reference voltage Vr1 to the limit pulse generation circuit 124. When the lighting state monitoring signal S2 is negated, the reference voltage generation circuit 122 generates the second reference voltage Vr2 corresponding to the second current limit value Ith2, and supplies the second reference voltage Vr2 to the limit pulse generation circuit 124. Since the first current limit value Ith1 is smaller than the second current limit value Ith2, in an example, the first reference voltage Vr1 is lower than the second reference voltage Vr2.

The limit pulse generation circuit 124 outputs a current limit signal S8. The limit pulse generation circuit 124 acquires the first node voltage V _N1 of the first connection node N 1 between the switching element M 1 and the primary winding L 1 of the input transformer 14, and supplies it to the reference voltage supplied from the reference voltage generation circuit 122. Compare with voltages Vr1 and Vr2.
During the ON period of the switching element M1, the first node voltage V _N1 is a primary current I _L1 (= current flowing in the switching element M1) flowing in the primary winding L1 of the input transformer 14 and an ON resistance Ron of the switching element M1. The product of Since the on-resistance Ron of the switching element M1 can be easily measured, the first node voltage V _N1 is a voltage representing the primary current I _L1 during the on-period of the switching element M1.

When the lighting state monitoring signal S2 is asserted, the limit pulse generation circuit 124 compares the first node voltage V _N1 and the first reference voltage Vr1, and when the primary current _IL1 exceeds the first current limit value Ith1. A pulse is generated in the current limiting signal S8.
When the lighting state monitoring signal S2 is negated, the limit pulse generation circuit 124 compares the first node voltage V _N1 with the second reference voltage Vr2, and when the primary current _IL1 exceeds the second current limit value Ith2 A pulse is generated in the current limiting signal S8.

  Duty ratio limit circuit 110 receives pre-limit PWM signal S6, voltage limit signal S7, and current limit signal S8. When the voltage limiting signal S7 is negated and no pulse is detected in the current limiting signal S8, the duty ratio limiting circuit 110 generates a control pulse signal S1 having the same waveform as the pre-limit PWM signal S6, and the switching element M1. Supply to the control terminal.

The duty ratio limiting circuit 110 sets the control pulse signal S1 to low level when the voltage limiting signal S7 is asserted, and fixes the control pulse signal S1 to low level while the voltage limiting signal S7 is asserted.
When the duty ratio limiting circuit 110 detects a pulse in the current limiting signal S8 while the control pulse signal S1 is at high level, the duty ratio limiting circuit 110 sets the control pulse signal S1 to low level. Next, the duty ratio limiting circuit 110 sets the control pulse signal S1 to the high level at the timing when the pre-limitation PWM signal S6 becomes the high level.

  The above is the configuration of the discharge lamp lighting circuit 100. Next, the operation will be described according to the drive sequence. FIG. 2 is a time chart showing the operating state of the discharge lamp lighting circuit 100. In the following time charts, the vertical axis and the horizontal axis are appropriately enlarged or reduced for easy understanding, and each waveform shown is also simplified for easy understanding.

1. When the user turns on the power switch 8 at power-on time t1, the discharge lamp lighting circuit 100 is activated. The control circuit 10 activates the DC / DC converter CONV and puts the inverter circuit 30 in the reset state. The DC / DC converter CONV boosts the battery voltage Vbat.

2. At breakdown time t2, starter circuit 20 typically generates a high voltage pulse of 20 kV or higher. As a result, the driving voltage VL of the discharge lamp 4 rises to about 15 kV, breaks down, and glow discharge starts.

3. DC period After breakdown, the control circuit 10 first controls the lamp current IL in the direction of the first polarity for about 10 ms. Next, the control circuit 10 performs control to flow the lamp current IL in the direction of the second polarity for about 10 ms. In this DC period, the glow discharge is shifted to the arc discharge.

  When the DC period ends and the arc discharge is stabilized, the control circuit 10 controls the inverter circuit 30 to alternately switch the polarity of the lamp current IL in the lighting cycle T2. Thereby, the discharge lamp 4 is turned on by alternating current.

  3A and 3B are time charts showing an operation state after the DC period of the discharge lamp lighting circuit 100. FIG. FIGS. 3A and 3B show waveforms during the run-up process and steady lighting, respectively.

4). As the run-up arc discharge grows, the light output of the discharge lamp 4 increases. The rise of the light output is determined by the standard, and the control circuit 10 monitors the drive voltage Vo, the lamp current IL, and the like so as to obtain a light output (power) that matches the standard, and by feedback, the switching element M1. Adjust the on / off duty ratio. In order to rapidly increase the light output of the discharge lamp 4 during the run-up period, the discharge lamp lighting circuit 100 temporarily supplies overpower that is higher than the rated power, and then the lamp voltage VL is set to 45 V, and the lamp current IL is set to 0. It stabilizes to 8A and approaches the rated power (35W) (FIG. 3 (a)).

5. Steady lighting After the run-up process, when the light output of the discharge lamp 4 is stabilized, the power supplied to the discharge lamp 4 is stabilized to a rated value of 35 W (FIG. 3B). Note that the waveforms of the lamp voltage VL and the lamp current IL shown in FIGS. 3A and 3B are simplified for ease of viewing, and actually have a frequency of 250 Hz to 750 Hz. .

  FIG. 4 is a time chart showing changes in the drive voltage Vo and the voltage Vst across the starter capacitor of the starter circuit 20 from power-on to breakdown. FIG. 4 shows a case where the discharge lamp 4 breaks down by the third high voltage pulse generation.

  After the power is turned on at time t1, the drive voltage Vo and the both-end voltage Vst begin to rise. When the drive voltage Vo reaches the vicinity of the upper limit voltage Vth2, it is stabilized to the upper limit voltage Vth2 by the action of the voltage limiting circuit 116. When the both-end voltage Vst reaches the dielectric breakdown voltage Vb, a high voltage pulse is applied to the discharge lamp 4. If the discharge lamp 4 does not break down even when the high voltage pulse is applied, the charging circuit 22 starts to recharge the starter capacitor, and the both-end voltage Vst starts to rise again from around 0V. In FIG. 4, the first pulse interval is referred to as a first pulse interval TP1, the second pulse interval is referred to as a second pulse interval TP2, and the third pulse interval is referred to as a third pulse interval TP3.

  FIG. 5 is a time chart showing an operation state of the discharge lamp lighting circuit 100 before the drive voltage Vo reaches the vicinity of the upper limit voltage Vth2. FIG. 5 corresponds to a region 202 surrounded by an alternate long and short dash line of the drive voltage Vo in FIG.

Since the drive voltage Vo is higher than the lighting determination voltage Vth1, the lighting determination circuit 106 determines that the discharge lamp 4 is turned off and sets the lighting state monitoring signal S2 to the high level. Further, since the drive voltage Vo has not reached the vicinity of the upper limit voltage Vth2, the voltage limiting circuit 116 keeps the voltage limiting signal S7 at a low level. At time t3 after the control pulse signal S1 becomes high level, the current limiting circuit 118 generates a pulse in the current limiting signal S8 when the magnitude of the primary current _IL1 reaches the first current limiting value Ith1. . When the duty ratio limiting circuit 110 detects the pulse, it sets the control pulse signal S1 to a low level. The same applies to time t4 and time t5.
In the case shown in FIG. 5, the duty ratio of the control pulse signal S1 or the length of the ON period of the switching element M1 is determined by the first current limit value Ith1.

  FIG. 6 is a time chart showing an operation state of the discharge lamp lighting circuit 100 after the drive voltage Vo reaches the vicinity of the upper limit voltage Vth2. FIG. 6 corresponds to a region 204 surrounded by a two-dot chain line of the drive voltage Vo in FIG.

Since the drive voltage Vo is higher than the lighting determination voltage Vth1, the lighting determination circuit 106 determines that the discharge lamp 4 is turned off and sets the lighting state monitoring signal S2 to the high level. At time t6 after the control pulse signal S1 becomes high level, the current limiting circuit 118 generates a pulse in the current limiting signal S8 when the magnitude of the primary current _IL1 reaches the first current limiting value Ith1. . When the duty ratio limiting circuit 110 detects the pulse, it sets the control pulse signal S1 to a low level. When the control pulse signal S1 becomes low level and the switching element M1 is turned off, the energy (1/2 · Lp · Ith1 ² ) stored in the primary winding L1 of the input transformer 14 is transferred to the output capacitor Co. The drive voltage Vo increases.

  At time t7 when the drive voltage Vo exceeds the upper limit voltage Vth2, the voltage limit circuit 116 sets the voltage limit signal S7 to a high level. Thereafter, the voltage limiting circuit 116 keeps the voltage limiting signal S7 at the high level until time t8 when the driving voltage Vo falls below the upper limit voltage Vth2 due to the natural loss of the driving voltage Vo due to the loss of the capacitor, the voltage detection resistor, or the like. In a period between time t7 and time t8 when the voltage limiting signal S7 is at a high level, the duty ratio limiting circuit 110 fixes the control pulse signal S1 at a low level regardless of the pre-limitation PWM signal S6.

  At time t9 when the rising edge appears in the unrestricted PWM signal S6 for the first time after time t8 when the voltage limiting circuit 116 sets the voltage limiting signal S7 to low level, the duty ratio limiting circuit 110 sets the control pulse signal S1 to high level. The subsequent processes at times t10, t11, t12, and t13 are the same as the processes at times t6, t7, t8, and t9, respectively.

  The increment ΔV1 of the drive voltage Vo after time t6 becomes a value corresponding to the energy stored in the primary winding L1 of the input transformer 14, that is, the first current limit value Ith1. Here, in the discharge lamp lighting circuit 100 according to the present embodiment, the first current limit value Ith1 is set to a constant value smaller than the second current limit value Ith2. Therefore, the increase ΔV1 of the drive voltage Vo can be suppressed as compared with the case where the first current limit value Ith1 is not set small. Thereby, after the drive voltage Vo reaches the vicinity of the upper limit voltage Vth2, the length of the period during which the switching element M1 is forcibly turned off by the action of the voltage limiting circuit 116 is reduced. As a result, a decrease in drive frequency f1 is suppressed.

  FIG. 7 is a time chart showing the operating state of the discharge lamp lighting circuit when the same current limit value is used regardless of whether the discharge lamp 4 is turned on or off in the current limiting circuit. In this current limiting circuit, a common third current limiting value Ith3 is used regardless of whether the discharge lamp 4 is turned on or off.

  As described above, in the discharge lamp lighting circuit, after the breakdown of the discharge lamp, an overpower higher than the rated power is temporarily supplied in order to rapidly increase the light output of the discharge lamp. Therefore, in order to enable such control, it is necessary to set the third current limit value Ith3 in the current limit circuit to a large value corresponding to overpower, particularly a value greater than the first current limit value Ith1.

At a time t14 after the control pulse signal S1 becomes a high level, a current limiting circuit, when the magnitude of the primary current I _L1 reaches a third current limit value Ith3, it generates a pulse to the current limit signal S8. When the duty ratio limiting circuit 110 detects the pulse, it sets the control pulse signal S1 to a low level. When the control pulse signal S1 becomes low level and the switching element M1 is turned off, the energy (1/2 · Lp · Ith3 ² ) stored in the primary winding L1 of the input transformer 14 is transferred to the output capacitor Co. The drive voltage Vo increases.

  Here, since the third current limit value Ith3 is set to a value larger than the first current limit value Ith1, the increment ΔV2 of the drive voltage Vo is relatively larger than that in the case of FIG. Therefore, the length of the period during which switching element M1 is forcibly turned off by the action of voltage limiting circuit 116 (the length of the period from time t15 to time t16) becomes longer.

  The above is the operation of the discharge lamp lighting circuit 100 according to the first embodiment. The discharge lamp lighting circuit 100 has the following advantages over the conventional discharge lamp lighting circuit.

  According to the discharge lamp lighting circuit 100 according to the present embodiment, the control circuit 10 determines that the primary energy is turned on when the discharge lamp 4 is turned off. The on / off of the switching element M1 is controlled so as to be smaller than the primary energy in the case. Therefore, as described above, it is possible to suppress a decrease in drive frequency after the drive voltage Vo has reached the vicinity of the upper limit voltage Vth2.

  Unlike when the discharge lamp 4 is lit, the main role of the DC / DC converter CONV when it is not lit (before lighting) is to store charges in the starter capacitor of the starter circuit 20. The speed at which charges are stored depends on the ON / OFF cycle (driving frequency f1) of the switching element M1, particularly when the Cockcroft-Walton-type charging circuit 22 is used. In the discharge lamp lighting circuit 100 according to the present embodiment, the drive frequency does not decrease so much even after the drive voltage Vo reaches the vicinity of the upper limit voltage Vth2, and it is approximately the same as before reaching depending on the setting. Therefore, fluctuations in the charging speed to the starter capacitor are suppressed, and the pulse interval becomes more uniform. Further, the pulse interval corresponding to the second and subsequent high voltage pulses is shortened. In particular, the difference between the first pulse interval (first pulse interval TP1) and the second and subsequent pulse intervals (second pulse interval TP2, third pulse interval TP3) is reduced.

  If the electric power when the discharge lamp is lit is 35 W and the drive frequency f1 is 350 kHz, the energy stored in the primary winding L1 of the input transformer 14 by one switching is about 0.1 mJ. If the primary winding L1 of the input transformer 14 stores the same energy when the discharge lamp is not lit, the increase of the drive voltage Vo increases, that is, the length of the period during which the switching element M1 is forcibly turned off. Becomes longer. In this case, the pulse interval experimentally reaches several hundred milliseconds to several seconds. On the other hand, in the discharge lamp lighting circuit 100 according to the present embodiment, the pulse interval can be suppressed to about several tens of milliseconds.

  If the time from when the driver turns on the headlight switch to when it is turned on, or when the discharge lamp is extinguished for some reason until it is turned on again, it may cause discomfort and anxiety to the user. Absent. Therefore, by making the pulse interval more uniform and shorter, a discharge lamp lighting circuit that does not make the user feel uneasy can be realized.

  Further, in the discharge lamp lighting circuit 100 according to the present embodiment, when it is determined that the discharge lamp 4 is turned off, the control circuit 10 turns on and off the switching element M1 so that the primary energy is constant. Control. Therefore, even if the drive frequency f1, the state of the discharge lamp (hot / cold), the target power, or the battery voltage Vbat varies, the energy transferred to the output capacitor Co is made constant by turning the switching element M1 on and off once. Can do. As a result, the pulse interval is less susceptible to these variations.

  Further, in the discharge lamp lighting circuit 100 according to the present embodiment, the primary energy can be made smaller and constant more easily by setting the first current limit value Ith1 to a small and constant value. Specifically, the reference voltage used in the current limiting circuit 118 may be configured to be switchable, and it is not necessary to introduce a complicated and highly accurate circuit.

  Further, in the discharge lamp lighting circuit 100 according to the present embodiment, the lighting determination circuit 106 generates the lighting state monitoring signal S2 based on the drive voltage Vo. When it is determined that the lamp current IL is turned off when the lamp current IL becomes lower than a certain threshold value based on the lamp current IL, it can be immediately determined that the discharge lamp is turned off for some reason. Then, since control works in the direction of decreasing the primary energy, there is a possibility of further promoting the extinction of the discharge lamp. On the other hand, when the drive voltage Vo, which is the output voltage of the DC / DC converter CONV, is used as a judgment criterion, it is not judged that the lamp is extinguished unless the discharge lamp is completely extinguished and the electric charge is discharged from the output capacitor Co. Therefore, there is a time lag compared to the determination based on the lamp current IL. That is, it becomes difficult to prevent the discharge lamp from being turned on.

The present inventors have recognized the following problems with respect to device breakdown voltages such as switching elements, rectifying elements, and smoothing capacitors used in the DC / DC converter of the discharge lamp lighting circuit.
This device withstand voltage is normally determined based on the value of the upper limit value (the upper limit voltage Vth2 in the first embodiment) of the voltage output by the DC / DC converter before the discharge lamp is turned on. By setting the device withstand voltage to a level that is determined by the upper limit value of this voltage, an inexpensive element or a high-performance element can be used. However, with this setting, if the ringing voltage generated at the moment when the switching element is turned off in the process of increasing the output voltage of the DC / DC converter is large, the breakdown voltage may be temporarily exceeded. Also, if the same ringing voltage is large after reaching the upper limit value of the voltage, the breakdown voltage may be temporarily exceeded. Therefore, in consideration of the ringing voltage, a device having a higher breakdown voltage must be used. Since the magnitude of the ringing voltage increases as the current value immediately before the switching element is turned off, an inexpensive element or a high-performance element can be used as long as the current value can be kept small.

  In the first embodiment, the first current limit value Ith1 when determined to be before lighting is set to be smaller than the second current limit value Ith2 when determined to be after lighting. Accordingly, since the current value immediately before the switching element M1 is turned off before lighting can be made smaller than that after lighting, the ringing voltage before lighting can be further reduced.

(Second Embodiment)
In the first embodiment, the primary energy before lighting is made smaller than that after lighting by switching the current limiting value in the current limiting circuit 118 before and after lighting the discharge lamp. In the second embodiment, the primary energy before lighting is made smaller than that after lighting by adding an offset to the node voltage input to the limiting pulse generation circuit.
Hereinafter, the discharge lamp lighting circuit 300 according to the second embodiment will be described focusing on differences from the discharge lamp lighting circuit 100 according to the first embodiment.

  FIG. 8 is a circuit diagram showing a configuration of the discharge lamp lighting circuit 300 according to the second embodiment and members connected thereto. The discharge lamp lighting circuit 300 includes an input capacitor Cin, a DC / DC converter CONV ′, a control circuit 310, a starter circuit 20, an inverter circuit 30, a first current limiting resistor R1, a second current limiting resistor R2, a current detecting resistor Rd, and a lighting. An auxiliary circuit 340.

When the DC / DC converter CONV ′ is compared with the DC / DC converter CONV according to the first embodiment, the DC / DC converter CONV ′ has a current flowing through the switching element M1 between the source terminal of the switching element M1 and the ground. Both are different in that a primary current detection resistor R3 for detecting (= primary current I _L1 ) is provided.

The auxiliary lighting circuit 340 is provided for arc growth of the discharge lamp 4. The lighting auxiliary circuit 340 includes a lighting auxiliary capacitor Ca, a first lighting auxiliary resistor R4, a second lighting auxiliary resistor R5, a lighting auxiliary switch M2 that is an npn-type bipolar transistor, and a lighting auxiliary diode D2. The lighting auxiliary capacitor Ca, the first lighting auxiliary resistor R4, the second lighting auxiliary resistor R5, and the lighting auxiliary switch M2 are connected in series between both ends of the output capacitor Co in this order. The lighting auxiliary circuit 340 has a positional relationship in which the lighting auxiliary capacitor Ca and the first lighting auxiliary resistor R4 are on the high voltage side.
The cathode of the lighting auxiliary diode D2 is connected to a fourth connection node N4 between the first lighting auxiliary resistor R4 and the second lighting auxiliary resistor R5, and the anode is connected to a terminal of the current detection resistor Rd on the inverter circuit 30 side. .

The auxiliary lighting switch M2 is turned on before the discharge lamp 4 is turned on and turned off after the lighting.
The lighting auxiliary circuit 340 generates charges stored in the lighting auxiliary capacitor Ca as the output voltage of the DC / DC converter CONV ′ decreases immediately after the starter circuit 20 generates a high voltage pulse and the discharge lamp 4 is turned on. Discharging to the discharge lamp 4 assists arc discharge.

The control circuit 310 includes a detection circuit 302 and a drive circuit 304.
The detection circuit 302 includes a lighting determination circuit 306 and a lamp power calculation circuit 108.
The lighting determination circuit 306 generates a lighting auxiliary signal S9 supplied to the base of the lighting auxiliary switch M2. The lighting determination circuit 306 monitors the drive voltage Vo or the lamp current IL, and when it is determined that the discharge lamp 4 is turned off, the lighting auxiliary signal S9 is set to a high level and it is determined that the discharge lamp 4 is turned on. If it is, the lighting assist signal S9 is set to a low level.

The drive circuit 304 includes a power control circuit 112, a sawtooth wave generation circuit 114, a voltage limit circuit 116, a current limit circuit 318, a PWM comparator 120, an offset circuit 312, and a duty ratio limit circuit 110.
The current limit circuit 318 includes a limit pulse generation circuit 124 and a reference voltage generation circuit 322. The reference voltage generation circuit 322 generates a fourth reference voltage Vr4 corresponding to the fourth current limit value Ith4 and supplies it to the limit pulse generation circuit 124.
Limit pulse generation circuit 124 obtains the offset voltage Voff which is generated by the offset circuit 312 in place of the first node voltage _{V N1} of the first connection node N1 between the primary winding L1 of the switching element M1 and the input transformer 14 , It is compared with the fourth reference voltage Vr4 supplied from the reference voltage generation circuit 322.

The offset circuit 312 acquires the second node voltage V _N2 of the second connection node N2 between the primary current detection resistor R3 and the source terminal of the switching element M1. Since the second node voltage V _N2 is represented by the product of the current flowing through the switching element M1 and the primary current detection resistor R3 in a state where there is no offset described later depending on the drive voltage Vo and the charging current Ich, the switching element It can be said that the voltage indicates the magnitude of the current flowing through M1.
The charging current Ich is a current that flows into the lighting auxiliary capacitor Ca when the lighting auxiliary capacitor Ca is charged before the breakdown of the discharge lamp 4. It can be said that the third node voltage V _N3 generated at the third connection node N3 between the lighting auxiliary capacitor Ca and the first lighting auxiliary resistor R4 is a voltage indicating the magnitude of the charging current Ich.

The offset circuit 312 _applies an offset to the second node voltage V _N2 based on at least one of the drive voltage Vo and the third node voltage V _N3 . In the offset circuit 312, the relationship between at least one of the drive voltage Vo and the third node voltage V _N3 and the offset is such that the switching element M1 is turned on as the drive voltage Vo and the third node voltage V _N3 are higher. The energy stored in the primary winding L1 of the input transformer 14 during the period is set to be small.

The offset circuit 312 includes a first offset resistor R6, a second offset resistor R7, and a third offset resistor R8. One end of the first offset resistor R6 is connected to the second connection node N2. A drive voltage Vo is applied to one end of the second offset resistor R7. One end of the third offset resistor R8 is connected to the third connection node N3. In other words, the third node voltage V _N3 is applied to one end of the third offset resistor R8.

The other ends of the first offset resistor R6, the second offset resistor R7, and the third offset resistor R8 are connected in common and constitute the output of the offset circuit 312. Offset voltage Voff generated in the output is a voltage offset is applied to the second node voltage V _N2, it is supplied to the limiting pulse generating circuit 124.

  Typical resistance values of the first offset resistor R6, the second offset resistor R7, and the third offset resistor R8 are 300Ω, 1MΩ, and 1MΩ, respectively. In this case, for example, when the drive voltage Vo is 400V, the offset is approximately 0.3V.

  The discharge lamp lighting circuit 300 according to the present embodiment has the following advantages over the conventional discharge lamp lighting circuit.

According to the discharge lamp lighting circuit 300 according to the present embodiment, an offset is applied to the second node voltage V _N2 and input to the limit pulse generation circuit 124. The primary current _IL1 limiting action due to this offset is stronger before the discharge lamp 4 is turned on. Thereby, the primary energy before lighting is made smaller than that after lighting. Therefore, similarly to the first embodiment, it is possible to make the pulse interval shorter and uniform by suppressing the decrease in the drive frequency after the drive voltage Vo reaches the vicinity of the upper limit voltage Vth2.

In the discharge lamp lighting circuit 300 according to the present embodiment, the offset applied by the offset circuit 312 is based on the third node voltage V _N3 . The relationship between the offset and the third node voltage V _N3 is set so that the primary energy decreases as the charging current Ich increases.

FIG. 9 is a waveform diagram showing waveforms of the drive voltage Vo and the third node voltage _VN3 . Power is turned on at time t1. As the charge to the lighting auxiliary capacitor Ca is charged, the charging current Ich decreases and the voltage value of the third node voltage _VN3 decreases. That is, the action of the third node voltage V _{N3 on} the offset becomes stronger as the drive voltage Vo rises and acts most strongly before or immediately after the drive voltage Vo reaches the upper limit voltage Vth2. Therefore, the ringing voltage in the process of increasing the drive voltage Vo can be effectively suppressed, and in particular, the ringing voltage before or immediately after the drive voltage Vo reaches the upper limit voltage Vth2 can be more effectively suppressed.

Moreover, according to the discharge lamp lighting circuit 300 according to the present embodiment, the current resistance of the lighting auxiliary switch M2 can be reduced. In order to assist the arc discharge transition when the discharge lamp 4 breaks down, it is necessary to fully charge the lighting auxiliary capacitor Ca before the high voltage pulse is generated. Accordingly, the resistance values of the first lighting auxiliary resistor R4 and the second lighting auxiliary resistor R5 are set to values corresponding thereto. In the discharge lamp lighting circuit 300 according to the present embodiment, the primary energy in the process of increasing the drive voltage Vo is reduced due to the offset caused by the third node voltage _VN3 . Therefore, the degree of increase in the drive voltage Vo becomes moderate. As a result, the peak current can be suppressed from the current defined by the resistance values of the first lighting auxiliary resistor R4 and the second lighting auxiliary resistor R5, and a transistor having a small current resistance can be selected.

10, when the second node voltage V _N2 do not impart an offset is an explanatory view showing a time variation of transistor current Itr1 flowing the driving voltage Vo and the auxiliary lighting switch M2.
11, in the case of imparting an offset based on the third node voltage V _N3 to the second node voltage V _N2, is an explanatory view showing a time variation of transistor current Itr2 flowing the driving voltage Vo and the auxiliary lighting switch M2.
It can be seen that the peak current Ip2 of the transistor current Itr2 in FIG. 11 is smaller than the peak current Ip1 of the transistor current Itr1 in FIG.

Moreover, in the discharge lamp lighting circuit 300 according to the present embodiment, the offset applied by the offset circuit 312 is based on the drive voltage Vo. The relationship between the offset and the drive voltage Vo is set so that the primary energy decreases as the drive voltage Vo increases. That is, as the drive voltage Vo is higher, the offset applied to the second node voltage _VN2 is higher, and the maximum value of the current flowing through the switching element M1 during the ON period of the switching element M1 is smaller.
The higher the drive voltage Vo is, the smaller the margin for the breakdown voltage of the device is. However, since the value of the current that flows immediately before the switching element M1 is turned off is also reduced, the ringing voltage is also lowered. Therefore, it is possible to use an element with a lower breakdown voltage.

  Further, before lighting, the amount of consumption of the drive voltage Vo is reduced, so that intermittent oscillation is likely to occur. Since the primary energy before lighting with a high drive voltage Vo is minimized by the effect of this offset, intermittent oscillation can be avoided. Since the difference between the driving voltage Vo before lighting and after lighting is large, the influence on the limited current after lighting can be ignored.

The configuration and operation of the discharge lamp lighting circuit according to the embodiment have been described above. These embodiments are exemplifications, and it is understood by those skilled in the art that various modifications can be made to each component and combination of processes, and such modifications are within the scope of the present invention. . Combinations of the embodiments are also possible. For example, the auxiliary lighting circuit 340 and the offset circuit 312 of the discharge lamp lighting circuit 300 according to the second embodiment may be introduced into the discharge lamp lighting circuit 100 according to the first embodiment. In this case, the primary energy when it is determined that the discharge lamp 4 is turned off can be further reduced.
In the discharge lamp lighting circuit combining the first embodiment and the second embodiment, it has been confirmed by experiments conducted by the present inventors that the pulse interval can be a uniform value of about 10 milliseconds. It was.

  In the first embodiment, the case where the voltage generated in the on-resistance of the switching element M1 is used as the voltage indicating the current flowing through the switching element M1 has been described. However, the present invention is not limited to this, and as in the second embodiment A current detection resistor may be provided separately. In the second embodiment, a voltage generated in the on-resistance of the switching element M1 may be used instead of the voltage generated in the primary current detection resistor R3.

  In the second embodiment, the case where a starter circuit similar to the starter circuit 20 of the first embodiment is used has been described. However, the present invention is not limited to this, and the input transformer 14 of the DC / DC converter CONV ′ has a high-voltage auxiliary circuit. Winding may be provided. Further, a starter circuit that does not use the DC / DC converter CONV ′ may be provided.

  In the second embodiment, the case where the lighting auxiliary circuit 340 includes the lighting auxiliary switch M2 has been described. However, the present invention is not limited thereto, and the second lighting auxiliary resistor R5 is directly connected to the ground terminal without including the lighting auxiliary switch M2. May be.

  In the first and second embodiments, a discharge lamp lighting circuit for driving a vehicle discharge lamp has been described as an example. However, the present invention is not limited to this and is widely applied to a discharge lamp lighting circuit including a DC / DC converter. it can.

  In the first and second embodiments, the case where the discharge lamp is driven with an AC drive voltage has been described. However, the present invention is not limited to this, and the embodiment is applied to a discharge lamp lighting circuit that drives the discharge lamp with a DC drive voltage. You may apply the technical idea which concerns. In this case, a configuration in which the inverter circuit 30 is removed from the first or second embodiment may be used. Alternatively, the technical idea according to the first or second embodiment may be applied to a so-called double converter type discharge lamp lighting circuit. In this case, of the two DC / DC converters of the double converter type discharge lamp lighting circuit, the DC / DC converter to which the starter circuit is connected is controlled by the control method according to the first or second embodiment. May be.

  In the first and second embodiments, the case of using an insulated DC / DC converter has been described. However, the present invention is not limited to this, and a non-insulated DC / DC converter may be used.

  In 1st and 2nd embodiment, although the discharge lamp 4 and the discharge lamp lighting circuit were demonstrated as a different body, it is not restricted to this, A discharge lamp may be integrated in a discharge lamp lighting circuit.

  4 discharge lamp, 6 battery, 8 power switch, 10 control circuit, 20 starter circuit, 30 inverter circuit, 100 discharge lamp lighting circuit, 300 discharge lamp lighting circuit, CONV DC / DC converter, Cin input capacitor, R1 first current limit Resistor, R2 Second current limiting resistor.

Claims (6)

A DC / DC converter for generating a drive voltage to be applied to a discharge lamp to be driven;
A control circuit that controls the DC / DC converter so that the drive voltage does not exceed a predetermined upper limit when the lighting state monitoring signal indicates that the discharge lamp is extinguished,
The DC / DC converter is
An input transformer,
A switching element connected in series with the primary winding of the input transformer,
The control circuit turns off the switching element while the drive voltage exceeds the upper limit value, and when the lighting state monitoring signal indicates that the discharge lamp is extinguished, the control circuit turns on the input transformer during the on period of the switching element. of the energy stored in the primary winding, the lighting state monitor signal is smaller than the corresponding energy when on the lighting of the discharge lamp, the switching element when the driving voltage reaches the vicinity of the upper limit value The discharge lamp lighting circuit is characterized in that on / off of the switching element is controlled so as to suppress a decrease in driving frequency . A starter circuit for generating a high-voltage pulse for causing the breakdown of the discharge lamp using energy stored in a primary winding of the input transformer during an ON period of the switching element;
When the lighting state monitoring signal indicates that the discharge lamp is extinguished, the control circuit is configured so that energy stored in a primary winding of the input transformer is constant during an ON period of the switching element. The discharge lamp lighting circuit according to claim 1, wherein on / off of the lamp is controlled. The control circuit includes:
A voltage limiting circuit for turning off the switching element when the drive voltage exceeds the upper limit;
A current limiting circuit that turns off the switching element when the magnitude of the current flowing through the switching element exceeds a predetermined limit value, and
In the current limiting circuit, the limit value of the magnitude of the current flowing through the switching element when the lighting state monitoring signal indicates that the discharge lamp is turned off is the limit value when the lighting state monitoring signal indicates that the discharge lamp is turned on. The discharge lamp lighting circuit according to claim 1, wherein the discharge lamp lighting circuit is set to a value smaller than a corresponding limit value.   The discharge lamp lighting circuit according to claim 1, wherein the control circuit generates the lighting state monitoring signal based on the driving voltage. A lighting auxiliary capacitor connected between both ends of the output capacitor of the DC / DC converter;
The control circuit is configured such that, prior to the breakdown of the discharge lamp, the larger the current flowing into the lighting auxiliary capacitor when the lighting auxiliary capacitor is charged, the larger the primary winding of the input transformer during the ON period of the switching element. The discharge lamp lighting circuit according to claim 1, wherein energy stored in the lamp is reduced. A lighting auxiliary capacitor connected between both ends of the output capacitor of the DC / DC converter;
The control circuit includes:
An offset circuit for applying an offset to the second voltage indicating the magnitude of the current flowing through the switching element, based on at least one of the drive voltage and the first voltage indicating the magnitude of the current flowing into the lighting auxiliary capacitor; ,
A comparison circuit that turns off the switching element based on a comparison result between a second voltage to which an offset is applied by the offset circuit and a reference voltage, and
In the offset circuit, the relationship between at least one of the drive voltage and the first voltage and the offset is such that the higher the at least one of the drive voltage and the first voltage, the higher the on-period of the switching element. 6. The discharge lamp lighting circuit according to claim 1, wherein the energy stored in the primary winding of the input transformer is set to be small.

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