JP6821835B2 - Lighting circuit and vehicle lighting equipment using it - Google Patents

Description

The present invention relates to a vehicle lamp used for an automobile or the like, and particularly to a failure detection thereof.

Conventionally, halogen lamps and HID (High Intensity Discharge) lamps have been the mainstream as light sources for vehicle lamps, especially headlights, but in recent years, semiconductor light sources such as LEDs (light emitting diodes) and laser diodes have been replaced. The development of vehicle lamps using the above is in progress.

Vehicle lighting fixtures that use a semiconductor light source are required to have a function of detecting an open abnormality caused by open destruction of the semiconductor light source, disconnection of a harness, disconnection of wiring, etc., and notifying the vehicle side. 1 (a) and 1 (b) are circuit diagrams of a vehicle lamp provided with a lighting circuit having an open abnormality detection function. It should be noted that these are circuits examined in advance by the present inventor and must not be certified as known techniques. The lighting circuit 10r of FIG. 1A includes a step-down converter (Buck converter) 20 and an open detection circuit 30r. The voltage V _BAT from the battery 4 is supplied to the lighting circuit 10r via the switch 6. The step-down converter 20 steps down the voltage V _BAT and supplies the output voltage V _OUT to the light source 2. The buck converter 20 is feedback-controlled by a converter controller (not shown) so that the drive current I _DRV flowing through the light source 2 approaches the target value I _REF that defines the target light amount of the light source 2.

The open detection circuit 30r of FIG. 1A includes a sense resistor _RS for current detection and a comparator 32r. Sense resistor _{R S} is inserted on the path of the driving current _{I DRV,} a voltage drop proportional to the driving current _{I DRV} (current detection _{signal) V IS} occurs across it. Comparator 32r compares the current detection signal _{V IS} with a predetermined threshold voltage _{V TH.} When the vehicle lamp 1r shown in FIG. 1 (a) is normal, the sense resistor _{R S} flow normal driving current _{I DRV,} the voltage drop _{V IS} to generate in excess of the threshold voltage _{V TH.} On the contrary, when an open abnormality occurs, the drive current _IDRV does not flow, so that the voltage drop _VIS becomes substantially zero, which is lower than the threshold voltage _VTH . Therefore, the output signal of the comparator 32r takes the first level (for example, high level) indicating normality when V _IS > V _TH , and the second level (low level) indicating open abnormality when V _IS <V _TH .

The open detection circuit 30s of FIG. 1B includes resistors R11 and R12 and a comparator 32s. The resistors R11 and R12 divide the output voltage V _OUT of the step-down converter 20. Comparator 32s is the after dividing the output voltage (voltage detection _{signal) V VS,} is compared with the threshold voltage _{V TH.}

When the vehicle lighting fixture 1s of FIG. 1B is normal, the output voltage V _OUT is feedback-controlled to an optimum voltage level for supplying the target current I _REF to the light source 2. When an open abnormality occurs, the drive current I _DRV does not flow, but the controller of the buck converter 20 increases the switching duty ratio in order to bring the drive current I _DRV closer to the target value I _REF , whereby the output voltage V _OUT is increased. To rise. As a result, the voltage detection signal _{V VS} exceeds the threshold voltage _{V TH.}

Therefore, the output signal of the comparator 32s _is, V VS _{<When V TH} takes a first level indicating a normal (e.g., high _level), V _VS> take the second level (low level) showing the open fault when the _{V TH.}

Japanese Unexamined Patent Publication No. 2004-134147

1. 1. As a result of examining the lighting circuits 10r and 10s of FIGS. 1A and 1B, the present inventor has come to recognize the following problems.

In vehicle lamps in which the laser diode is the light source 2, a low-luminance mode (test mode) in which the light source 2 emits light with low brightness may be required for maintenance and testing. In this case, the lighting circuit 10r of FIG. 1 (a), it is necessary to set lower than the voltage detection signal V _IS a threshold voltage V _TH in the low luminance mode, the driving current supplied to the light source 2 in the low luminance mode Since the I _DRV is weak and the voltage detection signal _VIS is extremely small, the threshold voltage _VTH must be set very low, which makes it susceptible to errors.

The light source 2 may be composed of a plurality of LEDs connected in series and some bypass switches provided in parallel with some LEDs. When this light source 2 is used, it is possible to control the lighting and extinguishing of the LED in parallel with the bypass switch according to the on / off. Here, the output voltage V _OUT of the step-down converter 20 is determined when the number of lit LEDs is N.
_{V OUT} ≒ V _F × N
_Therefore , the output voltage V _OUT dynamically fluctuates according to the number of lights N. In the lighting circuit 10s of FIG. 1 (b), when the output voltage _{V OUT} fluctuates dynamically, it is difficult to determine appropriately the threshold voltage _{V TH.}

2. 2. In addition, the present inventor has come to recognize the following problems in relation to open failure.
Semiconductor light sources are vulnerable to overcurrent, and especially in laser diodes, overcurrent can cause COD (Catastrophic optical damage) failure, so it is necessary to prevent current exceeding the absolute maximum rating from flowing even momentarily. Yes, more severe overcurrent protection is required than other light sources.

Chattering that repeats contact (normal state) and non-contact (open state) may occur at the connector contact between the lighting circuit 10r (or 10s) and the light source 2. In an open state, since the current detection signal V _IS lighting circuit 10r (10s) is zero, the duty ratio is increased so as to approach the target value. As a result, the voltage of the output capacitor increases. After that, when the connector contacts return to the contact state, the excess charge stored in the output capacitor flows into the light source 2, and an overcurrent occurs.

The present invention has been made in such a situation, and one of the exemplary objects of the embodiment is to provide a lighting circuit capable of appropriately detecting an open abnormality. Further, one of the exemplary objects of an aspect of the present invention is to provide a lighting circuit capable of suppressing an overcurrent.

1. 1. One aspect of the present invention relates to a lighting circuit. The lighting circuit supplies a drive current to the light source and feedback-controls the drive current so that it approaches the target current. An open detection that compares the potential difference between the input voltage and output voltage of the buck converter with a predetermined threshold voltage. It is equipped with a circuit.

When the load of the buck converter becomes an open abnormality, the drive current becomes zero, feedback is applied in the direction in which the output voltage rises to increase the drive current, and when an open abnormality occurs, the potential difference between the input and output of the buck converter becomes zero. Get closer. According to this aspect, an open abnormality can be detected based on the potential difference between the input and output of the buck converter.

The open detection circuit may include a PNP type bipolar transistor in which the emitter is connected to the input terminal of the buck converter and the base is connected to the output terminal of the buck converter. Since the on / off of the bipolar transistor corresponds to the result of abnormality detection and the voltage comparator is not required, the cost can be reduced.

The open detection circuit may further include a first resistor provided between the collector of the bipolar transistor and ground.

The open detection circuit may include a P-channel FET (Field Effect Transistor) in which the source is connected to the input terminal of the buck converter and the gate is connected to the output terminal of the buck converter. In this case, turning the FET on and off corresponds to the result of abnormality detection, and the voltage comparator becomes unnecessary, so that the cost can be reduced.

The open detection circuit may further include a clamping element provided between the gate and source of the FET. As a result, the voltage between the gate and source can be suppressed to be smaller than the withstand voltage.

The open detection circuit may further include a second resistor provided between the drain of the FET and ground.

2. 2. Another aspect of the present invention also relates to a lighting circuit. The lighting circuit has an output inductor, supplies a drive current to the light source via the output inductor, and feedback-controls the drive current so that it approaches the target current, and the output terminal of the converter returns from the open state to the normal state. When it detects that it has done so, it includes a protection circuit that stops the switching of the converter during the downtime.
In the open state, the detected value of the drive current becomes zero, so that the duty ratio of the converter increases and the output voltage rises. Then, when the normal state is restored, the excess charge stored in the output capacitor is supplied to the light source via the output inductor. The output inductor forms a resonance circuit together with the output capacitor, and a limited resonance current flows to the light source, so that an overcurrent is suppressed.
When this resonance current is superimposed on the drive current generated by the feedback control, it may become an overcurrent. However, when returning from the open state to the normal state, the resonance circuit is delayed by delaying the restart of the switching operation of the switching converter. Since the drive current is generated after the current is reduced, overcurrent can be suppressed.

The protection circuit may determine that the normal state is restored when the output voltage of the converter drops sharply. "The output voltage has dropped sharply" means that the slope of the output voltage exceeds a predetermined threshold value, the change width of the output voltage for a predetermined time exceeds a predetermined threshold value, and the output voltage is a predetermined value. This includes that the time required to change the width is shorter than a predetermined threshold value. As a result, the return from the open state to the normal state can be detected.

The protection circuit may slowly increase the switching duty ratio of the converter after the outage time has elapsed. As a result, the drive current gradually increases after the switching is restarted, so that overcurrent can be further suppressed.

The protection circuit may set the target current to zero during the stop time and gradually increase the target current after the stop time has elapsed.

When the protection circuit detects that the output terminal of the converter has returned from the short state to the normal state, the switching of the converter may be stopped during the stop time.
As a result, when the short state returns to the normal state, the restart of the switching operation of the switching converter is delayed, so that the drive current is generated after the current of the resonance circuit becomes small, so that the overcurrent can be suppressed.

The protection circuit may determine that the short state returns to the normal state when the output voltage of the converter rises sharply. As a result, the return from the short state to the normal state can be detected.

The protection circuit may include a differentiating circuit or a high-pass filter that receives the output voltage. When the output signal of the differentiating circuit or the high-pass filter exceeds a predetermined value, the protection circuit may determine that it returns to the normal state.

The protection circuit includes a capacitor whose one end is grounded, a charging resistor which is connected to the other end of the capacitor and applies a target voltage that defines the target current in a normal state, and a discharge switch provided in parallel with the capacitor. It may be included. The discharge switch may be turned on when a return to normal is detected.

The converter may be step-down. The lighting circuit may further include an open detection circuit that compares the potential difference between the input voltage and the output voltage of the converter with a predetermined threshold voltage.

Another aspect of the present invention relates to a vehicle lamp. Vehicle lighting fixtures include a light source and any of the lighting circuits described above that drive the light source.

According to an aspect of the present invention, an open abnormality can be appropriately detected. Further, according to a certain aspect of the present invention, overcurrent can be suppressed.

1 (a) and 1 (b) are circuit diagrams of a vehicle lamp provided with a lighting circuit having an open abnormality detection function. It is a circuit diagram of the vehicle lighting equipment which concerns on 1st Embodiment. It is an operation waveform diagram of the lighting circuit of FIG. 4 (a) and 4 (b) are circuit diagrams showing a specific configuration example of a vehicle lamp. 5 (a) and 5 (b) are circuit diagrams showing another configuration example of the vehicle lamp. It is a circuit diagram of the lighting equipment for a vehicle which concerns on 2nd Embodiment. It is an operation waveform diagram of the lighting circuit of FIG. It is a concrete circuit diagram of the lighting circuit of FIG. It is an operation waveform diagram of the lighting circuit of FIG. It is a circuit diagram which shows the specific configuration example of a protection circuit. It is a waveform figure which shows the return from a short state to a normal state. It is a circuit diagram of the protection circuit which concerns on modification 2.2. It is a perspective view of the lamp unit provided with the vehicle lighting equipment which concerns on embodiment.

Hereinafter, the present invention will be described with reference to the drawings based on preferred embodiments. The same or equivalent components, members, and processes shown in the drawings shall be designated by the same reference numerals, and redundant description will be omitted as appropriate. Further, the embodiment is not limited to the invention but is an example, and all the features and combinations thereof described in the embodiment are not necessarily essential to the invention.

In the present specification, the "state in which the member A is connected to the member B" means that the member A and the member B are physically directly connected, and that the member A and the member B are electrically connected to each other. It also includes the case of being indirectly connected via other members, which does not substantially affect the connection state, or does not impair the functions and effects performed by the combination thereof.
Similarly, "a state in which the member C is provided between the member A and the member B" means that the member A and the member C, or the member B and the member C are directly connected, and their electricity. It also includes the case of being indirectly connected via other members, which does not substantially affect the connection state, or does not impair the functions and effects performed by the combination thereof.

Further, in the present specification, the reference numerals attached to electric signals such as voltage signals and current signals, or circuit elements such as resistors and capacitors have their respective voltage values, current values, resistance values and capacitance values as required. It shall be represented.

According to those skilled in the art, it is possible to replace bipolar transistors, MOSFETs, and IGBTs (Insulated Gate Bipolar Transistors), replace the P channel (PNP type) and N channel (NPN type) of the transistor, and invert the power supply and ground. Is understood.

(First Embodiment)
FIG. 2 is a circuit diagram of the vehicle lamp 1 according to the first embodiment. The vehicle lamp 1 includes a light source 2 and a lighting circuit 10. The lighting circuit 10 includes a buck converter 20, a controller 22, and an open detection circuit 40.

The voltage V _BAT from the battery 4 is supplied to the lighting circuit 10 via the switch 6. The buck converter 20 steps down the input voltage V _IN corresponding to the battery voltage V _BAT , and supplies the output voltage V _OUT to the light source 2. The buck converter 20 is feedback-controlled by the converter controller 22 so that the drive current I _DRV flowing through the light source 2 approaches the target value I _REF that defines the target light amount of the light source 2. The buck converter 20 includes an input capacitor C1, an output capacitor C2, a switching transistor M1, a rectifier diode D1, and an inductor L1. The controller 22 generates a pulse signal S _PWM whose duty ratio changes so that the drive current I _DRV approaches the target value I _REF , and drives the switching transistor M1. The control method of the controller 22 is not particularly limited, and may be hysteresis control (Bang-Bang control) or feedback control using an error amplifier.

The open detection circuit 40 compares the potential difference ΔV between the input voltage V _IN and the output voltage V _OUT of the buck converter 20 with a predetermined threshold voltage V _TH . Then, when ΔV> V _TH , it is determined to be normal, the abnormality detection signal S1 is set to the first level (for example, high level), and when ΔV <V _TH , it is determined to be an open abnormality, and the abnormality detection signal S1 is set to the second level (for example, high level). Low level).

The above is the basic configuration of the lighting circuit 10. Next, the operation will be described. FIG. 3 is an operation waveform diagram of the lighting circuit 10 of FIG. Before the time t0, the vehicle lamp 1 is normal, and the drive current I _DRV is stabilized to the target amount I _REF . At this time, the output voltage V _OUT is stabilized at a certain voltage level.

When an open abnormality occurs at time t0, the drive current I _DRV is cut off and becomes zero. The controller 22 increases the duty ratio of the pulse signal S _{PWM in} order to bring the drive current I _DRV closer to the target value I _REF . In response to this, the output voltage V _OUT rises and eventually reaches the input voltage V _IN . Open detection circuit 40 _monitors the potential difference [Delta] V ₌ V _{IN -V OUT} between _{V IN} and _{V OUT,} when the [Delta] V _{<V TH} at time t1, the abnormality detection signal S1 to the low level.

The above is the operation of the lighting circuit 10. According to the lighting circuit 10, an open abnormality can be detected based on the potential difference ΔV between the input and output of the step-down converter 20.

When this lighting circuit 10 is used for a vehicle lamp 1 in which the light source 2 is a laser diode, even if the lighting circuit 10 is set to the low brightness mode and the drive current I _DRV is made minute, the open abnormality Can be detected properly. Alternatively, even when the lighting circuit 10 is used for a vehicle lamp that includes a plurality of LEDs to which the light source 2 is connected in series and controls turning on and off by a bypass switch, it is open regardless of the dynamic fluctuation of the output voltage V _OUT. Abnormalities can be detected appropriately.

The present invention extends to various devices and circuits grasped as the block diagram and circuit diagram of FIG. 2 or derived from the above description, and is not limited to a specific configuration. Hereinafter, more specific configuration examples will be described not for narrowing the scope of the present invention, but for helping to understand the essence of the invention and circuit operation, and for clarifying them.

4 (a) and 4 (b) are circuit diagrams showing a specific configuration example of the vehicle lamp 1. The open detection circuit 40a of FIG. 4A includes a PNP type bipolar transistor 42, a first resistor R1, and a base resistor R3. The emitter of the bipolar transistor 42 is connected to the input terminal of the buck converter 20, and the base thereof is connected to the output terminal of the buck converter 20 via the base resistor R3. The first resistor R1 is provided between the collector of the bipolar transistor 42 and the ground. The first resistor R1 may be omitted and used as an open collector output. Further, the base resistor R3 may be omitted.

A potential difference ΔV between the input and output of the buck converter 20 is input between the base and emitter of the bipolar transistor 42. Since the potential difference ΔV is sufficiently large in the normal state, the bipolar transistor 42 is turned on, and the abnormality detection signal S1 becomes a high level (V _IN ). When an open abnormality occurs and the potential difference ΔV between the input and output becomes smaller than the threshold voltage (0.6 to 0.7V) between the base and emitter of the bipolar transistor 42, the bipolar transistor 42 is turned off and the abnormality detection signal S1 Is low level. That is, the on / off of the bipolar transistor 42 corresponds to the presence / absence of abnormality detection. According to the vehicle lamp 1a of FIG. 4A, since the voltage comparator is unnecessary, the circuit cost can be reduced.

The open detection circuit 40b of FIG. 4B is understood to have a configuration in which the bipolar transistor 42 of FIG. 4A is replaced with the FET 44 of the P channel. The source of the FET 44 is connected to the input terminal of the step-down converter 20, and the gate thereof is connected to the output terminal of the step-down converter 20 via the gate resistor R4. The second resistor R2 is provided between the drain of the FET 44 and the ground. The clamping element 46 is provided between the gate and source of the FET 44, and clamps the voltage between the gate and source so as not to exceed a predetermined value. For example, the clamp element 46 can be composed of a Zener diode, a Schottky diode, or the like.

A potential difference ΔV between the input and output of the step-down converter 20 is input between the gate and source of the FET 44. Since the potential difference ΔV is sufficiently large in the normal state, the FET 44 is turned on, and the abnormality detection signal S1 becomes a high level (V _IN ). When an open abnormality occurs and the potential difference ΔV between the input and output becomes smaller than the threshold voltage VGS _(TH) (for example, 1.5V) of the FET, the FET 44 is turned off and the abnormality detection signal S1 becomes low level. That is, the on / off of the FET 44 corresponds to the presence / absence of an abnormality. According to the vehicle lamp 1b of FIG. 4B, since the voltage comparator is unnecessary, the circuit cost can be reduced.

5 (a) and 5 (b) are circuit diagrams showing a vehicle lamp 1c of another configuration example. In the vehicle lamp 1c shown in FIG. 5A, the open detection circuit 40c is configured by using the voltage comparator 48. The voltage comparator 48 may compare the voltage obtained by V _TH- shifting the input voltage V _IN to the low potential side and the output voltage V _OUT . The voltage shift _VTH is introduced by the level shifter 49. FIG. 5B is a circuit diagram showing a configuration example of the level shifter 49. For example, the level shifter 49 includes a resistor R5 and a current source 50. One end of the resistor R5 is connected to the input terminal of the step-down converter 20, and the current source 50 is connected to the other end. Current source 50 generates a predetermined constant current I _C. The junction of resistors R5 and the current source _50, voltage V IN -R5 × _{I C} is generated. That, R5 × _{I C} becomes _{V TH.} According to the vehicle lamp 1c of FIG. 5A, since the voltage comparator is used, accurate voltage comparison can be performed in exchange for the cost increase. Further, when a comparator circuit including a plurality of voltage comparators is used and there are surplus voltage comparators, no cost increase occurs.

(Second Embodiment)
FIG. 6 is a circuit diagram of the vehicle lamp 1d according to the second embodiment. For example, a connector 12 is provided between the light source 2 and the lighting circuit 10d, and the light source 2 and the lighting circuit 10d are detachably connected to each other. The lighting circuit 10d includes a step-down converter 20d, a controller 22, and a protection circuit 60. The technique described in the second embodiment can be used in combination with the technique described in the first embodiment, and therefore is omitted in FIG. 6, but the lighting circuit 10d has the above-mentioned open detection. The circuit 40 can be further provided.

The lighting circuit 10d includes an output inductor L2 in addition to the lighting circuit 10 of FIG. The output inductor L2 is inserted between the output capacitor C2 and the light source 2. When the protection circuit 60 detects that the output terminal of the step-down converter 20d has returned from the open state to the normal state, the protection circuit 60 stops switching of the converter 20d during the stop time τ1.

For example, the protection circuit 60 may detect a return from the open state to the normal state based on the output voltage V _OUT of the step-down converter 20d. After the stop time τ1 elapses, the protection circuit 60 may gradually increase the switching duty ratio of the converter 20d from zero (soft start).

The above is the basic configuration of the lighting circuit 10d. Next, the operation will be described. FIG. 7 is an operation waveform diagram of the lighting circuit 10d of FIG. The connector 12 is in the open state before the time t1. In the open state, the drive current I _DRV becomes zero. The controller 22 drives the switching transistor M1 with a large duty ratio in order to bring the zero drive current I _DRV closer to the target current I _REF by feedback control. As a result, the current of the inductor L1 flows into the output capacitor C2, and the output voltage V _OUT takes a higher voltage level than in the normal state.

At time t1, the disconnection of the connector 12 is resolved, and the contact state (that is, the normal state) is restored. As a result, the excess charge stored in the output capacitor C2 is supplied to the light source 2 via the output inductor L2. The output inductor L2 forms an LC resonance circuit 14 together with the output capacitor C2, and the limited resonance current _IRES flows to the light source 2, so that an overcurrent is suppressed. It should be noted that when the output inductor L2 does not exist, the lamp current I _LAMP flowing through the light source 2 increases without limitation as shown by the alternate long and short dash line, resulting in an overcurrent.

The lamp current I _LAMP is the sum of the drive current I _DRV generated by the buck converter 20d based on feedback control and the resonance current I _RES flowing through the resonance circuit 14. Resonance current _{I RES} flows a loop output capacitor C2 and the output inductor L2 is formed, thus the current detection signal _{V IS} to the controller 22 does not include the resonant current _{I RES.} Therefore, if the protection circuit 60 immediately resumes the switching operation of the switching converter 20d by omitting the stop time τ1 when returning from the open state to the normal state, the resonance current _IRES is generated by the feedback control. The lamp current I _LAMP superimposed on the current I _DRV and flowing through the light source 2 can be an overcurrent.

On the other hand, in the present embodiment, the protection circuit 60 restarts the switching operation of the switching converter 20d after the stop time τ1 elapses when returning from the open state to the normal state. The stop time τ1 may be determined in consideration of the relaxation time at which the resonance current I _RES becomes sufficiently small. As a result, the drive current I _DRV is generated after the resonance current I _RES of the resonance circuit 14 becomes small, so that the overcurrent can be suppressed.

Further, if the soft start control is not performed when the switching operation is restarted after the stop time τ1 has elapsed, an overcurrent may occur due to the resonance of the inductor L1, the output capacitor C2, and the output inductor L2. In this embodiment, on the other hand, by increasing gradually the output current I _DRV of the converter 20d by a soft-start, it is possible to suppress such an over-current.

Subsequently, a specific configuration example of the lighting circuit 10d of FIG. 6 will be described. FIG. 8 is a specific circuit diagram of the lighting circuit 10d of FIG. The controller 22 is a hysteresis control (Bang-Bang control) controller, and includes a current sense amplifier 70, a hysteresis comparator 72, and a driver 74. For example, the detection resistor _RS is inserted in the path of the drive current _IDRV generated by the converter 20d. The current sense amplifier 70 amplifies the voltage drop _{V IS} generated in the detection resistor _{R S.} Hysteresis comparator 72, the voltage drop V _IS, and compared with a threshold voltage V _H, V _L that varies 2 value in accordance with its own output, generates a modulated control pulses. The threshold values V _H and _VL are defined based on the reference voltage V _REF indicating the target value I _REF of the drive current I _DRV . The driver 74 drives the switching transistor M1 based on the control pulse generated by the hysteresis comparator 72. The control method of the controller 22 may be feedback control using an error amplifier.

As shown in FIG. 7, when the open state is restored to the normal state at time t1, the output voltage V _OUT drops instantly. The protection circuit 60 may detect the return to the normal state by utilizing this phenomenon. That is, the protection circuit 60 may determine that the return from the open state to the normal state is made when the output voltage V _OUT drops sharply.

For example, the protection circuit 60 can include a first differentiating circuit 62 or a low-pass filter. For example, the output signal _VA of the first differentiating circuit 62 increases as the slope of the downward slope of the output voltage V _OUT increases. After that, the output signal _VA returns to 0 with a slope corresponding to the time constant TC1 inside the first differentiating circuit 62. The time constant TC1 defines the above-mentioned stop time τ1.

The target current controller 64 adjusts the reference voltage V _REF that defines the target value I _REF of the drive current I _DRV according to the output _VA of the first differentiating circuit 62. Specifically, the target current controller 64 sets the reference voltage V _REF to the normal value V _NORM when the output V _A of the first differentiating circuit 62 is lower than the predetermined threshold value V _B. Target current controller 64, in a state where the output _{V A} of the first differentiating circuit 62 exceeds the threshold value _{V B,} the reference voltage _{V REF} i.e. the target current _{I REF} and zero. As a result, the switching of the buck converter 20d is stopped.

When the output V _A of the first differentiating circuit 62 falls below the threshold value V _B , the target current controller 64 gradually increases the reference voltage V _REF, that is, the target current I _REF toward the normal value V _NORM . This enables soft start control after the stop time τ1 has elapsed.

FIG. 9 is an operation waveform diagram of the lighting circuit 10d of FIG. FIG. 9 shows the operation of the connector 12 when the contacts are restored. When the contact of the connector 12 returns at time t1, the output voltage V _OUT drops sharply, the output V _A of the first differentiating circuit 62 rises, and exceeds the threshold value V _B. As a result, the reference voltage V _REF drops from the normal value V _NORM to zero, and the switching of the buck converter 20d is stopped.

Then gradually the voltage _{V A} in accordance with constant TC1 time within the first differentiating circuit 62 is lowered, lower than the threshold value _{V B} at time t2. Then, the target current controller 64 gradually raises the reference voltage V _REF . That is, the delay time at times t1 to t2 is the stop period τ1.

FIG. 10 is a circuit diagram showing a specific configuration example of the protection circuit 60. The first differentiating circuit 62 mainly includes a bipolar transistor Q11, a capacitor C21, and a resistor R21. With this configuration, a signal _VA corresponding to the slope of the downward slope of the output voltage V _OUT is generated. The time constant TC1 of the first differentiating circuit 62 is defined by the resistor R21 and the capacitor C21. The first differentiating circuit 62 can also be grasped as a high-pass filter.

The target current controller 64 mainly includes a capacitor C22, a charging resistor R22, and a discharge switch Q12. One end of the capacitor C22 is grounded. The charging resistor R22 applies a voltage V _CNT that defines a normal value V _NORM of the reference voltage V _REF to the capacitor C 22. When discharge switch Q12 is turned off, the voltage _{V C22} of the capacitor C22 is equal to the voltage _{V CNT.} Voltage _{V C22} of the capacitor C22 is by the buffer 66, is applied to the voltage divider circuit R23, R24, the reference voltage _{V REF} is generated.

The output signal _{V A} 'is the first differential circuit 62, is input to the base of the discharge switch Q12 is an NPN type bipolar transistor. Base voltage _{V A} of the transistor Q11 in the first differential circuit 62 is lower than the threshold voltage (above the threshold) _{V B} of the transistor ON / OFF, the output signal _{V A} of the first differentiating circuit 62 'is the base When the threshold V _BE between the emitters is exceeded, the discharge switch Q12 is turned on, the voltage VC22 of the capacitor _C22 becomes zero, that is, the reference voltage V _REF becomes zero. The discharge switch Q12 is a voltage comparison means and has a function of resetting the reference voltage V _REF to zero.

When the base voltage _{V A} of the transistor Q11 exceeds the threshold value _{V B} transistor Q11 is turned off, the discharge switch Q12 is turned off. Then, the capacitor C22 is charged via the resistor R22. At this time, the voltage _{V C22} of the capacitor C22 is gradually increased in the CR time constant TC1. As a result, the above-mentioned soft start can be realized. Since the transistor Q11 is a PNP type bipolar transistor and the input voltage V _IN is supplied to its emitter, it operates with reference to the input voltage V _IN . Therefore, it should be noted that the transistor Q11 turns on when the voltage _VA falls below the threshold voltage _{VA B} and turns off when the voltage _VA exceeds the threshold voltage _VA .

If the response delay of the buffer 66 is large, the transistor Q13 is added. Transistor Q13, the signal _{V A} is turned on exceeds the threshold _V BE (= _{V B),} the voltage divider circuit R23, pulling down directly to zero reference voltage _{V REF} generated at node R24. When the buffer 66 has a high speed, the transistor Q13 may be omitted, and the resistors R23 and R24 may be omitted.

(Modified example of the second embodiment)
(Modification example 2.1)
In the above description, the technique for suppressing the overcurrent in returning from the open state to the normal state has been described, but this technique can also be used for suppressing the overcurrent occurring in the return from the short state to the normal state. In this case, when the protection circuit 60 detects that the output terminal of the step-down converter 20d has returned from the short-circuit state to the normal state, the switching of the step-down converter 20d may be stopped during the stop time τ2. The stop period τ2 may be the same as or different from the stop period τ1.

FIG. 11 is a waveform diagram showing a return from the short state to the normal state. In the short-circuit state, the output voltage V _OUT is fixed near zero. The drive current I _DRV generated by the buck converter 20 is stabilized at the target value I _REF even in the short-circuit state. When the short state is restored to the normal state at time t1, the output voltage V _OUT greatly jumps up. Therefore, the protection circuit 60 may determine that the short circuit state is restored to the normal state when the output voltage V _OUT of the step-down converter 20d rises sharply. The protection circuit 60 may include a second differentiating circuit 62s (for example, shown in FIG. 12) or a low-pass filter instead of the first differentiating circuit 62 described above. The output signal of the second differentiating circuit 62s increases as the slope of the ascending slope of the output voltage V _OUT increases. After that, this output signal returns to 0 with a slope corresponding to the time constant TC2 inside the second differentiating circuit 62s. According to this modification, it is possible to suppress an overcurrent when returning from the short-circuit state.

(Modification example 2.2)
Furthermore, the circuit can be configured to support both the return from the open state and the return from the short state. For example, two protection circuits 60 may be provided, one for opening and one for shorting. Alternatively, in FIG. 8, two systems of the first differentiating circuit 62 for opening and the second differentiating circuit 62s for shorting may be provided to share the target current controller 64.

FIG. 12 is a circuit diagram of the protection circuit 60e according to the modified example 2.2. The protection circuit 60e further includes a capacitor C23 in addition to the protection circuit 60 of FIG. The capacitor C23 forms a second differentiating circuit 62s for short circuit together with the base resistor R12 of the transistor Q12 and the base resistor R13 of the transistor Q13. The second differentiating circuit 62s generates a voltage _{V C1, V C2} corresponding to the inclination of the positive edge of the output voltage _{V OUT.} Transistors Q12, Q13, the output signal _{V C1, V C2} of the second differentiating circuit 62s is turned on exceeds a predetermined value _{V B.}

Output signal _{V C1, V C2} of the second differentiating circuit 62s is increased as the inclination of the ascending slope of the output voltage _{V OUT} increases. Thereafter, the output signal _{V C1, V C2} returns to 0 with a gradient corresponding to the time constant TC2 inside of the second differentiating circuit 62s. The time constant TC2 defines the stop time τ2 at the time of short return.

According to the protection circuit 60e of FIG. 12, overcurrent can be suppressed in both the return from the open state to the normal state and the return from the short state to the normal state. If the first differentiating circuit 62 is omitted from FIG. 12, the overcurrent in returning from the short state to the normal state can be suppressed.

(Use)
Finally, the use of the vehicle lamp 1 will be described. FIG. 13 is a perspective view of a lamp unit (lamp assembly) 500 including the vehicle lamp 1 according to the first or second embodiment. The lamp unit 500 includes a transparent cover 502, a high beam unit 504, a low beam unit 506, and a housing 508. The vehicle lamp 1 described above can be used, for example, in the high beam unit 504. A vehicle lamp 1 may be used for the low beam unit 506 in place of or in addition to the high beam unit 504.

Although the present invention has been described using specific terms and phrases based on the embodiments, the embodiments merely indicate the principles and applications of the present invention, and the embodiments are defined in the claims. Many modifications and arrangement changes are permitted without departing from the ideas of the present invention.

1 ... Vehicle lighting, 2 ... Light source, 4 ... Battery, 6 ... Switch, 10 ... Lighting circuit, 12 ... Connector, 14 ... Resonance circuit, 20 ... Step-down converter, 22 ... Controller, 30 ... Open detection circuit, 32 ... Comparator , 40 ... open detection circuit, 42 ... bipolar transistor, R1 ... first resistance, R2 ... second resistance, R3 ... base resistance, R4 ... gate resistance, 44 ... FET, 46 ... clamp element, 48 ... voltage comparator, 500 ... Lamp unit, 502 ... cover, 504 ... high beam unit, 506 ... low beam unit, 508 ... housing, M1 ... switching transistor, L2 ... output inductor, C1 ... input capacitor, C2 ... output capacitor, 60 ... protection circuit, 62 ... 1 differential circuit, 64 ... target current controller, 66 ... buffer, 70 ... current sense amplifier, 72 ... hysteresis comparator, 74 ... driver, Q12 ... discharge switch, C22 ... capacitor, R22 ... charge resistance.

Claims (6)

A buck converter that supplies a drive current to the light source and feedback-controls the drive current so that it approaches the target current.
An open detection circuit that compares the potential difference between the input voltage and output voltage of the buck converter with a predetermined threshold voltage.
A lighting circuit characterized by being provided with. The lighting circuit according to claim 1, wherein the open detection circuit includes a PNP type bipolar transistor in which an emitter is connected to an input terminal of the buck converter and a base is connected to an output terminal of the buck converter. The lighting circuit according to claim 2, wherein the open detection circuit further includes a first resistor provided between the collector of the bipolar transistor and the ground. The open detection circuit according to claim 1, wherein the open detection circuit includes a P-channel FET (Field Effect Transistor) in which a source is connected to an input terminal of the buck converter and a gate is connected to an output terminal of the buck converter. Lighting circuit. The open detection circuit
A clamp element provided between the gate and source of the P-channel FET and
A second resistor provided between the drain and the ground of the P-channel FET,
The lighting circuit according to claim 4, further comprising. Light source and
The lighting circuit according to any one of claims 1 to 5 for driving the light source.
A vehicle lighting fixture characterized by being equipped with.

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