JP2017034970A - Lightning circuit and lighting fixture for vehicle using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lightning circuit capable of suppressing generation of an overcurrent.SOLUTION: A step-down converter 20d includes an output inductor L2 and supplies a driving current Ito a light source 2 through the output inductor L2, and feedback control is performed so that a driving current Iapproaches a target current I. A protective circuit 60, upon detecting that an output terminal of the step-down converter 20d has returned from an open state to a normal state, stops switching operations of the step-down converter 20d during a stopping period.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a vehicular lamp used in an automobile or the like, and more particularly to its failure detection.

  Conventionally, halogen lamps and HID (High Intensity Discharge) lamps have been mainstream as light sources for vehicle lamps, particularly headlamps, but in recent years, instead of these, semiconductor light sources such as LEDs (light emitting diodes) and laser diodes have been used. Development of vehicular lamps that use lanterns is underway.

A vehicular lamp using a semiconductor light source is required to have a function of detecting an open abnormality caused by open break of a semiconductor light source, disconnection of a harness, disconnection of wiring, and the like and notifying the vehicle side. FIGS. 1A and 1B are circuit diagrams of a vehicular lamp including a lighting circuit having an open abnormality detection function. Note that these are circuits that have been studied in advance by the present inventor and should not be recognized as a known technique. The lighting circuit 10r in 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 step-down 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 a target value I _REF that defines the target light amount of the light source 2.

The open detection circuit 30r in 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 vehicular lamp 1r in FIG. 1A is normal, a normal drive current I _DRV flows through the sense resistor _RS , and a voltage drop V _IS exceeding the threshold voltage V _TH is generated. On the other hand, when the open abnormality occurs, the drive current I _DRV does not flow, so the voltage drop _VIS becomes substantially zero and becomes lower than the threshold voltage _VTH . Therefore, the output signal of the comparator 32r takes a first level (eg, high level) indicating normality when V _IS > V _TH and takes a second level (low level) indicating open abnormality when V _IS <V _TH .

The open detection circuit 30s in FIG. 1B includes resistors R11 and R12 and a comparator 32s. Resistors R11 and R12 divide the output voltage _VOUT of step-down converter 20. The comparator 32s compares the divided output voltage (voltage detection signal) V _VS with the threshold voltage V _TH .

When the vehicular lamp 1s shown in 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 open fault occurs, the driving current _{I DRV} does not flow, the controller of the step-down converter 20, to close the driving current _{I DRV} to the target value _{I REF,} increasing the duty ratio of the switching, thereby the output voltage _{V OUT} 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 takes a first level (eg, high level) indicating normality when V _VS <V _TH , and takes a second level (low level) indicating open abnormality when V _VS > V _TH .

JP 2004-134147 A

  1. As a result of studying the lighting circuits 10r and 10s shown in FIGS. 1A and 1B, the present inventor has recognized the following problems.

In the vehicular lamp 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 luminance 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 I _DRV is weak and the voltage detection signal V _IS is very small, the threshold voltage V _TH must be set very low, and is susceptible to errors.

The light source 2 may be composed of a plurality of LEDs connected in series and several bypass switches provided in parallel with several LEDs. When this light source 2 is used, it is possible to control the lighting and extinguishing of LEDs in parallel with the bypass switch according to the on / off state of the bypass switch. Here, the output voltage _VOUT of the step-down converter 20 is N, where N is the number of lit LEDs.
V _OUT ≒ V _F × N
_Therefore , the output voltage _VOUT varies dynamically according to the number N of lighting. 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. In connection with the open failure, the inventor has recognized the following problems.
Semiconductor light sources are vulnerable to overcurrents, especially in laser diodes. Overcurrents can cause catastrophic optical damage (COD) failure, so it is necessary to prevent currents that exceed the absolute maximum ratings from flowing instantaneously. There is a need for more severe overcurrent protection than other light sources.

Chattering that repeats contact (normal state) and non-contact (open state) may occur between the lighting circuit 10r (or 10s) and the connector contact of 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. Thereafter, when the connector contact returns to the contact state, the excessive charge stored in the output capacitor flows into the light source 2 and an overcurrent is generated.

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

1. One embodiment of the present invention relates to a lighting circuit. The lighting circuit supplies a drive current to the light source, and a buck converter that is feedback-controlled so that the drive current approaches the target current, and an open detection that compares the potential difference between the input voltage and the output voltage of the buck converter with a predetermined threshold voltage A circuit.

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

  The open detection circuit may include a PNP bipolar transistor whose emitter is connected to the input terminal of the step-down converter and whose base is connected to the output terminal of the step-down converter. The on / off state of the bipolar transistor corresponds to the result of abnormality detection, and the voltage comparator is not necessary, so that the cost can be reduced.

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

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

  The open detection circuit may further include a clamp element provided between the gate and the source of the FET. As a result, the gate-source voltage can be kept smaller than the breakdown voltage.

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

2. Another aspect of the present invention also relates to a lighting circuit. The lighting circuit has an output inductor, supplies the drive current to the light source via the output inductor, and the converter is feedback controlled so that the drive current approaches the target current, and the output terminal of the converter returns from the open state to the normal state And a protection circuit that stops switching of the converter during the stop time.
In the open state, the detected value of the drive current becomes zero, so the duty ratio of the converter increases and the output voltage rises. When the normal state is restored, 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 overcurrent is suppressed.
When this resonance current is superimposed on the drive current generated by feedback control, it can become an overcurrent, but by resuming the switching operation of the switching converter when returning from the open state to the normal state, the resonance circuit Since the drive current is generated after the current becomes small, overcurrent can be suppressed.

  The protection circuit may determine that the converter has returned to the normal state when the output voltage of the converter has suddenly decreased. “A sudden drop in output voltage” means that the slope of the output voltage has exceeded a predetermined threshold, that the output voltage change width for a predetermined time has exceeded a predetermined threshold, and that the output voltage has This includes that the time required to change the width is shorter than a predetermined threshold. Thereby, the return from the open state to the normal state can be detected.

  The protection circuit may gradually increase the switching duty ratio of the converter after the stop time has elapsed. As a result, the drive current gradually increases after switching is restarted, so that the 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 protection circuit may stop the switching of the converter during the stop time.
Accordingly, when the return from the short-circuit state to the normal state is delayed, 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 is reduced, so that the overcurrent can be suppressed.

  The protection circuit may determine that the short-circuit state is restored to the normal state when the output voltage of the converter rapidly increases. Thereby, the return from the short state to the normal state can be detected.

  The protection circuit may include a differentiation circuit that receives the output voltage or a high-pass filter. The protection circuit may determine that the output is returned to the normal state when the output signal of the differentiation circuit or the high-pass filter exceeds a predetermined value.

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

  The converter may be a step-down type. 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 vehicular lamp. The vehicular lamp includes a light source and any one of the lighting circuits described above that drives the light source.

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

FIGS. 1A and 1B are circuit diagrams of a vehicular lamp including a lighting circuit having an open abnormality detection function. 1 is a circuit diagram of a vehicular lamp according to a first embodiment. FIG. 3 is an operation waveform diagram of the lighting circuit of FIG. 2. FIGS. 4A and 4B are circuit diagrams illustrating a specific configuration example of the vehicular lamp. FIGS. 5A and 5B are circuit diagrams illustrating another configuration example of the vehicular lamp. It is a circuit diagram of the vehicular lamp which concerns on 2nd Embodiment. FIG. 7 is an operation waveform diagram of the lighting circuit of FIG. 6. FIG. 7 is a specific circuit diagram of the lighting circuit of FIG. 6. It is an operation | movement waveform diagram of the lighting circuit of FIG. It is a circuit diagram which shows the specific structural example of a protection circuit. It is a wave form diagram showing return from a short state to a normal state. FIG. 10 is a circuit diagram of a protection circuit according to Modification 2.2. It is a perspective view of a lamp unit provided with the vehicular lamp concerning an embodiment.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. The embodiments do not limit the invention but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

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 to each other in addition to the case where the member A and the member B are physically directly connected. It includes cases where the connection is indirectly made through other members that do not substantially affect the general connection state, or that do not impair the functions and effects achieved by their combination.
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 their electric It includes cases where the connection is indirectly made through other members that do not substantially affect the general connection state, or that do not impair the functions and effects achieved by their combination.

  Further, in this specification, electrical signals such as voltage signals and current signals, or symbols attached to circuit elements such as resistors and capacitors indicate the respective voltage values, current values, resistance values, and capacitance values as necessary. It shall represent.

  Also, those skilled in the art can replace bipolar transistors, MOSFETs, IGBTs (Insulated Gate Bipolar Transistors), replace P-channel (PNP type) and N-channel (NPN type) transistors, and invert the power supply and ground. It is understood.

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

A voltage V _BAT from the battery 4 is supplied to the lighting circuit 10 via the switch 6. The step-down converter 20 steps down the input voltage _VIN corresponding to the battery voltage V _BAT and supplies the output voltage V _OUT to the light source 2. The step-down 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. Step-down 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 the 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 _VIN and the output voltage _VOUT of the step-down converter 20 with a predetermined threshold voltage _VTH . Then, when ΔV> V _TH 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 is determined to be open abnormality, the abnormality detection signal S1 is determined to be the second level (for example, 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. Prior to time t0, the vehicular lamp 1 is normal, and the drive current I _DRV is stabilized at the target amount I _REF . At this time, the output voltage _VOUT 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 _VOUT rises and eventually reaches the input voltage _VIN . 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 the lighting circuit 10 is used in the vehicular lamp 1 in which the light source 2 is a laser diode, even if the lighting circuit 10 is set to the low luminance mode and the drive current I _DRV is made minute, an open abnormality Can be detected appropriately. Alternatively, when the lighting circuit 10 includes a plurality of LEDs in which the light source 2 is connected in series and is used for a vehicular lamp that is controlled to be turned on and off by a bypass switch, the lighting circuit 10 is open regardless of the dynamic fluctuation of the output voltage _VOUT. Abnormalities can be detected appropriately.

  The present invention is understood as the block diagram and circuit diagram of FIG. 2 or extends to various devices and circuits derived from the above description, and is not limited to a specific configuration. Hereinafter, more specific configuration examples will be described in order not to narrow the scope of the present invention but to help understanding and clarify the essence and circuit operation of the present invention.

  4A and 4B are circuit diagrams illustrating a specific configuration example of the vehicular lamp 1. FIG. The open detection circuit 40a shown in 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 step-down converter 20, and the base thereof is connected to the output terminal of the step-down converter 20 via the base resistor R3. The first resistor R1 is provided between the collector of the bipolar transistor 42 and the ground. Note that the first resistor R1 may be omitted and an output of an open collector may be used. Further, the base resistor R3 may be omitted.

A potential difference ΔV between the input and output of the step-down converter 20 is input between the base and emitter of the bipolar transistor 42. Since the potential difference ΔV is sufficiently large at the normal time, the bipolar transistor 42 is turned on, and the abnormality detection signal S1 is at 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 0.7 V) between the base and emitter of the bipolar transistor 42, the bipolar transistor 42 is turned off, and the abnormality detection signal S1 Becomes low level. That is, ON / OFF of the bipolar transistor 42 corresponds to the presence / absence of abnormality detection. According to the vehicular lamp 1a of FIG. 4A, the voltage comparator is unnecessary, so that the circuit cost can be reduced.

  The open detection circuit 40b in FIG. 4B can be understood as a configuration in which the bipolar transistor 42 in FIG. 4A is replaced with a P-channel FET 44. 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 R 4. The second resistor R2 is provided between the drain of the FET 44 and the ground. The clamp element 46 is provided between the gate and source of the FET 44 and clamps the gate-source voltage 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 at the normal time, the FET 44 is turned on, and the abnormality detection signal S1 is at 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 V _{GS (TH)} (for example, 1.5 V) of the FET, the FET 44 is turned off and the abnormality detection signal S1 becomes a low level. That is, the ON / OFF of the FET 44 corresponds to the presence / absence of an abnormality. According to the vehicular lamp 1b of FIG. 4 (b), the voltage comparator is unnecessary, so that the circuit cost can be reduced.

FIGS. 5A and 5B are circuit diagrams showing a vehicular lamp 1c having another configuration example. In the vehicular lamp 1 c of FIG. 5A, the open detection circuit 40 c is configured using a voltage comparator 48. The voltage comparator 48 may compare the output voltage _VOUT with the voltage obtained by shifting the input voltage _VIN to the low potential side by _VTH . The voltage shift V _TH is introduced by the level shifter 49. FIG. 5B is a circuit diagram illustrating 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 is, R5 × I _C becomes V _TH . According to the vehicular lamp 1c of FIG. 5 (a), since a voltage comparator is used, an accurate voltage comparison is possible in exchange for an increase in cost. In addition, when a comparator circuit including a plurality of voltage comparators is used and there are surplus voltage comparators, the cost does not increase.

(Second Embodiment)
FIG. 6 is a circuit diagram of the vehicular 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. The lighting circuit 10d includes a step-down converter 20d, a controller 22, and a protection circuit 60. Note that the technique described in the second embodiment can be used in combination with the technique described in the first embodiment, and thus is omitted in FIG. 6, but the lighting circuit 10 d includes the above-described open detection. A circuit 40 may further be 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 the switching of the converter 20d during the stop time τ1.

For example, protection circuit 60 may detect the return from the open state to the normal state based on output voltage _VOUT of step-down converter 20d. The protection circuit 60 may gradually increase the switching duty ratio of the converter 20d from zero after the elapse of the stop time τ1 (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. Prior to time t1, connector 12 is in an open state. In the open state, the drive current I _DRV is zero. The controller 22 drives the switching transistor M1 with a large duty ratio in order to bring the zero drive current I _DRV close 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 _VOUT takes a voltage level higher than that in the normal state.

At time t1, the disconnection of the connector 12 is eliminated, and the contact state (that is, the normal state) is restored. As a result, excess electric charge stored in the output capacitor C2 is supplied to the light source 2 via the output inductor L2. The output inductor L2 forms the LC resonance circuit 14 together with the output capacitor C2, and the limited resonance current I _RES flows to the light source 2, so that 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 rises 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 step-down converter 20d based on feedback control and the resonance current I _RES flowing in the resonance circuit 14. The resonance current I _RES flows in a loop formed by the output capacitor C2 and the output inductor L2, and therefore the current detection signal V _IS to the controller 22 does not include the resonance current I _RES . Therefore, if the protection circuit 60 omits the stop time τ1 and immediately restarts the switching operation of the switching converter 20d when returning from the open state to the normal state, the resonance current I _RES is generated by feedback control. The lamp current I _{LAMP that} is superimposed on the current I _DRV and flows to the light source 2 can be an overcurrent.

On the other hand, in the present embodiment, protection circuit 60 resumes the switching operation of switching converter 20d after elapse of stop time τ1 when returning from the open state to the normal state. The stop time τ1 may be determined in consideration of a relaxation time in which the resonance current I _RES is 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 overcurrent can be suppressed.

If soft start control is not performed when the switching operation is resumed after the lapse of the stop time τ1, overcurrent may occur due to resonance of the inductor L1, the output capacitor C2, and the output inductor L2. On the other hand, in the present embodiment, such an overcurrent can be suppressed by gently increasing the output current I _DRV of the converter 20d by soft start.

Next, a specific configuration example of the lighting circuit 10d in FIG. 6 will be described. FIG. 8 is a specific circuit diagram of the lighting circuit 10d of FIG. The controller 22 is a controller for hysteresis control (Bang-Bang control), and includes a current sense amplifier 70, a hysteresis comparator 72, and a driver 74. For example, the detection resistor R _S is inserted on the path of the drive current I _DRV generated by the converter 20d. The current sense amplifier 70 amplifies the voltage drop V _IS generated in the detection resistor _RS . The hysteresis comparator 72 compares the voltage drop V _IS with threshold voltages V _H and V _L that change in binary according to its output, and generates a modulated control pulse. The threshold values V _H and V _L are defined based on the reference voltage V _REF that indicates 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 _VOUT decreases instantaneously. The protection circuit 60 may detect the return of the normal state using this phenomenon. In other words, the protection circuit 60 may determine that the output state is returned from the open state to the normal state when the output voltage _VOUT decreases rapidly.

For example, the protection circuit 60 can include a first differentiation circuit 62 or a low-pass filter. For example, the output signal V _A of the first differentiation circuit 62 increases as the slope of the down slope of the output voltage V _OUT increases. Thereafter, the output signal V _A returns to 0 with a slope corresponding to the time constant TC 1 inside the first differentiating circuit 62. The stop time τ1 is defined by the time constant TC1.

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 V _A of the first differentiation circuit 62. Target current controller 64 specifically, when the output _{V A} of the first differentiating circuit 62 is lower than a predetermined threshold value _{V B,} and sets the reference voltage _{V REF} to the normal value _{V NORM.} The target current controller 64 sets the reference voltage V _REF, that is, the target current I _REF to zero when the output V _A of the first differentiating circuit 62 exceeds the threshold value V _B. Thereby, the switching of the step-down converter 20d is stopped.

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 when the output _VA of the first differentiating circuit 62 falls below the threshold value V _B. 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 when the contact of the connector 12 is restored. When the contact of the connector 12 at time t1 is restored, the output voltage _{V OUT} decreases rapidly, the output _{V A} of the first differential circuit 62 is increased, exceeding a threshold value _{V B.} As a result, the reference voltage V _REF decreases from the normal value V _NORM to zero, and the switching of the step-down 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 increases the reference voltage _VREF . That is, the delay time from time t1 to t2 is the stop period τ1.

FIG. 10 is a circuit diagram illustrating 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, the signal _VA corresponding to the slope of the downward slope of the output voltage _VOUT 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. Charging resistor R22, the voltage _{V CNT} defining the normal value _{V NORM} reference voltage _{V REF,} is applied to the capacitor C22. When discharge switch Q12 is turned off, the voltage _{V C22} of the capacitor C22 is equal to the voltage _{V CNT.} The voltage V _{C22 of the} capacitor C22 is applied to the voltage dividing circuits R23 and R24 by the buffer 66, and the reference voltage _VREF is generated.

The output signal V _A ′ of the first differentiation circuit 62 is input to the base of the discharge switch Q12 that is an NPN bipolar transistor. The base voltage V _A of the transistor Q11 in the first differentiating circuit 62 is lower than the on / off threshold voltage (the above-mentioned threshold value) V _B of the transistor, and the output signal V _A ′ of the first differentiating circuit 62 is When the threshold value V _BE between the emitters is exceeded, the discharge switch Q12 is turned on, the voltage VC22 of the capacitor _C22 is zero, that is, the reference voltage _VREF is zero. The discharge switch Q12 is a voltage comparison unit and has a function of resetting the reference voltage _VREF 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. Thereby, the above-mentioned soft start can be realized. The transistor Q11 is a PNP bipolar transistor, and the input voltage _VIN is supplied to the emitter of the transistor Q11. Therefore, the transistor Q11 operates on the basis of the input voltage _VIN . Therefore, when the voltage V _A falls below the threshold voltage V _B wants transistor Q11 is turned on, be noted that the off exceeds.

When the response delay of the buffer 66 is large, the transistor Q13 is added. The transistor Q13 is turned on when the signal V _A exceeds the threshold value V _BE (= V _B ), and directly pulls down the reference voltage V _REF generated at the nodes of the voltage dividing circuits R23 and R24 to zero. When the buffer 66 is high speed, the transistor Q13 can be omitted, and further, the resistors R23 and R24 may be omitted.

(Modification of the second embodiment)
(Modification 2.1)
In the above description, the technique for suppressing the overcurrent in the return from the open state to the normal state has been described. However, this technique can also be used to suppress the overcurrent that occurs when the short state returns 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 state to the normal state, the protection circuit 60 may stop the switching of the step-down converter 20d 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 the return from the short state to the normal state. In the short state, the output voltage _VOUT is fixed near zero. The drive current I _DRV generated by the step-down converter 20 is stabilized at the target value I _REF even in a short state. When the normal state is restored from the short state at time t1, the output voltage _VOUT jumps greatly. Therefore, the protection circuit 60 may determine that the short-circuit state is restored to the normal state when the output voltage _VOUT of the step-down converter 20d rapidly increases. The protection circuit 60 can include a second differentiation circuit 62s (for example, shown in FIG. 12) or a low-pass filter instead of the first differentiation circuit 62 described above. The output signal of the second differentiating circuit 62s increases as the slope of the upward slope of the output voltage _VOUT increases. Thereafter, the output signal returns to 0 with a slope corresponding to the time constant TC2 inside the second differentiation circuit 62s. According to this modification, it is possible to suppress overcurrent when returning from the short state.

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

FIG. 12 is a circuit diagram of a protection circuit 60e according to Modification 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 shorting 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 voltages V _C1 and V _C2 corresponding to the slope of the positive edge of the output voltage V _OUT . The transistors Q12 and Q13 are turned on when the output signals V _C1 and V _{C2 of} the second differentiating circuit 62s exceed a predetermined value V _B.

The output signals V _C1 and V _C2 of the second differentiating circuit 62s increase as the slope of the up slope of the output voltage _VOUT increases. Thereafter, the output signals V _C1 and V _C2 return to 0 with a slope corresponding to the time constant TC2 inside the second differentiation circuit 62s. This time constant TC2 defines the stop time τ2 at the time of short circuit recovery.

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

(Use)
Finally, the use of the vehicular lamp 1 will be described. FIG. 13 is a perspective view of a lamp unit (lamp assembly) 500 including the vehicular 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 the high beam unit 504, for example. Instead of or in addition to the high beam unit 504, the vehicular lamp 1 may be used for the low beam unit 506.

  Although the present invention has been described using specific terms based on the embodiments, the embodiments only illustrate the principles and applications of the present invention, and the embodiments are defined in the claims. Many variations and modifications of the arrangement are permitted without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 1 ... Vehicle lamp, 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 is an open detection circuit, 42 is a bipolar transistor, R1 is a first resistor, R2 is a second resistor, R3 is a base resistor, R4 is a gate resistor, 44 is a FET, 46 is a clamp element, 48 is a voltage comparator, and 500 is a resistor. 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 ... No. 1 differentiation circuit, 64 ... target current controller, 66 ... buffer, 70 ... current Nsuanpu, 72 ... hysteresis comparator, 74 ... driver, Q12 ... discharge switch, C22 ... capacitor, R22 ... charging resistor.

Claims (17)

A converter having an output inductor, supplying a drive current to the light source via the output inductor, and feedback-controlled so that the drive current approaches a target current;
When detecting that the output terminal of the converter has returned from an open state to a normal state, a protection circuit that stops switching of the converter during a stop time;
A lighting circuit comprising:   The lighting circuit according to claim 1, wherein the protection circuit determines that the open state is restored to the normal state when the output voltage of the converter is drastically decreased.   The said protection circuit stops switching of the said converter during stop time, if it detects that the output terminal of the said converter returned to the said normal state from the short state, The converter of Claim 1 or 2 characterized by the above-mentioned. Lighting circuit. A converter having an output inductor, supplying a drive current to the light source via the output inductor, and feedback-controlled so that the drive current approaches a target current;
When detecting that the output terminal of the converter has returned from a short state to a normal state, a protection circuit that stops switching of the converter during a stop time;
A lighting circuit comprising:   5. The lighting circuit according to claim 3, wherein the protection circuit determines that the short state is restored to the normal state when the output voltage of the converter is rapidly increased.   6. The lighting circuit according to claim 1, wherein the protection circuit gradually increases or decreases the switching duty ratio of the converter after the stop time has elapsed. The protection circuit is
Including a differentiating circuit or a high-pass filter that receives the output voltage of the converter,
The lighting circuit according to claim 1, wherein when the output signal of the differentiating circuit or the high-pass filter exceeds a predetermined value, it is determined that the normal state is restored. The protection circuit is
A capacitor with one end grounded;
A charging resistor connected to the other end of the capacitor and applying a target voltage defining the target current in the normal state;
A discharge switch provided in parallel with the capacitor;
Including
The lighting circuit according to any one of claims 1 to 7, wherein when the return to the normal state is detected, the discharge switch is turned on. A converter having an output inductor, supplying a drive current to the light source via the output inductor, and feedback-controlled so that the drive current approaches a target current;
A protection circuit that stops switching of the converter during a stop time when the output voltage of the converter changes suddenly;
A lighting circuit comprising: The converter is a step-down type,
The lighting circuit according to claim 1, further comprising an open detection circuit that compares a potential difference between an input voltage and an output voltage of the converter with a predetermined threshold voltage.   11. The lighting circuit according to claim 10, wherein the open detection circuit includes a PNP-type bipolar transistor having an emitter connected to the input terminal of the converter and a base connected to the output terminal of the converter. A step-down converter that supplies a drive current to a light source and is feedback-controlled so that the drive current approaches a target current;
An open detection circuit for comparing a potential difference between an input voltage and an output voltage of the step-down converter with a predetermined threshold voltage;
A lighting circuit comprising:   The lighting circuit according to claim 12, wherein the open detection circuit includes a PNP-type bipolar transistor having an emitter connected to an input terminal of the step-down converter and a base connected to an output terminal of the step-down converter.   The lighting circuit according to claim 13, wherein the open detection circuit further includes a first resistor provided between a collector of the bipolar transistor and a ground.   The open detection circuit includes a P-channel FET (Field Effect Transistor) having a source connected to an input terminal of the step-down converter and a gate connected to an output terminal of the step-down converter. Lighting circuit. The open detection circuit includes:
A clamp element provided between the gate and source of the P-channel FET;
A second resistor provided between the drain of the P-channel FET and ground;
The lighting circuit according to claim 15, further comprising: A light source;
The lighting circuit according to any one of claims 1 to 16, which drives the light source;
A vehicular lamp characterized by comprising:

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